INFLUENCE Of AGGREGATE ON PROPERTIES OF SELF-CONSOLIDATING CONCRETE

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1 INFLUENCE Of AGGREGATE ON PROPERTIES OF SELF-CONSOLIDATING CONCRETE Youjun Xie, Yanguang Li, Guangcheng Long School of Civil Engineering and Architecture, Central South University, , Changsha, China Abstract: This paper deals with the influences of volume content, type and maximum size of aggregate size on properties of self-consolidating concrete (SCC) such as workability, strength, elastic modulus and shrinkage. Results indicates the flowability, filling capability and stability of fresh SCC are greatly influenced by ratio of volume between coarse aggregate and fine aggregate and there exists an optimum ratio to achieve the best workability of SCC best. Compared with crushed stone, gravel is more effective in improving flowability, filling capability and stability of fresh SCC. The decrease of maximum size of coarse aggregate also benefits to stability of fresh SCC. Under the same aggregate volume condition, the compressive strength and split strength and elastic modulus of SCC reduce, however, the drying shrinkage increase with the increase of fine aggregate volume. The compressive strength, split strength, elastic modulus and drying shrinkage of SCC made with crushed stone were higher than those of SCC with gravel. 1. INSTRUCTION Aggregate plays an important role in concrete. With the need of high performance for concrete, properties of aggregate gradually become an main factor during concrete design [1~3]. For example, the maximum size of aggregate has a limitation in high strength concrete. Self-consolidating concrete, as a high performance concrete with a special requirement in workability, has a specific demand for content of aggregate and other properties of aggregate [4~6]. However, the effects of aggregate on properties of fresh and hardened self-consolidating concrete (SCC) were not systematically documented due to the variety of factors and complexities of concrete in properties and structure. Based on above considerations, in present paper, the influences of aggregate on properties of self-consolidating concrete in fresh and hardened state were investigated through extensive experiments, aimed at obtaining a clear and overall knowledge related to the role of aggregate in SCC. 1

2 2.EXPERIMENT 2.1 Raw materials Ordinary Portland cement produced by Hunan Xiangxiang Cement Plant, was used. Its strength in standard mortar is given in Table 1. Fly ash from Hunan Power Plant meeting Chinese specifications for Grade I was used. Density, specific area and ignition loss of the fly ash are 2.32g/cm 3, 600 m 2 /kg and 4.12% respectively. Table 2 give the chemical compositions of cement and fly ash. River sand with a density of 2630 kg/m 3 and a bulk density of 1585kg/m 3 was used as fine aggregate Its fineness modulus is Two coarse aggregates were employed, one is crushed limestone with a density of 2700 kg/m 3 and a bulk density of 1450kg/m 3 and the other is river gravel with a density of 2680 kg/m 3. The particle distributing of coarse aggregate was listed in Table 3. A naphthalene sulphate superplasticizer produced by Shanghai huawang chemistry Lt. Co., was used. Table 1 Standard Mortar Strength of the Cement specimen Compressive strength /MPa Flexural strength /MPa Item 3d 28d 3d 28d Table 2 Chemical composition of cement and fly ash Item Chemical composition (%) Type SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 I.L cement Fly ash Table 3 particle distribution of crushed stone and river gravel ( % by mass) Item D max =20mm D max =20 mm Type Crushed stone Gravel Experimental method In order to obtain a better understanding to the effect of aggregate on properties of SCC in fresh and hardened state, eight mixing proportions given in table 4 were designed. Among them, five groups (1#~5#) was designed for the coarse aggregate to fine aggregate ratio factor, two groups (6#~7#) for the maximum size of aggregate factor and two groups (4#, 8#) for aggregate type factor. 2

3 Table 4 Mixing proportion of concrete samples (kg/m 3 ) No. Water Cement Fly Ash Sand Coarse Aggregate Superplasticizer (16mm) (gravel) Experimental method The slump and slump flow value of fresh self-consolidating concrete were measured through slump cone tests. Flowing velocity and time were measured in the L-box (see Fig.1) experiment. Also the elapsed time denoted as T s, which take concrete to flow out converting slump barrel, was measured;the difference of height denoted as Δ h of fresh concrete between two sides in the U-box test shown in Fig.2 was measured in order to evaluate filling capability and passing capability under its own weight condition. Stability may be defined as the uniform degree of coarse aggregate distribution in fresh concrete under a certain condition such as vibrating condition. Therefore, the stability of fresh concrete was evaluated by a parameter of aggregate vibrating segregation ratio in present test. The lower the index of stability is, the better the stability of fresh concrete is. A device mainly including cylinder used to fill fresh concrete and a vibration table was used to measure stability of fresh concrete. The cylinder is a disassemble system consisting of three parts made of metal and each parts has same height, 100mm high. The details of cylinder show in Fig.1. The experimental method for stability test was described in detail in another publication [7]. Specimens were cast by stainlessness steels with three dimensions of 100mm 100mm 100mm for compressive strength test, 100mm 100mm 300mmfor elastic modulus test and 100mm 100mm 500mm for shrinkage test and flexural strength test. The mechanical properties such as strength and elastic modulus of concrete were measured at 28 days. The condition for drying shrinkage test is a temperature of 20±3 and 55%±5%RH. The shrinkage test was performed at 2d after casting. 3

4 Fig.1 L-box Unit:mm Unit: mm Fig.2 U-box Fig.3 Apparatus for stability test 3.RESULT AND DIASCUSSION 3.1 Influence of aggregate parameter on workability of fresh SCC The value of The experimental results of influences of coarse aggregate to fine aggregate ration, expressed as parameter:, on flowability, filling capability, passing capability and stability of fresh SCC were shown in Fig.4, Fig.5 and Fig.6 respectively. means the volume of coarse aggregate and V m means the volume of mortar (paste and fine aggregate). During the experiment, no occurrence of segregating and bleeding were observed. From Fig.4 and Fig.5, It can be seen that the slump value of SCC almost keeps constant as increases The slump flowing value increases firstly and then reduces as increases There is an optimal value of to make slump flow value maximum. Meanwhile, one can also find there are good the slump value and slump flow value for five samples with various value, respectively. The slump values for all five samples (1#~5#) range from 240mm to 250mm and the slump flow values are 540mm to 640mm. Results in Fig.5 indicate that there also are two different optimal values of for the shortest T s and the highest velocity during the L-box testing. The values of T s for five samples are all short which agrees with the velocity in L-box test of five samples. 4

5 Fig.6 shows that the filling capability of fresh SCC decreases with increased volume of coarse aggregate. The value of Δ h increases with increasing. However, The index of stability firstly reduces with increasing, and then increases after value is more than one that makes index of stability minimum. It is found that, in investigated area, there exists an optimal value resulting in stability of fresh SCC best. Based on experimental result shown in Fig.4~Fig.6 and above analysis, one can find that, in order to obtain a excellent workability, including flowability, passing capability, filling capability and stability, there is a appropriate value. The appropriate value is 0.43 in present experiment. This is reasonable. Because when the total volume of coarse aggregate and fine aggregate keeps constant a appropriate coarse aggregate to fine aggregate Slump /mm Slump flowability /mm Fig.4 Effect of on slump and slump flowability of SCC Time /s Velocity of flow in L-box (mm/s) Fig.5 Effects of on T s and Velocity in L-box of SCC 5

6 40 11 Difference of High /mm Index of stability /% Fig.6 Effects of on stability and Δ h of SCC ratio results in a minimum porosity for aggregate system and therefore also have enough excess paste besides filling the porosity, which assure a suitable viscosity and a yield stress and therefore a good workability of fresh SCC Maximum size of coarse aggregate Table 5 gives the result of effects of coarse aggregate maximum size on workability of fresh SCC. No. Table 5 Effects of coarse aggregate maximum size on workability of fresh SCC. Slump (mm) Slump flow (mm) T s (s) Velocity (cm/s) Filling capacity h ( mm ) Stability index (%) Results in Table 5 indicate that the flowability, filling capability and the velocity all reduce with decreased maximum size of coarse aggregate. However, the stability of fresh SCC increases with decreasing maximum size of aggregate. This may result from the increasing specific area of coarse aggregate to lead to a increment of paste wrapped in aggregate surface and a decrease of corresponding excess paste, which reduce workability when maximum size. of coarse aggregate was decreased. Meanwhile, owing to the reduction of maximum size of coarse aggregate, the number of coarse aggregate increases resulting in increasing friction and occlusive stress between aggregate particles. On the other hand, less maximum size of coarse aggregate results in the decrement of difference of moving velocity between aggregate 6

7 particles and increase of uniformity of aggregate particles distribution of fresh SCC. Therefore, the stability of fresh SCC was improved.3.2. The effect of aggregate parameters on strength of SCC. 3.2 Influence of aggregate parameter on compressive strength of SCC The value of The effect of value on compressive strength of SCC is given in Fig.7. From the results in Fig.7, it can be seen that, under considered three ages condition, the compressive strength generally decreases with increasing value. Results in Fig.8 indicate that, with increased value, flexural strength of SCC reduces greatly. This may result from the increment of stress convergence tendency and more occurrence of the transition zone between matrix (paste and sand)-aggregate so as to reduce strength when increasing. Compressive strength /MPa d 28d 90d Fig.7 Effects of on compressive strength of SCC Flexural strength /MPa Flexural to compressive strength ratio 7

8 Fig.8 Effects of on flexural strength and flexural to compressive strength ratio of SCC Maximum size and type of aggregate Fig.9 and Fig.10 show the effects of aggregate maximum size and aggregate type on compressive strength of SCC. The results indicate that the compressive strength of SCC increases with decreasing maximum size of aggregate. Compared with SCC prepared with gravel, the compressive strength of SCC with crushed stone is higher. In addition, compressive strength of SCC increases with ages. These results agree with that of ordinary concrete. 70 Compressive strength /MPa mm 16mm 20 7day 28day 56day Fig.9 Effects of maximum size of coarse aggregate on compressive strength of SCC 70 Compressive strength /MPa crushed stone gravel 20 7day 28day 90day Fig.10 Effects of aggregate type on compressive strength of SCC 8

9 3.3 Effects of on elastic modulus of SCC The effects of coarse aggregate to fine aggregate ratio denoted as under a constant total volume of coarse aggregate and fine aggregate condition on elastic modulus of SCC are given in Fig.11. It can be seen that, from the experimental result, elastic modulus of SCC varies with increasing coarse aggregate to fine aggregate ratio. There is an optimal value to make elastic modulus best. This result may result from role of elastic modulus of coarse aggregate and the porosity of aggregate system decided by coarse aggregate to fine aggregate ratio on elastic modulus of SCC. 50 Elastic modulus /MPa 40 7d 28d V S Fig.11 Influence of on elastic modulus f SCC 3.4 Effects of aggregate parameter on drying shrinkage of SCC The result of effects of content of coarse aggregate ( ) on shrinkage of SCC was shown in Fig.12. The results indicate that drying shrinkage of SCC decreases with the increase of value. Meanwhile, it can be seen that the effect of value on drying shrinkage at later ages is more remarkable than that at early age. This is because that coarse aggregate restricts shrinkage was increased with increasing value, and decrease drying shrinkage of SCC. This result is in consistent ordinary concrete. Fig.13 are the effects of maximum size of coarse aggregate on shrinkage of SCC. The result shows that the drying shrinkage of SCC with larger maximum size of coarse aggregate is lower than that of SCC with smaller aggregate. This may be the reason that the lager the maximum size of coarse aggregate is, the more effective the role to restrict drying shrinkage is. In order to make drying shrinkage of SCC lower, larger maximum size of coarse aggregate is top-priority. However, it can be seen that, from above result, the workability of SCC will be worse when the maximum size of coarse aggregate is larger than certain size. Therefore, 9

10 combining workability and drying shrinkage requirements, the maximum size of coarse aggregate should be less than 20 mm Shrinkage / µm/m d 28d 90d Fig.12 Effect of on drying shrinkage of SCC 400 Shrinkage / µm/m d max =20mm d max =16mm 4.CONCLUSION Age /d Fig. 13 Effects of aggregate type on drying shrinkage of SCC From above analysis and experimental results, following conclusions can be obtained. 1). There is an optimum value to obtain excellent workability including good flowability, passing capability, filling capability and high stability. The optimum value is 0.43 in present experiment. 2). Flowability and filling capability of fresh SCC reduce with the decrease of themaximum size of coarse aggregate. Stability of fresh SCC increases with decrease of the maximum size of coarse aggregate. 10

11 3). Compressive strength of SCC increases with decreased maximum size of coarse aggregate. Compared with SCC prepared with gravel, the compressive strength of SCC with crushed stone is higher. 4). Compressive strength and flexural strength of SCC generally decreases with increasing value. There is an optimal value to make elastic modulus best. 5). Drying shrinkage of SCC decreases with increasing value. Larger maximum size of coarse aggregate is effective in decreasing drying shrinkage of SCC. References [1] Larrard F. de. and A. Belloc. The influence of aggregate on the compressive strength of normal and high strength concrete, ACI Materials Journal, Sept.~Octo, 1997, pp417~426 [2]WU Zhong-wei, LIAN Huizhen. High performance concrete (Chinese), Beijing: China Railway Publishing House, [3] Giaccio G. et al. High strength concretes incorporating different coarse aggregates, ACI Materials Journal, May~June, 1992, pp242~246 [4] Hajime Okamura. Self-Compacting High-Performance Concrete. Concrete International. 1997,19 (7): [5]Steffen Gru newald, Joost C. Walraven. Parameter-study on the influence of steel fibers and coarse aggregate content on the fresh properties of self-compacting concrete. Cement and Concrete Research, 2001,31(12): [6] Bertil Persson. A comparison between mechanical properties of self-compacting concrete and the corresponding properties of normal concrete. Cement and Concrete Research, 2001, 31(2): [7] LI Yan-guang. Properties of self-compacting concrete (Dissertation), Central South University, Changsha, China,