Introduction Mendis

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1.0 INTRODUCTION

1.1 High-strength Concrete (HSC) and High-Performance Concrete (HPC) -

Background

The advancement of material technology and production methods has led to the development

of high performance concrete (HPC) and particularly higher grades of concrete strengths. The

use of high-strength concrete (HSC) elements for concrete structures has proven most

popular, with economy, superior strength, stiffness and durability being the major reasons for

its popularity. With future advances in concrete technology, the definition of high-strength

concrete will undoubtedly change, however for the purposes of this seminar series HSC is

considered to be concrete with a 28 day characteristic compressive strength of at least 50

MPa. Without any doubt, HSC differs in many respects from normal or ordinary strength

concrete (NSC). NSC is concrete with f’c < 50 MPa. It must be noted that with the improved

quality control and the quality of materials, concrete up to 65 MPa can be considered as

normal strength concrete. New applications of HSC have been introduced in last 2 decades.

This trend will continue in the future and many other new applications will be developed in the

1950s 35 MPa was considered high strength, and in the 1960s compressive strengths of up to

50 MPa were being used commercially. In Australia HSC (up to 60 MPa) was introduced to

high-rise buildings in 1980s. Presently, concretes with strengths up to 130 -140 MPa are used

in projects around the world. These concretes can be produced using conventional production

procedures. ACI363 (2010) committee has changed the definition of high-strength concrete

from 41 MPa to 55 MPa.

Quality control is an essential part of the production of high-strength concrete; hence full

collaboration between the material and ready-mixed concrete suppliers, engineers and

builders is required for the effective application of this valuable product. High-strength

concrete will normally consist of not only Portland cement, aggregates and water, but also

superplasticisers and supplementary cementitious material.

A high-strength concrete is always a high-performance concrete, but a high-performance

concrete is not always a high-strength concrete.

Pre-Conference Workshop on “High Performance Concrete Design and Applications”� January 2012

2nd International Conference on Sustainable Built Environment (ICSBE) 201214th, 15th, 16th December 2012Kandy, Sri Lanka

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High Performance Concrete

A high-performance concrete is something which demands much higher performance from

concrete as compared to performance expected from conventional concrete. The American

Concrete Institute (ACI) defines high-performance concrete as concrete meeting special

combinations of performance and uniformity requirements that cannot always be achieved

routinely when using conventional constituents and normal mixing, placing and curing

practices. Some promising technologies have recently been developed that should make HPC

a more attractive material to owners, architects, engineers and contractors from both an

economical and a sustainable perspective (Aitcin, 2011).

My definition

A high-performance concrete provides much higher performance from concrete as compared

to performance expected from conventional concrete in improving both safety and

sustainability of structures.

High-performance concrete will provide superior performance in characteristics that may be

considered critical for a particular application. However the performance should be tested not

only in normal conditions but severe environmental and other loading conditions. Durability

under severe environmental conditions is also should be a mandatory requirement for HPC.

HPC is designed to provide several benefits in the construction of concrete structures as

listed below:

Ease of placement

Compaction without segregation

Early age strength

Long-term strength and mechanical properties

Permeability

Density

Heat of hydration

Toughness

Volume stability

Long life in severe environments



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Modulus of Elasticity

Shrinkage

Creep

Abrasion Resistance

Chloride Permeability

Cost Benefits

Sustainability and lower life-cycle costs

High-performance concretes are also more sensitive to changes in constituent material

properties than conventional concretes. This means that a greater degree of quality control is

required for the successful production of high-performance concrete.

How is HPC obtained in practice?

By careful selection of raw materials (including chemical/mineral admixtures) & appropriate

mix design to achieve the desired performance objectives. Most high-performance concretes

have a high cementitious content and a water-cementitious material ratio of 0.40 or less

(http://www.concretebasics.com).

How is HSC obtained in practice?

Manufacture of high-strength concrete involves making optimal use of the basic ingredients

that constitute normal-strength concrete. Producers of high-strength concrete know what

factors affect compressive strength and know how to manipulate those factors to achieve the

required strength. In addition to selecting a high-quality portland cement, producers optimize

aggregates, then optimize the combination of materials by varying the proportions of cement,

water, aggregates, and admixtures. When selecting aggregates for high-strength concrete,

producers consider the strength of the aggregate, the optimum size of the aggregate, the

bond between the cement paste and the aggregate, and the surface characteristics of the

aggregate. Any of these properties could limit the ultimate strength of high-strength concrete.

Admixtures

Pozzolanic admixtures, such as fly ash, ground granulated iron blast furnace slag and silica

fume, are the most commonly used supplementary cementitious materials (SCMs) in high-



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strength concrete. These materials impart additional strength to the concrete by reacting with

portland cement hydration products to create additional C-S-H gel, the part of the paste

responsible for concrete strength.

It would be difficult to produce high-strength concrete mixtures without using chemical

admixtures. A common practice is to use a superplasticiser in combination with a water-

reducing retarder or accelerator. The superplasticiser gives the concrete adequate workability

at low water-cement ratios, leading to concrete with greater strength. The water-reducing

retarder slows the hydration of the cement and allows workers more time to place the

concrete. More details are given later.

1.2 Advances in Concrete Technology

Numerous advances in all areas of concrete technology including materials, mixture

proportioning, recycling, structural design, durability requirements, testing and specifications

have been made. Innovative contracting mechanisms have been considered, explored and

tried. Some progress has been made in utilizing some of these technology innovations in

routine practice.

The following sections describe some of the innovations.

1.2.1 Concrete materials

The development of chemical admixtures has revolutionized concrete technology in the last

fifty years. The use of air entraining admixtures, accelerators, retarders, water reducers and

corrosion inhibitors are commonly used for bridges and pavements. The use of self-

consolidating concrete is becoming popular (mostly used for precast elements and mass

concrete). Supplementary cementitious materials e.g. fly ash, ground granulated blast furnace

slag (GGBFS) and amorphous silica (especially silica fume) are routinely used.

1.2.2 Use of recycled materials in concrete

Recycled concrete aggregate, slag aggregates and manufactured sand are being used where

appropriate. As the useable sources for natural aggregates for concrete are depleted



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utilization of these products will increase. Utilization of fly ash and GGBFS in concrete

addresses this issue in addition to improving concrete properties. The replacement of

Portland cement by fly ash or GGBFS reduces the volumes of cement utilized which is a

major benefit since the cement manufacture is a significant source of carbon dioxide

emissions worldwide (Vanikar, 2004). Silica fume is a comparatively expensive product and it

is added in smaller quantities in concrete mixture rather than as a cement replacement,

usually for sprayed concrete or high strength-high durability applications.

1.2.3 Concrete mixture proportioning

Continuous gradation and consideration of workability during laboratory testing are slowly

gaining acceptance in practice. The utilization of laboratory as well as full scale trial batches

are used on major projects.

1.2.4 Concrete durability properties

Concrete durability requirements are specified on some project specifications now

(particularly for most major bridge and pavement projects). Typically the requirements are

based on “Rapid Chloride Permeability” tests or other test methods. There are many guides

available on durability including those published by the Concrete Institute of Australia and the

American Concrete Institute. The lack of better laboratory and field tests has hindered

progress in this area.

1.2.5 Concrete tests

The utilization of advanced test procedures e.g. various shrinkage tests, air-void analyzer and

non-destructive tests have become widespread. The non-destructive tests including maturity

test are gaining wider acceptability. New types of Workability test are being evaluated by

several organizations. Koheler and Fowler (2003) identified 61 different types of workability

tests. They have described the key principles and trends in the measurement of workability.

Based on the successes and failures of past test methods and the current needs of the

concrete industry, requirements for new test methods were proposed.



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1.2.6 Concrete construction control

In-situ concrete testing, effective curing practices and utilization of computer software to

monitor concrete strength development as well as minimizing cracking potential are used on

major projects.

1.2.7 Specifications

Performance related specifications rather than prescriptive specifications for concrete are

becoming popular (Hootan et al.,2011). The use of incentive/disincentive clauses in

specifications tends to improve concrete quality.

1.3 Engineering Properties

1.3.1 Compressive Strength

Enhanced compressive strength is the most important of HSC’s functional properties.

Admixtures such as silica fume or fly ash are not essential to the manufacture of high-strength

concrete; however, the incorporation of these mineral admixtures, particularly silica fume does

facilitate the process. The main reason for the spectacular increase in concrete strength in

silica fume concrete is the creation of a dense concrete matrix enabled by the uniformly

distributed fine silica fume particles in between larger cement particles. The use of

superplasticisers and good compaction by vibration aids in the densification process lead to

the higher strength. The microstructure of high-strength concrete is very dense and

amorphous and contains very little free water. It has a very low-porosity and lacks the

accumulation of lime crystals, as in the case of NSC.

Test methods to determine the characteristic compressive strength of NSC are applicable to

high-strength concrete cylinders. The only exception being that in some circumstances the

limitations in the capacity of testing machines or other facilities may necessitate the use of

smaller cylinders of size 100(D) x 200(H). In November 2005, ACI Subcommittee 318-A

voted to allow the use of 4 x 8 in. (100 x 200 mm) cylinders (commonly known as 4 x 8s) for

acceptance testing of concrete. Section 5.6.2.4 of ACI is 318 as follows:



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A strength test shall be the average of the strengths of at least two 6 by 12 in. [150 by

300 mm] cylinders or at least three 4 by 8 in. [100 by 200 mm] cylinders made from

the same sample of concrete and tested at 28 days or at test age designated for

determination of fc'.

The reason behind requiring at least three 4 x 8s (100x200 mm) instead of two 6 x 12 in. (150

x 300 mm) cylinders (commonly known as 6 x 12s) is explained in the Commentary: Testing

three 4 by 8 in. [100 by 200 mm] cylinders preserves the confidence level of the average

strength because 4 by 8 in. [100 by 200 mm] cylinders tend to have approximately 20 percent

higher within-test variability than 6 by 12 in. [150 by 300 mm] cylinders.

A discussion about Compressive Strength Testing of Concrete with Different Cylinder

Specimen Sizes is given by Taghddos et al. (2010).

The important factors that affect the strength of HSC are:

1.3.2 Curing

Whilst there are conflicting reports on the relative importance of curing on strength gain of

HSC cylinders compared with NSC cylinders, curing procedures similar to those followed for

NSC members are sufficient and should be adopted. However, the possibility of self-

desiccation and high early shrinkage of HSC requires continuous moist curing for a longer

period.

De Larrad (1988) reports that self-desiccation is probable in HSC and hence specimens cured

in water will absorb water, thus increasing the strength of the concrete. An opposite view is

expressed by some who argue that water evaporation from a NSC cylinder is greater than

that from a HSC cylinder. Therefore, the strength development of a NSC cylinder will be more

affected by deficient curing than the strength development of a HSC cylinder.

Studies by Aitcin (1998) on curing of HSC show that HSC members have a delayed response

to strength gain. Aitcin suggests that due to the low permeability of high-strength concrete it

takes considerable time for water to penetrate the concrete and contribute to the hydration

process, hence longer periods of moist curing of HSC specimens is recommended.



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Carino and Meeks (2001) summarized the results of an exploratory study on the effects of

curing duration on the variation of mortar strength with distance from the drying surface of

high-performance concrete specimens. The data tended to show that 1 day of moist curing

might be sufficient to ensure adequate strength development at a depth of 25 mm from the

exposed surface.

1.3.3 Water/binder (w/b) ratio and cement content

HSC usually contains one or two mineral additives which are used as partial replacement for

cement. Therefore, the term water/cement (w/c) ratio used in reference to NSC is replaced by

w/b ratio, where the binder is the total weight of the cementitious materials (cement +

additives). HSC’s have been made with w/b ratios as low as 0.2. However, high dosages of

superplasticisers are required to maintain workability.

Silica fume is a pozzolan that is commonly used in HSC but others such as fly ash and slag

can also be used. Typically, such high performance concrete mixtures results in lower

porosity, permeability and bleeding because resulting from higher binder contents when

compared with normal strength (Normal Class) concretes making the paste structure denser.

The use of ultra-fine mineral admixtures like silica fumes, generate the higher strength due to

the following two actions:

a) Filler effect - the fine grains fill up the larger voids in between the cement particles,

thus reducing the water demand.

b) Pozzolanic effect - The siliceous particles react with lime released by the cement in

the presence of water, to generate products of hydration.

Importantly, the above can also be achieved by other binder systems and achieving a low w/b

ratio at an appropriate workability is key to having a usable HPC system in the field.

1.3.4 Influence of Mineral Admixtures on Compressive Strength

As mentioned earlier, the incorporation of mineral admixtures such as silica fume, fly ash or

slag facilitates the manufacture of high-strength concrete. However, the incorporation of

these mineral admixtures is not mandatory.



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Silica fume is a by-product of electric arc furnaces. These are used to manufacture ferro-

silicon and silicon metal alloys. Silica fume contains mainly non-crystalline silica in the form of

very fine particles (0.1 microns average diameter). Due to its finess as compared to other

mineral admixtures, silica fume is by far the most effective mineral admixture that can be used

to manufacture HSC as compared with other commercially available mineral admixtures. As

described later, it is essential to produce very high-strength concrete. AS3582.3 “Condensed

Silica Fume” (CSF) has been renamed “Amorphous Silica” in 2002 version, thus recognizing

that there are other Amorphous Silica materials other than silica fume that satisfy all the

chemical and physical requirements in that standard.

Fly ash is a by-product of the combustion of pulverised coal in thermal power plants. It is

removed as a fine dust by mechanical extractors, electrostatic precipitators or fabric filters. Fly

ash can be included into concrete either blended with cement or directly introduced as an

additional cementitious material at the concrete mixing plant. Typical applications are in

pumped or in superplasticized concretes, particularly where heat of hydration is considered to

be a problem. AS3582.1-1998 (with an amendment in 1999) covers fly ash. Ultrafine Fly Ash

(Special Grade) is a potential substitute for Silica Fume to produce HSC.

Slags are by-products of the metallurgical industry. Slags that are most commonly used in

concretes for building and construction applications usually originate from iron blast furnace

facilities. These slags are glassy (amorphous) materials and are obtained by melt

waterquenching. In fresh concrete, slags tend to improve the workability of the concrete due

to their angular shape and smooth surface texture. However, their relatively slow reaction

rates [when compared to ordinary Portland cement (OPC)] cause increases in setting time

and bleeding tendencies. Slag cement concretes generally produce less heat of hydration

than do equivalent OPC concretes. Reductions in temperature rise and the associated

likelihood of thermal cracking can be reduced with slag inclusion into concrete, but such

reductions only become significant at higher slag replacement levels. AS3582.2-2001 covers

Slag—Ground granulated iron blast-furnace.

1.3.5 Superplaticizers

Superplasticisers, also known as high range water reducers, are chemicals used as

admixtures where well-dispersed particle suspension is required. These polymers are used as

dispersants to avoid particle aggregation, and to improve the flow characteristics (rheology) of

suspensions such as in concrete applications. Their addition to concrete or mortar allows the

reduction of the water to cement ratio, not affecting the workability of the mixture, and enables



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the production of self-consolidating concrete and high performance concrete. This effect

drastically improves the performance of the hardening fresh paste. Indeed the strength of

concrete increase whenever the amount of water used for the mix decreases. However, their

working mechanisms lack of a full understanding, revealing in certain cases cement-

superplasticiser incompatibilities.

Superplasticisers are essential to produce good workable high-strength concrete. Adding 1-

2% superplasticiser per unit weight of cement is usually sufficient. However, note that most

commercially available superplasticisers come dissolved in water, so the extra water added

has to be accounted for in mix proportioning. Adding an excessive amount of superplasticiser

will result in excessive segregation of concrete and is not advisable. Some studies also show

that too much superplasticiser will result in a retarding effect.

Plasticizers are commonly manufactured from lignosulfonates, a by-product from the paper

industry. High Range Superplasticiser have generally been manufactured from sulfonated

naphthalene condensate or sulfonated melamine formaldehyde, although new-generation

products based on polycarboxylic ethers are now available. Traditional lignosulfonate-based

plasticisers, naphthalene and melamine sulfonate-based superplasticisers disperse the

flocculated cement particles through a mechanism of electrostatic repulsion. In normal

plasticizers, the active substances are adsorbed on to the cement particles, giving them a

negative charge, which leads to repulsion between particles. Lignin, naphthalene and

melamine sulfonate superplasticisers are organic polymers. The long molecules wrap

themselves around the cement particles, giving them a highly negative charge so that they

repel each other.

Polycarboxylate ethers (PCE) or just polycarboxylate (PC), the new generation of

superplasticisers, are not only chemically different from the older sulfonated melamine and

naphthalene-based products, but their action mechanism is also different, giving cement

dispersion by steric stabilisation, instead of electrostatic repulsion. This form of dispersion is

more powerful in its effect and gives improved workability retention to the cementitious mix.

Furthermore, the chemical structure of PCE allows for a greater degree of chemical

modification than the older-generation products, offering a range of performance parameters

that can be tailored to meet specific needs. The amount of superplasticiser to be added to a

mix is governed by the required workability. With a relatively low dosage (0.5–1.5% by cement

weight) they allow a water reduction up to 40%, due to their chemical structure which enables

good particle dispersion. Some commercial examples are GLENIUM 27 and RHEOBUILD

1000 MSDS.



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Superplasticisers are covered in AS 1478.1-2000 Chemical admixtures for concrete, mortar

and grout - Admixtures for concrete and AS 1478.2-2005 Chemical admixtures for concrete,

mortar and grout - Methods of sampling and testing admixtures for concrete, mortar and

grout.

1.3.6 Coarse Aggregates

As compared to NSC, the mineralogy and the crushing strength of coarse aggregates has a

significant effect on the strength of a HSC mix. For an optimum compressive strength with a

high cement content and low water/cement ratio, the maximum aggregate size should be kept

to a minimum of about 10 mm (Donnaes, 2000). According to Setunge (1994), higher

strength aggregates do not necessarily produce higher-strength concretes. A more desirable

property is the compatibility of the stiffness of the aggregates and the mortar. The ideal

material will be crushed rock with low stiffness and high strength. It is generally observed that

smaller aggregates are desirable to produce high strength concrete due to:

a) reduction in the water accumulating near the coarse aggregates

b) larger available surface area for bonding with cement matrix

For commercial applications, taking into account the economy of production, workability and

shrinkage and creep, well graded aggregates of 14-20mm size are recommended.

In another experimental study, Mak and Sanjayan (1990) report that:

(a) the compressive strength of HSC is dependent on the type of aggregate used.

(b) gap graded aggregates yield higher strength but lower workability

(c) a higher percentage of sand increases the workability of the HSC mix but reduces the

compressive strength and

(d) 25% increase in strength was observed with a 8% replacement of cement by silica

fume.

Aitcin (1992), for the purposes of discussion divided high-strength concrete and very high-

strength concrete into five categories and discussed the relative importance of various factors

on the strength of concrete.



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Category I - 50 -75 MPa - This grade of concrete can be manufactured using good quality but

generally used materials, existing production technology and a w/b ratio of about 0.40. There

is no particular need to incorporate mineral admixtures to attain this grade of concrete.

Superplasticisers may be used to achieve the required workability at the low w/c ratio and

also for slump recovery at the job site.

Category II - 75-100 MPa - To achieve this strength, high quality, generally used materials are

required. However, due to the very low w/b ratio of about 0.25 - 0.30, superplasticisers are

required to achieve adequate workability. The use of mineral admixtures is also strongly

recommended. The coarse aggregates have a significant influence on the resulting strength

of the concrete and they must be round or cubic in shape.

Category III - 100-125MPa - Very high quality materials, efficient mixing techniques and

stringent quality control are needed. The w/b ratio must be lowered to 0.22 - 0.25. If high

quality materials are not locally available they must be imported. High dosages of

superplasticisers are essential in conjunction with silica fume.

The other two categories refer to 125 MPa and beyond and will not be discussed here as they

are not commonly used, difficult to achieve in the field and beyond the scope of this seminar

series.

Manufacturers of high-strength concrete need to have in place a quality assurance

programme which controls the process from contract review through to onsite delivery (Munn,

1994). Some typical commercial mix proportions taken from Shah et. al. (1994) and Malier et.

al. (1992) are given in Table 1.1 A. A typical high-strength concrete mix used in Australia is

given in Table 1.1 B. This mix was used for the 53 storey “Century Tower” in Sydney. As seen

the concrete was required to have an elastic modulus of 45 GPa at 90 days in addition to the

high strength.

Aggregates are covered in AS 2758.1-1998: Aggregates and rock for engineering purposes -

Concrete aggregates.



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Table 1.1 A Examples of commercially produced high strength concrete mixes

Mix Number 1 2 3 4 5 6 7 8 9 10

water (kg/m3) 170 195 158 165 145 160 135 151 144 130

cement (kg/m3) 425 505 564 451 315 475 500 475 564 513

fly ash (kg/m3) - 60 - - - 59 - 104 - -

slag (kg/m3) - - - - 137 - - - - -

silica fume (kg/m3) 30 - - - 36 24 30 74 89 43

coarse aggregate

(kg/m3) 1033 1030 1068 1030 1130 1068 1110 1068 1068 1080

fine aggregate 705 630 647 745 745 659 700 593 593 685

water reducer L/m3 4 0.98 - - 0.9 - - - - -

retarder L/m3 1.0 - 1.12 4.5 1.8 1.04 4.5 1.51 1.47 -

superplasticizer

L/m3 3 - 11.61 11.25 5.9 11.61 14 16.45 20.12 15.7

w/cementious ratio 0.4 0.35 0.28 0.37 0.31 0.29 0.27 0.23 0.22 0.25

f’c 28-day (MPa) -

moist cured 59 65 78.6 80 83 88.5 42.5 107 118.9 119

f’c 91-day (MPa) -

moist cured 66 79 86.5 87 93 100.4 106.5 119.3 131.8 145

1. La Grande Arche, Paris 6. HSC mixture 2 in the Chicago area

2. Water Tower Place (1975) 7. Scotia Plaza, Toronto (1987)

3. HSC mixture 1 in the Chicago area 8. HSC mixture 3 in the Chicago area

4. Joigny Bridge, France (1989) 9. HSC mixture 4 in the Chicago area

5. La Laurentienne Building, Montreal (1984) 10.Two Union Square, Seattle (1988)



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Table 1.1 B Specification Requirements for Concrete and Achieved Field Performance for Century Tower

Sydney

Mix Constituent Lower Columns and Core Level 9 Transfer Slab

or Property Specification

Requirement

Achieved Field

Performance

Specification

Requirement

Achieved Field

Performance

Strength Data (f’c) 80 MPa 105 MPa 60 MPa 72 MPa

90 Day Ec (GPa) 45 48 - -

56 Day Drying

Shrinkage (�strain)

500 650 500 - 530

Creep (AS 1012) 15 /MPa @ 3

months

- -

Temperature Control - - 10C reduction

in concrete

temp

Liq. nitrogen cooled

Target - 14 2C

at time of discharge

Total binder - 640 kg/m3 - 440 kg/m3

Binder Type - SL cement ,

Micropoz & fly

ash

- SL cement,

Micropoz & fly ash

Aggregate inclusion 10 mm max

Basaltic

aggregates*

10 mm max

Penrith Crushed

River Gravels

Penrith Crushed

River Gravels

Admixture inclusion Use of

superplasticisers

Water

reducer/retarder

and

superplasticiser

Water red./retarder

and

superplasticiser

Max. W:B - 0.25 to 0.28 -

Initial slump (mm) - < 30 - 60

Final slump (mm) - 120-180

(depending on

element)

- 120

*This table was taken from the document “A guide for Designers and Specifiers –CSR Constrcution

Materials, NSW Project Case Study 1”



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1.3.7 Tensile strength

The tensile strength of HSC is significantly greater than that of NSC, though to a lesser extent

than the compressive strength. Fracture surfaces are smooth, indicating the homogeneity of

the material. The densification of the concrete matrix and the aggregate-matrix transition zone

explains the improvement of the tensile strength. Tensile strength of concrete is highly

dependent on the specimen conditioning and the method of test. The tensile strength is

determined either by direct tensile tests or by indirect tensile tests such as flexural, also

known as modulus of rupture, and split cylinder tests. The direct and indirect methods for

evaluating the tensile strength in accordance with the appropriate parts of AS1012- Methods

of Testing Concrete are adequate for the tensile testing of HSC specimens.

AS3600-2009 recommends a lower and upper bound values of the characteristic flexural (f′cf)

and uniaxial tensile (f′ct) strength of concrete in the absence of more accurate data. The lower

characteristic flexural tensile strength and the lower characteristic uniaxial tensile strength of

concrete are given by Eq. (1.1) and Eq. (1.2) respectively.

f fcf c' . ' 0 6 (1.1)

cct ff '36.0' (1.2)

The mean and upper characteristic values can be estimated by multiplying these lower bound

values by 1.4 and 1.8 respectively.

1.3.8 Modulus of Elasticity

The modulus of elasticity (Ec) of HSC is dependent on parameters such as the volume of

aggregates, the modulus of the paste and the modulus of the aggregates. Complex equations

incorporating these factors and as suggested by various researchers are reviewed by

Setunge (1994). These equations are not easily or readily applied and are therefore not

suitable for a design office. Previous versions of AS3600 suggested a simplified, empirical

formula to predict the elastic modulus.

Setunge (1993) has shown that the existing AS3600-2001 formula of:

Ecm = 0.0431.5cmf ' 20% (1.3)

has the tendency to overestimate the elastic modulus of HSC.



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AS3600-2009 code recommends Eq. (1.3) to estimate the mean modulus of elasticity of

concrete up to 40 MPa. However, for concrete strength greater than 40 MPa, the code

recommends Eq. (1.4) to predict the elastic modulus of concrete.

Ecm = 1.5 (0.024 cmf ' + 0.12) 20% (1.4)

Mendis (2001) proposed that the following expression is appropriate to predict the elastic

modulus of all grades of NSC and HSC concretes with various types of aggregates:

Ec = 0.431.5 c'f 20% (1.5)

where, = 1.1-0.002f’c 1.0

The term has the value of 1 for NSC and is less than 1 for HSC. This formula was derived

by calibrating experimental results and comparing them with the widely used Carrasquillo et

al. (1981) formula given in Eq. (1.6).

2320

6900332051.

ccc 'fE

(1.6)

Results from standard elastic modulus tests on samples taken from site delivered concrete to

World Tower in Sydney and Q1 Tower on the Gold Coast are shown in Figure below. The

results indicate that both the Gilbert (2002) and Mendis (2001) equations perform

satisfactorily at different levels. Equation (1.4) gives good elastic modulus prediction of World

Tower data particularly for concretes greater than 65 MPa, but seems to under-predict Q1

data between 60 and 90 MPa. Equation (1.5) gives a good average elastic modulus prediction

of Q1 data for concrete strengths great than 60 MPa, but seems to overestimate World Tower

data. It is clear that the previous AS3600 equation over-predicts and therefore a justification of

modification for higher strengths is obvious. Importantly, further analysis of the data shown in

Fig 1.1 has shown an age effect on modulus versus strength where a higher rate of stiffening

occurs after 28 days when compared with corresponding increases in compressive strength.



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Fig 1.1 Elastic Modulus results of World Tower and Q1 concretes

1.3.9 Shrinkage and Creep in HSC

Shrinkage

Shrinkage in concrete comprises three distinct components viz. Plastic shrinkage,

Autogenous shrinkage and Drying shrinkage. Most codes and procedures do not differentiate

between the three types of shrinkage. The empirical procedures laid out in most codes

determine the total shrinkage at the end of a stipulated time or design period.

For HSC, which is expected to be produced with low w/b ratios and better aggregates, the

total shrinkage should decrease in comparison with NSC specimens. Because of the high

paste content in HSC, autogenous shrinkage is expected to be higher, and in some cases

doubled (De Lallard et. al., 1994). However, since the specimens contain very little water, the

drying shrinkage will be significantly reduced. All experimental results point to the fact that the

total shrinkage values for HSC specimens are less than those obtained for NSC specimens.

Although several potential benefits such as improved surface abrasion resistance, reduced

chloride penetrability, and improved resistance to freezing and thawing damage are

associated with the use of HSC, these mixtures may exhibit increased sensitivity to early-age

10,000

20,000

30,000

40,000

50,000

60,000

30 50 70 90 110 130 150

fcm (MPa)

Ec

(MP

a)

World Tower

Q1

AS3600

Mendis (2001)

Gilbert (2002)



18

shrinkage cracking. Inclusion of silica fume at high replacement levels significantly increases

the autogenous shrinkage of HSC due to refinement of pore size distribution that leads to a

further increase in capillary tension and more contraction of the cement paste, whereas

decreases its drying shrinkage Sato et al. (1999) found that at early age the autogenous

shrinkage strains develop more rapidly in concrete without any admixtures, and in concrete

containing silica fume, than in concrete containing blast-furnace slag. One of the ways to

minimize autogenous shrinkage of high-strength concrete is to add not more than 10% silica

fume to the mix. Also shrinkage-reducing admixture (SRA) may be used to decrease the

potential for early-age shrinkage cracking in HSC while sustaining the advantageous

mechanical and durability properties associated with HSC.

Autogenous shrinkages occur with the loss of water used in hydration of cementitious

materials. This shrinkage occurs in the short term. Since the autogenous shrinkage is higher

for HSC, the early shrinkage in HSC is higher, and this aspect should be carefully considered

by the structural engineer. In particular, this behaviour is important in the design of

indeterminate structures consisting of many restraints at supports and also has implications

for prestress losses in prestressed concrete design. If adequate precautions are not taken,

shrinkage cracking in the structure is liable to occur. The structural engineer should consider

incorporating the support restraints after the high early shrinkage has occurred. Loukili et al.

(1999) believe that autogenous shrinkage in very high-strength concrete stops after 10 days.

Swamy (1986) reported that for HSC 20 - 50% of the total 700 day shrinkage occurred in the

first 7 days. De Larrard and LeRoy (1992) have carried out measurements of autogeneous

shrinkage of HSC mixes and proposed a relationship between the composition of the mix and

the autogeneous shrinkage. Some results are presented in Table 1.2. As can be seen, HSC

specimens show high early shrinkage and a lower final shrinkage.

Table 1.2 Comparative shrinkages in NSC and HSC test specimens [23]

Mixes No. 1 2 3 4 5 6

Cement, kg/m3 350 450 456 453 453 421

Silica Fume, kg/m3 - - 36 36 36 42

Super-plasticizer, l/m3 - 4.5 7 6.6 3.6 7.9

Retarder, l/m3 - 0.9 0.5 0.5 0.5 -

Water, kg/m3 195 168 151 175 188 112

W/C ratio 0.56 0.37 0.33 0.39 0.42 0.27



19

f’c (MPa) 40 78 94 83 74 101

Autogeneous Shrinkage

(at 1 yr)x10-6

90 90 290 200 140 150

Drying Shrinkage (at 1 yr)x10-6 290 90 120 190 260 110

Total Shrinkage (at 1 yr)x10-6 380 180 410 390 400 260

Investigations by Yue and Taerwe (1993) indicate that the lower limit of 600x10-6 is a

reasonable estimate of the basic shrinkage strain of HSC. However, caution must be

exercised in using the basic shrinkage strain values in design of structural elements which are

sensitive to differential settlement (like columns in tall buildings). Every HSC mix has different

characteristics and if the project is sufficiently important and sensitive to shrinkage,

appropriate tests and monitoring should be carried out to determine the actual shrinkage

values.

Gilbert (1998) has suggested basic shrinkage versus concrete strength relationships for

normal and high-strength concrete. The AS3600-2001 procedure assumed a tolerance of

40% for the basic shrinkage strain, cs.b, of 850x10-6. AS 3600-2009 modifies the existing rules

of design shrinkage strain for HSC. The newly proposed rules consider the design shrinkage

strain cs, to be the sum of autogenous shrinkage cse and drying shrinkage csd. The

autogenous shrinkage is taken as Eq. (1.7)

e-1.0 -0.1t csecse (1.7)

Where t is the time (in days) after setting and cse is the final autogenous shrinkage given by

Eq (1.8).

610500.106.0 ccse f (1.8)

The basic drying shrinkage bcsd. is given by Eq (1.9).

bcsdcbcsd f .. 008.00.1 (1.9)

where bcsd . has been considered to be 800x10-6 for Sydney and Brisbane, 900x10-6 in for

Melbourne and 1000x10-6 elsewhere depending on the quality of the local aggregates. It is

recommended in the code that in the absence of any other data, a value of 1000x10-6 be used



20

as the final drying shrinkage strain to determine the typical design shrinkage strain within a

tolerance of 30%. However from studying actual data, the author is convinced that these

values can be lower with the incorporation of present quality control measures.

The shrinkage strain at time t measured from the start of drying (after 7 days for moist cured

concrete), as recommended by ACI 209R-92 is

t

tt shsh

*

35)(

(1.10)

where sh* is the final shrinkage strain at time infinity and depends on the following

parameters: relative humidity; size and shape of member; slump; aggregate, air and cement

content; and period of initial moist curing. Huo et al. (2001) recommends a modification to the

shrinkage strain to cover HSC which depends on the concrete compressive strength. A

correction factor was also introduced in the proposed modification to consider the lower

ultimate shrinkage strain due to HSC.

The treatment of shrinkage in MC90 is almost identical to that of EC2 and appears to cover

the high-strength concrete. The code procedure for calculation of shrinkage takes into

account the increase in shrinkage with reduction in compressive strength. The total shrinkage

strain is calculated from;

)(),( sscsoscs tttt (1.11)

where, βs is the coefficient to describe the development of shrinkage with time

t = age of concrete

t = age of concrete at the beginning of shrinkage

εcso = basic shrinkage coefficient

After analysing the shrinkage of standard specimens taken from site delivered concrete at

three locations in Melbourne, Fragomeni and Baweja (2008) concluded that the updated

shrinkage model in AS3600-2009 gives better prediction of shrinkage compared to the current

code shrinkage model.



21

Creep

Creep is defined as the increase in strain under a sustained constant stress. Basic creep is

the creep encountered when no drying is involved and the drying creep is the additional creep

that occurs with simultaneous drying of concrete. Aggregates in concrete do not creep at the

stress levels encountered in concrete.

Creep decreases with an increase in strength of concrete. Thus, it is expected and

experiments confirm that HSC creeps to a lesser degree than NSC. A resulting advantage is

that higher proportions of the maximum stress can be endured by HSC for similar values of

specific creep.

The single most significant factor that affects creep is the stress level. The higher the stress

level, the higher will be the creep strains. At 0.3f’c, 50% higher creep strains were observed as

compared to 0.2f’c, and at 0.6f’c the creep strains were 3 times of those at 0.2f’c. Other than

the compressive strength, the creep coefficients for concretes of varying strengths also

depend on the mix components and proportions; water/cement ratio; supplementary

cementitious materials and other additives. In an experimental study, Suksawang et al. (2005)

concluded that while fly ash increases the compressive creep of HSC, silica fume decreases

it. Mendis et al.(1997), while reviewing the work carried out by others, reported that a

reduction in w/c ratio in the HSC mix will lead to a reduction in creep.

The basic creep coefficient is defined as the ratio of total long term strain and initial elastic

strain at a particular level of stress. In AS3600-2001 values of basic creep co-efficient (cc.b)

are given for concrete with compressive strengths up to 65 MPa and for a stress level of 0.4f’c

(Cl. 6.1.8). AS3600-2009 recommends a modified equation for the design creep coefficient,

cc, which is applicable for concrete up to 100 MPa within a tolerance of ±30% (Cl. 3.1.8).

Based on an experimental and analytical study Setunge and Padovan (1997) proposed basic

creep factors for HSC (Fig 1.2). Gilbert (1998) has suggested slightly higher creep factors

(compared to Setunge and Padovan) for high-strength concrete. Suggestions of Setunge and

Padovan (1997) and Gilbert (1998) are given in Table 1.3. It must be noted that the creep

coefficients for concretes of varying strengths depend on many factors other than the

compressive strength, including mix components and proportions, water/cement ratio,

supplementary cementitious materials and other additives.



22

Fig 1.2 Basic Creep Factor [25]

Table 1.3 Basic creep factors

Characteristic Compressive strength (MPa)

20 25 32 40 50 60 80 100

Basic creep factor

-AS3600-2001

5.2 4.2 3.4 2.5 2.0 1.7 (at 65 MPa)

- -

Basic creep factor

AS3600-2009

5.2 4.2 3.4 2.8 2.4 2.0 (at

65 MPa)

1.7 1.5

Basic creep factor

-Setunge and Padovan (1997)

5.2 4.2 3.4 2.5 2.0 1.5 0.9 0.5

Basic creep factor

-Gilbert (1998)

5.2 4.2 3.4 2.8 2.5 2.4 2.2 2.0

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Compressive Strength MPa

Bas

ic C

reep

Fac

tor

AS3600

Setunge & Padovan(1997)

Other reported data

Proposed



23

The basic creep coefficient for moist cured concrete as recommended by the ACI 209R-92

does not depend on the concrete strength. The code allows for the following variation to the

standard conditions: concrete composition, age at loading, ambient relative humidity, size

factor, and ambient temperature. Huo et al. (2001) found that creep coefficients of HSC were

lower than those of conventional concrete resulting in over-estimation of those values of HSC

by ACI 209. He suggested a modification to the creep coefficient given in ACI 209 to cover

HSC

The methods given for calculating the creep coefficient in MC90 and Annex B.1 of EC2 are

almost identical. The relations to calculate the creep coefficient are empirical and calibrated

on the basis of laboratory tests subjected to stress levels below 0.4f′c. These two codes only

take into account parameters that are generally known to the designers in the design stage.

The parameters those are required to predict creep coefficient are: mean or design strength of

the concrete, member dimensions, mean relative humidity RH of the ambient atmosphere,

age at loading and duration of loading. The creep coefficient φ(t,t0) in MC90 and EC2 may be

calculated from:

φ(t,t0) = φ0 . βc(t,t0) (1.12)

where, t is the age of concrete and t0 is the age of concrete at loading and, φ0 = φRH β(f′c) β(t0)

1.3.10 Stress Strain Models

The stress-strain behaviour of HSC differs from that of NSC [Fig 1.3]. The reasons for the

variance in behaviour are as follows:

For HSC, the stress-strain curve remains linear up to the highest value of the

stress/ultimate stress ratio. The internal microcracking that occurs in concrete as load

is applied is delayed until a large proportion of the ultimate load is reached. Therefore

the elastic response to compression is extended.

the strain at the peak stress increases with strength

the post peak branch becomes steeper as the strength increases (or reduced

ductility). The extensive ductility exhibited by lower strength concretes beyond

maximum stress, caused by the spreading of microcracks which form an

interconnected network with large redundancies to dissipate the energy is not

observed in HSC.

the typical ultimate compressive strain decreases as the strength increases



24

the typical fracture surface of an ordinary concrete is rough. The fracture occurs along the

aggregate-matrix interface; the aggregates are not broken. For HSC, a typical fracture surface

is smooth and the cracks pass without any discontinuity through both the matrix and

aggregates. The fracture mode of HSC is more sudden than that for NSC

Fig 1.3 Uniaxial stress-strain curve for concrete (Pendyala et. al., 1997)

It is important to know the details of stress-strain behaviour of HSC in order to determine the

full-range moment-curvature behaviour of HSC columns and beams. Various stress-strain

relationships for HSC have been proposed in the literature. A model developed at the

University of Melbourne is described below.

Modified Scott Model (Pendyala et. al., 1997 and Mendis et. al., 2000)

The Modified Scott Model has been based on the model for confined concrete developed by

Scott et al. (1982) for NSC members. Mendis (1986), based on an experimental and

theoretical investigation, recommended the Scott Model as being the most suitable to predict

the full range stress-strain behaviour of concrete beams. In order for the model to be

applicable for both NSC and HSC appropriate modifications were made to the Scott Model.

The Modified Scott model has been experimentally validated for HSC members (Pendyala et.

al., 1997 and Kovacic, 1995).

The proposed Modified Scott Model is described by Eqs (1.13) - (1.22) and Fig 1.4.



25

For confined concrete:

The equation which defines the parabolic ascending portion of the curve is:

2

2

cccc

c'fKf for cc (1.13)

The equation for the linear descending portion is:

resccmc fZ'fKf 1 for cc (1.14)

But not less than resc ffK '

Where,

c

l

'f

fK 31 (1.15)

And,

0

00204

3

1000145

2903

50

K.s

h

f

'f.

.ZZ

h

"

s'c

c

m (1.16)

c*

ccc K.K. 760 240 3 , and (1.17)

0 00320 280 ccres 'f..'Kff (1.18)

where:

fl = the confinement pressure (MPa) using the Mander method

s = volumetric ratio of hoop reinforcement to concrete core measured to outside

of the hoop

h'' = width of concrete core measured to outside of peripheral hoop (mm)

sh = centre-to-centre spacing of hoop sets (mm).

c = 4 26

4

.

'

'

f

f

Ec

c

c , where Ec is the Modulus of Elasticity of concrete



26

f

Kf ’c

cc

K f’c fres Zm

ConfinedUnconfined

0.004c

f ’c

r

f’c fres

Z’m

Fig 1.4 The Modified Scott Model

For unconfined concrete:

The equation which defines the parabolic ascending portion of the curve is:

2

2

cc

c'ff for c (1.19)

the equation for the linear descending portion is:

rescmc 'f'Z'ff 1 for c (1.20)

but not less than resc ff '

where:

0

1000 145

290 3 0.5

c

c

cm

'f

'f.Z'Z (1.21)

Z = 0.018f’c + 0.55 (1.22)

[Note: ,'f

'f.

c

c0

1000 145

290 3c

indicates a very steep slope beyond the ultimate load,

hence a very large value (e.g. 1x105) should be allocated to m'Z .



27

Application of the Modified Scott model for high-strength concrete columns is presented in

detail by Mendis (2001).

1.3.11 Fire Resistance of High-Strength Concrete

Fire resistance of concrete members is normally accomplished by structural adequacy and

insulation for a specified fire resistance period (C & CA, 2010). Several researchers have

concluded that with the exception of spalling, which is defined as the detachment of pieces of

concrete when a concrete member is exposed to fire; there is no apparent reason to treat

high-strength concrete differently from lower strength concrete. Adverse effects of HSC in fire

have been described by Sanjayan (2011). The pieces can be large or small and detachment

can either occur explosively or pieces may dislodge and subsequently fall (Fig 1.5). Spalling

can take place over the whole surface area of a member or in localised areas (Sanjayan,

1994). The risk of spalling is higher in high-strength concrete due to the following reasons:

(i) Low permeability of HSC retains the moisture inside the concrete resulting in a high

moisture content being present for prolonged periods.

(ii) Low porosity of HSC creates higher pore pressure.

(iii) HSC tends to be subject to higher compressive stresses than lower strength concrete.

Fig 1.5 Behaviour of HSC in Fire (Breitenbucker, 1996)

Sudden burst due to the steam-pressure

Water bound chemically and physically in cement paste

Sudden burst due to the steam-pressure

Water bound chemically and physically in cement paste



28

Conflicting observations have been reported in the literature on spalling of HSC. Some tests

have shown explosive spalling of HSC, while others have reported no difference to the

behaviour of NSC. In 1996, a comprehensive investigation was conducted at the National

Institute of Standards and Technology, on experimental and analytical studies on fire

performance of HSC. The key findings and a literature review are given by Phan (1996). Of

the ten materials test programs reviewed by Phan (1996), five reported explosive spalling. Of

the five element test programs reviewed, three reported spalling. Also, it was observed that

explosive spalling does not occur for every specimen tested under identical conditions. The

reported temperature range when explosive spalling occurs is between 300°C to 650°C.

Concrete with dense pastes resulting from the addition of silica fume are more susceptible to

explosive spalling. HSC made with lightweight aggregate appears to be more prone to

explosive spalling than HSC made of normal weight aggregate concretes. Chan et al. (1999)

showed that moisture content and strength are the two main factors governing explosive

thermal spalling of concrete. Spalling in all their specimens occurred between 480-500°C.

Also, HSC specimens heated at higher heating rates, such as hydrocarbon fire which

occurred in WTC collapse on September 11, and larger specimens are more prone to spalling

than specimens heated at lower rates and of smaller size. Ta (2010) showed that, for HSC,

hydrocarbon fire produces explosive spalling compared to standard fire.

The effectiveness of polypropylene fibres and carbonate ;aggregates in order to improve fire

resistance has been reported in the literature [Phan, 1996, Ta, 2010]. Sanjayan (1994)

suggests the following methods to reduce the risk of spalling in HSC:

(i) Reduction of concrete covers (20 - 25 mm) where possible;

(ii) Where it is not possible to provide small cover for structural steel, sacrificial steel with

small concrete covers (20-25 mm) may be provided to prevent spalling of cover to the

main steel bars. The sacrificial steel is not to be considered in structural design for

carrying loads; and

(iii) Use of fibre reinforcement to prevent spalling [see also Breitenbucker, 1996]. The

length of fibres may be of importance. Short fibres, combined with a high fibre

content may even give adverse spalling effects.

According to the review by Phan (1996), the material properties of HSC vary differently with

temperature as compared to those of NSC. The differences are more pronounced in the

temperature range of between 25°C to about 400°C, where higher strength concretes have

higher rates of strength loss than lower strength concretes. These differences become less

significant at temperatures above 400°C. Compressive strengths of HSC at 800°C decrease

to about 30% of the original room temperature strengths. The difference between the

compressive strength versus temperature relationships of normal weight and lightweight



29

aggregate appears to be insignificant, based on the limited amount of existing test data. The

tensile strength versus temperature relationships decreases similarly and almost linearly with

temperature for HSC and NSC. HSC mixtures with silica fume have higher strength loss with

increasing temperatures than HSC mixtures without silica fume. The failure of HSC is more

brittle than NSC at temperatures up to 300°C. With further increase in temperature,

specimens exhibit a more gradual failure mode.

Phan and Carino (1998), by considering several experimental results on mechanical

properties of concrete when exposed to rapid heating as in a fire showed that Eurocode

provisions and the CEB design curves are unsafe for HSC. They also measured the elastic

modulus and compressive strength of HSC by heating 100 x 200 mm cylinders at 5°C/min to

temperatures of up to 600°C. The properties were measured at elevated temperatures as well

as after cooling to room temperature. Results indicated that losses in relative strength due to

high-temperature exposure were affected by the test condition and w/c ratio, but there were

significant interactions among the main factors that resulted in complex behaviours. The

presence of silica fume did not appear to have a significant effect. Measurements of

temperature histories in the cylinders revealed complex behaviours that were believed to be

linked to heat-induced transformations and transport of free and chemically combined water.

There are only a few studies reported recently on the structural behaviour of HSC members

subjected to fire. Meda et al. (2002) studied the ultimate behaviour of HSC sections at high

temperature and after cooling subjected to several fire durations. They concluded that HSC

sections are more temperature-sensitive than NSC sections. However, it is not significant.

Kodur (2003) recommended design guidelines for mitigating spalling and enhancing fire

endurance of HSC columns. A recently concluded project at the University of Melbourne

investigated the behaviour of HSC members under hydro-carbon fire conditions. In this project

10 large-scale concrete wall panels were tested. Four walls were of normal strength concrete

(NSC), two of which were axially loaded at an eccentricity of 10 mm (0.39 in.) and two with no

load, and exposed to either standard or hydrocarbon fires. Four identically dimensioned high

strength concrete (HSC) walls were also tested using these variables. A further two HSC

walls, with polypropylene fibres added, were tested under hydrocarbon fire conditions only. All

walls were tested in a vertical position in a large furnace, and supported top and bottom only.

The results indicate that all concrete wall panels exposed to the standard fire tests survived

the 120 minute fire period, with low to moderate spalling evident. The NSC walls exposed to

hydrocarbon fires also survived the 120 minute test whereas the HSC walls experienced

severe spalling under these fire conditions with failure at 31 minutes. The Addition of

polypropylene fibres in the concrete improved the fire resistance of HSC walls in hydrocarbon

fire to 65 minutes.



30

The performance of concrete structures in fire has become increasingly significant in the past

decade. This is due in part to the increased incidence of accidental fires, explosions and

terrorist attacks. The increased use of high strength concrete (HSC) with concrete strengths

more than 50 MPa, which is perceived to have less adequate fire performance compared to

normal strength concrete (NSC), has necessitated dedicated research in this area.

According to the results of laboratory focused fire tests conducted recently, there are

remarkable differences between the properties of HSC and NSC in terms of;

• the loss of cross-section

• the timing of loss of strength; and

• the degrees of deformation and spalling at elevated temperatures

The most notable finding is that HSC is considered to suffer more seriously from spalling due

to fire than NSC.

Rules for High-strength Concrete in Eurocode

(Although AS3600-2009 is based on this code, this latest clauses are not included in

AS3600-2009)

Section 6 of EN1992-1-2 2004 gives additional rules for high strength concrete. Structural

elements shall be designed at elevated temperature with the properties of that type of

concrete and the risk of spalling shall be taken into account. Strength properties are given in

three classes and recommendations against spalling are given for two ranges of HSC. Where

the actual characteristic strength of concrete is likely to be of a higher class than that specified

in design, the relative reduction in strength for the higher class should be used for fire design.

A reduction in strength, fc,q/ fck, at elevated temperature should be made according to the

value given in following Table.

Class 1, f’c between 50 and 70 MPa

Class 2, f’c between 70 and 90 MPa

Class 3, f’c is equal to 90 MPa

Note: Simplified Calculation Method presented in Appendix B (500 degree isotherm method,

zone method, curvature method) can be applied for high-strength concrete with some

modification factors.



31

Table 1.4 Reduction of strength at elevated temperature

Spalling

(1) For concrete strengths between 55 to 80 MPa, fire design rules provided for normal

strength concrete apply if the maximum content of silica fume is less than 6% by weight of

cement. For higher contents of silica fume the rules given in (2) apply.

(2) For concrete strengths between 80 and 90 (and can be assumed to extrapolate up to 100

MPa) spalling can occur in any situation for concrete exposed directly to the fire and at least

one of the following methods should be provided:

Method A: A reinforcement mesh with a nominal cover of 15 mm. This mesh should have

wires with a diameter ≥ 2 mm with a pitch ≤ 50 x 50 mm. The nominal cover to the main

reinforcement should be ≥ 40 mm.

Method B: A type of concrete for which it has been demonstrated (by local experience or by

testing) that no spalling of concrete occurs under fire exposure.

Method C: Protective layers for which it is demonstrated that no spalling of concrete occurs

under fire exposure.

Method D: Include in the concrete mix more than 2 kg/m3 of monofilament propylene fibres.



32

1.4 HSC in Other Standards

1.4.1 HSC in Bridge Code (AS 5100, 2004)

Traditionally the AS5100 and the AS3600 codes have contained many equivalent provisions

and it is anticipated that AS5100 will be revised in due course to be compatible with AS3600.

In the interim, specifications in many road authorities mention that most up to date information

should be used for bridge design and consequently the requirements in AS3600-2009 and not

the equivalent section in AS5100 are to be utilised.

The applications of HPC in bridges have been discussed in different forums around the world.

For example, as early as in October 1997, the Precast/Prestressed Concrete Institute (PCI)

organized the PCI/FHWA Symposium on HPC in New Orleans, Louisiana.

Mendis et al. (2000) studied the use of high performance concrete in precast, prestressed

concrete Super-T highway bridge beams. Jenkins (2011) has reviewed the international use

of high performance concrete in bridge decks including reported benefits and problems,

restrictions placed on strength grade, and special design provisions required for higher

strength grades. He has also presented some Case studies comparing the design benefits

under Australian conditions, of using the current maximum strength grade permitted under the

Bridge Code AS5100 (2004), and higher strength grades that may be introduced in the future.

Finally he has given some recommendations for situations in which the use of high

performance concrete is likely to be of overall benefit to the quality and economy of the

structure.

A parametric study is given in Chapter 6 to show the advantages of using HSC for super-T

beams in Bridges. It is shown that as concrete strength increases, the prestressing level can

be increased. This can reduce the cost of steel as well as decrease the mass of the section,

because prestressing steel requires less space to provide the same tensile force as passive

reinforcing steel and hence, the bottom flange thickness is reduced



33

1.4.2 HSC in Piling Code (AS 2159, 2009)

The piling code (AS2159, 2009) requires that where a bending moment exists, the pile be

designed in accordance with the principles of AS3600. However, the pile may be designed as

an unreinforced section below the level where the bending moments become zero.

It also requires that when concrete strength exceeds 65 MPa, the requirements of AS 3600

shall be considered, together with any confining action provided by the soil or rock around the

pile shaft, when calculating the lateral reinforcement.

Precast concrete piles are frequently used in the footings of buildings, bridges and marine

structures and are known for their toughness, durability and economy. Precast prestressed

piles are designed to safely support vertical loads together with bending moment arising from

lateral loads such as wind, waves and earthquake. The level of prestress is chosen to ensure

crack-free performance is achieved for the service load. Piles can be designed for aggressive

soil or marine applications.

Prestressed concrete piles are widely used as deep foundation elements. The perception still

exists that there is little advantage of increasing concrete strength since the existing design

loads are usually much less than code allowable loads in piles. Two examples were given by

Goble and Hussein (2000) to show possible advantages of the use of high-performance

concrete, particularly high-strength concrete in driven piles. It was shown that design loads

are usually limited by driving stresses. Therefore if higher strength concrete is used,

increased driving stresses could produce higher design loads with associated reduced

installed cost.

In substructural applications (e.g., deep foundations), construction materials such as concrete

are subjected to deterioration due to environmental impacts. Using innovative and new

materials for foundation applications makes the AASHTO objective of 75 years service life

achievable (TR 558, 2008). Ultra High Performance Concrete (UHPC) with compressive

strength of 180 MPa and excellent durability has been used in superstructure applications but

not in geotechnical and foundation applications. This study by CTRE (TR 558, 2008) explored

the use of precast, prestressed UHPC piles in future foundations of bridges and other

structures. An H-shaped UHPC section, which is 250 mm deep with weight similar to that of

an HP10×57 steel pile, was designed to improve constructability and reduce cost. In this

project, instrumented UHPC piles were cast and laboratory and field tests were conducted.



34

Laboratory tests were used to verify the predicted analytical response of UHPC pile section.

In the field, two UHPC piles were successfully driven in glacial till clay soil and load tested

under vertical and lateral loads. Test results, durability, drivability, and other material

advantages over normal concrete and steel indicate that UHPC piles are a viable alternative

to achieve the goals of AASHTO strategic plan (TR 588, 2008).

1.4.3 Maritime Structures

Marine structures are those structures built on sea or near it, properly designed to withstand

internal, external loads and aggressive environmental exposures both physical and chemical

in order to prolong service-life.

Reinforced concrete structures as part of marine structures are exposed to severe physical

and chemical exposure such as wave impact, sulphate and chloride exposure. A combination

of above deleterious effects may cause severe defects in a concrete structure only in very few

years.

In order to delay these detrimental effects, high durability, chloride and sulphate resistant

high-performance concrete become a necessity in constructing marine structures. The use of

silica fume with a high water reducing admixture with retarding effect then become

increasingly popular to produce a low permeability concrete and high sulphate resistant

concrete.

Marine structures are subjected to very severe exposure conditions and their durability is

directly related to the quality of concrete used. Concrete must possess low to very low

permeability characteristics for these applications and its durability can be assured with the

use of proper mix compositions. Since high-strength concrete has low permeability such a

concrete grade is needed to achieve high-performance in a marine environment. In order to

withstand the extremely hostile environment, compressive strength of concrete above 60MPa

is required, although this level of strength may not be required to satisfy the design loads.

The cement paste matrix in high-strength concretes is dense owing to the absence of large

capillary voids and a relative strong aggregate-cement paste interfacial zone, which has a

much lower tendency for micro-cracking. As discussed previously HSC has relatively high

binder material content, a superplasticiser, and very low water content to achieve the water to



35

binder ratio below 0.30, by weight. They are capable of achieving a discontinuous capillary

pore structure and low permeability within a few days of hydration of cement.

Design Considerations in Maritime Structures Code (AS 4997, 2005)

Section 6.1 of in Maritime Structures Code (AS 4997, 2005) describes the requirements for

Durability. Maritime structures are generally sited in very aggressive environments for normal

structural materials, and the design of maritime structures should include consideration of the

requirements to withstand the aggressive environment while the structure remains

serviceable.

The effect of extreme events on the structure’s durability should also be considered. For

example, the effect on concrete structures, which may be heavily stressed and cracked in an

extreme event early in the life of the structure, should be considered, where such cracking

may then lead to accelerated corrosion of steel reinforcement.

Design life is defined as the period for which a structure or a structural element remains fit for

use for its intended purpose with appropriate maintenance. The design life of maritime

structures will depend on the type of facility and its intended function. This design life will

depend on the owner’s requirements.

As well as determining loads for a facility, it is necessary to decide on a realistic design life for

the structure. This design life should be based on consideration of capital and maintenance

expenditure. Durability is to be realized either by a maintenance program, or, in those cases

when maintenance cannot (or is not expected to) be carried out, by design such that

deterioration will not lead to failure. In the latter case the initial capital cost is expected to be

high.

The designer should determine an appropriate maintenance regime consistent with the

adopted design and materials that will achieve the design life. Particular care should be taken

when considering design life and maintenance regimes for inaccessible members. Sections or

components of the structure that have limited access or are inaccessible after construction

should have a design life (with no maintenance) equal to the design life of the structure.



36

At the end of the design life, the structure should have adequate strength to resist ultimate

loads and be serviceable, but may have reached a stage where further deterioration will result

in inadequate structural capacity.

Material Considerations in AS 4997

According to AS4997, the following items should be considered when selecting concrete as a

material in the design of a maritime structure:

(a) Concrete deterioration is usually a result of corrosion of reinforcing steel due to chloride

ingress.

(b) Reinforced concrete may not be a ‘lifetime’ maintenance-free material. Reinforced

concrete structures require regular condition inspection and maintenance of deteriorated

sections. Recent history has shown some maritime concrete structures experiencing

significant premature deterioration as a result of an inappropriate selection of materials for

the required design life.

(c) Improved performance of concrete structures will be achieved by a combination of the

following:

(i) Limiting design stresses in reinforcing steel.

(ii) Appropriate selection of member sizes, shapes and detail.

(iii) Using high performance concrete and reinforcement

(v) Closely controlled construction methods.

Specifications for Concrete Work in Marine Structure Projects

(Note: This is an example only. The specifications vary with individual project requirements)

a) Objective:

High Performance Concrete including High strength concrete and durable concrete

b) Design Criteria:

Sulphate and chloride resistance

Low W/C ratio (splash & atmospheric zone < 0.40, submerged < 0.45) target 0.3 –

0.38 W/C

High strength

Abrasion resistance



37

Low permeability

Durability

c) Concrete Production:

Objectives: Produce high strength concrete (HSC) & durable concrete (HPC)

Targets: Maximization of concrete density

Actions: Adherence to good construction practices

d) Concrete Mix Design:

Performance for both fresh & hardened

Maintenance – free during design life

Proper selection of material

High binder content

Use of supplementary cementitious materials

Use of superplasticiser or other chemical admixtures required for construction

e) Concrete Curing:

For HSC and other concrete structures, proper curing is essential. In order to

maintain a satisfactory moisture content and temperature in concrete during its early

stages so that desired properties may develop. The strength and durability of

concrete will be fully developed only if it is cured.

Failure to prevent such excessive evaporation frequently causes plastic shrinkage

and loss of strength in the material near the concrete surface.

As mentioned earlier, marine environment requires the use of a very low permeability

concrete for the construction of durable concrete structures. Mix compositions as well as the

binder material type influence the permeability of concrete. Use of blended cements with

pozzolanic materials is known to reduce the permeability. Ravindrarajah et al. (2000)

presented the results on an investigation into the effects of cement replacement with fly ash or

ground granulated blast furnace slag with and without silica fume on the properties of high-

strength high-performance concrete. Cement replacement reduced the early age strength and

no serious effects were noted beyond 28 days. The use of silica fume to replace part of either

fly ash or slag helped to improve the strength at all ages. The 90-day free drying shrinkage of

high-strength concrete varied between 370 and 460 microstrains. The results showed that

high-performance concrete suitable for marine environment can be economically produced by

replacing 25% of the cement by weight with either fly ash or slag and no significant long-term

advantage is achieved by using more expensive silica fume.



38

Technical Note 4 of Ash Development Association of Australia (ADAA) (2009) states that for

marine or high chloride environment (Classification C per AS 3600) HPC should possess:

The concrete strength grade shall not be less than 50 MPa.

The binder shall consist of Portland cement, fly ash, and one or a combination of

additional supplementary cementitious materials conforming to AS 3582 to provide

the high concrete resistivity characteristics that will ensure improved resistance to

chloride induced corrosion of steel.

Cover to reinforcement shall be appropriate for the design life, and as per the

requirements of AS 3600, or other relevant standard or specification.

The curing treatment shall be equivalent to at least seven days of wet curing at 23oC

without loss of moisture from the concrete.

1.4.4 HSC in Other International Standards

Most national and international standards, rules and regulations for concrete structures have

raised the upper limit of the concrete strengths to take into account the higher strength of

modern concrete. A number of different standards and national codes have been examined

and compared with respect to maximum concrete strength and application of high strength

concrete. A summary of major codes of practice, including the allowable concrete strengths

and comments on proposed extensions to cover HSC, is presented in Table 1.4.

Table 1.4: Summary of Major Codes of Practice

Code Year Country Maximum

Strength (MPa)

Notes

EC2 2004 European

Standard

105 (200 mm cube)

90 (cylinder)

EC2 replaced BS8110 in 2010

ACI 318 2011 USA Not specified

(150/300 mm or

100/200 mm cylinders)

AC I 318-2014 will be reorganized

to move from the behavior based

format to a member based format.

AS3600 2009 Australia 100 (150/300 mm

cylinder)

Limitation for clause on concrete

contribution to shear and on bond



39

(65 MPa)

CEB-FIP

MC90

1993 International 80 (150/300 mm

cylinder)

Extensions proposed in new draft

version 2010

1.5 Applications of HSC

A significant cost saving can be achieved by utilizing High-strength Concrete since member

sizes can be reduced. Table 1.5 presents typical applications of HSC in recently constructed

infrastructure.

Table 1.5 Typical Applications of High-strength Concrete

Type of

Structure Applications Advantages of HSC

High Rise

Buildings

Columns in parking bays/high rise

buildings

High rise frames

Residential buildings

Deck slabs and beams

Service cores

Shear walls and outriggers

Strength, workability and pumpability

Reduction of cross-sectional dimensions

Greater rigidity of the frames

Overall economy and faster construction,

most cost effective building material in

many cases

Reduced wall and column thickness

results in a larger rentable area

Shallower floor system leading to reduced

height of building

Reduced column size leads to reduced

dead load and reduced total load on the

foundation system

Increased punching shear resistance in

slabs

Reduction in the amount of steel

reinforcement

Precast/

Prestressed

industry

Spun transmission poles

Prestressed roof elements

Allows for a higher degree of prestress

Reduction in weight; improved handling



40

Precast beam and column elements

Prestressed piles and sheet piles

Precast tube elements for sewerage

Tunnel lining

Micro-tunneling segments

Tiltup panels

Very low permeability of spun concrete

reduce the risk of corrosion of

reinforcement

Higher cracking loads

Reduced stripping time for removal of

formwork and reduced time for transfer of

prestress, reduced time for lifting, etc.

Improved durability in aggressive

environment (sewerage and allied

applications)

Improved punching shear resistance in

slabs

Higher radial pressure capacity

Case studies show that HSC is more cost

effective than cast iron hoops or normal

strength concrete in the case of tunnel

lining

Slabs and

Pavements

Residential slabs on ground

Road/ Highway pavement slabs

Garage floors

Heavy duty industrial floors

Foundries

Warehouses

Provides a low relative humidity due to

self-desiccation to avoid moisture damage

Shorter period of drying

Low maintenance needs; improved

durability

Better abrasion resistance

Increased wear resistance to steel studed

tires

Improved resistance to chemical attack

Improved freeze-thaw durability

Overall a more favourable life cycle cost

Bridges

17 - 35 m span beam girders and

super-structure of multiple span

bridges, in precast, insitu and

prestressed HSC

Submerged concrete bridges for

underwater pipelines

Cable stayed bridges 150 - 465m span

Case studies show that precast -

pretensioned bridges are more economical

than steel-concrete composite girders

Low permeability and improved durability

The use of HSC results in smaller cross-

sections and savings in foundations and

material handling costs

Ability to increase span and spacing of



41

Highway overpasses

Repairs (strengthening of bridge piers)

girders

Higher torsional strength and cracked

stiffness

Low creep and shrinkage

Marine

structures and

other structures

in contact with

water

Offshore oil production platforms

Floating bridge (pontoon)

Sea-wall panels

Slabs and walls of sludge digesters

Water tanks

Tunnels

Increased strength, durability and

buoyancy (in case of offshore platforms

and pontoons)

Reduced wave and wind loading by virtue

of reduced dimensions

Low shrinkage (reduced shrinkage cracks)

Earthquake

resistant design

Frames

Walls

Reduced inertial loads due to reduced

dimensions

Enhanced ductility under flexure

Higher stiffness for sway control

Blast and

Impact

resistant design

Blast wall barriers

Protective structures subject to

projectile or aircraft impact

Transportation structures subject to

vehicle crash impact

Offshore structures exposed to ice

impact

Enhanced resistance against dynamic

impact loads

Higher energy absorption capacity

Ultra HSC panels do not break into

fragments when subject to blast

Can eliminate the risk of injury or damage

caused by concrete debris



42

Fig. 1.1 The Petronas twin towers at construction stage (up to 80 MPa concrete was used for these

columns)

Fig 1.2 A typical application of High-strength Concrete in early 1990s

(Core structure of Telecom Corporate Building in Melbourne)



43

Fig 1.3 A typical application of High-strength Concrete in high-rise structures

(Core shaft of 300 m tall Eureka Tower in Melbourne under construction)

Fig 1.4 Joigny Bridge in France (80 MPa concrete was used to construct the bridge)

Fig 1.5 Annacis Bridge in British Columbia (55 MPa concrete was used in precast concrete deck panels)



44

Fig 1.6 The precast superstructure and column segments of the Roslyn Viaduct, USA used 70 MPa

concrete

Fig 1.7 The Confederation Bridge in Canada used HSC for both durability and strength



45

Fig 1.8 HSC used for durability and strength for the Coal Loader Wharf Kooragang Island, Newcastle

1.5.1 Precast Applications of HSC/HPC

Precast applications are described in various sections of this document. High strength,

factory produced, long life prestressed/precast concrete offers the ultimate outcome with

minimal maintenance. For example, factory-produced precast concrete is better able to satisfy

the very rigid specifications required for the durability of marine structures. Precast concrete

offers a very durable, low-maintenance product, benefiting from fabrication in a controlled

factory environment, use of quality materials and proven manufacturing techniques.

Sophisticated mix designs characterised by low water-cement ratios, good compaction and

adequate curing (all associated with durable concrete) are synonymous with precast products.

HSC is an obvious choice for precast concrete as listed in Table 1.5. Some innovative

additional applications are given below.

Light Weight Concrete (LWC) Panels

The density of concrete can be reduced by using lightweight aggregates, with strengths as

high as 105 MPa (15 ksi). LWC has a lower unit mass, saving costs in handling, transporting,



46

placing and in the foundations. In-situ LWC is used mainly for cantilever, cable-stayed

bridges.

An aerated cellular LWC with a density of 300 – 1000 kg/m3 and strength of less than 10 MPa

could not be used in structural applications since they are mainly produced for non-load-

bearing purposes. New technologies that allow the production of high strength lightweight

cellular concretes without the need for autoclaving could provide significant advantages by

increasing the functionality of cellular concrete whilst potentially reducing manufacturing and

construction costs. In particular, the capital cost of establishing autoclaves and the operational

energy requirements can be very significant.

Manufacturing and Infrastructure Technology Division of CSIRO has developed an advanced

non-autoclaved cellular concrete product (HySSIL). This Load-Bearing Cellular Concrete

(LBCC) technology is being used to produce pre-cast panel elements. The LBCC technology,

commercially named HySSIL, is a pore-graded cellular material system manufactured using a

patented low energy non-autoclaved process. The unique pore gradation characteristic of this

material means that the product has very dense near surface zones and a relatively porous

core zone, all in one monolithic element, i.e. without the need for layering or laminating

materials of varying densities. The presence of this very dense cover zone provides important

advantages in providing the superior strength and durability. The compressive strength of the

cellular material for load-bearing applications typically ranges between 15 – 30 MPa over a

dry density range of 1270 – 1620 kg/m3. A joint PhD project was conducted between CSIRO

and University of Melbourne (with Dr Mak and Prof Mendis as Supervisors) to investigate the

load bearing properties of this innovative product.

In general, the composition of the LBCC includes fine graded sand, type GP (general-

purpose) cement, silica fume, water and chemical admixtures. The main difference between

LBCC and normal density concrete is that there is no coarse aggregate in LBCC. Aluminium

powder is used as the gas-forming agent, generating bubbles from chemical reactions to

create the porous structure of LBCC.

Many studies have been carried out to investigate the relationship between porosity and

strength of cellular concrete (CC), in which CC’s strength was expressed as a function of its

bulk porosity and the theoretical strength at zero porosity.

This study showed a new approach in determining strength of CC on the basis of its own

properties; that is, matrix properties and relative porosity. Polystyrene beads were used to



47

simulate air bubbles inside CC, such that size composition of bubbles was easily monitored.

The results on strength of lightweight concrete, made of 60, 80 and 100 MPa mortars mixed

with polystyrene beads, were reported (Phan et al., 2008). This study showed that the

compressive strength of cellular concrete increases with reduction in moisture content, and

tends to stabilize when moisture content level reaches either upper or lower limits.

Reactive Powder Concrete (RPC)

RPC is an ultra-high-strength concrete (UHSC) patented jointly in 1990. It is usually heavily

dosed with special steel or organic fibres, along with Portland cement, silica fume, quartz

flour, fine silica sand, high-range water reducer, and it contains no coarse aggregate. It

possess a very high compressive strength, ranging from 200-800 MPa; high flexural strength

ranging from 45-140 MPa; high resistance to abrasion and acids and to freezing and/thawing;

very little sensitivity to creep and shrinkage, with enhanced durability, sea-water and low

water permeability, and it facilitates faster construction.

Ultra High-strength Reactive Powder Concrete (RPC) is an ideal material for precast

applications. APTES group at the University of Melbourne worked with VSL Ltd. to explore

and test, precast applications of this material for protective structures starting in 2004 (Ngo et

al., 2007). Other applications in Bridges etc. are described in a paper presented by VSL in

UHPFRC conference (2009).

Compact High-Strength Composite Concrete (CHSC)

CHSC is a derivative of Compact Reinforced Composite (CRC), which is a very dense silica

particle (DSP) cement high performance concrete with fibre reinforcement, developed by

Aalborg Portland in Denmark in 1986. The CRC concrete consists of a large volume of steel

fibres (from 4% to 12%) in a dense silica particle cement matrix. A typical CHSC mix includes

high quantities of superplasticisers, high volumes of microsilica, and water/binder ratios of

typically 0.16 or lower. The resulting composite possesses very high compressive strength

ranging from 140-400 MPa, high flexural strengths ranging from 50-100 MPa, as well as

toughness and ductility.

The addition of steel fibres and a large content of microsilica makes the material more

homogeneous with a very high bond strength, and transforms it from a brittle to a more ductile

material. The CRC panels can be used as protective layers in protecting buildings and critical



48

structures in confined city centres, where it is not feasible to increase the stand-off distance.

CRC concrete is only half of the weight of steel, so it can also be used effectively to protect

the underside of military and civilian vehicles (such as those used in peace-keeping

operations) against mine blasts. The high-strength of CRC, with its mortar-like consistency,

and the fact that it does not contain coarse aggregates, allows for very close rebar spacing,

making it possible to precast thin, lightweight structural elements for structures such as

balcony slabs and slender staircases. A collaborative project between University of Melbourne

and Denmark Technical University explored the application of CRC in Australia.

1.5.2 High Performance Concrete Pavements

The Test and Evaluation Project 30 (TE-30) on High Performance Concrete Pavement

(HPCP), conducted by the Federal Highway Administration (FHWA) is exploring the

applicability of innovative Portland cement concrete (PCC) pavement design and construction

concepts in USA. These innovative concepts, ranging from the use of trapezoidal cross

sections to alternative dowel bar materials to fiber-reinforced concrete, all share the same TE-

30 goal of providing long lasting, economical, PCC pavements that meet the specific

performance requirements of their particular application.

Several innovation areas for the program have been suggested:

Increasing pavement service life.

Decreasing construction time.

Lowering life-cycle costs.

Lowering maintenance costs.

Constructing ultra-smooth riding concrete pavements.

Incorporating recycled or waste products while maintaining quality.

Utilizing innovative construction equipment or procedures.

Utilizing innovative quality initiatives.

In each of these applications, emphasis is given to an integrated design approach in which

site influences (traffic loading, climate, and subgrade), concrete mix design, structural design,

joint details, and construction are considered together to develop the appropriate pavement

design. Consequently, the term “high performance” does not necessarily refer to high strength

concrete, but rather to any of the materials and mix design, structural design, or construction

components of the pavement that are expected to contribute to long-term performance.



49

High-performance concrete with high durability featuring low permeability, improved curing,

high air content, well-graded aggregate, high-quality aggregate, low w/c ratio (less than 0.40),

and ground granulated blast furnace slag cement have been used for pavements. A

significant amount of additional research is currently underway to optimize concrete mix

designs for the wide variety of individual site conditions. A number of projects looked at the

effect of adding fibers to improve PCC (Portland Cement Concrete) pavement performance. A

complete list of projects is given in the report (FHWA 32, 2006).

Another application in HPC is, the “Next Generation Concrete Surface (NGCS)” low noise

diamond ground surface. The NGCS, resembling a “manufactured surface,” provides its low

noise benefits when initially constructed and does not require a wear in period to break the

fins down.

1.5.3 Other Types of HPC

Self-Compacting Concrete (SCC)

This type of concrete, originally invented in Japan, has been used in a number of projects in

Australia. SCC is based primarily on the properties of the concrete during placement. They

can be compacted into the formwork by means of its own weight and without vibration.

Despite high flowability there is no segregation. The mix may incorporate steel, polycarboxilic-

based superplasticisers with mineral/chemical admixtures and microsilica. With proper quality

control, 30-40 MPa (4.35-5.8 ksi) overnight strengths can be achieved for SCC, whereas the

2-day strengths can break the 100 MPa (14.5 ksi) barrier. Experience has shown that a high

degree of quality control is required, but there are benefits, especially where access is

difficult; where noise of vibration is undesirable; and where labour savings for placing,

compacting and finishing can be achieved. The high quality finish is possible with virtually no

‘bug holes’, making it particularly suitable for quality architectural exposed finishes.

Geopolymer Concrete (Inorganic Polymer Concretes-IPC)

Inorganic polymer concretes (IPCs) can be made predominantly from industrial waste

materials such as fly ash (a coal combustion by-product), granulated blast furnace slag

(GBFS), mine tailings and contaminated soil. These materials are also commonly referred to

as geopolymer concretes or alkaline cements. While pozzolanic cements generally depend on

the presence of calcium, inorganic polymers do not utilise the formation of calcium-silica-



50

hydrates (CSH) for matrix formation and strength. Instead, IPCs utilise the polycondensation

of silica and alumina precursors and a high alkali content to attain structural strength. Details

are given elsewhere (Lukey et al., 2006).

As in the case of OPC concrete, the aggregates occupy about 75-80 % by mass, in

geopolymer concrete. The silicon and the aluminium in the low-calcium (ASTM Class F) fly

ash react with an alkaline liquid that is a combination of sodium silicate and sodium hydroxide

solutions to form the geopolymer paste that binds the aggregates and other unreacted

materials. An example of a Geopolymer concrete mix (Lloyd and Rangan, 2010) is given

below. Hardjito and Rangan (2005) have shown the following:

Higher concentration (in terms of molar) of sodium hydroxide solution results in higher

compressive strength of geopolymer concrete.

Higher ratio of sodium silicate solution-to-sodium hydroxide solution ratio by mass,

results in higher compressive strength of geopolymer concrete.

The slump value of the fresh geopolymer concrete increases when the water content

of the mixture increases. Superplasticisers may assist in improving workability.

As the H2O-to-Na2O molar ratio increases, the compressive strength of geopolymer

concrete decreases. Figure below shows the compressive strength vs.

water/Geopolymer solids ratio. The test specimens were 100x200 mm cylinders, heat-

cured in an oven at various temperatures for 24 hours. The mass of geopolymer

solids is the sum of the mass of fly ash, the mass of sodium hydroxide solids used to

make the sodium hydroxide solution, and the mass of solids in the sodium silicate

solution (i.e. the mass of Na2O and SiO2).

Fig 1.9 Effect of Water-to-Geopolymer Solids on Compressive Strength



51

Table 1.6 Geopolymer Mix proportions

The main environmental benefits of geopolymer concrete lie in its novel manufacturing

process, which reduces the CO2 emissions into the atmosphere caused by the cement

industries by about 80%. It can achieve a high early strength of around 80 MPa during the first

2 days of curing with several benefits, such as low thermal conductivity and water

permeability, high durability and fire resistance, acid and chemical resistance and a high

capacity for toxic metal immobilization. One of the main drawbacks of Geopolymer Concrete

is the low early-age strength and the need for heat curing (Vijai et al., 2010). Low flexural

tensile strength is another problem.

The engineering properties of inorganic polymer (geopolymer) concretes (IPCs) having a

design compressive strength of 50 MPa have been studied by Sofi et al. (2007). The study

includes the determination of static chord modulus of elasticity, Poisson’s ratio, compressive

strength, splitting tensile strength and flexural strength of IPCs formulated using three

different sources of Class-F fly ash. The results showed that these engineering properties of

the IPC mixes compare favourably with the models presented by the standards for OPC

based concretes. Elastic Modulus was closer to the lower limit provided by AS3600.



52

Fig 1.10 Elastic Modulus for Geopolymer Concrete Mixes

Structural applications of reinforced IPC depend on the bond performance of the material to

the reinforcement. Due to their difference with ordinary Portland cement (OPC) based

concrete in terms of chemical reaction and matrix formation it is not known whether IPC

exhibit different bonding performance with the reinforcement. Simply relying on compressive

strength of the material and extrapolating models and equations meant for OPC based

concrete may lead to unsafe design of structural members. To that end, 27 beam-end

specimens, 58 cubic direct pullout type specimens and number of laboratory test specimens

were tested at the University of Melbourne by Sofi et al. (2007) to evaluate bonding

performance of IPC with reinforcement. The results of beam-end specimens and direct pullout

type specimens correlated favourably, although the results of direct pullout tests are in

general more conservative than those of beam-end specimens. Overall, it was concluded that

bond performance of IPC mixes are comparable to OPC based concrete and therefore IPC

and steel can be used as a composite material to resist tension in addition to compression.

Performance of geopolymer concrete under elevated temperatures has been studied by Kong

et al. (2008). The work conducted on Geopolymer Concrete at CSIRO is described by

Steveson and Sagoe-Crentsil (2005).



53

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Contents 1.0 INTRODUCTION ...................................................................................................................... 1

1.1 High-strength Concrete (HSC) and High-Performance Concrete (HPC) -Background ....................................................................................................................................... 1

1.2 Advances in Concrete Technology .................................................................................... 4

1.2.1 Concrete materials ....................................................................................................... 4

1.2.2 Use of recycled materials in concrete ....................................................................... 4

1.2.3 Concrete mixture proportioning .................................................................................. 5

1.2.4 Concrete durability properties .................................................................................... 5

1.2.5 Concrete tests ............................................................................................................... 5

1.2.6 Concrete construction control ..................................................................................... 6

1.2.7 Specifications ................................................................................................................ 6

1.3 Engineering Properties ........................................................................................................ 6

1.3.1 Compressive Strength ................................................................................................. 6

1.3.2 Curing ............................................................................................................................. 7

1.3.3 Water/binder (w/b) ratio and cement content ........................................................... 8

1.3.4 Influence of Mineral Admixtures on Compressive Strength ................................... 8

1.3.5 Superplaticizers ............................................................................................................ 9

1.3.6 Coarse Aggregates .................................................................................................... 11

1.3.7 Tensile strength .......................................................................................................... 15

1.3.8 Modulus of Elasticity .................................................................................................. 15

1.3.9 Shrinkage and Creep in HSC ................................................................................... 17

1.3.10 Stress Strain Models .................................................................................................. 23

1.3.11 Fire Resistance of High-Strength Concrete ........................................................... 27

1.4 HSC in Other Standards.................................................................................................... 32

1.4.1 HSC in Bridge Code (AS 5100, 2004) .................................................................... 32

1.4.2 HSC in Piling Code (AS 2159, 2009) ...................................................................... 33

1.4.3 Maritime Structures .................................................................................................... 34

Design Considerations in Maritime Structures Code (AS 4997, 2005) .............................. 35

Material Considerations in AS 4997 ........................................................................................ 36

1.4.4 HSC in Other International Standards .................................................................... 38

1.5 Applications of HSC ........................................................................................................... 39



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1.5.1 Precast Applications of HSC/HPC ........................................................................... 45

1.5.2 High Performance Concrete Pavements ................................................................ 48

1.5.3 Other Types of HPC ................................................................................................... 49



Documents

Introduction Mendis