INFLUENCE OF THE DEGREE OF FLAKINESS OF LOCAL AGGREGATE ON THE PROPERTIES OF SELF-COMPACTING

CONCRETE

Manu Santhanam*, lIT Madras, India P.J. Amal Raj, lIT Madras, India

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29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore

INFLUENCE OF THE DEGREE OF FLAKINESS OF LOCAL AGGREGATE ON THE PROPERTIES OF SELF-COMPACTING

CONCRETE

Manu Santhanam*, liT Madras, India P.J. Amal Raj, liT Madras, India

Abstract

Self-compacting concrete (SCC) offers numerous benefits that include easy casting of congested reinforcement sections, minimization of hearing related damages, reduction in time for casting, and enhanced a ppearance and hardened properties of concrete. Researchers across the world have been able to produce self-compacting concrete with the locally available aggregate. Although there have been a number of studies on the effect of coarse aggregate content on the flow behaviour of SCC, enough attention has not been paid to quantify the effect of the shape of the aggregate. Aggregates constitute the bulk of a concrete mixture, and are responsible for the dimensional stability of concrete. Among the important aggregate characteristics are the shape and gradation of aggregate. In the case of see, the aggregate shape may be expected to playa major role. For example, perfectly rounded aggregates would provide a better flowability and lesser blocking potential for a given water to powder ratio, as compared to angular and semi­rounded aggregates. Moreover, the presence of flaky and elongated particles can be expected to give rise to blocking problems in confined areas. It is also possible that the highly flowable nature of see could make it tolerate a higher proportion of flaky aggregates compared to normal concrete. This paper presents the results of laboratory investigations performed with a number of samples of coarse aggregates with differing amounts of flaky particles. Results indicate that it is possible to incorporate a relatively high degree of flaky particles in sec. It was possible in this study top roduce sec with good fresh and hardened properties with as much as 25% flaky aggregate.

Keywords: Self-compacting, flaky aggregate, flowability, blocking

1. Introduction

Aggregates constitute the bulk of a concrete mixture, and give dimensional stability to concrete. Among the various properties of aggregate, the important ones for see are the shape and gradation.

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Many researchers have been able to produce self-compacting concrete with locally available aggregate. It is observed from these studies that self-compactability is achievable at lower cement (or fines) content when rounded aggregates are used, as compared to angular aggregates. Although there have been several studies on the effect of coarse aggregate content on the flow behaviour of

1 2 3 see • . , enough attention has not been paid to quantify the effect of the shape of the aggregate. In the case of see, rounded aggregates would provide a better f10wability and less blocking

potential for a given water-to-powder ratio, compared to angular and semi-rounded aggregates. Moreover, the presence of flaky and elongated particles may give rise to blocking problems in confined areas, and also increase the minimum yield stress (as depicted in Figure 1). Incorporation of aggregate shape in the mixture design would enable the selection of appropriate paste content required to overcome these difficulties. It is possible that the highly f10wable nature of see could allow a higher proportion of flaky aggregates compared to normal concrete. However, this aspect needs to be checked.

Conventlonal flow behaviour Flow behaviour in the presence of flaky and elongated particles - could increase yield stress

Conventional blocking due to 'arch' formation

Risk ofblocking could increase in the presence offlaky and elongated particles

Figure 1. Possible scenarios with flaky and elongated aggregates

In this study, an attempt was made to quantify the effect of increasing the proportion of flaky particles in a coarse aggregate sample used for producing self-compacting concrete. see mixtures with coarse aggregates having different levels of flakiness were prepared and evaluated with respect to slump flow (total spread as well as T 50 - time to spread 500 mm) and U-box test methods. A description of these test methods is available elsewhere4

, and the specifications for see performance in these tests is given by EFNARes. As per these specifications, the acceptable slump flow range for see is 600 - 750 mm, while the T50 is 2 - 4 sec, and the difference in the levels of the two limbs of the U-box should be less than 30 mm.

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2. Materials and experimental methods

53 grade cement conforming to IS:122696 was used for the study. Apart from cement, limestone powder (of cement fineness) was used for all the mixtures as a mineral admixture. The specific gravity of limestone powder was 2 .6. Locally available coarse and fine aggregates were used . River sand with a fineness modulus of 2.3 and a specific gravity of 2.63 was used as fine aggregate.

Crushed granite of maximum size 12.5 mm and specific gravity of 2.9 was used as coarse aggregate. The sample of coarse aggregate was sieved and only the portion between 12.5 mm and 6.3 mm was used. The portion below 6.3 mm was discarded because of the large quantity of dust. The portion between 12.5 and 10 mm was then separated into flaky and non-flaky particles as per the standard test for flakiness according to IS 2386 (Part 1)7. The three samples of aggregate (12.5 - 10 mm non-flaky, 12.5 - 10 mm flaky, and 10 - 6.3 mm) were then combined in the proportions given in Table 1. It should be stated here that the portion of coarse aggregate between 10 and 6.3 mm was assumed to be completely free of flaky particles. The overall mixture design, presented in Table 2, was similar for all mixtures.

Table 1. Coarse aggregate proportioning for SCC mixtures (quantities in kg/m3)

Designation 12.5 -10 mm Non-flaky

12.5-10 mm Flaky

10-6.3 mm Flakiness Index (%)

M1 244 0 431 0 M2 219 25 431 3.70 M3 207 37 431 5.50 M4 183 61 431 9.03 M5 152 92 431 13.70 M6 98 146 431 21.63 M7 79 165 431 24.50 M8 49 195 431 28.89 M9 0 244 431 36.15

Table 2. Mixture design

Ingredient Quantity (kg/m") Cement 360

Limestone powder 290 Fine aggregate 825

Coarse aggregate 675 Water 220

Superplasticizer (Polycarboxylate ether)

3.6

Viscosity modifier (Diutan gum)

0.07

.C>Note: Paste volume = 450 htres/m , Aggregate volume = 550 htres/m3

The dry ingredients (cement, limestone powder, and aggregate) were mixed in a pan-type mixer for 30 sec, after which period water (mixed with the superplasticizer) was added to the mixer. Mixing was continued for 2 min, and then the viscosity modifier was added to the mixer. After 2 more minutes of mixing, concrete was discharged from the mixer and used for testing.

Slump flow (total spread as well as T50 - time to spread 50 cm) test and U-box test were conducted, following which three 15 cm cube specimens were prepared for testing of the 1 day compressive strength (as per IS 516\

3. Results and discussion

The results of all tests are presented in Table 3. It is evident from this table that although the slump flow (both in terms 0 f total spread and T 50) iss atisfactory, the presence 0 f excess flaky particles

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seems to affect the passing ability of the see, as measured using the U-box test. The 1-day compressive strengths for all mixtures are similar, with values ranging between 10.9 and 13.1 MPa.

Table 3. Measurements of see properties

Mixture No. Flakiness index (%)

Slump flow (mm)

T50 (sec) U-Box Difference

(mm)

1 day compressive

strength (MPa) M1 0 720 3 5 13.1 M2 3.70 725 3 8 12.2 M3 5.50 750 2 16 10.9 M4 9.03 735 3 15 11.2 M5 13.70 750 2 15 11.8 M6 21.63 680 4 16 11.5 M7 24.50 750 2 52 12.3 M8 28.89 740 2 260 12.2 M9 36.15 720 2 306 12.2

.::>Note. As per EFNARe , satisfactory performance of see IS Indicated by a slump flow of 600 - 750 mm, T50 of 2 - 4 sec, and U-Box difference in levels of less than 30 mm

The results in Table 3 indicate that an increase in blocking due to flaky particles possibly occurs, leading to failure in the U-box test. On the other hand, the yield stress and viscosity of the mixture (indicated indirectly by the slump flow and T50 values) are not adversely affected by excessive flaky particles. Theoretically, yield stress is dependent mainly on the intergranular friction, which in turns depends on the quantity and shape of coarse aggregate. Rounded aggregates result in lower friction as compared to angular aggregate. However, within angular aggregates, the presence of flaky particles possibly does not contribute to an increase in the friction. The viscosity is more a function of the paste composition and overall volume fraction of aggregate, both of which are constant for the mixtures investigated. Hence, there is not much of an effect on the T50 values.

In the case of the U-Box test, however, the property of see being measured is its ability to pass around an obstacle. In the apparatus used, the obstacle was simulated by 10 mm diameter reinforcing bars paced at 30 mm clear spacing. As discussed earlier, aggregates have a tendency to form an arch around obstacles. This tendency is possibly increased in the presence of excess flaky particles, causing a large difference in levels in the test. Figure 2 shows a plot 0 f t he variation 0 f difference in levels with the flakiness index of the see mixtures. The dashed line (30 mm difference) indicates passing criteria for see. It can be observed that it is possible to produce satisfactory see with as much as 23 - 25% f laky a ggregate. In 0 rder to produce see with larger amounts of flaky particles, studies on the optimal paste content need to be conducted.

3aI

:m

I25l 1/1Qi ii200

...j

.E <II

g151e ~ °100 )( 0 III::,

5)

0

Fall

Pass

0 5 10 15 20 25 35

Flakiness IndlK ('/~

Figure 2. Variation in U-box results for different flakiness indices

458

Conclusions

• Excessive flakiness in coarse aggregate does not affect the flowability of SCC; satisfactory performance was observed in the slump flow and T50 tests for SCC mixtures with flakiness indices ranging between 0 and 36%.

• Beyond a value of around 23 - 25 %, increasing the amount of flaky particles caused excessive blocking to occur, leading to failure in the U-Box test. Thus, the passing ability of SCC is affected by the presence of excessive flakiness .

• The results from this investigation indicate that it is possible to produce satisfactory SCC (at the given paste volume) using coarse aggregate with as much as 23 - 25 % of flaky particles.

Acknowledgments

The assistance of Ms. K. R. Mohanapriya from NIT Tiruchirapalli in conducting the laboratory work is greatly appreciated . Support from the Department of Science and Technology, Government of India, under the Fast Track Scheme for Young Scientists (No. SRiFTP/ETA-14/2002) is gratefully acknowledged.

References

1. N. Mishima, Y. Tanigawa, H. Mori, Y. Kurokawa, K. Terada, and T. Hattori, "Study on Influence 0 f Aggregate Particle on Rheological Property of Fresh Concrete," Journal of the Society of Materials Science, Japan, Vol. 48, No. 8, 1999, pp. 858 - 863.

2. Y. Kurokawa, Y. Tanigawa, H. Mori, and K. Nishinosono, "Analytical Study on Effect of Volume Fraction of Coarse Aggregate on Bingham's Constants of Fresh Concrete," Transactions of the Japan Concrete Institute, Vol. 18, 1996, pp. 37 - 44.

3. S. Grunewald and J. C. Walraven, "Parameter-Study on the Influence of Steel Fibres and Coarse Aggregate Content on the Fresh Properties of Self-Compacting Concrete," Cement and Concrete Research, Vol. 31, No. 12,2001, pp. 1793 - 1798.

4. A. Skarendahl and O. Petersson, "State of the Art Report of RILEM Technical Committee 174-SCC, Self-Compacting Concrete," Paris, RILEM Publications S.A.R.L, 2000, 154 p.

5. EFNARC: Specifications and Guidelines for SCC, EFNARC, Hampshire, UK, 2001, 29 pp. 6. Indian Standard Designation IS12269-1987, "Specification for 53 Grade Ordinary Portland

Cement," Bureau of Indian Standards, New Delhi, 2003. 7. Indian Standard Designation IS2386-Partl : 1963, "Methods of Test for Aggregates for

Concrete - Part I: Particle Size and Shape," Bureau of Indian Standards, New Delhi, 2003. 8. Indian Standard Designation IS516: 1959, "Methods of Test for Strength of Concrete," Bureau

of Indian Standards, New Delhi, 2003.

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