lecture2-2015

Spring 2015

Part 1: Concrete mix design

Part 2: Concrete problems

Contents:

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Part 1: Concrete mix design

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1-Cement

Cement Type Use Type 10

Normal Portland

cement General purpose cement suitable for

all uses (where special properties are not requires)

Type 20

Moderate Portland cement

Used where precaution against moderate sulphate attack is important

Type 30

High early strength Portland cement

High strength at early period (usually a week or less)

Type 40

Low heat of hydration Portland

cement

When rate and amount of heat generated from hydration must be

minimized Type 50

Sulphate resistant Portland cement

For concrete exposed to severe sulphate action

According to CSA

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1-Cement ASTM C 150 provides 8 types:

-Normal

-Normal, air-entraining

-Moderate sulphate resistance

-Moderate sulphate resistance, air-entraining

-High early strength

-High early strength, air-entraining

-Low heat of hydration

-High sulphate resistance

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2-Aggregates: i- Generally, they must be:

-Clean, hard, durable -Free of absorbed chemicals -Free from coatings of clay -Free of fine materials in amounts that could affect the hydration and bond to the cement paste

ii-The nominal max size versus the maximum size (difference)

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2-Aggregates:

Nominal maximum size of an aggregate should not exceed: -1/5 the narrowest dimension between the sides of forms -3/4 of the minimum clear spacing between reinforcing bars and

forms -1/3 depth of the slab -For pumped concrete:

-1/3 smallest internal diameter of the hose or pipe -40 mm

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2-Aggregates: Property Effect

Potentially harmful material (as Organic impurities, fine

materials, soft particles, clay lumps)

pop outs, delayed setting , volume change of concrete, weak bond

between the cement paste and the aggregate

Resistance to freeze and thaw ( frost resistance of an

aggregate)

D-cracking of concrete

Wetting and drying properties Alternate wetting and drying cycles can cause severe strain in some aggregates leading to permanent increase in volume and concrete

breakdown Abrasion and skid resistance heavy duty floors and pavements

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3-Admixtures The main reasons for using admixtures:

1- To achieve certain properties in concrete

2- To maintain the quality of concrete during the stages of mixing,

transporting, placing and curing

3-To overcome certain emergencies No admixture of any type or amount can be considered a

substitute for good concreting practice

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3- Admixtures

Admixture Use Air entraining Improve durability in freeze and thaw

Water reducing To reduce water Typically 5-10%

Accelerating accelerate setting and early strength development

Retarding retard setting time Corrosion inhibitors Reduce steel corrosion in chloride

environment

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4-Supplementary cementing materials -They may be used in addition to or as partial replacement of the

portland cement . -They are used to improve a particular concrete property

Material Description

Fly ash Fine residue that results from the combustion of coal in electric generator

Slag made from iron blast furnace slag

Silica fume By product of the reduction of high purity quartz with coal in an electric arc furnace

Natural pozzolans

include calcined clay and calcined shale

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4-Supplementary cementing materials They are used to improve a particular concrete property

Property Effect Fly ash Silica fume Slag Natural pozzolans

water demand

dec. inc. dec. Little effect, may inc or dec

workability improve admixtures needed to maintain

workability

improve

improve

Bleeding and seggregation

dec dec

Bleeding inc.,

No effect on segregation

Little effect

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4-Supplementary cementing materials

Property Effect Fly ash Silica fume Slag Natural pozzolans

Heat of hydration

dec. Little effect ; may or may

not dec.

dec.

dec.

Permeability and absorbtion

dec.

dec. Very effective

dec.

dec.

Alkali aggregate reaction

dec.

dec.

dec.

dec.

Sulphate resistance

Inc. Inc.

Inc.

Inc.

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5-Concrete Mix design:

A proper proportioned concrete mix should posses the following: 1-Acceptable workability of fresh mix 2-Durability, strength, and uniform appearance of the hardened

concrete 3-Economy The design of the concrete mixture involves: 1- Establishing specific concrete characteristics 2-Selection of proportions of available materials to produce

concrete of required properties

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5- Concrete Mix design:

i-Strength: -The specified compressive strength (fc' ) at 28 days is the strength that is expected to be equal to or exceeded by the average of any set of 3 consecutives tests.No individual test (average of 2 cylinders) can be more than 3.5 MPa below the specified strength. -The average strength (fcr ' ) = the specified strength (fc' ) + allowance to account for variations in materials/mixing/placing..etc -The average strength (fcr ' ) is the strength required in the mix design

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5- Concrete Mix design

i-Strength: CSA A23.1:

fcr' =fc '+1.4S

fcr '=fc '+(2.4S-3.5) MPa

Where: S is the standard deviation (MPa)

If standard deviation is not available, Table 9-11 is used instead

to determine fcr

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ii-Classes of exposure: -Two classes for air content category 1: exposed to freeze and thaw, category 2: not exposed to freeze and thaw -Four classes of exposure for structures to Chlorides (CL) C1: Structurally RC exposed to CL (category 1 or 2) C2:Plain concrete exposed to CL (category 1) C3: Continuously submerged concrete exposed to CL (category 2) C4: P.C. exposed to CL (category 2) Three other classes for (air content and chlorides combination) F1 , F2 and N (page 155-156)

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ii-Classes of exposure:

-Three classes for Sulphate exposure:

S1, S-2 and S-3 as very severe, severe and moderate.

The degree of severity is determined based on the sulphate

concentration in soil or water.

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5- Concrete Mix design: It is the mass of water divided by mass of cementing materials selection of w/c ratio: 1-Must be the lowest value required to meet anticipated exposure conditions (Tables 9-1 and 9-2 co-relate the maximum w/c with class of exposure) 2-If durability and exposure are not the governing factors, it should be selected based on compressive strength ( from Table 9-3 and Figure 9-2)

iii-Water-Cementing Materials ratio (w/c):

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iii-Water-Cementing Materials ratio (w/c): a-amount of water in (kg/m3)

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5- Concrete Mix design: b-Supplementary cementing materials: as Fly ash, natural pozzolans, Slag, silica fume


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5- Concrete Mix design: -The maximum size is determined as explained earlier. -From Table 9-4, the volume of dry coarse aggregate per unit

volume of concrete is obtained

iv- Aggregates:

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5-Concrete Mix design: -How can we get the volume of fine aggregates!!! - We need weights, so how can we translate volume to weight!!

iv- Aggregates:

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5-Concrete Mix design:

Additional information The code provides values for: - Recommended slump ( Table 9-6) -Minimum cement content:

To ensure satisfactory durability and guarantee suitable appearance of vertical surfaces

Concrete under water=390kg /m3 of cementing materials

Severe freeze and thaw and sulphate exposure=335kg /m3 of cementing materials

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-See an EFFECT then determine the CAUSE

-The EFFECT of undesirable behaviour can be seen as: cracking, scaling, disintegration, spalling , erosion, seepage, distortion,

delamination, or popouts

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i- Cracks in concrete:

Types of cracks

After hardening

Physical Drying shrinkage

Crazing

Chemical Corrosion of RFT, AAR, Carbonation

Thermal Freeze/thaw, Temp.

variations, early thermal contraction

Structural Overload, Creep, Design loads

Before hardening

Plastic Shrinkage/settlement

Constructional movement formwork, subgrade 30


i- Cracks in concrete: Cracks occur manifest themselves at different times and locations; For instance:

Plastic shrinkage: 30 minutes to 6 hours In roads , slabs, RC slabs due to rapid early drying ( random or diagonal). It could also occur over the RFT in the R.C. slabs if the RFT is near the surface.

Plastic settlement: 10 minutes to 3 hours In deep sections and top of columns ( Over RFT and arching).

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Corrosion: More than 2 years Beams and columns.

AAR: more than 5 years mainly damp locations (reactive aggregate + high alkali cement)

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Crazing: 1-7 days slabs

Early thermal cracking: 1 day-2/3 weeks thick wall

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Load induced crack:

Pure exure

Pure tension

Shear

Shear + moment

Bond

Compression load

Torsion

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ii-Other Symptoms of undesirable behaviour:

Scaling:

is local flaking or peeling away of the near surface portion of concrete.

Spalling:

deeper surface imperfection extending to the top layers of reinforcing steel

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Delamination:

-is a separation along a plane parallel to a surface, in the case of a concrete slab, a horizontal splitting, cracking, or separation within a slab in a plane roughly parallel to, and generally near, the upper

-caused by the corrosion of reinforcing steel or freezing and thawing; similar to spalling, scaling, or peeling except that delamination affects large areas

- can often only be detected by non destructive tests,

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Erosion

progressive disintegration caused by the actions of fluids or solids in motion

Seepage

Movement of water through pores and cracks

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Popout

is the breaking away of small portions of a concrete surface;

small popouts leave holes up to 0.4 in. (10 mm) in diameter;

medium popouts leave holes 0.4 to 2 in. (10 to 50 mm) in diameter;

large popouts leave holes greater than 2 in. (50 mm) in diameter.

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ii-Cracks Widths Generally; Crack width is affected by: -tensile stress in reinforcement (most important variable) -thickness of cover -area of concrete around each bar

Fine crack

< 0.3 mm

No repair

Medium cracks

0.3-0.5 mm

loss monitor

Wide cracks

>0.5 mm

considerable loss, immediate repair

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Exposure Condition Crack width (mm)

Dry air or protective membrane 0.41

Humidity, moist air, soil 0.30

Deicing chemicals 0.18

Seawater and sea water spray wetting and drying

0.15

Water retaining structures 0.1

ii- Cracks Widths

According to CEB-FIP 1990 provisions

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How do we calculate the crack width for a given element?

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i-Crack control parameter

-Beams and One way slabs ( section 10.6 in CSA)

Bars in tension zone shall be spaced as follows: z = fs (dc A)1/3

less than 30 kN/mm for interior exposure less than 25 kN /mm for exterior exposure

What is Fs, dc, A!!!

Limiting (z) corresponds to crack widths of 0.4 mm and 0.33 mm,

respectively. ii-Skin Reinforcement

ii- Cracks Widths

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lecture2-2015