Insight into Thermal Stress Distribution and Required Reinforcement Reducing Early-Age Cracking in Mass Foundation Slabs.

Barbara Klemczak,Aneta Żmij

doi:10.3390/ma14030477

Abstract

The heat released during cement hydration results in temperature-induced non-uniform volume changes in concrete structures. As a consequence, tensile thermal stresses of significant values may occur. The level of these stresses can be lowered by using various technological measures during the construction process and a proper concrete mix composition. Nevertheless, the application of an appropriate reinforcement is a reliable method for controlling the width and spacing of possible cracks. The rules for calculating this reinforcement are not precisely detailed in the standards devoted to concrete structures. Additionally, the correct calculation of the reinforcement requires the identification of the tensile stress distribution in a mass slab. The presented study provides insight into stress distribution and relevant reinforcement for controlling early-age cracks of thermal origin. The existing standards and guidelines are discussed and clarified. The possible paths for calculating the reinforcement are proposed through the example of mass foundation slabs with different levels of external restraints. The results indicate a significant impact of the calculation method as well as the restraint conditions of the slab on the area of required reinforcement.

Highlights

Foundation slabs are exposed at an early age to elevated temperatures resulting from the heat released during cement hydration
The results indicate that the existing near-surface reinforcement of the slab is too small for early thermal cracks
The temperature caused by the cement hydration can be estimated using the iterative temperature caused by the cement hydration can be estimated using the iterativemethod method provided providedbybyCIRIA

Summary

Introduction

Foundation slabs are exposed at an early age to elevated temperatures resulting from the heat released during cement hydration. These thermal loads coming from the material itself can induce a significant level of stress. Self-induced stresses result from the nonlinear temperature and strain distribution at the slab cross-section. The external limitations of thermal deformations induce restrained stresses. Cracks in reinforced concrete structures are acceptable, their width is limited due to serviceability reasons. Early cracks require special attention, as they may expand during the later life of the structure under the impact of mechanical loads. The crack width is limited to protect reinforcement against corrosion hazards. The level of induced stresses and the cracking risk depend on many factors, which have recently been categorized into five basic groups [1]

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Conclusion