Applicability of the Maturity Method as a Means of Estimating Concrete Strength

Abstract

In Kenya, quality control of concrete involves taking samples of poured concrete, preparing specimens under controlled conditions, and subjecting the specimens to strength testing to ensure that the design strength is achieved in 28 days. However, depending on prevailing in-situ temperature conditions, concrete strength prediction based on the 28-day rule may be inaccurate. Research has shown that this conventional concrete quality control protocol, which is easily circumvented, is not effective on ensuring structural reliability of new or existing buildings, as evidenced by the collapse of seventeen buildings between 2006 and 2014 in Kenya. The maturity method of estimating concrete strength, which was developed by the US National Bureau of Standards, has been successfully used to predict the strength of concrete prepared according to American standards and without admixtures, resulting in tighter quality control of concrete. This research set out to demonstrate that the method is equally applicable to concrete prepared according to the locally used British standards, and concrete containing a plasticizer - a chemical admixture which is gaining widespread use in the production of concrete. To this end, three mixes were prepared, one according to American standards (mix A) and two according to British standards (mixes B and C). Modified lignosulphonate (Sika Plastiment BV-40), a locally available plasticizer, was used to enhance the workability of mix C. Concrete mix proportioning according to British standards resulted in a denser mix (2400 kg/m3) than American standards (2342 kg/m3); this was the main difference between the two standards. Cylindrical specimens (each measuring 150 mm in diameter by 300 mm deep) and beam specimens (each measuring 150 mm wide by 530 mm long by 150 mm deep) were made and cured at 23 ºC. The compressive and splitting tensile strengths of the cylinders and the flexural strength of the beams were almost identical for the three mixes. Also, the internal temperature (and hence the calculated maturity) of concrete was the same for all mixes. These findings indicate that: (1) the choice of standards used to prepare concrete (American or British) has no effect on the strength and maturity of the resulting concrete mix; and (2) the maturity method may be applied to concrete containing a plasticizer.