Optimization of the Concrete Mix Proportions Centered on Performance After Exposure to High Temperature

A. Chaboki-Khiabani1, a, M. Bastami 1, b M. Baghbadrani2, c and M. Kordi 2, d 1Assistant Professor, Engineering faculty, University of Kurdistan, Sanandaj, Iran

2Engineering faculty, University of Kurdistan, Sanandaj, Iran

aa.chaboki@uok.ac.ir, bm.bastami@uok.ac.ir, cm.baghbadrani@uok.ac.ir, dm.kordi@uok.ac.ir

Key words: High Strength Concrete, mix proportion, Taguchi approach, retained, elevated temperature.

Abstract. This paper presents the results of an experimental and statistical study on the effect of

high temperatures on the retained mechanical properties of high-strength concretes (HSC). The

mechanical properties of HSC significantly change during and later than exposure to elevated

temperature. The compressive and splitting tensile strength of more than 400 HSC cylindrical

specimens with sixteen mix proportion have investigated to study the effect of mix proportion on

the retained mechanical properties of HSC specimens after heating. According to these results, a

considerable loss was observed for all mixes and specimens in strength particularly in tensile

splitting strength. In addition, these experimental data were investigated using Taguchi approach to

find the effective parameters of mix proportion. Also, the most optimum mix proportion was found

and checked experimentally. According to our results, by controlling some factors in the mix

proportion, it is possible to reduce the retained destructive effects of elevated temperature on HSC

specimens.

Introduction

Some studies have found that the chemical composition and physical structure of high strength

concrete (HSC) significantly change during and after exposure to high temperature [1]. this

exposure leads to transformations and reactions that cause the progressive breakdown of cement gel

structure and aggregates and consequent loss in its mechanical properties such as load bearing

capacity[2, 3]. High temperatures also cause chemical and micro-structural changes, such as water

migration (diffusion, drying), increased dehydration, interfacial thermal incompatibility, and

chemical decomposition of hardened cement paste and aggregates[2]. These changes decrease the

strength and stiffness of HSC members permanently and increase the deformation of HSC

members[4, 5]. The behavior of high strength concrete under and after fire exposure is determined

by the numerous factors such as the aggregates, the cement matrix, its moisture, content, pore

structure and loading, in addition to the rate of heating and maximum temperature attained[6].

Among these factors, the mix proportion of HSC is one of the key factors that could efficiently

control its permeability, bearing capacity, moisture content, density, aggregate interlock, pore

structures and even durability[7, 8, 9].

Finding the optimum mix proportion of HSC using Taguchi optimization method

Due to the wide variability of material contents of HSC, in general, a large number of experiments

are usually required to find a suitable mixture for attaining the targeted requirements. Therefore;

utilizing a reliable optimization method is necessary. Thus, the fractional factorial experiments

using an orthogonal array were investigated by Taguchi, which can substantially decrease the

number of experiments required to study the effects of factors and their interactions[10, 11]. Four

control factors, including water to binder material, the ratio of fine aggregate to total aggregate

silica fume percentage and the supper plasticizer were selected for this study. Each of these factors

was treated at four levels, as shown in Table 1.

Advanced Materials Research Vols. 268-270 (2011) pp 372-376Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.268-270.372

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Table 1 Optimization parameters and their variation levels

Level Variables

Levels

1 2 3 4

A Water to Binder Material 0.21-0.23 0.23-0.25 0.25-0.27 0.27-0.29

B Fine aggregate to total aggregate 0.32-0.36 0.36-0.40 0.4-0.44 0.44-0.48

C Silica Fume to cement % 2-6 6-10 10-14 14-18

C Supper Plasticizer (kg/m3) 0-5 5-10 10-15 15-20

Preparation and heating of the specimens

The experimental program included making more than 400 cylindrical HSC specimens with sixteen

different mix proportions and then applying the standard ISO 834 elevated temperature to them.

The details of each HSC mix proportion used and the average retained strength data for each mix

proportion are listed in Table 2.

Table 2 The components of each Mix proportion and the their average retained strength

Mix Name

water (kg/m3)

cement (kg/m3)

Cilicia fume

(kg/m3)

Large aggregate

(kg/m3)

Small aggregate

(kg/m3)

Super plasticizer

(kg/m3)

Ave. comp.

strength (MPa)

Ave. tensile.

strength (MPa)

Retained comp.

strength (Mpa)

Retained comp.

strength %

Retained tensile

strength (MPa)

Retained tensile

strength%

Mix1 151 520 64 1153 600 11 77 5.9 21 27 1.3 22

Mix2 130 513 43 1080 685 16 86 6.4 16 19 1.3 21

Mix3 144 564 89 1068 593 20 82 6.0 15 18 1.5 25

Mix4 125 545 52 1103 589 18 78 6.0 21 27 1.3 22

Mix5 135 500 30 1110 700 14 90 6.4 19 21 1.2 19

Mix6 151 519 62 1122 579 9 94 6.6 16 17 1.5 22

Mix7 165 535 80 1153 597 5 81 5.7 14 17 1.3 23

Mix8 150 500 25 1004 822 9 78 5.8 20 26 1.2 20

Mix9 150 500 75 1016 760 3 82 6.0 25 30 1.4 24

Mix10 150 500 25 1233 617 8 81 5.9 21 26 1.1 19

Mix11 150 475 24 1066 710 3 76 5.7 17 22 1.1 19

Mix12 156 487 47 1068 676 11 66 5.5 14 21 1.2 21

Mix13 150 500 25 1233 617 8 81 6.4 22 27 1.3 20

Mix14 150 475 24 1066 710 3 76 6.1 21 28 1.2 19

Mix15 156 487 47 1068 676 11 64 5.4 17 27 1.1 21

Mix16 150 500 0 1234 617 8 72 5.9 17 24 1.1 18

Analysis the results with Taguchi approach

The best possible levels of mix proportions of HSC were investigated for the maximization of

compressive strength and splitting tensile strength values using the Taguchi technique. The

compressive retained strength and splitting tensile strength are a ‘larger the better’ type of quality

characteristic since the goal is to maximize these strengths. The standard S/N ratio computing

formula for this type of response is:

]11

[log101

210 ∑=

−=

n

j iji YnN

S

(1)

Where ‘i’ is the number of a trial; ‘Yij’ is the measured value of quality characteristic for the ith

trial and jth experiment; ‘n’ is the number of repetitions for the experimental combination. A

statistical analysis is performed to determine the statistically significant factors and the data analysis

Advanced Materials Research Vols. 268-270 373

is presented in Tables 3 and 4. In these tables, the degree of contribution of each significant factor is

obtained so as to determine the level of its statistical importance in the model.

Table 3 Response Table for Signal to Noise Ratios for compressive strength (Larger is better)

Supper

Plasticizer

Silica

fume %

Fine aggregate to total

aggregate

Water to

binder Level

27.16 26.92 28.31 26.98 1

27.34 26.77 26.86 25.99 2

26.86 27.97 26.3 27.78 3

27.85 27.54 27.74 28.45 4

0.99 1.2 2.01 2.46 Delta

4 3 2 1 Rank

Table 4 Response Table for Signal to Noise Ratios for tensile splitting strength (Larger is better)

Supper

Plasticizer

Silica

fume %

Fine aggregate to total

aggregate

Water to

binder Level

26.53 26.09 26.51 27.03 1

26.02 26.23 26.11 26.42 2

26.05 26.79 26.8 26.3 3

26.94 26.42 26.1 25.79 4

0.92 0.7 0.7 1.24 Delta

2 4 3 1 Rank

Since all parameters had interaction between them, the best possible testing conditions of the HSC

properties can be determined from the main effect plot graphs which is shown in Figure 1 and 2.

Figure 1 Main effect plot for retained compressive strength

374 Computational Materials Science

Figure 2 Main effect plot for retained splitting tensile strength

According to these figures, the water to binder ratio has the most effective effect on the retained

compressive strength of specimens. Moreover, the impact of fine aggregate ratio is also significant.

However the effect of Silica fume and super plasticizer is not that significant.

In addition, the water to binder ratio has the most effective effect on the retained tensile strength of

specimens. Furthermore, the impact of super plasticizer is also significant. However the effect of

Silica fume and fine aggregate ratio is not that significant. Using Taguchi approach, the best mix

proportion for the retained stresses is tabulated in Table 5.

Table 5 The optimum factor’s value according to the Taguchi approach

SP (kg/m3) Silica

fume%

Fine agg.

to all agg. W/b Optimization goal

20 12% 0.34 0.28 The retained comp. strength

20 12% 0.42 0.22 The retained tensile strength

Therefore, just only one mix proportion is not most advantageous proportion for both retained

purposes: compressive and tensile strength of HSC specimens. In order to verify the optimum mix-

design proportion obtained using the Taguchi method, laboratory experiments were carried out to

determine whether the retained compressive strength and tensile splitting strength can be maximize

by the proposed optimum mixture proportions. To obtain meaningful results, the same materials and

the same conditions were used as in the Taguchi analysis. The results are shown in Table 6 and

table 7.

Table 6 The optimum mix proportion verification on retained compressive strength

HSC mix

Water

(kg/m3)

Cement

(kg/m3)

Cilicia

fume (kg/m3)

Gravel

(kg/m3)

Sand

(kg/m3)

Super

plasticizer (kg/m3)

Ave. comp.

strength

(Mpa)

Average retained

strength

(Mpa)

Retained strength

reduction

%

Taguchi

Method prediction

Mix17 157 500 60 1140 590 20 81 24 30 31

Advanced Materials Research Vols. 268-270 375

Table 7 The optimum mix proportion verification on retained tensile strength

HSC

mix

Water

(kg/m3)

Cement

(kg/m3)

Cilicia

fume (kg/m3)

Gravel

(kg/m3)

Sand

(kg/m3)

Super

plasticizer (kg/m3)

Ave. tensile

strength

(Mpa)

Average retained

strength(

Mpa)

Retained strength

reduction

%

Taguchi

Method prediction

Mix18 124 500 60 1004 726 20 6.2 1.6 26 26

The verification study results showed that the proposed optimum mix proportions concurred well

with result obtained by Taguchi’s approach in both compression and tension.

Conclusion

On the basis of the experiments carried out on the various specimens and the Taguchi optimization

approach, the following conclusions could be made.

• There is no direct relationship between the compressive strength reduction and tensile

splitting strength in different mix proportion. Even thought, both decrease considerably,

their reduction percentage is not associated.

• The verification study results showed that the proposed optimum mix proportions concurred

well with result obtained by Taguchi’s approach. However, this mix proportion optimization

increased the retained compressive and tensile strength of HSC up to 10 percent which is not

very significant when compared to the strength reduction percentage.

• Using these data, we have checked the data with Taguchi approach and it is found that the

water to binder ratio is the most important factor in mix proportion in both compressive and

tensile strength.

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376 Computational Materials Science

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