Controlling Shrinkage and Crack Widths in RC Structures Using Sustainable Construction Materials

Durability and sustainability of reinforced concrete (RC) infrastructure depends on the materials and techniques used and the extent to which the inevitable cracking is controlled. Serviceability limit state design (SLS) is currently being used to control crack widths to achieve the desired life span of the structure. Crack control is achieved by using relatively large amounts of steel reinforcement near the surface. Alternative cementitious materials and fibres, from more sustainable sources, can also be used to control crack widths. In large structures, restrained shrinkage can increase tensile strain and amplify crack widths and to address that additional amounts of reinforcement and cover are needed. Recent studies also show that shrinkage is non-uniform through the section, possibly due to uneven aggregate distribution. Thus, the correct determination of strains induced by shrinkage are necessary for the accurate prediction of structural deformations.
The aim of this study is to contribute to the understanding of how shrinkage and fibres impact short- and long-term crack widths determination in RC structures so as to lead to better predictions that can enhance the durability and sustainability of urban infrastructure. Experimental work is used to study: a) the development of non-uniform shrinkage strains in specimens made with different sizes and configurations of reinforcement, as well as the impact of shrinkage and fibres on the development of crack widths and deflections of elements tested in flexure and b) the effect of steel fibre blends in controlling shrinkage cracks in restrained elements, internally or externally, and propagation of crack widths under pure bending tests.
It was found that non-uniform shrinkage strains develop through sections due to non-uniform distribution of aggregates and a model to predict the stiffness centroid of sections and shrinkage induced strains is developed. This is used to modify crack widths predictive equations using an effective strain approach. A refined design equation, based on a modification of the fib Model Code 2010, is proposed.
Replacement of ordinary Portland cement (OPC) with 50% granulated ground blast-furnace slag (GGBS) was found to reduce shrinkage on average by about 35%, but the steel fibres do not affect free shrinkage strains. However, fibre blends of manufactured and recycled steel fibres can successfully control cracks in restrained and RC elements. For typical RC elements with about 1% of longitudinal bars, the addition of about 1% of steel fibres can reduce crack widths by more than 30%.