EFFECT OF EARLY CURING ON CONCRETE DURATION BY EVAPORATION REDUCER

Transcription

1 EFFECT OF EARLY CURING ON CONCRETE DURATION BY EVAPORATION REDUC Jiaping Liu, Lei Li, Qian Tian, Cangwen Miao, Yujiang Wang State Key Laboratory of High Performance Civil Engineering Materials (Jiangsu Research Institute of Building Science Co., Ltd) Nanjing 28, China Abstract In present paper, typical high performance concrete with low water/cement ratio and mineral admixture was cured by evaporation reducer () at plastic stage. Effect of on concrete duration was characterized by carbonation, electric flux and freeze thawing experiment. In order to illustrate the reason of duration improvement, neat cement with water/cement=.4 was prepared and the microstructure with and without curing was discussed. Results pointed out that concrete with early curing at its plastic stage by could improve concrete durability through increasing cement hydration degree, optimizing pore structure and decreasing porosity. 1. Introduction Curing of concrete is important with respect to both strength and durability properties, especially for modern high performance concrete with low water/cement ratio and mineral admixture [1-2]. According to ACI (38R-1), concrete curing is not a single-step process, which could divide into three stages: initial curing, intermediate curing and final curing. However, nowadays most of conventional curing methods, such as spraying water, plastic cover or membrane curing are aimed at intermediate curing and final curing [3]. It has been pointed out water loss from fresh and young concrete without adequate curing might result in undesirable effects including plastic shrinkage cracking, strength reduction and permeability increase [4]. In our previous research, a novel curing method by evaporation reducer () aimed at early age curing had been developed []. It had been pointed out that could effectively reduce water evaporation and reduce plastic cracking on concrete surface. Capillary force testing indicated could delay the time when the capillary force starting to increase which might be the reason of cracking decreasing [6]. In this paper, effect of on concrete carbonation, electric flux and freeze thawing performance was discussed to illustrate its effect on concrete durability, and concrete surface microstructure was characterized to explain why the durability was improved. 2. Experimental

2 2.1 Concrete mixture proportion Concrete mixture proportion was given in Table 1. Table 1. Concrete mixture proportion Cement Sand Stone Water Fly ash Water reducer Notes: Cement, Normal Portland cement ( Jing Ning Yang, 42.RP II ) from Jiangnan Cement Co., LTD.; Fly ash, First grade fly ash from Nanjing Power Plant; Water reducer (JM-B), Jiangsu Bote New materials Co Carbonation, electric flux and freeze thawing experiment Carbonation, chloride diffusion and freeze thawing experiment were taken as GB/T ( for test methods of long-term performance and durability of ordinary concrete). 2.3 Cement hydration degree Cement hydration degree was characterized by testing content of chemically bonded water. The temperature rising was divided in two stages: for 3h and then for 1h. Every sample was tested three times and average result was taken. 2.4 Pore distribution and porosity The pore distribution and porosity of cement surface was tested by an automatic mercury analyzer pore master (Quantachrome Instruments, USA). 3. Results and Discussion According to section 2.1, concrete specimen was prepared and the standard specimen was cured under dry conditions for 28days after demoulding. Specimen cured with was first cured with for one time and then cured as the standard specimen. Results from carbonation, electric flux and freeze thawing experiment as shown in table 2 indicated that could effectively improve concrete durability. As the result from our previous research, this improvement might come from the less plastic shrinkage and cracking. Table 2. Effect of on concrete performance Carbonation depth (mm) Relative dynamic elastic modulus (%) Electric flux (C) 1d 7d times 2times / In order to reveal the reason of such improvement on concrete durability, mm thicker neat cement specimen was prepared to characterize microstructure change on cement surface. To simplify the study object, the neat cement (water/cement ratio=.3 and.4) was prepared without any mineral mixing material or water reducing agent. The standard specimen was prepared without any curing and the monolayer curing specimen was cured by for only one time without any further curing.

3 The result of on cement hydration degree showed in table 3 indicated that though effect on cement hydration degree at 1 day was not obvious, it could greatly improve hydration degree at 28d. The hydration degree could be increased nearly more than 2% (w/c=.4, 28d).The results seemed to be strange because could only reduce water evaporation at the plastic stage of cement as indicated in our previous research, the hydration degree at 1d should be increased more greatly than 28d. The reason might be discussed in the following part. Table 3. Hydration degree on cement surface Water/cement=.3 Water/cement=.4 standard standard Hydration (1d) 19.9% 2% 2.4% 21.2% Hydration (28d) 49.1% 9.9% 9.9% 74% The cement pore distribution and porosity was tested by mercury intrusion porosimetry (MIP) method. Results from MIP illustrated for both water/cement=.3 and.4 the content of pores with diameter less than nm was increased (Fig. 1) and the total porosity (Table 4) was decreased. As it had been pointed out that pores with diameter less than nm had almost no harmful effect on concrete durability, but pores with diameter larger than nm might lead to concrete damage at harsh environment. could heighten concrete durability through improving pore distribution and porosity. Additionally, the low porosity might lead to less water evaporation compared with high porosity. So, more water could react with cement. That might be the reason of high hydration degree of cement covering with monolayer at 28d. Hence, the higher hydration degree, lower porosity and improved pore size distribution on the concrete surface which came from curing might be the reason of better performance in carbonation, electric flux and freeze thawing experiment compared to specimen without curing.

4 2 1d, w/c=.3 28d, w/c=.3 nm<ps<2 nm<ps< d, w/c=.4 28d, w/c=.4 nm<ps<2n nm<ps<2 Fig. 1. Monolayer effect on cement pore distribution Table 4. Monolayer effect on cement porosity Water/cement=.3 Water/cement=.4 standard standard Porosity (1d) 34.87% 34.97% % % Porosity (28d) 28.28% 26.8% 31.71% 28.88% 4. Conclusion In this paper, effect of on concrete durability was characterized. Results clearly showed that could improve concrete carbonation, electric flux and freeze thawing performance. Microstructure test of neat cement surface revealed that could increase cement hydration, decrease porosity and improve pore distribution. The improved microstructure might be the reason of concrete durability improvement. It was proven that curing at early age was very important for concrete durability and was an effective method for early curing. Reference [1] Bushlaibi, A.H. and Alshamsi, A.M., Efficiency of curing on partially exposed high-strength concrete in hot climate, Cem. Con. Res. 32 (22) [2] Tan, K.F. and Gjorv, O.E., Performance of concrete under different curing conditions, Cem. Con.

5 Res. 26 (1996) [3] Lura, P., Breugel, K.V. and Maruyama, I., Efeect of curing temperature and type of cement on early-age shrinkage of high-performance concrete, Cem. Con. Res. 31 (21) [4]Ozer, B. and Ozkul, M.H., The influence of initial water curing on the strength development of ordinary portland and pozzolanic cement concrete, Cem. Con. Res. 34 (24) [] Liu, J.P., Li, L., Miao, C.W., Tian, Q., Ran, Q.P. and Wang, Y.J. Characterization of the monolayers prepared from emulsions and its effect on retardation of water evaporation on the plastic concrete surface, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 366 (2) [6] Liu, J.P., Li, L., Tian, Q., Ran, Q.P. and Miao, C.W., Monolayers construction by amiphiphilic mccules self-assembly and its application on plastic concrete, The 7th international symposium on cement and concrete, (Jinan, China, 2).