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:

• Cracking

• Bond strength

• Flexural strength

• Shear strength

• Column behaviour

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.

INTRODUCTION

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^[2] 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.

CRACKING

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]	The amount of bar expansion to cause cracking increases with increasing concrete tensile strength However, increasing the tensile strength is not as effective as increasing the c/D ratio
Concrete elastic modulus[4]	The amount of bar expansion to cause cracking increases with decreasing concrete elastic modulus
c/D ratio[4, 5, 6, 7,  8, 9, 10, 11]	The amount of corrosion required to cause cracking increases linearly with increasing c/D ratio
Section loss and crack width[5, 6, 7]	At lower levels of corrosion the crack width can be related to the section loss in each of the tests
Corrosion rate[5, 6, 12]	At lower corrosion rates the amount of corrosion required to cause cracking was found to be larger than at higher corrosion rates However, beyond a certain corrosion rate that trend reverses
Concrete pore structure[5, 8, 9]	The measured amount of corrosion to cause cracking was found to be significantly higher than that predicted by analytical or numerical methods Corrosion products were observed in the concrete pore structure surrounding the corroding bar This could explain the discrepancy
Corrosion of adjacent reinforcement[11, 13]	The corrosion of adjacent bars may produce a horizontal delamination plane depending on the relative size of the bar spacing and the cover
Crack propagation[4, 7, 11, 13]	Cracks propagate first in the direction of the smaller cover for single bars The second crack will either propagate to the same face or normal to the first crack depending on bar spacing and relative cover sizes For multiple bars the cracks propagate first in the direction of either the smaller cover or the spacing between adjacent bars depending on the relative size of the two

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

BOND STRENGTH

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

Phase	Typical behaviour
Uncorroded	Behaviour is as assumed in design codes
Pre-cracking[10, 14, 15]	Expansive corrosion products are resisted by the surrounding concrete The corrosion induces extra confinement to the bar Light rusting on the bar surface increases the frictional resistance The two together combine to increase the bond strength This increase can typically be up to 1.5 times the uncorroded value
Cracking[10, 14, 15 16]	When the first crack appears, much of the confinement is lost and there is a drop in bond strength from the pre-cracking peak Plain bars appear to exhibit a larger drop than ribbed bars Bond strengths in the region of 0.9 to 1.2 times the uncorroded strengths have been observed in tests
Post-cracking[10, 14, 15, 16, 17, 18]	The bond strength has been observed to reduce with increasing corrosion As the ribs of deformed bars deteriorate, there is little difference between them and plain bars Some tests have shown the residual bond strength to be 0.15 times the uncorroded values at 8% corrosion However, other tests have shown the residual to be 0.6 at 25% corrosion (but at a lower corrosion rate)

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]	An increase in c/D generally increases the time to cracking, and thus time to loss of bond strength An increase in the c/D ratio appears to increase bond strength at lower levels of corrosion, but bond strengths begin to converge at higher levels corrosion
Links[19]	Links appear to offer greater benefit to corroded members than uncorroded ones In tests 70 to 80% of the uncorroded bond strength was maintained when corroded with links present compared to 20 to 30% when they were not
Bar position[10, 18]	Once cracking has taken place, both top and bottom cast bars have similar bond strengths That is, bottom cast bars suffer a greater proportional reduction
Concrete tensile strength[18]	The bond strength appears to increase with increasing tensile strength
Applied transverse stress[16, 18]	With the applied load so close to the support, the test set-up appeared to be a little too unrealistic to make a definitive judgement
Corrosion rate[19]	Increasing the corrosion rate initially leads to increases in bond strength Further increases in corrosion rate lead to reductions in bond strength for the same amount of corrosion
Bar type[10, 14]	At first cracking, ribbed bars showed a smaller drop in bond strength than plain bars Post-cracking, ribbed bars showed a larger drop.  Ribbed bars appear to require less corrosion to cause cracking than plain bars

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.

FLEXURAL STRENGTH

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]	Approximately linear losses of flexural strength up to a limiting value where bond or shear become critical The critical section loss will vary with individual details (i.e. the presence of links or not)
Longitudinal corrosion cracks[20, 21, 22, 23, 24, 25]	These only appear to influence load-carrying capacity when anchorage or bond become critical When longitudinal cracks are in the bending region and bond or shear failure is prevented, bending capacity still appears to be controlled by reinforcement cross-section
Reduction in length of plateau of load-deflection curve<[20, 22, 24, 25]	Changes in failure mechanism from flexure to bond, fracture or shear leads to brittle failure mechanisms that exhibits little or no load-deflection plateaux
Corrosion at laps[26]	Ductility of failure increases with decreasing link spacing Abrupt failure at peak load is possible at large link spacing
Link corrosion[18, 26, 27]	There is a possibility that members with corroded links will not achieve their full flexural capacity due to premature shear failure Considerable link corrosion is required for this to happen

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

SHEAR STRENGTH

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]	The loss of shear strength is not linear with the reduction in tension and link reinforcement area
Longitudinal corrosion racks[18, 28]	The presence of longitudinal corrosion cracks appears to influence load-carrying capacity when anchorage bond becomes critical
Link corrosion[18, 28]	It is possible that members with corroded links will not achieve their full bending capacity despite being designed to fail in flexure Considerable link corrosion is required for this to happen A more ductile failure mechanism is apparent when links are present
Bar type[28]	Beams with corroded plain bars (and no links) demonstrated a significant enhancement in load-carrying capacity over the uncorroded control beams This was not evident with the beams with ribbed bars
Failure mode – beams designed to fail in flexure[17, 18, 22, 23, 24, 28]	The failure mode can change from flexure to shear with increasing corrosion
Failure mode – beams designed to fail in shear[28]	It appears that there may be the possibility of tied arch behaviour occurring due to reduction in bond strength in the beams with no links particularly when plain bars are used
Cover to bar diameter ratio[28]>	Increases in the c/D ratio appear to cancel out much of the potential reductions in shear strength due to corrosion It is unclear whether the bottom or side cover is the significant factor

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^[28].

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 V_cis A_s.  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

COLUMNS

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]	Leads to a reduction in load-bearing area in both axial compression and buckling In addition, the rust product leads to cracking along the line of the reinforcement followed by spalling
Spalling of concrete[24, 29]	Spalling of the concrete cover from the corner bars causes a reduction in confinement of the main compression reinforcement Spalling of the concrete cover from the corner bars causes a reduction in the concrete cross-section available to resist the axial load
Loss of link reinforcement cross-section[24, 29]	When the links corrode the lateral restraint to the main compression reinforcement is likely to be lost This will increase the effective length of the main compression reinforcement leading to a possibility of the reinforcement failing in buckling before it fails in compression

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^[29].

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

CONCLUSIONS

The following conclusions can be drawn from the work described in this paper:

1. The main parameter in determining the amount of corrosion to cause cracking is the cover to bar diameter (c/D) ratio.

2. 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).

3. 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.

4. 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.

5. 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.

REFERENCES

1. WEBSTER, M. P.: The Assessment of Corrosion-Damaged Concrete Structures, PhD Thesis, University of Birmingham, July 2000
2. CROPPER, D., JONES, A. K. and ROBERTS, M. B.  ‘A risk based maintenance strategy for the Midlands links motorway viaducts’,The Management of Highway Structures, ICE, 22-23 June 1998.
3. MORINAGA, S.  ‘Prediction of Service Lives of Reinforced Concrete Buildings Based on Rate of Corrosion of Reinforcing Steel’, Special Report of Institute of Technology, Shimizu Corporation, No 23, June 1988, pp 82.
4. UEDA, T, SATO, Y, KAKUTA, Y and KAMEYA, H.  ‘Analytical study of concrete cover cracking due to reinforcement corrosion’, Concrete in severe environments, CONSEC 1998, Norway, E F N Spon, pp 678-687.
5. ANDRADE, C, ALONSO, C and MOLINA, F J.  ‘Cover cracking as a function of bar corrosion: Part 1 – Experimental test’, Materials and structures, 26, 1993, pp 453-464.
6. ALONSO, C, ANDRADE, C, RODRIGUEZ, J and DIEZ, J M.  ‘Factors controlling cracking of concrete affected by reinforcement corrosion’, Materials and Structures, Vol 31, August-September 1998, pp 435-441.
7. CABRERA, J G.  ‘Deterioration of concrete due to reinforcement steel corrosion’, Cement and Concrete Composites, 18, 1996, pp 47-59.
8. LIU, Y.  Modelling the time-to-corrosion cracking of the cover concrete in chloride contaminated reinforced concrete structures, PhD, Virginia Tech, October 1996, pp 117.
9. MOLINA, F J, ANDRADE, C and ALONSO, C.  ‘Cover cracking as a function of bar corrosion: Part II – Numerical model’, Materials and structures, 26, 1993, pp 532-548.
10. SAIFULLAH, M.  Effect of reinforcement corrosion on bond strength in reinforced concrete, PhD, University of Birmingham, April 1994, pp 258.
11. DAGHER, H J and KULENDRAN, S.  ‘Finite Element Modelling of Corrosion Damage in Concrete’, ACI Structural Journal, Vol. 89, November-December 1992, pp 699-708.
12. SAIFULLAH, M and CLARK, L A.  ‘Effect of corrosion rate on the bond strength of corroded reinforcement’, Corrosion and corrosion protection of steel in concrete, Proceedings of an International Conference, Sheffield University, 24-28 July 1994, pp 591-602.
13. BAZANT, Z P.  ‘Physical model for steel corrosion in concrete sea structures – Application’, Journal of the Structural Division, Proceedings of the American Society of Civil EngineersVol. 105, ST6, June 1979, pp 1155-1166.
14. AL-SULAIMANI, G J, KALEEMULLAH, M, BASUNBAL, I A and RASHEEDUZZAFAR.  ‘Influence of corrosion of cracking on bond behaviour and strength of reinforced concrete members’, ACI Structural Journal, Vol. 87, No. 2, March-April 1990, pp 220-231.
15. ALMUSALLAM, A A, AL-GAHTANI, A S, AZIZ, A R and RASHEEDUZZAFAR.  ‘Effect of reinforcement corrosion on bond’, Construction and Building Materials.  Vol. 10, No. 2, 1996, pp 123-129.
16. CABRERA, J G and GHODDOUSSI, P.  ‘The effect of reinforcement corrosion on the strength of the steel/concrete – bond’, Proceedings of the International conference on Bond in Concrete, Riga, Latvia, 1992, pp 10/11-10/24.
17. LIN, C Y.  ‘Bond deterioration due to corrosion of reinforcing steel’, Performance of Concrete in the Marine Environment, ACI SP-95, 1980, pp 255-269.
18. RODRIGUEZ, J, ORTEGA, L M and CASAL, J.  ‘Corrosion of reinforcing bars and service life of reinforced concrete structures: Corrosion and bond deterioration’, International Conference on Concrete Across Borders, Odense, Denmark, Vol. II, 1994, pp 315-326.
19. SAIFULLAH, M and CLARK, L A.  ‘Effect of corrosion rate on the bond strength of corroded reinforcement, Corrosion and corrosion protection of steel in concrete’, Proceedings of an International Conference, Sheffield University, 24-28 July 1994, pp 591-602.
20. ALMUSALLAM, A A, AL-GAHTANI, A S AZIZ, A R , DAKHIL, F H and RASHEEDUZZAFAR.  ‘Effect of reinforcement corrosion on flexural behaviour of concrete slabs’, ASCE Journal of materials in Civil Engineering, Vol. 9, No. 3, August 1996, pp 123-127.
21. LEE, H. S., NOGUCHI, T, and TOMOSAWA, F.  ‘FEM analysis for structural performance of deteriorated RC structures due to rebar corrosion’, Concrete under severe conditions: Environment and loading, CONSEC ’98, E & F N Spon, 1998, pp 327-336.
22. TACHIBANA, Y., MAEDA, K., KAJIKAWA, Y, and KAWAMURA, M.  ‘Mechanical behaviour of RC beams damaged by corrosion of reinforcement’, Third international symposium on corrosion of reinforcement in concrete construction, 21-24 May 1990, pp 178-187.
23. UOMOTO, T., TSUJI, K. and KAKIZAWA, T.  ‘Deterioration mechanism of concrete structures caused by corrosion of reinforcing bars’, Transactions of the Japan Concrete Institute, Vol. 6, 1984, pp 163-170.
24. UOMOTO, T. and MISRA, S.  ‘Behaviour of concrete beams and columns in marine environment when corrosion of reinforcing bars takes place’, Concrete in marine environments, ACI SP 109-6, 1990, pp 127-146.
25. DALY, A. F.  Effects of accelerated corrosion on the flexural strength of small scale beams, TRL Research Report PR/CE/15/94, Crowthorne, 1994, Unpublished.
26. KAWAMURA, A., MARUYAMA, K., YOSHIDA, S. and MASUDA, T.  ‘Residual capacity of concrete beams damaged by salt attack’, Concrete under severe conditions: Environment and loading, E & F N Spon, pp 1448-1457.
27. MARUYAMA, K. and SHIMOMURA, T.  ‘Effect of rebar corrosion on the structural capacity of concrete structures’, Concrete in severe environments, CONSEC ’98, Norway, E & FN Spon, pp 364-371.
28. DALY, A. F.  Effects of accelerated corrosion on the shear behaviour of small scale beams, TRL Research Report PR/CE/97/95, Crowthorne, 1995, Unpublished.
29. RODRIGUEZ, J., ORTEGA, L. M. and CASAL, J.  ‘Load carrying capacity of concrete columns with corroded reinforcement’, Fourth international symposium on the corrosion of reinforcement in concrete structures, Cambridge, UK, 1996, pp 220-230.

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 mike.webster@mpwrandr.co.uk or give him a call on 07969 957471.