Concrete Carbonation and Chloride Resistance Under Initial Hot Water Curing

Three concrete mix proportions were designed and prepared, respectively, such as fly ash concrete (abbreviated as “FAC”) with 30% fly ash replacement ratio of cement, fly ash, and slag concrete (abbreviated as “FSC”) with each of 20% fly ash and slag replacement ratio and ordinary Portland cement concrete (abbreviated as “OPC”) for the research of carbonation and chloride resistance of concrete under different initial hot water curing. Specimens with precuring were put into 20°C water tank for curing firstly until a certain compressive strength of 14 MPa reached after demolding, while specimens without precuring were put into 40, 60, and 80°C water tanks for curing directly just after demolding. Hot water curing of each specimen was finished when the designed compressive strength of 35 MPa was reached, then specimens were taken out into indoor natural environment. High concentration CO2 carbonation and Coulomb electric flux experiments were carried out at specimens’ 100-day age. Results show that with the addition of fly ash or slag, the carbonation resistance of concrete declines, whereas the resistance to chlorides is improved. With the increasing of initial water-curing temperature from 40 to 80°C, the carbonation and chloride resistance of OPC concrete all decrease, whereas for FAC and FSC concretes, the carbonation resistance declines and chloride resistance goes up. Precuring at the normal temperature before the elevated temperature water curing is beneficial for concrete long-term carbonation and chloride resistance. As one of the hydraulic cementing materials, concrete needs suitable environment temperature and humidity for its setting and hardening. Initial curing conditions not only determine the development of concrete mechanical properties but also concrete micropore structures and long-term durability performances (Haque, Al-Khaiat, & Kayali, 2007; Shafig & Cabrera, 2004). Elevated temperature curing is always one of the important curing methods for prefabricated concrete elements, which can shorten the curing time needed, realize concrete strength designed, and increase the production efficiency. At the same time, it is also recognized that initial high temperature curing will play adverse effects on concrete long-term mechanical properties (Abd-El Aziz, Abd El Aleem, & Mohamed Heikal, 2012; Balendran & Martin-Buades, 2000; Sha & Huang, 2011a). However, it has still no consistent conclusions about initial high temperature curing on concrete durability performances (Das & Pandey, 2011; Lo, Nadeem, Tang, & Yu, 2009; Mehta & Gerwick, 1982; Ramezanianpour, Khazali, & Vosoughi, 2013; Yazıcı, Aydın, Yiğiter, & Baradan, 2005). Mehta and Gerwick (1982) investigated the San Mateo bridge over San Francisco bay after being exposed for 17 years in the environment, which is composed of both steam-cured and moist-cured concrete beams with the same mixture proportions and materials. The study demonstrated that steam-cured beams had to be repaired according to the corrosion damage, while moist-cured beams showed no signs of deterioration. Other studies also illustrated that accelerated curing by excessively increased temperature leads to porous concrete with coarse and continuous pore structure and heterogeneous distribution of hydration products. Consequently, it increases the permeability of concrete against carbonation and aggressive ions, such as chlorides or sulfates (Lo et al., 2009; Ramezanianpour et al., 2013). However, with the addition of pozzolanic materials, such as silica fume, fly ash, or slag, to Portland cement, it is known that its heat resistance is improved (Das & Pandey, 2011; Feng, Chen, Ye, et al., 2010; Tan & Liu, 2006; Yazıcı et al., 2005). Lu, Li, Yuan, et al. (2011) studied the carbonation of fly ash concrete cured in 20, 30, and 40°C water for 3, 7, 14, and 28 days, respectively, and found that prolonging initial curing time and increasing curing temperature is beneficial for concrete carbonation resistance. For the same curing conditions, carbonation rate of fly ash concrete is usually higher than OPC concrete, but with the increase of initial curing temperature, the difference can be reduced. Sha & Huang (2011b) studied the effects of precuring on steam-cured concrete durability and found that the frost resistance, impermeability, and carbonation resistance of concrete without precuring decreased more greatly than that of concrete with precuring. It suggests that precuring has significant effects on concrete durability after steam curing. CoNCRete CARboNAtIoN AND CHLoRIDe ReSIStANCe UNDeR INItIAL Hot WAteR CURINg 61 To the authors’ knowledge, it is still very limited of research works carried out on the effects of elevated temperature curing on concrete long-term durability. Concrete carbonation and chloride resistance are its important durability performances. The objective of this paper is to minimize or avoid the negative effects of initial hot water curing on concrete long-term carbonation and chloride resistance through some measures, such as addition of mineral admixtures, changing the initial conditions before hot water curing, or setting the temperature upper limit of hot water curing.