Early age cracks in concrete walls

Abstract

During the development of construction industry, people looked forward on building their houses by Construction of a concrete structure requires a large volume of concrete. Due to the small surface area-to-volume ratio, concrete structures are often subjected to high potential of thermal cracking, caused by the heat generation from cement hydration. To reduce the thermal cracking and ensure structural integrity, a good understanding about the crack patterns in concrete elements is required. These kinds of cracks mostly occur during the early age state of concrete. Since Sri Lanka is very near to the equator, the probability of early age crack occurring is even higher. The purpose of this research is to explore the potential early age crack patterns in vertical concrete walls. Main reason for the early age cracking in vertical walls are shrinkage and thermal contraction. This research focuses on the understanding of early age thermal cracking in concrete and developing a simple method to model this phenomenal computationally. Series of boundary conditions were modelled to obtain stress distributions using walls 3m high and 4m & 8m long. Boundary conditions were imposed according to guidelines in BS8007 and wall thickness maintained as 300mm during the analysis. All the analysis was carried out using FEM commercial software Sap 2000 (V19.1). Two approaches were followed initially to identify the best method to represent the restraint conditions as per BS8007. End restraints reduced by using roller supports up to a 2.4m distance from the free edge of the wall with gradually increasing applied horizontal forces proved to be the better technique than that of using reduced E-values. The case studies yielded the following general findings that agree with the literature and field observations; (i) 4m walls can have possible vertical and horizontal cracks. (ii) 8m walls can have possible cracks approximately 2.4m away from the free edges with an inclination of approximately 45º-60º. (iii) 8m walls get the highest stress close to 2.4m from the free edges whereas the 4m walls get the highest stress at the centre. (iv) In 8m walls higher stresses are distributed over a central length whereas in 4m walls the higher stress is concentrated at the centre. (v) 4m wall with top movement can cause possible inclined cracks. (vi) 8m wall with top movement can lead to two possible dominant cracks and two minor cracks. This validation was done qualitatively using the literature and on-site observations.