CARBONATION OF MEDIUM STRENGTH SCC

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1 SCC 9-China, June 5-7 9, Beijing, China CARBONATION OF MEDIUM STRENGTH SCC Youjun Xie (1),Kunlin Ma (1,) and Guangcheng Long (1) (1) Civil Engineering and Architecture College, Central South University, China () Hunan Industry Polytechnic, China Abstract: Generally speaking, self-compacting concrete (SCC) contains a large amount of powder materials, a superplasticizer and viscosity modifying admixtures mineral admixtures such as fly ash, slag etc, are often used as powder materials. In this paper, rapid carbonation test was used to investigate the effects of factors, such as water to binder ratio (w/b), volume of coarse aggregate in SCC per m 3 (Va) and sand ratio of mortor Rs (Rs=Vs/Vm, Vs refers to the volume of sand in SCC per m 3 and Vm refers to the volume of mortar in SCC per m 3 ), on the carbonation resistance of medium strength self-compacting concrete (SCC). The relationship between carbonation depth of SCC and its -day compressive strength was also studied. The service life of SCC under carbonation condition was predicted by diffusion theory model. Results indicated that w/b, Va and Vs/Vm have great influence on the carbonation depth of SCC under rapid carbonation conditions. Carbonation depth and compressive strength of SCC was well linearly related. The carbonation behaviour of SCC was the same as that of ordinary concrete. Reasonable design and adequate curing conditions can ensure the service life of medium strength SCC under carbonation conditions. Keywords:self-compacting concrete (SCC); carbonation; Va; Rs; carbonation coefficient 1. INSTRUCTION Carbonation of concrete is a chemical reaction between carbon dioxide and calcium hydroxide presented in hydrated concrete matrix, which lowers the alkalinity of concrete to such an extent that reinforced steel bar become rust and spall the concrete cover [1, ]. Steel reinforcement in concrete is normally in a passive state due to the high alkalinity of the concrete pore solution. Chemical reactions of penetrating carbon dioxide (CO ) with the cement matrix reduce the alkalinity of the concrete pore solution. Thus, the protecting property of the concrete diminishes, and the steel becomes susceptible to corrosion. There are many researches on concrete carbonation [3~], and some results reveal that 9

2 SCC 9-China, June 5-7 9, Beijing, China SCC has good capability of carbonation resistance [7,]. However, raw material and mixed proportion of SCC and ordinary concrete varied greatly, especially medium strength SCC, and there is no consensus about service life of medium strength SCC in carbon dioxide ambience environment. Therefore, in this paper, carbonation of medium strength SCC was investigated.. EXPERIMENT.1 Raw material and mixture proportion The cement was Grade of.5 ordinary Portland cement according to the Chinese standard. The sand used in the experiment had a fineness modulus of. and apparent density of.7 kg/m 3. The coarse aggregate used in the experiment was crushed pebble with the size of 5mm~5mm and apparent density of. kg/m 3. Graded Ⅱ fly ash has a density of.1g/cm 3 and specific surface area of about m /kg was used in this study. A polynaphthalene sulphonate superplasticizer was used. Water used in this study was deminerialized water. Chemical composition of cement and fly ash was given in table 1. Table 1 chemical composition of cement and fly ash (%) SiO Al O 3 Fe O 3 CaO MgO SO 3 Cement Fly ash Experimental procedures According to characteristic of SCC, factors affecting carbonation of SCC, such as water to binder ratio (w/b), volume of coarse aggregate (Va) and sand volume ratio (Rs) and workability, were taken into account. Experimental mixture proportions and related parameters are shown in Table. Carbonation of SCC was tested according to Chinese standard GBJ-195. Compressive strengths of SCC at d were carried out according to Chinese standard GB/T51-. All SCC specimens were molded without vibrating, and concrete samples were cured in water at (±) after being demolded at 1d. 3. RESULTS AND DISCUSSION 3.1 Influence of w/b Carbonation depth of SCC with additive and ordinary concrete without additive at different ages is shown in Fig.1. There was no strong vibration in the process of SCC specimen preparation, and 5% fly ash was used to replace cement in both types of concrete. As can be seen from Fig.1, for concretes with different w/b, concrete carbonation depth increases with increased carbonation ages. With w/b decreasing, concrete carbonation depth decreases at different carbonation ages. When w/b of concrete is above., carbonation depths of different concretes increase quickly with increasing carbonation ages, especially concrete with w/b of.5. In contrast, when w/b of concrete is under., carbonation depths of different concretes increase slowly. Water to binder ratio has great influence on concrete pore structure and porosity. When water to binder ratio is under., porosity of concrete decrease obviously, and diffusion rate of carbon dioxide and moisture in concrete is reduced effectively. 5

3 SCC 9-China, June 5-7 9, Beijing, China 1 1 w/b=. w/b=.5 w/b=.5 w/b= w/b=.,sp=.% w/b=.35,sp=.% w/b=.3,sp=.% a) Carbonation depth of SCC b) Carbonation depth of Ordinary concrete Fig.1 Influence of w/b on carbonation depth of SCC and ordinary concrete Factors 1 w/b SP Va Rs Table Mixture proportions of SCC and related parameters Raw material (kg/m 3 ) Admixture (SP) (%) slump flow (mm) f cu, (Mpa) Cement Fly Crushed ash pebble Sand Water w/b Note: 1, mean SCC without additive and with additive respectively. 3. Influence of workability High workability is one of the characteristics of SCC, which is often obtained by additive. In order to improve workability of SCC, the slump flow of SCC with different levels of 51

4 SCC 9-China, June 5-7 9, Beijing, China additive was examined. Fig. shows the carbonation depths of concrete with different slump flows. As can be seen from Fig., carbonation depth of SCC with.5 % additive is the highest among all tested concrete. In fact, when the addition of superplasticizer reaches.5%, concrete admixture starts to show segregation. Porosity of concrete may increase, carbonation depth of concrete with high slump flow also show the rising tendency. All the facts illustrate that excessive additive has detrimental effect on carbonation depth of concrete. Carbonation depth(mm) SP=%, Slump flow:3mm SP=.%, Slump flow:5mm SP=.%, Slump flow:5mm SP=.5%, Slump flow:mm w/b=., 5%FA 7 1 Fig. Influence of slump flow on carbonation depth of SCC 3.3 Influence of volume of coarse aggregate Fig.3 is the influence of volume of coarse aggregate on carbonation depth of SCC. As can be seen from Fig.3-a, when volume of coarse aggregate (Va) is in the range of.~.3, carbonation depth of SCC decreases slightly with increasing Va, but all carbonation depth of SCC are under 15mm at d. As can be seen from Fig.3-b, carbonation depth of SCC increases with increasing age, but carbonation depth of SCC decreases with increasingva. Carbonation reaction takes place in hydrated products of concrete. On the one hand, the volume of hydrated products decreases with increasing Va,which influences the process of carbonation reaction in concrete. On the other hand, because of high Va interface area between coarse aggregate and hydrated products increases, which affects the diffusion of carbon dioxide and moisture in concrete[9,1]. Therefore, the increase of Va can improve the capability of carbonation resistance of SCC to some extent. 5

5 SCC 9-China, June 5-7 9, Beijing, China 1 1 Concrete carbonated at d 1 Concrete carbonated at d Volume of coarse aggregate Va=.3 Va=.3 Va=.3 Va=.3 Va=. Va=. w/b=., 5%FA 7 1 (a)influence of carbonation ages to b) Relationship between Va and carbonation depth carbonation depth Fig.3 Influence of volume of coarse aggregate on carbonation depth of SCC 3. Influence of consistency of mortarsand volume ratio Rs Fig. is the influence of Rs on capability of carbonation resistance of SCC, and all the Va of concrete samples is kept at.3. As can be seen from Fig., first, carbonation depth of SCC increases with increasing carbonation ages; secondly, carbonation depth of SCC decreases with the increasing consistency of mortar. When consistency of mortar increases from. to.51, carbonation depth of SCC decreased. mm at d. The proportion of hydrated products of mortar in concrete decreases with increased Rs. The decrease in concrete porosity results in the increasing capability of carbonation resistance of SCC Rs=. Rs=. Rs=. Rs=.7 Rs=.51 w/b=.,5%fa,va= Concrete carbonated at d 1 Concrete carbonated atd Consistency of mortar (Rs) a) Influence of carbonation ages on b) Relationship between Rs and carbonation depth of SCC carbonation depth of SCC Fig. Influence of Vs/Vm on capability of carbonation resistance of SCC 3.5 Relationship between carbonation depth of SCC and related parameters Compressive strength of concrete is usually used to evaluate the concrete quality, and is a major parameter in concrete structure design. Now, as the importance of concrete durability is generally realized and accepted, the durability design of concrete structure is much desired. 53

6 SCC 9-China, June 5-7 9, Beijing, China Mechanism of concrete carbonation indicates that capability of carbonation resistance of concrete are greatly related to composition characteristic of concrete. Compressive strength is an important index to show concrete quality, so there must be some relationship between carbonation depth and compressive strength of concrete. Nevertheless, there are seldom studies and analysis about carbonation depth of SCC and related parameters. Fig.5 is the results of relationship between compressive strength of SCC at d and carbonation depth in different carbonation ages. It is clear shown in Fig.5 that carbonation depth of SCC decreases with increasing compressive strength. Despite of the carbonation ages, there are close relationship between carbonation depth and compressive strength. It is obvious there are good linear relationship between them. According to relationship between compressive strength and concrete porosity and the diffusion mechanism of carbon dioxide in concrete, it is concluded that porosity is the key factor to control the capability of carbonation resistance of SCC. 1 Concrete carbonated at d Y=.3-.39X, R= Y=.7-.7X, R=.9 Concrete carbonated atd Compressive strength at d (MPa) Fig.5 Relationship between carbonation depth and compressive strength of SCC 3. Carbonation rate of SCC Some investigations proposed a square root rule for the carbonation depth of ordinary concrete and carbonation ages, which is given in Eq. (1): X = k t (1) where X is carbonation depth (m); t is carbonation ages (s);k is carbonation coefficient (m/s 1/ ). Obviously, carbonation coefficient can be calculated according to carbonation depth from experimental results, and then carbonation rate of concrete can be predicted. Fig. is the calculating results of carbonation coefficient of concrete with different water to binder ratios and carbonation ages. Ordinary concrete without additive, and with water to binder ratios of.,.5 and. and SCC with additive, and with water to binder ratios of.,.35 and.3 were tested,respectively. 5

7 SCC 9-China, June 5-7 9, Beijing, China Carbonation coefficient (1 - m/s 1/ ) 1 1 w/b=. w/b=.5 w/b=.5 w/b=. w/b=.35 w/b= Carbonation coefficient (1 - m/s 1/ ) 1 1 7d d Water to binder ration (w/b) a)carbonation coefficient at different b)carbonation coefficient in different w/b carbonation ages Fig. Carbonation coefficient of SCC in different conditions As can be seen from Fig., carbonation coefficients of concrete with different water to binder ratios increase gradually with increased carbonation ages. Carbonation coefficients increases quickly when rapid carbonation experiment is under 7d, and when rapid carbonation experiment is beyond d, carbonation coefficients show a decrease tendency. Carbonation coefficients decrease with increasing water to binder ratio and carbonation ages. In the case of the same carbonation age, carbonation coefficient of concrete increases with increased water to binder ratio. During the carbonation experiment test, carbonation coefficient of concrete changes with time, and shows an attenuation tendency. This can be explained that in the process of carbonation, some changes take place in concrete pores, and diffusion of carbon dioxide and moisture in concrete is blocked. There are similar changes in both SCC and ordinary concrete..conclusions - Carbonation depth of SCC increases with increased water to binder ratio, and carbonation depth of SCC decreases slightly with increased volume of coarse aggregate or consistency of mortar. In some scopes, the change of workability of SCC has little effect on capability of carbonation resistance. -The rapid carbonation experiment test shows that carbonation depth of SCC is linearly related to compressive strength at d. -With proper design and curing conditions, medium strength self-compacting concrete can ensure adequate service life in carbon dioxide ambience environment. 55

8 SCC 9-China, June 5-7 9, Beijing, China REFERENCES [1] NIU D.T., Durability and life forecast of reinforced concrete, ISBN: , Chinese science press, 3 [] Barya, B., Sellierb, A., Coupled moisture carbon dioxide calcium transfer model for carbonation of concrete, Cement and Concrete Research, Vol 3, No.9 pp ,. [3] Song, H.W., Kwon, S.J., Byun, K.J., Predicting carbonation in early-aged cracked concrete, Cement and Concrete Research, Vol 3,No.5 pp ,. [] Steffens, A., Dinkler, D., Ahrens,H., Modeling carbonation for corrosion risk prediction of concrete structures, Cement and Concrete Research, Vol 3,No. pp ,. [5] Khan, M.I., Lynsdale, C.J., Strength, permeability, and carbonation of high-performance concrete, Cement and Concrete Research, Vol 3,No.1 pp ,. [] Khunthongkeawa, J., Tangtermsirikul, S., Leelawat,T., A study on carbonation depth prediction for fly ash concrete, Construction and Building Materials, Vol,No.9 pp.7-753,. [7] Liu, Y.H. Study on preparation and application technology of SCC.Ph.D.Thesis, Changsha: Central South University,, 91. [] Corinaldesi, V., Moriconi, G., Durable fiber reinforced self-compacting concrete, Cement and Concrete Research, Vol 3,No. pp9 5,. [9] Gonne, T., Yazicioglu, S., The influence of compaction pores on sorptivity and carbonation of concrete, Construction and Building Materials, Vol 1,No.5 pp.1-15,7. [1] Basheer,L., Basheer, P.A.M., Long,A.E., Influence of coarse aggregate on the permeation, durability and the microstructure characteristics of ordinary Portland cement concrete, Construction and Building Materials, Vol 19,No. pp. 9,5. 5

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