Experimental Study On The Influence Of Curing Conditions On Capillary Absorption Of Concrete

Curing condition of concrete plays a vital role in both the hardening of cement and the formation of its micro-pore structure. The mechanical property and durability of concrete could be significantly improved by an adequate curing condition. In general, the division of concrete strength grade, the measurement of mechanical property, the evaluation of durability and long-term performance are carried out based on the test results obtained under standard curing condition. However, the curing environment of concrete in practical engineering is quite complicated, which is not only affected by climate and weather conditions, but also restricted by construction technology and operating conditions. Consequently, the performance of concrete measured under standard curing condition could not represent the concrete of practical engineering. As a carrier of migrant corrosive medium, water could cause the expansion/cracking of reinforced concrete structures and the corrosion of reinforcement. Generally, water could penetrate into the interior of the concrete material through the capillary absorption driven by surface tension of pore liquid. Sorptivity has recently been used as an important index for characterizing the concrete durability. It is closely related to the pore structure and its distribution. In order to estimate the effect of curing condition on the performance of concrete, the sorptivity S is regarded as the main evaluation index to conduct a series of comparative experiments. The effect of various curing conditions on mechanical property and durability of normal and high-strength concrete under external and internal curing methods is studied. The distribution of relative water content in concrete is predicted through theoretical method based on the test results above. The main contents and conclusions in this paper can be briefly summarized as follows:(1) The cylinder and pie specimens are drilled out and cut from plate specimens with dimensions of 500mmx500mmx 150mm. Compared to that cured under standard curing condition, normal strength concrete (NSC) specimens cured under three curing conditions, i.e. water, natural and sealed curing, are used to measure the strength, ultrasonic pulse velocity, porosity and capillary absorption, respectively. The results show that the compressive strength of specimens cured in standard, water, natural and sealed curing condition orderly decreases after the curing age of 28 days. The negative correlation can be obtained between ultrasonic pulse velocity and porosity from top to bottom of the plate specimens. The curve of cumulative water content exhibits the bilinear trend, and the corresponding turning time point decreases as curing age elapses.(2) In order to quantitatively assess the influence of external and internal curing methods on mechanical property and durability of high-strength concrete (HSC), the specimens with normal weight aggregate (NWA) under three types of immerged water, natural and sealed curing condition as well as that with pre-wetted lightweight aggregate (LWA) cured in natural and sealed condition are used. In this paper, the influence of curing conditions on the strength, ultrasonic pulse velocity, porosity and capillary absorption of HSC is experimentally investigated by means of external and internal curing methods. Quantitative evaluation of curing effect of those concrete are conducted by using relative curing efficiency (RCE). The results indicate that the compressive strength of concrete with LWA is lower than NWA concrete, but sealed curing condition can significantly improve the durability of concrete. Furthermore, it is lower than to the porosity, cumulative water content and sorptivity of NWA concrete. The order of average relative curing efficiency of NWA concrete from high to low is water, sealed and natural curing while average relative curing efficiency of LWA concrete cured in sealed condition is higher than that under natural curing condition. Initial sorptivity and the cumulative water content of plate specimen from top to bottom gradually decrease while its splitting tensile strength and ultrasonic pulse velocity increase. The linear correlation can be thus obtained between initial sorptivity and porosity.(3) The test error caused by wiping and weighing in traditional gravimetric method can be avoided by using the improved test setup of capillary absorption by concrete. It can solve the problem of discontinuous measurement and no real-time records. Based on unsaturated flow theory of capillary absorption by concrete, the initial and second sorptivities of NSC and HSC specimens obtained by testing are used to predict the water distribution of concrete under various curing conditions by means of exponential and power laws. The results indicate that the wetting front position of NSC specimens descend with an increase of the curing age. Moreover, wetting front position of concrete specimens with pre-wetted LWAs under sealed curing condition is about 0.75 times than that under natural curing condition. Additionally, the wetting front position of specimens descends as the average relative curing efficiency increases.