Mechanical characteristics of self-compacting concretes with different ﬁller materials, exposed to elevated temperatures

8/8/2019 Mechanical characteristics of self-compacting concretes with different filler materials, exposed to elevated temperat

1/13

O R I G I N A L A R T I C L E

Mechanical characteristics of self-compacting concretes

with different filler materials, exposed to elevatedtemperatures

N. Anagnostopoulos K. K. Sideris A. Georgiadis

Received: 7 April 2008 / Accepted: 11 December 2008 / Published online: 24 December 2008

RILEM 2008

Abstract In this paper, the studies concern the

influence that different fillers have on the properties

of SCC of different strength classes when exposed to

high temperatures. A total of six different SCC and

two conventional concrete mixtures were produced.

The specimens produced are placed at the age of

180 days in an electrical furnace which is capable of

reaching 300C at half an hour and 600C at 70 min.

The maximum temperature is maintained for an hour.

Then the specimens are let to cool down in the

furnace. The hardened properties measured after fireexposures are the compressive strength, splitting

tensile strength, water capillary absorption and the

ultrasonic pulse velocity. Explosive spalling occurred

in most cases when specimens of higher strength

class are exposed to high temperatures. The spalling

tendency is increased for specimens of higher

strength class C30/37 irrespective of the mixture

type (SCC or NC) and the type of filler used.

Keywords Filler materials Glass filler

Mechanical characteristics Self-compacting concrete Slag Temperature

1 Introduction

1.1 General

An SCC is due to its various advanced properties

most useful regarding to the structure industry. The

ability to self compact without the use of any vibrator

allows SCC to pass through dense reinforcement and

fill in restricted sections, guaranteeing time superior

quality of the cast structure at the same. Moreover the

fact that the compaction takes place while casting,without any further delay, ensures a tight and

accurate construction schedule. The feature of self

consolidation is partly based on a new method for the

production and quality control of SCC [1] which

involves lower water to binder ratio, accumulates the

use of filler materials and the addition of superplast-

icizer in order to achieve the desired workability.

Many scientists have reported the similarity of SCC

with high performance concrete (HPC), which is also

produced with decreased water to cement ratio and

certain chemical admixtures [2, 3]. The problem thatoccurs is the behavior of such concrete mixtures

when exposed to high temperatures.

In general, concrete as a building material has a

reasonably good fire resistance. But when SCC or

HPC is used there are some complications. These

complications concern microstructure changes which

grow along with the increasing temperature [4]. At

certain temperatures there is apparent deterioration

mostly due to the dehydration of CSH gel and the

N. Anagnostopoulos K. K. Sideris (&) A. GeorgiadisLaboratory of Building Materials, Democritus University

of Thrace, P.O. Box 252, Xanthi 67100, Greece

e-mail: [email protected]

Materials and Structures (2009) 42:13931405

DOI 10.1617/s11527-008-9459-6


2/13

increasing pore water pressure. The finer pore

distribution along with the poor pore connectivity

that characterizes SCC and HPC keeps the free and

chemically bound water trapped inside the structure,

leading to growing pore pressure [2, 3, 5]. When high

temperature and high heating rate are applied, the

concretes fire resistance is most likely to decreaseand thus spalling to occur.

1.2 Objective

Considering that SCC is a newer type of concrete

compared to the traditional concrete or even HPC, the

research performed on SCC after fire exposure is yet

limited. As reported in [2] the SCC mixtures which

are subjected to fire have an explosive spalling

tendency which is evident in concrete mixes of higher

strength classes, while SCC of lower strength classeshas a rather good fire resistance. In this contribution

the efforts are focused on producing SCC of different

strength classes which incorporate different filler

materials, in order to investigate their performance

after exposure at gradually up scaled temperature.

2 Materials and methods

2.1 Materials and mixtures

A total of six SCC and two NC mixes are produced

for this study. The same class of blended cement

(CEM II 42, 5 N) is used in all cases to produce

strength classes such as C25/30 and C30/37, accord-

ing to EN206-1 [6]. Coarse aggregates consisting of

crushed granite and limestone sand are used. A high

range water reducing carboxylic either polymer

admixture is added in different dosages to achieve

slump of 190 mm in the case of NC, or self

compactibility in the case of SCC. The filling

materials used for the production of all SCC mixesare respectively: limestone filler, slag and glass filler.

The cement-filler material chemical compositions as

well as the aggregate grading curves are listed in

Tables 1, 2 respectively. In all cases the water/

cement ratios as well as the cement content are kept

relatively the same for each strength class. Moreover

the slump flow tests and slump tests with reference to

SCC and NC correspondingly were attempted to be of

the same order of value and thus to present respective

properties while in fresh state. The mix proportions of

all concretes are presented in Table 3.

2.2 Specimens and temperatures

The mixing is carried out in a pan mixer according tothe European Guidelines for SCC [7]. Right after the

mixing is completed the SCC is tested accordingly as

instructed in EFNARC specifications [8]. A number

of 150-mm cubes are prepared in order to assess the

compressive strength and the water capillary absorp-

tion at the age of 28 days. The water capillary

absorption is measured according to the procedure

described by RILEM TC116 [9]. The 150-mm cubes

are tested for compressive strength after a period of

Table 1 Chemical composition of cement and filling materials

(%)

Sample CEMII-A/M

42.5 N

Limestone

filler

Ladle

furnace slag

Glass

filler

SiO2a 23.85 1.8 32.5 62.1

Al2

O3

5.22 0.45 2.5 1.6

Fe2O3 4.13 0.08 0.1

FeO 1.72

CaO 58.2 54.8 54.1 18

MgO 3.2 0.68 5.55 2.4

SO3 3.3 0.05 0.2

K2O 0.68 0.04 0

Na2O 0.32 0.34 12.4

TiO2 0.24 0.17

P2O5 0.06 0.02 0.1

SrO 0.03

Cr2O3 0.02 ZnO 0.01

LoIb 1.57 40.5 3.19 0.4

SGc (g/cm3) 3.1 2.65 2.59 2.51

a All the samples are expressed by weigh percentageb Loss of ignitionc Specific gravity

Table 2 Composition of materials used (%)

Aggregate passing 0.25 0.5 1 2 4 8 16 32Limestone sand 6 18 56 81 100 100 100 100

Coarse aggregates 0 0 0 0 12 57 100 100

All fine materials pass through the 0.125-mm sieve

1394 Materials and Structures (2009) 42:13931405


3/13

28 days of moist curing (20 2C, RH C 95%), as

the mean value of three specimens. Fire resistance is

measured on 100-mm cubes and 150 9 300-mm

cylinders. Those specimens also go through moist

curing for 28 days and are then left at ambience

(20 2C, RH C 65%), and not tested till the age of

fire tests (180 days).

Right before fire exposure at the age of 180 days

three 100-mm cubes of each mixture are dried to

constant mass at 105C. Table 4 shows the moisture

content that is afterwards determined as following:

W= (m0 - md)/mdmd = mass of the test specimen after drying at

105C

m0 = mass of the specimen before drying

At the age of 180 days the specimens which meant

to be exposed to high temperatures are heated in an

Table 3 Mix design proportions and fresh properties of self-compacting concretes (SCC) and normal concretes (NC)

Mix design proportions (kg/m3) Self compacting concrete Normal concrete

L-filler Slag Glass filler

SCC SCC SCC SCC SCC SCC NC NC

C25/30 LF C30/37 LF C25/30 SL C30/37 SL C25/30 GF C30/37 GF C25/30 NC C30/37 NC

CEMII-A/M 42.5 N 335 375 340 375 340 380 330 375

Filler 135 100 0 0 0 0 0 0

Slag 0 0 135 100 0 0 0 0

Glass filler 0 0 0 0 130 100 0 0

Sand 915 900 825 862 845 862 940 870

Coarse aggregates 800 800 800 800 800 800 927 955

Water 185 186 188 189 190 194 183 186

Super plasticizer (%)a 1.63 1.88 1.29 1.74 1.16 1.17 1.0 1.0

W/C 0.55 0.50 0.55 0.50 0.56 0.51 0.55 0.50

W/P 0.39 0.39 0.40 0.40 0.40 0.40 0.55 0.50

Air content (%) 1.70 1.60 1.90 1.70 1.40 1.20 2.10 1.80Slump (cm) 19 20

Slump flow D (cm) 75.5 75 75.5 75.5 74 73.5

t50 (s) 2 1.72 4.72 4.25 1.66 1.25

V funnel 1 (s) 10.5 10 8.49 9.18 4.38 6.06

V funnel 2 (s) 28 15 14.4 11.25 5.16 13

J ring H (cm) 0.3 0.3 1 0.9 0.6 0.5

J ring D (cm) 68 68 67 68 66 68

LBOX (h2/h1) 0.88 0.88 0.83 0.85 0.82 0.84

t200 (s) 1 1 2.5 3.41 1.2 1.35

t400 (s) 2.01 3 5.5 5.1 1.4 2.25

fc28 (Mpa)b 37.1 54 37.7 53.5 38.3 49 36 52.7

a SP (super plasticizer) value is measured by % percent by weight of the entire fines amount (cement and filler materials)b

Compressive strength at the age of 28 days is measured in specimens of 150-mm edge cubes

Table 4 Moisture content of all the mixtures at the age of 180 days

Mixture C25/30

CC-LF

C30/37

SCC-LF

C25/30

SCC-SL

C30/37

SCC-SL

C25/30

SCC-GF

C30/37

SCC-GF

C25/30

NC

C30/37

NC

Moisture content (%)a 4.41 3.39 3.17 3.18 4.01 4.11 3.98 3.93

a Moisture content is measured in specimens of 100-mm edge cubes

Materials and Structures (2009) 42:13931405 1395


4/13

electrical furnace Fig. 1. Two peak temperatures are

examined to determine the specimens fire resistance:

300 and 600C. The heating rate applied is 10C/min

until the target temperature is reached, and this is

maintained for a period of 1 h, Fig. 2. When the

heating period finishes, the furnace remains sealed for

24 h in order to cool down the specimens down to theambient temperature. Then the specimens are tested

to determine properties such as compressive strength,

splitting tensile strength and pulse velocity.

2.3 Compressive strength

The compressive strength is measured according to

EN 12390-3 [9]. The original compressive strength is

measured on 150-mm cubes which are tested at the

age of 28 days in order to specify the concretes

strength class. Residual compressive strength ismeasured after the fire tests on 100-mm cubes. A

Buehl & Fabel compression testing machine with

3000 KN capacity is used in all cases.

2.4 Splitting tensile strength

Splitting tensile strength is determined by measuring

the tensile strength on 150 9 300-mm cylinders at

different peak temperatures so that the residual

tensile strength is assessed in each case. According

to EN 12390-6 [10] a splitting attachment (CON-TROLS Model 50-C9000) is adjusted on the

laboratory compression testing machine.

2.5 Water capillary absorption

The water capillary absorption is measured according

to the procedure described by RILEM TC116 [11].

That property is measured on pre-weighted 150-mm

cubes. Specimens are placed on adjusted plastic

plates filled with water, so that only one surface of the

specimen is getting wet. Then the specimens areweighted in regular intervals and the absorbed water

quantity is estimated.

2.6 Stressstrain curves

100-mm cubes are used in order to calculate stress

strain curves at the age of 180 days. The specimensFig. 1 Indicative seating plan of the specimens in the furnace

Fig. 2 Temperature development of 300C (left) and 600C (right)



5/13

are placed on the laboratory compression testing

machine and stressstrain sensors are adjusted on the

upper metallic plate of the pressing device. As soon

as the specimen is loaded, the sensors transmit

electric signal to the data logger which is converted

through a computer programme into stressstrain

curves.

2.7 Pulse velocity

Pulse velocity is measured on 100-mm cubes accord-

ing to the procedure described by EN 12504-4 [12].

Specimens are tested at the age of 180 days before

and after fire tests using a PUNDIT ultrasonic pulse

velocity testing device.

3 Results and discussion

3.1 Original compressive strength

The compressive strength results at the age of

28 days are presented at Table 3 for all prepared

concrete mixes. While studying the compressive

strength results it emerges that in almost all cases

SCC develop higher values as compared with NC of

the same strength class. This is attributed to the

changes of the interfacial transition zone (ITZ)

caused by the different filler materials [13]. Asreported in Zhu and Bartos [14] ITZ is denser and

significantly more uniform in SCC than in NC.

Moreover as Traghard points out, the porosity of ITZ

is much lower in SCC than in NC of the same w/c

ratio, as the hydrated phases and unhydrited particles

appear to be more evenly distributed between the ITZ

and bulk density of SCC [15].

In the case of SCC, it appears that there are slight

variations as it regards their compressive strength

values. The use of different filler materials for SCC

mixture production has everything to do with these

deviations. SCC mixtures which are produced usingladle furnace slag as a filler material have higher

water absorption than expected, resulting to a viscous

and rather slow concrete, while in fresh state. On

the other hand, when limestone filler is used the

mixture performance is excellent in terms of rheo-

logical and mechanical characteristics. Similar

performance is noted in the case of glass filler with

even better rheological features. As mentioned

before, all mixtures are produced by keeping the w/c

ratio relatively the same. That means that in relation to

their absorption requirements, certain porosity isdeveloped in each case, which is finally reflected in

the compressive strength values.

3.2 Residual compressive strength

What would be of great importance in a fire scenario

is definitely the state of the concretes mechanical

properties. The residual compressive strength for all

SCC and NC mixtures after heating in 300 and 600C

is presented in Fig. 3. As Chan reports in his

investigation there are three temperature ranges fromthe point of strength loss: 20400, 400800 and

8001200C [2, 5]. After exposing to fire HPC

and NC mixtures prepared with ordinary Portland

cement, Chan concludes that only a small part of the

original strength is lost up to 400C, while severe

Fig. 3 Compressive strength of C25/30 (left) and C30/37 (right) SCC and NC, after heating at different temperatures (300 and

600C) compared with the compressive strength at room temperature (20C)



6/13

compressive strength loss occurs within the 400

800C range. That is mostly the case for the SCC and

NC mixtures prepared in this contribution, which are

exposed to slightly different temperature ranges,

though.

Regarding the 20300C range, there is a com-

pressive strength reduction for the SCC 25/30 whichfluctuates between 12% and 15% of its initial value,

while for the SCC 30/37 the strength loss percentage

reaches 18%. The corresponding strength loss per-

centages for the equivalent 25/3030/37 NC are 18%

and 17.6% respectively. At 600C the reduction in

compressive strength ranges from 52% to 57% for all

mixtures and explosive spalling occurs in cylindrical

specimens in all cases. The phenomenon is more

intense when ladle furnace slag is used as filler

material and spalling occurs in 100-mm cubes as

well. As there was no way for visible inspection,spalling is identified by hearing the series of pop outs,

happening in most cases when the distinctive tem-

perature of 300C is overrun.

The use of different filler materials in the case of

SCC does not seem to make any difference as it

regards explosive spalling Fig. 4. During fire the

humidity of the concrete increases along with tem-

perature and fluid water is formed [16]. The water is

transported inwards to the center of the specimen

where the space is limited [16]. At a certain point the

region becomes saturated and the entrapped water iseventually released in the form of steam by explosive

spalling [17, 18]. The filler content used is relatively

high (150 kg/m3) to ensure low levels of concrete

moisture and eventually to avoid spalling due to

steam pressure. The concept of using glass filler is the

formation of micro-cracks which develop because of

the thermal expansion of glass that is greater than the

concretes [16]. Thus greater pore connectivity is

thought to provide canals for the steam to escape.

Ladle furnace slag is used as a filler material as itpresents cementitious behavior mainly due to its high

content in CaO [19]. According to Piasta et al. the

development of micro-cracks increases beyond

300C and firstly occurs around calcium hydroxide

Ca(OH)2 crystals [20]. Hence slag due to its cemen-

titious properties is expected to ensure the

development of micro-cracks which will lead to

greater porosity.

3.3 Moisture content and water absorption

Since the use of different filler materials is expected

to alter the porosity of SCC mixtures and thus to

prevent the effect of spalling, the moisture content

and the capillary water absorption for all mixtures are

presented in Tables 4 and 5 respectively. As Bostrom

et al. points out, SCC has a high probability of

spalling when exposed to fire compared to conven-

tional concrete [21]. Considering the low

permeability of SCC due to its denser structure,

water vapor is very limited to evaporate out of SCC.

Lower moisture content is therefore of great signif-icance, since the accumulated pore pressure is

accordingly minimized.

The concrete mixtures produced in this investiga-

tion vary with reference to their capillary water

absorption since they belong to different strength

classes. Concrete mixtures of the lower strength

classC25/30appear to have greater water capillary

absorption values compared to C30/37. Lower w/c

ratios as well as the higher cement content that is

used for the C30/37 production forms a tighter

structure which eventually results in lower perme-ability. Water capillary absorption values are in all

cases lower in SCC compared to NC of the same

strength class, probably due to a more efficient

packing that is achieved by the use of filler materials.

Among SCC mixtures the one produced with glass

filler seems to have greater water absorption values,

since the w/c ratio in that occasion is somewhat

higher. SCC produced with ladle furnace slag appears

to be less permeable than the remaining mixturesFig. 4 Spalled specimens after exposure at high temperature

(600C)



7/13

mainly due to its denser microstructure, while

limestone filler performed similarly. In any occasion

there is explosive spalling when the peak temperature

of 600C is maintained when all concretes of both

strength classes are tested, and this is valid irrespec-

tive of the strength class and the filler material used.

3.4 Pulse velocity

The significant changes which concrete specimens

undergo as far as their pore structure is concerned,

when heated at different peak temperature are

assessed by the pulse velocity test. The results are

plotted in Fig. 5 where the residual pulse velocity is

expressed as the ratio of the pulse velocity after

exposure to each peak temperature to the initial value

at ambient temperature. Using pulse velocity propa-

gation is possible to figure out through a non

destructive method, the extent of deterioration due

to elevated temperatures or even the presence of

cracks and voids [22]. A decrease in velocity

indicates the initiation of cracks in the concrete mass

and increase in the porosity [2]. According to Piasta

the development of micro-cracks in cement paste

increases significantly beyond 300C [20]. Lin et al.

further confirms that the majority of the cracks and

the extremely large cracks are formed between 300

and 500C [23].

The relative pulse velocity results in this research

coincide well with the above made statements, as

there is a slight reduction in the slope up to 300C

and then the angle of gradient is greater for the

temperature range of 300600C for all mixtures

Fig. 5 (C25/30-left and C30/37-right). For the con-

crete mixtures which belong to the lower strength

classC25/30the relative pulse velocity is almost

identical for all mixtures after exposure at 300C. At

the following temperature range (300600C) more

severe degradation occurs. After exposure to 600C

SCC produced with limestone filler have the best

performance and SCC with ladle furnace slag appear

to have suffered greater deterioration. That is mostly

the case for the C30/37 mixtures. After exposure at

600C all SCC specimens containing slag suffer

severe deterioration due to explosive spalling. With

the exception of all mixture produced with slag,

SCCs of the higher strength class perform slightly

higher residual values compared to ordinary concrete

after exposure to 600C.

3.5 Residual tensile strength

With a close look at the tensile splitting strength in

Fig. 6 it becomes evident that there is a sharp loss of

tensile strength compared to a rather smoother declin-

ing curve which corresponds to the compressive

Table 5 Capillary water absorption (g/cm2

)

Time/mix C25/30

SCC-LF

C30/37

SCC-LF

C25/30

SCC-SL

C30/37

SCC-SL

C25/30

SCC-GF

C30/37

SCC-GF

C25/30

NC

C30/37

NC

T /10 min 0.1578 0.1022 0.1428 0.1122 0.1569 0.1448 0.1444 0.0933

T /24 h 0.5133 0.4133 0.5019 0.4003 0.5180 0.4536 0.5444 0.4667

Fig. 5 Ratio of residual pulse velocity [V(T)] after peak temperature to the pulse velocity at room temperature [V(20C)] of C25/30

(left) and C30/37 (right) for SCC and NC



8/13

strength loss at different peak temperatures [2, 5].

Chan et al. [5] attribute this to the presence of many

micro or macro cracks that are produced in the

specimens due to thermal incompatibility.

The tensile splitting strength results for the C25/30

mixtures are plotted in Fig. 6 consisting of two

descending branches, exhibiting the strength loss at

20300 and 300600C temperature ranges respec-

tively. All mixtures of this strength class tend to a

similar decrease from 20 to 300C. SCC with

limestone filler appear to have the best perfor-

mance, from 300 to 600C. On the other hand SCC

with ladle furnace slag in its composition along

with SCC using glass filler follows similar decline

with NC and suffers the greater loss of tensilestrength at the range of 300600C. Splitting

tensile strength results are only plotted from 20

to 300C regarding to the higher strength class

concrete, since all the cylinders which belong to

that strength category spall explosively. SCC with

limestone filler manage in this case also to

maintain greater percentage of its original tensile

strength compared to the rest SCC and NC of the

same strength class.

3.6 Stressstrain curves

The stressstrain curve, representing the deformation

and mechanical characteristics, is an important

material characteristic of concrete [24]. It is also

important to extract results for the concrete attributes

from the stressstrain curves at elevated tempera-

tures, although many coexisting effects determine the

shape of the curve. In this study, an attempt is made

to observe the difference of the stressstrain curves

among SCC mixtures prepared with several filler

materials compared to normal concrete.

As it is observed in Fig. 7, there is no noticeable

difference between SCC and NC in the shape of the

curves, either between SCC with different filler

materials. Generally, the ascending phase of all the

curves as the temperature increases becomes

smoother especially at high temperatures (600C)

while the peak strain increases and the peak strength

decreases. It must be mentioned that there could be a

pronounced concave-up curve at the beginning of

loading due to the pre-existing cracks caused by

heating and cooling [25]. Comparing the curves

between the two strength classes it can be mentioned

that the higher the strength class, the more rapid theascending phase and the more linear the descending

one, due to the stiffness softening of the specimens at

all temperatures respectively. Also, the percent of

peak strain increasing is for the C25/30 mixtures 12

15% and for the C30/37 mixtures 710% at 300 and

at 600C the respective percent is 6065% and 50

55%. Finally, SL mixtures give the impression of

having more linear phases and rough curve distribu-

tion before and after the peak strength compared to

the GF mixtures that have the smoother distribution.

3.7 Model suggestion

The model equations which are proposed in order to

evaluate the mechanical characteristics for both

unheated and heated concrete are shown in the

following equations [25]. In this paper the residual

mechanic characteristics (only the peak values) are

evaluated and compared with the values that have

been produced by the experimental program at

Fig. 6 Tensile strength of C25/30 (left) and C30/37 (right) for SCC and NC after heating at different temperatures



9/13

different temperatures (T). In particular the residual

peak strength value (fcr) is evaluated by Eq. 1 using

original compressive strength (fc). Equation 2

expresses the peak strain value (eor) using the

unheated peak strain. The residual tensile strength

(ftr) is calculated by Eq. 3 using the experimentally

measured tensile strength (ft), whilst Eq. 4 evaluates

the residual modulus of elasticity (Ecr).

fcr=fc 1:008T

450ln T5800

!0:0; 20C\T800C1

Fig. 7 Stressstrain curves of C25/30 (left) and C30/37 (right) of NC (a, b) and SCC (ch) after heating at different temperatures



10/13

ftr=ft 1:05 0:025 T;

0:80;1:02 0:0011 T!0;

8

Documents

Mechanical characteristics of self-compacting concretes with different ﬁller materials, exposed to elevated temperatures