Use of Accelerated Curing Methods & Non-Destructive Testing for Correlation of 28-day Compressive Strength of Concrete

Abstract

The compressive strength of cement concrete obtained after 28 days of moist curing is used in the quality control of concrete constructions. However, for a better quality control, for reworking before the concrete gets hardened and for reducing the waiting time, evaluating the 28-day compressive strength at early age of concrete with a reasonable accuracy is extremely necessary. The 28-day compressive strength of the concrete can be estimated by finding the strength at an early age by using the methodology of accelerated curing. The correlation suggested by IS: 9013-1978 between the accelerated curing for one day and 28-day compressive strength, 28-day compressive strength of concrete is generally predicted. It has been proved that substantial portion of compressive strength of concrete is attained due to curing. The accelerated curing techniques can be used for early prediction of the compressive strength of concrete. There is a limited amount of work done on the effect of curing period, curing techniques of concrete. Also, it is tried to evaluate the effect of accelerated curing on strength of concrete by varying cementitious material, age and type of curing method on different grades of concrete in the present investigation. Total 40 concrete mixes are cast. Among these mixes, 7 concrete mixes are directly obtained from RMC plant. On the other have 33 concrete mixes are cast in laboratory with mix design collected from different sites in Gujarat region. 21 concrete mixes consisted combination of OPC and fly ash in addition to other ingredients. Other 19 concrete mixes are cast using OPC along with other ingredients. As per provisions of IS: 9013-1978 for warm water method, after 1.5 hours to 3.5 hours of casting, the concrete cube moulds are placed in accelerated curing tank for 20 hours at 55 ± 20 C. The concrete cubes are placed in the cooling tank for a period of not less than 1 hour. Similarly for boiling water method after 23 hours of casting, the concrete cube moulds are placed in accelerated curing tank for 3.5 hours at 1000C, and after that the concrete cubes are placed in cooling tank for 2 hours. By using the methodology of accelerated curing, the compressive strength of all 40 concrete mixes is evaluated at 1 day. 28 day compressive strength is predicted in case of all 40 concrete mixes. The NDT by means of rebound hammer is employed for all the 40 concrete mixes. Concrete cube specimens are tested by keeping position of rebound hammer vertical. Also, NDT by means of UPV is employed for all the 40 concrete mixes. For concrete cube specimens, direct method is adopted to find the UPV results. Rebound Hammer, UPV and compressive strength results are measured for concrete cube. The results are measured at 7 and 28 days as well as 1day by means of accelerated curing test. From all the results, the correlation between rebound number result taken after 1 day of accelerated curing of concrete and 28 days compressive strength of concrete. Also, correlation is established between rebound hammer result and compressive strength of all the concrete mixes at 28- days. The correlation between compressive strength of concrete after accelerated curing by both methods and 28 day compressive strength of concrete by normal curing with OPC & OPC + fly ash of concrete mixes have been established. The established correlation between 1 day rebound number and 28 day compressive strength of concrete is helpful in predicting 28 day compressive strength of concrete. Similarly, correlation between rebound number and compressive strength of concrete at 28 days is helpful in cross checking 28 day compressive strength of concrete. Correlation between compressive strength of concrete evaluated at one day and 28 day compressive strength of concrete is helpful in predict 28 day compressive strength of concrete through accelerated curing in case of both the method.

For OPC based concrete the percentage difference in warm water method with respect to 28 day normal curing of concrete is 15% to 20% higher than 28 day normal curing of concrete. In boiling water method percentage difference is 25% to 30%. Similarly, for fly ash based concrete the percentage difference in warm water method with respect to 28 day normal curing of concrete is 2% to 5% higher than 28 day normal curing of concrete. In boiling water method percentage difference is 5% to 10%. From all the results it is conclude that the predicted 28 day compressive strength of concrete by warm water method the results is showing lower then boiling water method. So, warm water method is good to predict 28 day compressive strength for OPC based and fly ash based concrete.