Cracks in Concrete

Introduction

In order for concrete to crack, there has to be a tensile stress present, be it applied or induced. Concrete is actually designed to crack, as it is assumed that the steel reinforcement within it resists all of the tension stress within the element. The sight of cracks does cause concern however, and an understanding of what causes them is important when inspecting new works and/or existing structures.

This Guidance Note describes the causes of cracking in concrete. It does not extend to the numerical analysis of cracked concrete elements

Cracking in concrete

Cracks in concrete can be divided into three categories: non-structural, thermal and structural. Non-structural refers to cracking in concrete that has occurred due to effects that do not have a significant impact on the integrity of the structure. Thermal cracks are those that come about either during the curing process (soon after the concrete element has been formed) or from the movement of the structure in response to varying temperatures. Structural cracks are those that appear due to defects in the structure and are considered to be a failure of some sort.
Whatever the symptoms of cracking, the ultimate cause is always the same: the tensile strength of the concrete has been exceeded due to either an induced action or an applied one. If you are in any doubt as to why cracks are forming in a concrete element, always remember this fundamental point

Non Structural Failure

Non-structural cracking can easily be misidentified as structural in nature and in some extreme cases, this can lead to the condemning of perfectly sound structures. It is therefore important to recognise that these types of crack are defects to the surface of the concrete, not signs that the element is about to collapse.
In most cases, non-structural cracks can be prevented by employing good practice with regards to design, construction and especially detailing of reinforcement within concrete elements. Where no reinforcement is present, careful management of the placing and curing of concrete is required in order to prevent cracking. The formation of cracks within concrete elements can be considered from two perspectives. The first is the appreciation of the tensile strength of the concrete element and the extent of its restraint. If it is unrestrained then it is free to expand and contract without hindrance from any other agent. If on the other hand the concrete is restrained, which is usually the case, a tensile stress develops around the point of restraint, which leads to the concrete cracking. It is important to note that concrete changes over time. This change in material property affects the elastic modulus of the material and can have an impact on how it responds to the surrounding environmental conditions it is exposed to during its lifetime.

The second perspective is to consider the tensile strain capacity of concrete. This changes over time, because when the concrete is setting, its tensile capacity is very high as it is in a liquid phase and deforms quite readily. As it hardens, deformation becomes more difficult and in turn its tension strain capacity decreases. Over time, however, tension strain capacity does increase soon after the concrete has set, as it absorbs moisture and thus becomes more amenable to deformation. The increase is not enough to prevent it from cracking, hence it being more prone to do so in the earlier part of a concrete element’s existence. Either way, no crack that is formed in this manner can be considered to be structural. They are occurring due to the properties of the material and the environment within which it is placed. They can however lead to water ingress that can have an impact on the structure by causing the concrete to spall (and the steel reinforcement to corrode).

Grading of Cracks

As previously explained; concrete is designed to crack. Indeed it is preferable for it to do so in order for the reinforcement within it to be efficiently designed. The question arising, however, is what is acceptable in terms of cracking? Typically this comes down to a combination of both durability and aesthetics. Table NA.4 of the UK National Annex to BS EN 1992-1-1 states that for in situ concrete elements exposed to most environments, the maximum crack width that can be permitted to develop is 0.3mm. BS EN 1992-3 concerns water retaining structures and their limits on crack widths are reduced to a range between 0.05 and 0.2mm.
There is an issue with setting these limits on crack widths as they only concern the visible crack that is at the surface of the concrete. There is no recognition given to the nature of the crack underneath the surface, which can widen and spur off into more cracks that are not visible. Without carrying out intrusive investigations, however, it is not practical to determine the exact form and depth of a crack.

Water ingress and corrosion

Corrosion of reinforcement within concrete causes the spalling of concrete as the steel expands and degrades. To prevent this from occurring, certain environmental and chemical conditions need to be present within the concrete. Therefore, it is important that:

• the level of chloride within it is not exceeded beyond a critical level • the reinforcement is placed appropriately and surrounded by good quality concrete that is not carbonated • there are no stray electrical currents present

The limit of 0.3mm described in BS EN 19921-1 has been deemed to be acceptable in terms of preventing moisture ingress having an impact on the environmental conditions of the reinforcement. Water ingress can obviously impact on the reinforcement, therefore maintaining the limit of crack width will most likely reduce the risk of this occurring.

Cracking due to settlement

Cracking of concrete due to settlement concerns how the concrete is placed, and not the movement of the structure as a whole due to the deflection of supports. It is known as ‘plastic cracking’ and occurs soon after the concrete has been poured. As the concrete begins to set, it is vibrated into place and it is at this point plastic cracking can appear. As the concrete is vibrated the water within it rises to the surface while the heavier elements, such as the cement and aggregates, settle towards the bottom. This process is known as ‘bleeding’ and it influences the likelihood of plastic cracks forming. With a combination of elements of the concrete being restrained and a high amount of bleeding, plastic cracks occur in the surface of the concrete.
To overcome this, admixtures can be installed within the concrete to reduce the amount of bleeding. This can be done in combination with designing and detailing the concrete element to reduce the amount of restraint to the concrete as it sets. It is also possible to eliminate plastic cracks by re-vibrating the concrete 2-3 hours after it has been formed, as it is still relatively pliable at this stage.

Shrinkage cracking

A variant of the plastic crack is the shrinkage crack. These typically occur over large slab pours and walls, which can appear within 2-3 hours of the concrete setting. If the concrete is unprotected while it cures it can dry too quickly, due to wind passing over it if it is exposed. This can result in ‘crazing’, which is a form of cracking on the outer surface of the concrete that consists of cracks that when linked together form small polygonal shapes. Provided the finish to the concrete is appropriately troweled and/or floated, these cracks can be graded out. However, there are instances where on the surface no cracks can be detected but instead, they have formed underneath a layer of mortar that has been created by the troweling/foating action.

Various methods have been adopted to prevent such cracks from developing. They are primarily focused on reducing the rate at which the concrete sets. The faster it sets, the stiffer it becomes and thus it has a reduced capacity for tension stress. Again, it always returns to the key point of crack propagation in concrete, which is dependent upon the tensile capacity of the material

This occurs due to the water evaporating from the concrete. Reducing the rate of evaporation is key to the prevention of shrinkage cracking. One way to address this is to apply a compound to the surface of the concrete within a few hours of it being placed. This lessens the rate at which water escapes the concrete and thus significantly reduces the likelihood of cracks forming. A more traditional method is to cover the surface of the concrete with hessian or polythene sheeting in order to prevent the bleed water from evaporating too quickly, thus allowing the concrete to set in a more controlled manner.

Use of Polyethene Sheeting on Concrete to Prevent Shrinkage Cracking.

Thermal movement

Concrete structures should be allowed to move in response to the varying temperatures they are exposed to during their design life. If they are not, then cracks can appear as the structure expands and contracts, leading to tension being generated in the concrete. This is more likely to occur in lower temperatures, as that is when concrete shrinks. Where the concrete is restrained it tends to crack as the tension within the concrete exceeds its capacity. Smaller structures, such as those with a footprint dimension of no more than 50m, are less prone to thermal effects. Larger concrete structures however, do require either additional reinforcement within them or the inclusion of movement joints. These are joints which split the structure into separate structures. The integrity of each of these structures needs to be maintained. The amount of movement the structure is likely to undergo during its lifetime must be allowed for within the movement joint.

One way to address cracking as the concrete sets is to introduce construction joints i.e. a straight crack. If no continuity of reinforcement is present, then a crack

“Concrete is designed to crack. Indeed it is preferable for it to do so”

can be generated as the concrete shrinks towards the steel reinforcement. By creating a construction joint, the crack does not appear and hence the integrity of the structure is maintained