Civil Engineering Reference
In-Depth Information
contributing to the risk of the development of reinforcement corrosion, such
carbonation can also have a weakening effect that is related to the unusually
coarse texture of the calcium carbonate crystals formed in the 'carbonated'
cement hydrates.
Stress concentrations due to differences in the strength and thermal
expansion characteristics of the repair and substrate concretes
A very large range of proprietary repair materials are currently available,
ranging from very dense, low porosity materials with a high compressive
strength and modulus of elasticity through to relatively low strength and low
modulus repair materials such as those containing PFA cenospheres and with
high entrained air contents.
It is desirable for modulus of elasticity, compressive strength and
thermal expansion characteristics of repair materials to be compatible with
those of the substrate. For example, a dense, high modulus repair mortar
applied to a relative weak substrate with a low modulus of elasticity may
develop microcracking at the repair/substrate interface when the structure
is subjected to structural loading or large temperature variations due to
stress concentrations at the repair/substrate interface. With existing repairs,
identifying the composition, porosity distribution and binder types in the
concrete substrate and repair materials is an essential step to recognising
compatibility problems of this type.
2.5 Common causes of cracking in concrete repair materials
2.5.1 Shrinkage
Shrinkage cracking falls in to two main categories:
•
Drying shrinkage is a long-term phenomenon that will begin as soon as
the concrete has hardened. The rate of drying shrinkage will decrease
exponentially with time and a high proportion of the total possible
drying shrinkage will occur within the first year after the application of
the repair.
•
Plastic shrinkage develops before the concrete hardens and whilst the
concrete is still plastic.
Fine cracking and microcracking caused by drying shrinkage is one
of the most common forms of cracking seen in the surfaces of concrete
repairs. Cracking of this type is commonly most visible when the surface
of the repair begins to dry out after being wetted. The potential for drying
shrinkage is governed to a large extent by the cement and water contents of
the repair materials and the high cement contents of many repair materials
may contribute to the potential for shrinkage. Shrinkage-compensating
