J. Cent. South Univ. Technol. (2008) 15(s1): 221-224

DOI: 10.1007/s11771-008-350-y

Additional water use influencing strength and fluidity of recycled concrete

ZHANG Xue-bing(张学兵)^{1, 2}, FANG Zhi(方 志)², DENG Shou-chang(邓寿昌)³,

CHENG Ke(成 珂)¹, QIN Yin-hui(覃银辉)³

(1. Civil Engineering & Mechanics College, Xiangtan University, Xiangtan 411105, China;

2. Civil Engineering College, Hunan University, Changsha 410082, China;

3. College of Civil Engineering, Architecture and Mechanics, Central South University of Forestry and Technology, Changsha 410004, China)

Abstract: Through adding different additional water use, the compressive strength, splitting tensile strength and fluidity of recycled concrete of three aggregate combination forms were studied by experiment respectively. The experimental results show that with the increase of adding additional water use, the compressive strength and splitting tensile strength of recycled coarse aggregate concrete decrease, but that of recycled fine aggregate concrete and recycled all aggregate concrete increase firstly then decrease. When additional water use is added more 15% or 20% than that of basic ordinary concrete, the recycled coarse aggregate concrete and fine one can get pretty good fluidity. When it is added more 30%, the recycled all aggregate concrete has fluidity that is just satisfied.

Key words: additional water use; basic ordinary concrete; recycled concrete; compressive strength; splitting tensile strength; fluidity

1 Introduction

For the ordinary concrete, generally speaking, when water use increases over some value, the strength will decrease.

Owing to the surface coarseness of recycled aggregate and appearance of a lot of edge-angles in fragmentation, mechanical damage forming lots of microcracks inside, its specific surface area would be great, so its water absorption would be more than that of ordinary aggregate. In proportioning, if water use added is less, the workability of the allocated RC (recycled concrete) cannot reach the demand of construction, if more, strength of RC will be dropped and dry shrinkage will increase. All these are disadvantageous to structural bearing. Research in Refs.[1-4] shows that if the mixture proportion of RC is designed the same as that of ordinary concrete (that is, not increasing unit water use), it leads to slump constant reduction too much (decreasing by 130 mm). So the working performance of construction of RC is difficult to meet the demand. What causes slump constant reduction is that recycled aggregate is added in RC. Because of its large water absorbing capacity, actual mixing water use reduces and actual water-cement ratio also reduces. Thus causes the working performance of construction of RC cannot meet the demand. Therefore, in case of the workability and strength of RC that should meet the designing requirement at the same time, the allocation of RC cannot simply use the method of mixture ratio design of ordinary concrete indiscriminately^[1].

The water use in each stere of RC would add water more 10 kg than that of ordinary concrete^[2]. The basic concrete which is the contradistinctively ordinary concrete under other conditions is equal. In this experiment, the RC was mixed by recycled coarse aggregate (used as coarse aggregate) and natural sand (used as fine aggregate), the mixture proportions of the RC and basic concrete are the same, and the slump constants of them are about the same. If coarse and fine aggregates of RC are both used recycled aggregate, water increases to 25 kg/m^{3 [5-10]}.

2 Experimental

Coarse aggregate is usually two-thirds of concrete aggregate. In order to make good use of recycled aggregate, coarse aggregate should use all or part of recycled aggregate. In experiment three main kinds of aggregate combination were considered, as shown in Table 1. The water use in each cubic meter of RC mixed by these aggregate combinations is adopted the same as ① that of the basic concrete, increment ②5%; ③10%;④15%;⑤20% on the basis of the basic ordinary concrete respectively. For the recycled full aggregate concrete, additional ⑥30%; ⑦50%. The water use added more than that of the basic concrete is called the additional water use. A5 is used to denote the water use of the RC of type A which is added more 5% than that of the basic concrete, and so on.

3 Experimental mix proportion and results analysis

3.1 Raw material

3.1.1 Cement

Cement is 32.5 class Pan-feng brand ordinary silicate cement produced by Xiang-xiang Building Material Industry Limited Company of Hunan Province.

3.1.2 Water

Water is tap water of Xiangtan University for mixing and curing RC.

3.1.3 Ordinary aggregate

Ordinary coarse aggregate is crushed stone and ordinary fine aggregate is natural sand.

3.1.4 Recycled aggregate

Abandoned concrete is from old roadway of Xiangtan University, old structural member of demolished buildings and a few abandoned test specimens of concrete. It was broken up into pieces by hand, and sieved. Then recycled coarse and fine aggregates were obtained. Ordinary and recycled aggregates were both in dry state, whose gradations are listed in Table 2.

3.2 Mixture proportion of concrete and results analysis

Slump constant of the basic concrete is 30-50 mm. Side length of mixed cubic test specimen is 150 mm. Mixing by hand, rodding three-layer by steel bar, then concrete specimens were compacted by jolting table. After it was curing for 1 d in the standard curing room, the test block was detached from the mold plates. After it was cured for 28 d, its compressive strength and splittingtensile strength were tested. After it was mixed, its slump was measured. Experimental mixture proportion is shown in Table 3.

For the recycled coarse aggregate concrete and fine one, the additional water use doesn’t increase to 15% or 20% until it gets good fluidity. But for the all recycled aggregate concrete, it does not increase 30% until it gets good fluidity just satisfied.

The 28 d compressive and splitting tensile strength of concrete is illustrated in Figs.1-2.

From Fig.1, it can be seen that the basic concrete and type B concrete have almost the same variation law. That is, as the additional water use increases, the compressive strength of them reduces. The probable causes are as follows:

1) Because of the surface coarseness of aggregate and its great specific surface, a large part of additional water increased is absorbed by the recycled coarse aggregate, so that actual water joined in hydration reaction reduces and cement paste formed is also reduced.

2) As additional water use increases, the hydration reaction becomes intense. But at the same time, water-cement ratio becomes large, excessive water forms a lot of gas pore in the RC, which lowers the density of RC. The latter’s influence exceeds the former’s, so the strength of RC reduces. The mechanism needs to be researched further.

RC of type A and C show about the same variation trend. That is, as the additional water use increases from 0 to 5%, the compressive strength increases; from 5% to 10%, the strength reduces; when it increase to 10%, the strength reduces to the minimum value; from 10% to 15%, the strength increases; when it increases to 15%, the strength reaches the maximum value. Then as it increases, the strength reduces. But the strength of RC of type A declines more than that of type C. On the   whole of variation trend, the strength first increases then

Table 2 Gradations of ordinary coarse aggregate and recycled aggregate

Table 3 Experimental data of basic concrete and RC

Fig.1 Influence of additional water use on compressive strength of basic and recycled concrete

decreases. The reasons are that water absorption rate of recycled aggregate is high, the speed of water absorption is large and recycled aggregate absorbes a lot of water so that actual water-cement ratio of cement pastes reduces and the strength of RC increases. When the additional water use increases to 15%, water absorbing capacity of recycled aggregate and cement pastes basically tends to saturation. At this time continuing increasing water will form a lot of small gas pore in the RC, which makes compactness reduce, so the strength reduces.

Fig.2 Influence of additional water use on split-tension strength of basic and recycled concrete

From Fig.2, it can be seen that with increment of additional water use, the split-tension strength of the RC of type B reduces. But on the whole of variation trend, the strength of type A and C firstly increases then decreases. When additional water use increases to 10%, the strength reaches the maximum. Then with the increase of additional water use, the strength reduces. But the strength reduction of RC of type A exceeds that of type C. The reason is similar to that of the compressive strength.

It’s worth noting that whether compressive strength or split-tension strength, the RC of type A and C both shows similar variation trend. What’s the most important is that they both include recycled fine aggregate. But the RC of type B and C both include recycled coarse aggregate, they do not show the similar law. This shows that the influence of recycled fine aggregate on the strength of RC is of vital importance.

4 Conclusions

1) Additional water use has a great effect on the strength of RC. But the influence on the RC of different aggregate combinations is different.

2) With the increase of the additional water use, the compressive strength and split-tension strength of recycled coarse aggregate concrete reduce. While the ones of recycled fine aggregate concrete and recycled all aggregate concrete both show about the same variation trend. On the whole of view, the strength firstly increases then decreases. When it increases 15%, the compressive strength reaches the maximum; and when it increases 10%, the split-tensile strength reaches the maximum.

3) The influence of recycled fine aggregate on the strength of RC is of vital importance.

4) When the additional water use increases 15% or 20%, the recycled coarse aggregate concrete and recycled fine aggregate concrete can get good fluidity. But for recycled full aggregate concrete, it increases at least 30%, the satisfied fluidity can be gotten.

References

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(Edited by CHEN Can-hua)

Foundation item: Project(Xiangjianke(2007)No.425) supported by Scientific Research Fund of Hunan Provincial Construction Department; Project supported by the Youth Framework Teacher Fund of Xiangtan University(2006)

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: ZHANG Xue-bing; Tel: +86-732-8293854; E-mail: zhangxb00@163.com