Page 8 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

PART 2

Setting and hardening of
concrete

Plastic and thermal
cracking

2.1 Introduction

Reinforced concrete is a composite material where load-bearing and deformation properties
are determined by the behaviour between the elements – steel and concrete – as well as
the individual constituents of these elements, particularly those of the concrete.

Concrete, at all ages, has a low tensile strength compared to the compressive strength.
Under load, the tensile strain builds in the tensile zone. This tensile strain is taken up by
the reinforcement but it is inevitable that regular but controlled cracking will occur. This
is accounted for as part of the structural design process, where the crack widths are
limited by an appropriate area of reinforcement suitable to the working environment.

Tensile strain and the possibility of cracking (flexural, shear, torsion, anchorage failure
etc.) do not only occur due to structural loading. Micro-cracks will develop in the concrete
at the interfaces between the steel and cement paste and aggregate, although only visible
through a microscope, due to internal shrinkage etc. This type of cracking is not covered
here. Cracks can develop at the unformed surface of immature concrete due to a rapid
reduction of volume at the surface (plastic shrinkage). If a concrete bleeds excessively,
the denser particles tend to settle over embedded materials, e.g. the reinforcement, causing
the near surface to tear (plastic settlement). In these cases the reinforcement does not
generally take up any tensile strain, although may affect the crack pattern developed.
Cracks can also be associated with temperature cycles, either from the hydrating concrete
at early ages or solar gain (thermal), where restraint against movement prevents expansion
or contraction so that tensile strain is induced. Here the reinforcement accommodates the

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