Computation of Static Modulus of Elasticity and Poisson's Ratio of M20 Grade Self-Curing Concrete with PEG-400 as a Self Curing Agent Using IS Code and ASTM Standard

Abstract

The concrete does not attain its original shape after unloading as it is not an elastic material. It is a non-linear material as depicted by its stress-strain curve. Hence, the elastic constants like modulus and Poisson’s ratio are not strictly applicable to such a complex, heterogeneous and nonlinear construction material. However, an elastic behaviour is assumed for the analysis and design of concrete structures by making use of these constants. The modulus of elasticity of concrete is its inherent property of undergoing an elastic deformation. Higher value of modulus leads to an enhanced stiffness of the structural elements. Curing is one of the important parameters influencing the modulus value. Curing caters to maintaining the required temperature, preventing water loss and shrinkage associated with it. The Shrinkage Reducing Admixture (SRA) in the form of Polyethylene Glycol 400 (PEG 400) liquid, added as a self-curing agent, plays an important role of reducing the water evaporation from the concrete mass, resulting into an improved water retention capacity of the concrete. PEG 400 is hydrophilic in nature. It plays a dual role of controlling evaporation and attracting moisture from the atmosphere. Self-curing leads to uninterrupted and effective curing by retaining the required quantity of water for an adequate cement hydration and maintaining the high relative humidity (RH), thereby resulting in to hard and dense concrete, with reduced thermal and shrinkage cracks. To provide a practical solution for the nonavailability of better quality water or lack of proper curing due to negligence, producing a self-curing concrete is a sustainable option. Generally, self-curing is advocated for the High Strength Concrete (HSC) having low water-cement ratio (w/c), because of chemical shrinkage associated with Portland cement hydration and low permeability of these materials. Moreover, it is an effective way to reduce the self-desiccation, autogenous shrinkage, plastic shrinkage cracking, drying shrinkage cracking and water absorption. Though the structural elements like Reinforced Cement Concrete (RCC) slabs are cast using lower grade concrete like M20, generally with higher w/c ratio, they are directly exposed to the harsh environment while concreting and a considerable period after concreting. To add to this, many times, the curing is not sufficient on the construction site. This leads to the undesirable situation of concrete properties not getting developed to their full potential values; the vital parameters being strength and modulus. Modulus of elasticity of concrete is known to be a function of its compressive strength. Development of required modulus of elasticity, for a structural element with a particular concrete grade, makes it adequately stiff thereby satisfying the limit state of serviceability for deflection. Excessive deflection of any structural element is highly undesirable phenomenon; moreover, it is well established fact in RCC theory that the slab depth is governed by the deflection criterion. This paper discusses an experimental investigation of short term static modulus of elasticity and Poisson’s ratio of M20 grade of self curing concrete using PEG 400 as a self curing agent. Three different dosages of 1%, 1.5% and 2% of PEG 400, expressed as percentage of weight of cement, were used. The conventional cylindrical specimens were subjected to water curing for 28 days, whereas self cured cylinders were exposed to air curing in an open shaded area for 28 days. Indian Standard: IS 516- 1959 and American Society for Testing and Materials (ASTM) standard: C469/C469M-2014 were followed for testing conventional as well as self cured specimens. The laboratory investigation was aimed at comparing the values of the two elastic constants obtained for the self cured concrete with that for the conventional mix, by both the methods. Though conventional concrete had highest values of density and compressive strength, it was observed that concrete with 1.5% PEG 400 exhibited greatest values of modulus of elasticity, by both the methods. Dosages of 1.5% and 2% of PEG 400 resulted in to higher values of Poisson’s ratio in comparison with that for conventional mix and 1% PEG 400 mix.