Deterioration and defects – what are the risks to the safety and service life of concrete structures?
Concrete structures can be affected by a range of defects and deterioration. These can impact on both the safety and service life of structures.
The most common forms of defects and deterioration that may occur in concrete structures, how they occur and what the potential effects on structural safety and service life are discussed in this article. References are provided for further information.
The deterioration mechanisms and defects can be grouped broadly into five categories:
- Early age
- Design and construction
Discussion of the deterioration mechanisms and defects is kept to a basic level in the following sections. The main concerns in this article are the implications for safety and service life.
An article summarising the effects of corrosion on cracking, bond strength, flexural strength, shear strength and column behaviour is available in this post.
A summary of what designers should do to minimise the risk of deterioration in new structures is available in this post.
1. Early-age deterioration mechanisms
This refers to those forms of deterioration which appear in the first week after construction. They are generally not regarded as being serious problems in their own right but can help to promote other deterioration mechanisms in later life.
Further information is provided in Concrete Society Report 22.
1.1 Plastic settlement
This usually occurs within two to ten hours after placing. Bleeding and settlement, aggravated by incomplete compaction, can lead to plastic settlement cracking. Cracking usually occurs at positions of restraint such as over reinforcement or at a change in section. It can be a common occurrence at the top of deep pours.
The consequences include a reduction in bond between the top reinforcement and the surrounding concrete. The cracks can also provide a route into the cover concrete for moisture, oxygen and chlorides thus shortening the service life.
1.2 Plastic shrinkage
Plastic shrinkage cracking usually occurs within two to ten hours of placing. Typical locations include the exposed top surfaces of road slabs, suspended slabs and walls. It is primarily caused by the rate of water evaporation exceeding the rate of bleeding. The high temperatures in hot countries can exacerbate this. Cracking results when the restrained plastic contraction exceeds the tensile strength of the concrete. Cracks can be quite large (2-4mm) and run through the full depth of the section.
The consequences are that moisture, oxygen and chlorides have a direct path into the concrete. This may shorten the service life.
1.3 Early-age thermal cracking
Cracking can be caused by the restraint to thermal contraction resulting from excess heat generated within concrete members due to the hydration of the cement. Generally, the larger the concrete sections, and the higher the cement content and placing temperature, the more heat that is generated.
Early thermal cracking usually appears within two to ten days of construction. Thermal cracking generally does not penetrate the whole depth of a section although it can sometimes follow the line of the reinforcement depending upon how the steel has been detailed. As such, there is an enhanced risk of chloride-induced corrosion.
2. Environmental effects
These mechanisms result from an interaction between the concrete and the ambient conditions such as moisture, drying and thermal effects.
2.1 Drying shrinkage
Chemical and physical loss of water during hardening and subsequent loss of water to the atmosphere lead to a volume reduction during the lifetime of a concrete member.
The outer layers will try to contract more than the inside. Restraint to this contraction can lead to cracking in the surface layers. Any cracking usually appears at least a year after construction. These cracks are generally not very wide and appear as random map cracking.
The likely consequences are contractions and deflections. Deflections in beams can lead to cracking of partition walls and secondary load paths may be established.
Further information is contained in Reference 2.
Creep is a time-dependent phenomenon whereby stressed concrete deforms gradually under load. A stress that is applied and then sustained will cause creep. The creep effects tend to be more extensive at early ages due to less complete hydration.
Creep can have both beneficial and detrimental effects. Beneficially, it can relieve internally induced stresses such as those from corrosion products. Detrimentally, external applied loads cause contraction.
The likely consequences are contractions and deflections. Deflections in beams can lead to cracking of partition walls and secondary load paths could be established. In taller structures vertical shortening of heavily loaded members relative to other (non-load-bearing) members could cause problems. Examples may include concrete fins on the outer face of buildings that may spall.
In columns, load will be shed in the long term from the concrete section to the reinforcement. Creep has a non-linear relationship with stress making it more significant in under-strength concrete. If the concrete is of a particularly low strength, then the concrete section will be highly stressed leading to the potential for that reinforcement to yield in compression.
Further information is contained in Reference 3.
2.3 Moisture-sensitive aggregates
Some aggregates are sensitive to their internal level of moisture. Aggregates can expand when taking up water or shrink when losing water. The consequences include random or aligned cracking and increased deflections.
The problem is localised, resulting from some aggregates found in parts of Northern England, Scotland and Northern Ireland. Any problems are usually found in the first year.
As with shrinkage, the likely consequences are contractions and deflections.
2.4 Long-term thermal movements
Like most materials, concrete will expand when heated and contract when cooled. There are two primary categories of thermal movement:
- Axial movement –where the length of the member changes
- Differential thermal gradients –particularly in deeper members, can cause significant stresses at the top and bottom surfaces
Thermal movements are only a problem when there is restraint to that movement. Thermal movement can widen existing cracks, open poor construction joints and, in some cases, generate cracking. Multi-storey car park roofs with large exposed areas are particularly prone to long-term thermal movements unless that movement is catered for. The primary consequence of long-term thermal movements is that of speeding up the effect of other deterioration mechanisms such as corrosion.
2.5 Freeze-thaw attack: Surface scaling
Surface scaling generally results from the surface concrete undergoing freeze-thaw cycling whilst saturated with moderately saline water. The cement paste is gradually scaled away by this freezing and thawing action. This initially undermines the sand particles and then the coarse aggregate particles. Eventually these particles are loosened to the extent that they fall out.
The exposed surface is gradually scaled off, thus reducing the cover to the reinforcement and protection against corrosion. The bond strength between the reinforcement and concrete is reduced with the loss of cover concrete. The appearance of the member also suffers, but surface scaling is not usually accompanied by internal damage.
There are few occurrences in the UK. The most likely locations being in colder regions. It is unlikely to occur in air-entrained concrete.
Further information is contained in Reference 4.
2.6 Freeze-thaw attack: Internal damage
Internal freeze-thaw damage can result from water freezing in pores, voids and cracks within the concrete. The water content within the concrete is the most important factor in determining whether internal frost damage will occur. If the water content is above a critical value, then frost damage can be induced in one single freezing.
Internal freeze-thaw damage induces a random system of cracks in the heart concrete together with cracks parallel to the exposed surface of the concrete which decrease in severity away from the surface. The cracking parallel to the exposed surface is of particular concern as it will be parallel to and close to the reinforcement. A random crack pattern is formed on the surface.
Internal freeze-thaw damage leads to a loss of cohesion within the concrete as a result of the internal cracking. This has structural implications. The concrete’s tensile and compressive strengths are reduced as is the bond between reinforcement and concrete. Young’s modulus also suffers a reduction.
In the UK, freeze-thaw attack occurs in colder regions. Freeze-thaw attack is unlikely in air-entrained concrete.
Further information is contained in Reference 4.
2.7 Corrosion: Carbonation-induced
Atmospheric carbon dioxide will penetrate into concrete. In the presence of atmospheric moisture, calcium hydroxide within the cover concrete will be converted to calcium carbonate. This reduces the pH within the cover concrete, and when the carbonation front reaches the reinforcement the level of passivation is reduced around that reinforcing bar. If atmospheric moisture and oxygen are able to penetrate to the reinforcing bar, then there is a possibility that that bar will start to corrode. As the corrosion product occupies a greater volume than the reinforcing bar, tensile stresses will be developed within the cover concrete.
The ultimate outcome of this is cracking along the line of the reinforcing bar and possibly spalling of the cover concrete. Depending on the depth of cover and quality of the concrete this could take between 5 and 100-plus years. For buildings, the highest rates of carbonation occur internally where the risk of corrosion is lowest.
The likely consequences of corrosion also have safety implications. These fall into two categories:
- Loss in load-carrying capacity
Spalled concrete has the potential to inflict injury on passers-by. For instance, a small piece of cover concrete spalling off a cladding panel on a high-rise building could result in serious injury.
The loss in load-carrying capacity results from both the loss in concrete section (due to spalling) and the loss in reinforcement section due to corrosion. Loss of bond between steel and concrete is a result of cracking along the line and a reinforcing bar. Shear links have lower covers than flexural steel and will thus corrode first. A concern is that of a possible change in failure mode from ductile flexure to brittle shear.
Atmospheric carbon dioxide will penetrate into concrete. In the presence of atmospheric moisture, calcium hydroxide within the cover concrete will be converted to calcium carbonate. This reduces the pH within the cover concrete, and when the carbonation front reaches the reinforcement the level of passivation is reduced around that reinforcing bar.
Further information is on the material issues is contained in Reference 5 and further information on the structural implications is contained in Reference 6.
2.8 Corrosion: Chloride-induced
Chloride induced corrosion is initiated by chloride salts from a range of sources. Until the 1970s, calcium chloride was used as an accelerator in concrete mixes and provided an internal source of chlorides. Marine structures are subject to both atmosphere and water-borne salts. De-icing salts have been used on roads during winter. Bridges and multi-storey car parks have been particularly exposed to de-icing salts. In some countries, such as Hong Kong, the use of seawater for flushing toilets and washing down staircases has led to chloride ingress.
Chloride ions penetrate concrete by a combination of absorption and diffusion. The time taken for the chloride ions to reach the reinforcement is a function of both the depth and quality of the cover concrete. It can range from only a few years to in excess of one hundred years. The mechanism by which chlorides promote corrosion is not completely understood yet. The possibility of corrosion is likely to be dependent on the access of moisture and oxygen to the reinforcement and the chloride threshold level.
As with carbonation-induced corrosion, cracking, spalling and loss of load-carrying capacity are all possible. A discussion of the implications for safety and service life is available in this post.
Further information is on the material issues is contained in Reference 5, and further information on the structural implications is contained in Reference 6.
3. Chemical deterioration mechanisms
This section includes those deterioration mechanisms where the deterioration is a result of chemical reactions within the concrete. These chemical reactions are usually a product of the interaction between external chemicals and those within the concrete.
3.1 Alkali-silica reaction
The alkali-silica reaction (ASR) involves the reaction between hydroxyl ions in the pore water and disordered silica in the aggregate. The reaction product is a gel which swells as it takes in pore water. The swelling of this gel can cause both expansion and cracking, but it does not always do so.
Three components must be in place before the reaction can occur. There must be reactive silica within the aggregate, a high alkali content in the cement and a ready source of water.
If cracking does occur, it will be aligned at right angles to the restraint. That is parallel to reinforcement and axial loads. Where there is little or no restraint present random map cracking will result. In the UK, cracking usually appears between one and six years after construction.
Although, the cracking may appear unsightly, the macro-cracks only penetrate the surface and there are likely to be no major adverse effects upon structural performance. ASR does lead to a loss in both tensile and compressive strength within the concrete. However, any loss in load-carrying capacity can usually be explained by the loss of concrete strength. A proportion of any reduction in load-carrying capacity can be offset against the chemical prestress developed as a result of restraint to expansion.
ASR is rare in the UK. Most observed cases are in structures built between 1969 and 1972.
Further information is contained in Reference 7.
3.2 Sulphate attack
Sulphate attack occurs as a result of sulphates in ground water, flue gases, industrial waste and sea water. As with other deterioration mechanisms, sulphate attack also requires the presence of moisture. The reaction product occupies more space than its components. As a result, expansive stresses are set up and fine particles of concrete can be lost from the surface. Eventually, aggregate particles become dislodged and fall out.
Sulphate attack is very rare in the UK, localised in areas according to the geology of the area or the industrial products output.
The main consequence likely is loss of cover to any reinforcement that is present. This will gradually erode the bond strength which is a function of the cover-to-bar ratio. In structures where flue gases are the main source of sulphates, cover loss can lead to a reduction in the protection provided to the reinforcement against other deterioration mechanisms.
Further information is contained in Reference 4.
3.3 Delayed ettringite formation
The mechanisms of delayed ettringite formulation (DEF) have not been established clearly. The name DEF has been selected because bands of ettringite are formed around aggregate particles. DEF can occur in sections which have undergone abnormal temperature rises in excess of 70ºC and where a source of moisture is present. Typically, this is either heat-cured precast sections or massive in situ sections with high cement contents. Random and aligned cracks can occur with widths of up to 10mm.
In the UK these cracks take between eight and twenty years to occur. DEF is, however, very rare in the UK.
The likely consequences include reductions in tensile and compressive strength and elastic modulus. The structural effects have yet to be evaluated.
Further information is contained in Reference 4.
4. Design and construction defects
Defects are not strictly deterioration mechanisms in the same way as those described in previous sections. These mechanisms should not occur in properly designed, detailed and constructed structures.
4.1 Incorrect cover
Low cover to reinforcement has two effects on reinforced concrete:
- Durability is reduced –in particular the time to corrosion is reduced, as there is less of a barrier to carbonation or chlorides
- Bond strength – can be reduced
High cover is also of concern, particularly in cantilevers. If the reinforcement is placed too far from the tension face, then the lever arm will be reduced, and it will not be effective in carrying load.
4.2 Low concrete strength
One of the more common defects in some countries is low strength concrete. This can result from a number of causes, but the prime cause appears to be low cement contents.
Low concrete strength implies a high water-cement ratio (w/c). In addition to low strength, a high w/c ratio is likely lead to low durability, as the effects of most deterioration mechanisms are inversely proportional to the w/c ratio. This implies that the service life of low strength concrete is likely to be lower than that assumed at the design stage.
Generally, modern design codes limit the characteristic strength to no less than 25 N/mm2. The code rules are likely to be calibrated for normal strength concrete (i.e. a characteristic compressive strength greater than 25 N/mm2). Thus, both the mechanical and the durability rules in the code may be invalid and the assumptions regarding service life may be over optimistic.
There is a fundamental difference between low strength concrete where low strength was the intention, such as foam concrete, and low strength concrete that results from quality control problems and insufficient cement.
Further information is contained in Reference 8.
These are not strictly deteriorations mechanisms in the same way as those described in previous sections. However, the structural mechanisms can lead to cracking if the members are overloaded. These cracks can then provide a route into the concrete for the aggressive substances described in previous sections.
A point to bear in mind is that these mechanisms should not occur in properly designed, detailed and constructed structures where the occupier does not impose loads in excess of those for which the structure was designed. The important point is that loading should be considered in relation to that assumed in the approved design.
Flexural stresses result from the bending of a member. These stresses will either be tensile or compressive depending on the orientation of the member and its loading. For example, in a cantilever there would be tensile stresses in the top face. Compressive stresses are found in the bottom face.
Very fine tensile flexural cracks are to be expected in reinforced concrete under service loads. They should be barely visible.
Cracking or crushing in the compression zones is not to be expected. Beam and slab members should be designed to fail in a ductile tensile mode rather than a brittle compressive mode. Ductile failure modes are much safer as they give greater deflections and thus more warning before failure.
Flexural failures are very rare. There is a high degree of redundancy in most structures, which allows redistribution of stresses away from over-stressed zones. This is not the case for cantilevers.
The influence of deterioration mechanisms such as corrosion could increase the probability of flexural failure. Flexural load-carrying capacity is primarily a function of the area and yield strength of the reinforcement. Corrosion reduces the area of reinforcement and thus the flexural capacity.
Shear forces are at their highest in support regions. Shear cracks are usually inclined and stretch most of the way from top face to bottom face in the region of supports. Shear forces are resisted by a combination of longitudinal reinforcement, shear link reinforcement and the concrete.
Shear cracking is highly undesirable. It is also very rare as members should be designed to fail in a ductile tensile failure mode rather than a brittle shear mode.
Shear capacity can be affected by deterioration. For example, shear link reinforcement can corrode before longitudinal bending reinforcement as it has lower cover. Corrosion will lead to loss of section in the shear links. A member which is designed to fail in flexure within its as designed state may thus fail in shear in its deteriorated state.
Axial loads occur along the longitudinal axis of the member. Columns taking axial compression are the most common type.
It is unusual for axial loads to act in isolation. There is usually an element of flexure involved from asymmetric loading.
There have been cases of catastrophic failure due to axial overload in columns. An example is the New World hotel in Singapore. This was caused by a combination of overload and low strength concrete.
Failure tends to be preceded by longitudinal cracking and spalling of the cover concrete – cracks were visible in the New World Hotel in the weeks before collapse. There is then a combination of concrete crushing and reinforcement yielding (or buckling) in compression.
As with the other structural mechanisms, the axial load-carrying capacity can be reduced by the effects of deterioration. For example, corrosion of the longitudinal reinforcement can lead to longitudinal cracking and spalling of the cover concrete. This leads to a reduction in both the concrete and reinforcement area available to resist axial loads. It could also lead to a column being transformed from a stocky to a slender section.
Any deterioration mechanism which reduces concrete strength can reduce axial load-carrying capacity.
6. References for further information
- THE CONCRETE SOCIETY. Non-structural cracks in concrete, Technical Report No. 22, Fourth Edition, 2010
- HOBBS, D.W. and PARROTT, L.J. ‘Prediction of drying and shrinkage’, Concrete, 13, 1979, pp 19–24
- GILBERT, R. I. Time Effects in Concrete Structures, Elsevier, 1988
- HOBBS, D. W. ‘Concrete deterioration: causes, diagnosis and minimising risk’, International materials Reviews, Vol. 46, No. 3, 2001, pp 117-144
- HOBBS, D. W. (editor). Minimum requirements for durable concrete: Carbonation- and chloride-induced corrosion, freeze-thaw attack and chemical attack. Crowthorne, British Cement Association, 1998. 172 pp.
- WEBSTER, M. P. The Assessment of Corrosion-Damaged Concrete Structures, PhD Thesis, University of Birmingham, July 2000
- INSTITUTION OF STRUCTURAL ENGINEERS Structural effects of alkali-silica reaction. Technical guidance on the appraisal of existing structures, 1992 (including Addendum, April 2010)
- McNICHOLL, D. P., AINSWORTH, P. R., HARLEY, M. V., STUBBINGS, B. J. and WATKINS, R. A. M. Public housing blocks in Hong Kong: the identification, investigation and rectification of structural defects. The Structural Engineer. Vol. 68, No. 16. 21 August 1990. pp. 307-316.
About the author:
Dr Mike Webster is a chartered civil and structural engineer with over 30 years’ experience. He specialises in construction and structural safety, CDM and risk, and founded MPW R&R to provide Consulting, Forensic and Expert Witness services in those areas.
Mike has worked on the design, inspection, appraisal and site supervision of building, bridge and car park structures. He has worked at both the Cement and Concrete Association (C&CA) and the British Cement Association (BCA) where he developed guidance for assessing the safety and service life of deteriorating concrete structures.
Mike led an independent review of CDM 1994 and the independent evaluation of CDM 2007. He also led the review of the use of CDM 2007 in the construction of London 2012.
Mike has been instructed as an expert witness by both defence and prosecution teams in cases involving allegations of gross negligence manslaughter, breaches of the Health and Safety at Work Act and the CDM Regulations and the appeal of enforcement notices. These cases have involved the construction, maintenance and demolition of a range of building, bridge and industrial structures.
Mike is the author of around 20 published reports and papers on structural safety, construction health and safety and the CDM Regulations. He is also the author of a range of articles on CDM 2015. He is a member of Structural-Safety and the Institution of Structural Engineers Health and Safety Panel.
For more information email Mike at firstname.lastname@example.org or give him a call on 07969 957471.