Structural Implications of Concrete Cover on Reinforced Concrete Structures
Reinforcement of concrete with steel is done to strengthen the structural element in tension as concrete is weak in it, but structures do fail as a result of corrosion attack on steel, fire hazard or poor concrete-reinforcements bonding. These have created a serious concern worldwide, with costly repairs now in billions of dollars annually. In addition, there is numerous intangible losses such as the energy needed to manufacture replacements of corroded objects.
The aim of this blog is to accurately show the structural significance of concrete cover to reinforcements and to also recommend adequate methods the contracting engineer could employ to guarantee that a project is insured against hazards arising from cover to reinforcement.
Concrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete (ACI 130). The depth of a concrete cover could be measured using an instrument called the cover meter.
Significance of provision of concrete cover:
There are certain reasons for which minimum concrete cover to reinforcements is provided in reinforced concrete structure.
- Cover against corrosion
To protect the steel reinforcement bars (rebars) from environmental effects to prevent their corrosion. This provision is to ensure that durability requirements for reinforced concrete against the various exposure conditions is met.
As already noted, concrete is reinforced with steel to ensure that the element is capable of resisting tensile stress as concrete is weak in it, however structures do fail due to corrosion of steel embedded in it.
Corrosion in its simplest definition refers to an electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfates. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. In the event of rusting of steel, a chemically inert layer is formed on its surface.
Since corrosion is a phenomenon subject to diffusion, methods such as passivation, chromate conversion and concrete covering is employed to restrict activities of ingress water, carbon (IV) oxide and sulphates. In construction management, a sufficient thickness of concrete cover is thus required in order to prevent ingress water and subsequent carbonation process towards the rebar. The minimum concrete cover will depend on the nature of exposure of the member. A well compacted concrete made from lower water to cement ratio will resist diffusion rate hence protecting the embedded steel against corrosion.
In provision of concrete covering, care must be taken to ensure that excessively thick cover isn’t provided especially as thick cover leads to increased crack widths in flexural reinforced concrete members. As the crack width increases, embedded reinforcements become susceptible to corrosion therefore defeating the purpose for which the thick crack width was provided.
- Cover Against Fire
To provide thermal insulation, which protects the reinforcement bars from fire. Concrete is a versatile construction material with so many unique qualities, one of which is it’s fire resistance. This is due to its constituent materials which when chemically combined together forms a passive whole with low thermal conductivity. In case of fire outbreak, the exposed concrete surface temperatures rises to over 1500F. Farther away from the concrete surface the temperature is relatively cooler hence the embedded reinforcements at this point will be slightly cooler but for temperature of about 1200F when the steel loses over 50% of its design strength. At this juncture, explosive spalling begins to occur on the structure. Concrete cover plays an important role in reducing the rate of temperature increase of steel for a particular fire resistance which is always given in hours. This is to ensure that the strength and structural integrity of the system is maintained until help comes. There are ranges of design codes of practice the structural engineer is required to employ in carrying out design of any structural member in order to cover for fire resistance.
Finally in the event of a fire accident, in case the structure or elements does not collapse, the extent of damage due to the fire on the strength or load carrying capacity of the member is not the same prior to fire exposure. Therefore, a structural integrity check of the actual strength or load carrying capacity of the resulting structure need be established in order to ensure a good decision on economy and structural safety.
- To give reinforcing bars sufficient embedding to enable them to be stressed without slipping.
The bond strength between reinforcements and concrete comes mainly from friction and adhesion and it’s affected both by the properties of steel and those of concrete and relative movement due to volume changes such as shrinkage. However, there’s also mechanical interlock between reinforcements and concrete. Bond strength between concrete and steel is as important as the compressive strength of concrete. According to Neville bonding between concrete and steel is important both in terms of structural behavior as in relation to cracking caused by shrinkage and thermal effects at early ages. A simple pullout or push test conducted on reinforcements embedded in concrete will indicate that adequate surrounding concrete is necessary to provide bond shear strength required to withstand the stress of the applied load.
Achieving adequate concrete cover to reinforcements:
Concrete cover is usually maintained with the use of special stands called spacers. These must not be affected by corrosion and therefore they are usually specially formed chips of precast concrete or prefabricated plastics.
The simplest solution for providing the necessary cover thickness of the reinforcement is special plastic under-layers. Usage of steel rebar spacers is forbidden as they are highly susceptible to corrosion.
For slabs, these spacers cut with its thickness according to the design concrete cover are placed under the reinforcements at intervals of 1m. The same technique is also applied for beams. However for columns and other vertical members, the spacers are secured between the formwork and the reinforcements with the its thickness conforming to the design cover.
There are other forms of covering materials that could be used on structures. These materials could be in form of finishing materials like paint and tiles. However they can only serve most times as cover against exposure conditions and environmental effects.