CONCRETE CRACKING

The expectation of a crack free concrete finish is one which the concrete is poured, the walls and framing
of the most common sources of complaint and onto and into which the concrete is built all act to
eventual dispute on building projects. restrain freshly poured concrete from shrinking.
Cracking is commonly blamed on insufficient
Concrete is poured as a liquid, and gradually hardens reinforcement, but this is a misnomer. Reinforcement
as the result of a chemical reaction called hydration. In does not prevent cracking but it does restrict crack
doing so, it loses moisture and shrinks. It is in this widths, and in sufficient quantity assists in causing
initial part of its life that concrete is at greatest risk of many small cracks to appear rather than one large
cracking. There are many variables that influence the crack. This is usually more desirable, and is a measure
likelihood and severity of cracking - some design of how well controlled the cracking is.
related (poor detailing, insufficient reinforcement or
inappropriate jointing), some construction related Even the most careful design and detailing cannot
(poor preparation, placement, finishing and curing of prevent concrete from cracking. The engineer has to
concrete, poor batching, mixing and quality of find the best compromise which is compatible with
constituent ingredients) and some environment the desired architectural outcome (ie. the shape and
related (humidity, wind, temperature). size of the building), and the more functional aspects
such as the location of mechanical services
Whilst most cracking occurs early in the life of the penetrations, stair penetrations etc. Generally internal
concrete and is due to shrinkage, concrete will also (or re-entrant) corners, long slender shapes and well
crack when subjected to tensile stresses which exceed restrained concrete elements are more at risk of
its capacity. This occurs in flexural members (such as cracking than others.
spanning beams and slabs) and slabs on reactive clay
soils. More often than not this cracking is hidden If the best possible concrete finish is required, there
beneath floor coverings, or above ceilings. Cracking of are various ways that the risk of cracking can be
flexural members is not necessarily indicative of a minimised. In addition to the best efforts of all
structural problem, but if excessive, there is a good concerned, it is important to consider courses of
chance that the accompanying deflection of these action to deal with a crack should it occur. The
members is causing greater problems than the simplest repair involves filling cracks with epoxy, but
cracking itself. the repair will be visible in a polished concrete floor,
which may or may not be acceptable depending upon
Further discussion is limited to shrinkage cracking, and the desired aesthetic. Alternative floor coverings are
in that context it is important to define what is a an obvious solution, but means abandoning the
significant crack related defect and what is not. original intention. More expensive options include
Fortunately The Victorian Building Commission have pouring the polished concrete elements as screeds
published a comprehensive guide to construction within a setdown. This method results in less restraint,
standards and tolerances which deals with the issue. and if a crack occurs the screed can always be
We rely on this document to define acceptable and replaced at any stage.
unacceptable defects. The exception to this is where a
particular finish is desired, and this has been As with so many things, it is a matter of balancing cost
communicated by the client. Under these against risk and final quality. The important thing is to
circumstances further discussion is needed in advance make sure that all parties are aware of the risks and
so that all parties are aware of the likelihood of are able to make informed choices. We are well aware
success. of these risks, and are happy to discuss strategies for
dealing with them. We are also aware that it is
If a client asks Gandy & Roberts to design a polished preferable to have the conversation before rather
floor and guarantee that cracking will not occur we than after the problem occurs.
will seek to design and detail in such a way that the
risk of cracking is minimised, but cannot guarantee
that it will not occur. There are just too many variables
beyond our control that can cause cracking.

If concrete was free to shrink without any external