Why Concrete Cracks

Published: May 13, 2016 By: McIntosh Perry
While concrete has an impressive track record for durability, as demonstrated by such buildings as the Pantheon, it nonetheless has properties that make the development of cracks inevitable.

Drying Shrinkage

Concrete is a mixture of aggregate and cement paste; as the cement paste sets and hardens, it shrinks by as much as 25mm in 30cm. While most of this shrinkage occurs during the con- construction of a building, some will occur afterwards. This means that concrete always cracks; the number, size and location of the cracks is controlled by the amount of steel reinforcement embedded in the concrete and by the number and location of expansion joints and control joints.

Elastic and Creep Deformations

Concrete CracksHardened concrete also deforms. This process continues for a period of time that is estimated to be from 5 to 10 years following construction. Elastic and creep deformation is the gradual compression of concrete under load. It results  in the overall height of a 20 story building being reduced by up to 30mm and in floor slabs deflecting downwards by as much as 15mm at mid span locations. It will also cause balconies to deflect towards the outside edge. In this connection, it is of interest to note that brickwork may expand by as much as 30mm in 20 stories due to moisture and temperature related movement.

This means that the differential movement created by shrinkage of the building frame and expansion of the brickwork may be as much as 3mm per floor or 60mm in a 20 storey building.

The reduction in the height of the concrete frame of a building will not result in cracks developing in the concrete but does make essential the provision of horizontal control joints in the exterior and interior walls at each floor level to accommodate this movement. The deflection of the floor slabs may cause cracks to develop in the slabs at mid span and result in cracks developing in the panels of brickwork cladding a building. It may also cause a gap to open between the bottoms of panels of brickwork and the tops of exposed slab edges with the gaps being widest at mid span and disappearing two or three feet from the shear walls.

Floor slab deflection due to creep is also a major cause of the cracks in the exposed concrete forming the exterior spandrel walls of many apartment buildings constructed in the eighties and early nineties. Creep deflection of suspended floor slabs may causes tiles on the top surface to pop off unless measures are taken during construction to act. The deflection of the floor slabs may cause cracks to develop in the slabs at mid span and result in cracks developing in the panels of brick work cladding a building. It may also cause a gap to open between the bottoms of panels of brickwork and the tops of exposed slab edges with the gaps being widest at mid span and disappearing two or three feet from the shear walls.

The normal safeguard is to leave joints between the tiles empty of grout at intervals of approximately 3m in each direction and to subsequently fill them with a flexible sealant material coloured to match the grout Shortening of the overall height of a building and deflection of the outside edges of balcony slabs are important considerations to take into account when enclosing a balcony.

Both drying shrinkage and creep movement are irreversible processes with one being effectively completed within the first year and the other within the first five to ten years

Temperature and Moisture

Cracked ConcreteConcrete expands and contracts with changes in both temperature and moisture content. This movement is mainly accommodated by expansion joints which are incorporated into parking garage walls and suspended floor and roof slabs and sometimes into the buildings for this purpose. Unfortunately expansion joints often fail to accommodate all of the seasonal movement that occurs due to temperature changes; movement that may be increased by changes in moisture content. The movement that does not occur at the expansion joint is accommodated by cracks in the walls and slabs created by drying shrinkage and creep deformation. This causes the cracks to change in width between summer and winter.

These changes in crack width may cause split to develop in waterproofing membranes adhered to the surface of a slab. In repairing cracks it is important to first establish whether they are “active or “passive”. Active cracks change in width seasonally while passive cracks do not change in width.

Passive cracks are normally located either inside a building where the concrete remains dry or at a constant temperature of close to expansion joints which accommodate the thermal movement. They also may result from structural distress caused by impact or overloading.

Salt

Concrete prevents embedded steel rusting due to high level pH level of the cement paste. However, the presence of salt in the concrete above a certain threshold level, when activated by water, creates conditions that cause rusting of the tensile stresses gradually develop in the concrete and eventually the sections of concrete overlying the rusting bars crack and break loose.

Lack of Cover

A similar condition will develop without the presence of salt at locations at which reinforcing bars have very little concrete cover, such as often occurs at balcony and floor slabs edges, since under these conditions the cement paste component of the concrete no longer has the ability to protect the steel against rusting.

Carbonation

Vehicle exhaust fumes will chemically react with the cement paste at exposed concrete surfaces with the depth of the reaction below the surface of the concrete increasing with time. Carbonated concrete does not protect embedded steel from rusting and a gradual increase in the depth of carbonated cement paste may cause rusting of the steel and related cracking of the overlying concrete to develop at locations which, for many years may have been trouble free.

Crack Control

The control of cracks in concrete (and masonry) is achieved by incorporating into a building control (construction) and isolation (expansion) joints at predetermined locations in floors and walls where cracking or movement is anticipated. These joints must be kept sealed against water penetration. Construction joints also occur in concrete surfaces between adjacent pours, such as between floor slabs and shear walls. These joints, which normally have reinforcing steel passing through them, are prone to water penetration and in floor slabs are weak locations at which drying shrinkage cracks develop. They should be kept watertight.

Repair

It is important when repairing concrete to properly identify the factors causing the cracks and to design a repair that takes into account the natural properties of the concrete.For example, the procedure followed in repairing cracks due to creep movement should be different before movement is complete compared to afterwards. Similarly it is important to take into account whether cracks are active or passive are structural in origin are due to too little concrete cover over the reinforcing steel are caused by salt in the concrete or have developed for other reasons.

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