Impact of corrosion on the safety and strength of concrete structures – Summary of the mechanisms
Reinforced concrete structures degrade with time and exposure to the environment, particularly if that environment contains salts. That degradation can lead reinforcement to corrode, and this can affect the safety and load-carrying capacity of concrete structures.
The data from existing research were reviewed to produce an overview of the structural effects of chloride-induced corrosion on reinforced concrete structures. The effects on the following are covered in this article:
Suggestions for simple models are presented to allow both the assessment of corrosion-damaged concrete structures and the durability design of new structures.
The full details of the material discussed in this article are provided in Reference 1. This includes an appendix containing suggested modifications to UK codes. To download a copy, click here.
Reinforcement corrosion is probably the most serious durability problem affecting reinforced concrete structures. Whilst cost is an issue, safety is, in my opinion, the primary issue. As the reinforcement corrodes there is the potential for cracking, spalling of concrete and reductions in reinforcement cross-section. This leads to two safety concerns:
- Falling of loose concrete onto pedestrians, vehicles, users etc.
- Reductions in load-carrying capacity potentially leading to collapse
This implies that remedial action is required when corrosion is present. However, resources are scarce, and it is not always possible to take remedial action straight away. In the case of the Midlands Links viaducts the extent of corrosion was so great that the whole network could not be repaired in one go for reasons of cost, resources and disruption. This meant leaving corroded bridges for several years before they were repaired. To achieve this, estimates of the likely load-carrying capacity of each bridge would have been required before they could be left for any period of time.
In determining the current and future load-carrying capacity of a corroded concrete structure it is necessary to:
- Estimate the amount of deterioration of the materials
- Determine the structural effects
- Estimate the load-carrying capacity now and in the future
This article links together a range of issues to produce an overview of the effects of corrosion on reinforced concrete structures. Qualitative descriptions of the structural effects of corrosion are presented for a range of load-carrying mechanisms. Suggestions are then made for simple models that provide a quantitative means of assessing the safety and load-carrying capacity of corrosion-damaged concrete structures. Full details of the suggested modifications to UK code equations are contained in Appendix A of Reference 1 (click here to download).
An article summarising the impact of other forms of deterioration and defects on structural safety and service life is available in this post, a summary of the influence of design and detailing on the safety of deteriorating concrete structures is available in this post, and a summary of what designers should do to minimise the risk of deterioration in new structures is available in this post.
From a review of previous research, the important parameters controlling the amount of corrosion to cause cracking of the concrete surrounding a reinforcing bar were found to be those given in Table 1.
Table 1: The effects of various parameters on corrosion-induced cracking
|Parameter||Effect on cracking|
|Concrete tensile strength[3, 4]||
|Concrete elastic modulus||
|c/D ratio[4, 5, 6, 7, 8, 9, 10, 11]||
|Section loss and crack width[5, 6, 7]||
|Corrosion rate[5, 6, 12]||
|Concrete pore structure[5, 8, 9]||
|Corrosion of adjacent reinforcement[11, 13]||
|Crack propagation[4, 7, 11, 13]||
Many attempts have been made to predict the amount of corrosion required to cause cracking. These have typically used analytical or numerical solutions. However, most of these solutions ignored the amount of corrosion product that is accommodated within the pore structure surrounding the bar. Hence the amount of corrosion to cause cracking can be underestimated, often by an order of magnitude. Study of the available laboratory data indicates that such sophistication is not necessarily required, and that a simple empirical solution can be obtained by relating the amount of corrosion to cause cracking to the cover to bar diameter (c/D) ratio as shown in Figure 1.
Figure 1: The relationship between bar weight loss due to corrosion and the c/D ratio
Design and assessment code rules are derived on the assumption that the strains in both concrete and reinforcement are the same; that is perfect bond exists between the two materials. However, corrosion can reduce the bond strength.
Bond is required for the main load-carrying mechanisms of bending, shear and axial load. These mechanisms do not require explicit checks in design provided that the code detailing clauses on anchorage lengths, lap lengths and minimum reinforcement levels are complied with. In the assessment of existing, and deteriorated, structures many of these clauses may not be complied with. In such cases, explicit checks on bond strength may well be required in order to calculate load-carrying capacities.
Study of the available data from laboratory tests indicates that the effects of corrosion on bond strength can be categorised into the four phases shown in Table 2. The effects of a number of parameters observed on the corroded bond strength are summarised in Table 3.
Table 2: Effects of corrosion on the phases of bond strength
|Pre-cracking[10, 14, 15]||
|Cracking[10, 14, 15 16]||
|Post-cracking[10, 14, 15, 16, 17, 18]||
Table 3: Effects of various parameters on bond strength – corroded reinforcement
|Parameter||Effect on bond strength|
|Cover to bar diameter (c/D) ratio[10, 14, 15, 18]||
|Bar position[10, 18]||
|Concrete tensile strength||
|Applied transverse stress[16, 18]||
|Bar type[10, 14]||
These reductions in bond strength should not be viewed in isolation. To be realistic, any corroded bond strength has to be compared to the bond strength required of that bar. The ultimate bond strength has been calculated in accordance with BS 8110 for all of the test data on corroded bars reviewed. The partial safety factor is set at 1.0 to allow comparisons. In Figure 2 the BS 8110 values are compared to the measured bond strengths obtained with corroded reinforcement.
Figure 2: The effect of corrosion on the bond strength of reinforcing bars in comparison to the ultimate bond strength requirement of BS 8110
It is interesting to note that the characteristic (5%) value of the ratio of corroded to BS 8110 bond strength considering all of the specimens is 0.74. Considering just the corroded specimens reported as cracked the characteristic value is 0.69. The value given in BA 51/95 for the ratio of corroded to BD 44 bond strength is 0.7 (BD 44 and BS 8110 both have the same approach to bond). This value was obtained at a time when considerably less data was available and would thus appear to be validated to an extent by the additional data.
The effects of corrosion on a variety of laboratory specimens subject to accelerated corrosion were reviewed, and the results are summarised in Table 4.
Table 4: Effects of corrosion on flexure
|Parameter||Effect on flexural strength|
|Loss of reinforcement cross-section[17, 18, 20, 21, 22, 23, 24, 25]||
|Longitudinal corrosion cracks[20, 21, 22, 23, 24, 25]||
|Reduction in length of plateau of load-deflection curve<[20, 22, 24, 25]||
|Corrosion at laps||
|Link corrosion[18, 26, 27]||
Provided that premature failure does not occur due to shear or bond failure, flexure is relatively straightforward. The reduction in flexural load-carrying capacity is a function of the reinforcement area in singly reinforced members. In members with corroded reinforcement in the compression zone the effects of concrete spalling in the compression zone also have to be considered. The effect of carrying out these modifications is shown in Figure 3.
Figure 3: The variation in the ratio of test/predicted flexural load-carrying capacity with corrosion for (a) singly reinforced beams and (b) those with reinforcement in the compression zone
(a) Singly reinforced
(b) Reinforcement in compression zone
Corrosion can modify the failure mode by reducing the shear load-carrying capacity faster than it reduces the flexural load-carrying capacity. There are two reasons for this:
- By necessity, shear links have lower covers than flexural reinforcement and will start to corrode first
- Shear is more sensitive to reductions in anchorage bond of the tension reinforcement at supports
Many of the shear failures due to accelerated corrosion have arisen out of tests aimed at investigating the effects of corrosion on beams rather than the effects of corrosion on shear. These beams failed in flexure when little or no corrosion was present but failed in shear at higher levels of corrosion. Only one set of data is available where the effects of corrosion on shear was specifically investigated. A summary of the key parameters is given in Table 5.
Table 5: Effects of corrosion on shear strength
|Parameter||Effect on shear strength|
|Loss of reinforcement cross-section[18, 28]||
|Longitudinal corrosion racks[18, 28]||
|Link corrosion[18, 28]||
|Failure mode – beams designed to fail in flexure[17, 18, 22, 23, 24, 28]||
|Failure mode – beams designed to fail in shear||
|Cover to bar diameter ratio>||
The effects of corrosion on shear strength are complex and cannot be accounted for simply by using reduced reinforcement areas as with flexure. This can be illustrated by considering the corroded beams tested in shear by Daly.
The beams tested by Daly with no shear reinforcement are nominally identical. Assuming that there is no change in failure mechanism, for each c/D ratio the only parameter that varies in the BS 8110 expression for Vcis As. Using this equation, the shear strength of the corroded beams is related to the control by the following expression:
|where:||As||=||effective area of tension reinforcement in the uncorroded beam (mm2)|
|As.eff||=||effective area of tension reinforcement in the corroded beam (mm2)|
|b||=||breadth of the beam (mm)|
|d||=||effective depth to the tension reinforcement (mm)|
|Vc||=||shear load-carrying capacity in the control (uncorroded) beam (kN)|
|Vc.corr||=||shear load-carrying capacity in the corroded beam (kN)|
The effective areas of tension reinforcement calculated using this approach are given in Figure 4. It can be seen that using the corroded area of reinforcement would have underestimated the load-carrying capacity for the beams with a c/D of 1 but would have overestimated the load-carrying capacity for the beams with a c/D of 3.
Figure 4: Relationship between the corroded area of longitudinal reinforcement and the effective area for shear
The last of the key load-carrying mechanisms addressed in this paper is axial load-carrying capacity or, as it is referred to here, column behaviour. The term column behaviour is used to acknowledge that axial load on its own is not common in real structures. It usually occurs in conjunction with applied bending moments.
Only two sets of test data were available. Although these columns were axially loaded, nominal moments were present due to a combination of the non-uniformity of the corrosion, imperfections in the casting and testing regime and, at later stages, spalling. The effects of the main parameters on column behaviour are summarised in Table 6.
Table 6: Effects of corrosion on column behaviour
|Parameter||Effect on axial load-carrying capacity|
|Loss of longitudinal reinforcement cross-section[24, 29]||
|Spalling of concrete[24, 29]||
|Loss of link reinforcement cross-section[24, 29]||
The key conclusion from this review was that the concrete cover outside of the links should be ignored in any calculations to estimate the load-carrying capacity. By doing this and allowing for the corroded area of reinforcement a reasonable fit to the test data can be obtained using both the nominal moment expression and the interaction diagram method outlined in BS 8110. This is show in Figure5 for the series 1 columns tested by Rodriguez et al.
Figure 5: Comparison of predicted test and predicted load-carrying capacities of corroded columns calculated using two approaches
(a) Nominal moment method
(b) Interaction diagram
The following conclusions can be drawn from the work described in this paper:
- The main parameter in determining the amount of corrosion to cause cracking is the cover to bar diameter (c/D) ratio.
- A reasonable estimate of the bond strength of corroded reinforcement can be obtained by taking a value of 70% of that given in UK codes (BS 8110, BS 5400 and BD 44).
- The effects of corrosion on flexural strength can be catered for by using the corroded reinforcement area and reducing the concrete section in the compression zone to allow for any spalling associated with corrosion of the compression reinforcement.
- The effects of corrosion on the shear strength are not straightforward. Using the corroded area of reinforcement could lead to either over or underestimates of the load-carrying capacity. Further work is required.
- The effects of corrosion on column behaviour can be catered for by using the corroded area of reinforcement and ignoring the concrete area outside the links.
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About the author:
Dr Mike Webster is a chartered civil and structural engineer (FICE, FIStructE) 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.