The influence of design and detailing on the safety of deteriorating concrete structures

Concrete structures will deteriorate when exposed to severe environments. This deterioration can lead to risks to the safety of deteriorating concrete structures.  In this article, Mike Webster discusses the behaviour of deteriorating concrete structures to provide a qualitative understanding of what contributes to safety.  To illustrate the points, the resistance of concrete structures to deterioration is treated as a multi-layer system.  Within this, the design concept is the first layer, the design details are the second, the materials are the third and the structural detailing is the fourth. In essence, when one layer is overcome, the next one comes into play.  The layers are considered individually.  However, there is also an element of interaction between them.

To illustrate what this means in practice, a simple means of visualising the sensitivity of typical structural systems to deterioration is then presented in tabular form.

This article is aimed primarily at understanding the safety of existing structures.  However, many of the concepts discussed are applicable to the design of new structures.

1  Introduction

1.1  Deteriorating concrete structures

The author has inspected a range of deteriorating concrete structures over the years.  Typically, those which had deteriorated prematurely were not compatible with the exposure conditions.

The range of design and construction issues which have lead to premature deterioration include:

• Low cover to reinforcement
• Inadequately compacted cover concrete
• Failure of expansion joints above piers and abutments
• The presence of lapped joints, which can restrict compaction
• Cold joints at the ground level of bridge piers and columns
• Poorly designed concrete mixes
• Inadequate design of concrete mixes to resist freeze-thaw attack

Horizontal surfaces of reinforced concrete, such as the tops of pier heads, are particularly vulnerable to chloride-ingress problems.  This results from:

• Bleeding which increases the water-cement ratio of the surface layer and forms vertical bleed channels
• Plastic settlement cracks induced by reinforcement placed close to the top of a concrete pour

Design concept and detailing can have an influence on both how buildable a structure is and how it reacts to the effects of deterioration.  Good design concepts and detailing practice can minimise the chances of deterioration.  If deterioration does occur, they can also minimise the impact.

Design concept is largely a global issue whereas detailing is more of a local issue.

The environment to which the structure is exposed varies from one structure to another depending on its use and the design concept.

The aim of this article is to discuss the elements of structural design and detailing that influence the construction process and the response of concrete structures to deterioration.  In doing so, this article will provide a qualitative understanding of the safety of deteriorating concrete structures.

1.2  Related areas

Emphasis is placed on corrosion in this article – this is the deterioration mechanism of greatest interest in the UK.  However, where issues are relevant to other deterioration mechanisms, they are highlighted. An article summarising the impact of other forms of deterioration and defects on structural safety and service life is available in this post,

This article is aimed primarily at understanding the safety of existing structures.  However, many of the concepts discussed are applicable to the design of new structures.  Indeed, they allow the lessons learned from the behaviour of existing structures to be incorporated in new designs.   In particular, the discussions indicate that attempting to minimise concrete deterioration by the correct specification of concrete is unlikely to be effective on its own without changing design and detailing practices to maximise concrete durability performance.  In addition, a summary of what designers should do to minimise the risk of deterioration in new structures is available in this post.

2  Multi-layer system

In this article the resistance of concrete structures to deterioration is considered to be a multi-layer system.  Within this, the design concept is the first layer, the design details are the second, the materials are the third and the structural detailing is the fourth.  Essentially, when one is overcome, the next one comes into play.

2.1  Design concept

The choice of design concept is made on the basis of several factors; including use, aesthetics, cost and durability.  However, the influence of each will vary from structure to structure.  For example, where the structure is exposed to an aggressive environment, durability will be more significant.

The design concept can:

• Control the environment in some parts of the structure
• Determine the robustness of a structure through continuity and the provision of alternative load paths
• Minimise construction defects (such as low cover or poor compaction) if it is easy to build
• Influence the inspectability and maintainability of structures (for example, the inclusion of inspection chambers in bridge abutments allows both inspection of the structure and clearing of the drainage system)

In a typical office structure, the main structural elements are protected from the external environment by cladding.  As such, durability of the main frame is a lesser issue whilst durability of the cladding is more significant.

However, in a marine environment, structures (such as a jetty) are fully exposed.  As such, it is more difficult to use the design concept to minimise the exposure.  In such structures, the materials have to resist the environment.  In addition, robust design and detailing are required to minimise the impact of any deterioration.

An example of the use of design concept is in short span bridges where the Highways Agency⁽¹⁾ has specified that all bridges under 60m in length should be integral; that is with no movement joints.  This requirement is a result of the deterioration problems caused by de-icing salts seeping through leaking movement joints in simply supported bridges.

Two benefits can be gained from adopting the integral concept.  Firstly, eliminating movement joints removes the direct source of de-icing salts to the piers and abutments.  Secondly, moving from a simply supported design to an integral one, improves the robustness.  A simply supported structure only requires one hinge to form for failure, whilst an integral (continuous) structure requires three.  This is a little simplistic, as not all integral bridges will have continuity, but it illustrates the point.

A similar argument applies to multi-storey car parks (MSCP).  Based on American observations, larger simply-supported elements were performing better than the thinner continuous ones⁽²⁾.  The reason suggested for this was that the critical zone for continuous members tends to be on the top face in the hogging zone where salt-laden water can pond.  With simply supported members, the critical zone tends to be on the bottom face at mid-span where it is unlikely that chlorides will have direct access.

However, a beam should not be viewed in isolation.  The load path back to the ground needs to be considered.  The simply supported beam will be connected to either a column or a secondary beam.  The support may be a corbel if it frames into a column or the shelf of a secondary beam.  These supports tend to be highly stressed.  They also have horizontal top faces on which chlorides can pond.  As such, it is important to consider the whole load path; not only in terms of how loads are transmitted, but also in terms of how deterioration may occur.

2.2  Design details

These are design features that can also be used to control the environment to which the structure is exposed.  Typical examples include good drainage, overhangs, drip details, waterproofing and protective over-cladding.

Water flow should be controlled.  It is a common feature in many deterioration mechanisms including corrosion, ASR and frost.  For example, a MSCP⁽²⁾ will require floors laid to falls that disperse water quickly and efficiently.  Water proofing is also specified regularly for roofs to add an extra layer of protection.

In the same way that the load path down to the ground is considered, engineers should also consider how the water gets back to ground level and into the surface water drainage system.  Without proper drainage, salt-laden water will pond and act as a reservoir penetrating the top surface of the floor slabs.  This is particularly undesirable when chlorides are present as it can increase the surface exposure condition.

Maintenance is essential if a structure is to retain its resistance to deterioration.  There are both technical and cost reasons for this.  The designer assesses the environment at the design stage and aspects of the design, such as concrete grade and cover, are dependent on those assumptions.  Any change in the environment due to inadequate maintenance can invalidate the designer’s assumptions and increase the exposure class.  For example, if the drainage system in a MSCP is not maintained, ponding of salt water can lead to premature deterioration in areas that the designer did not assume to be exposed to significant levels of chlorides.

2.3  Materials

Once the environment is in direct contact with the concrete, it is the material that has to provide the barrier to deterioration.

Current design codes and standards recommend different grades of concrete for different environments⁽³⁾.  Generally, the more aggressive the environment, the lower the w/c ratio that is specified.  In the assessment of existing structures, those structures containing concrete with lower w/c ratios should be able to resist deterioration better than those with concrete containing higher w/c ratios.

The quality of materials is important where the structure is exposed directly to the environment.  However, in the author’s experience, many deterioration issues are not down to the materials on their own⁽⁴⁾.  Poor compaction, congested reinforcement, plastic cracking, cold joints and not achieving the specified cover all serve to negate material resistance.

2.4  Structural detailing

Once the materials begin to deteriorate, resistance is provided by the structure itself.  There are certain features that enhance resistance to a range of deterioration mechanisms.  Transverse reinforcement, particularly in the form of links, provides confinement to the concrete and restricts cracking.  As such, the presence of links enhances the bond strength substantially in deteriorating concrete structures.

Bond strength assumes greater significance in the assessment of existing structures.  In design, perfect bond between steel and concrete is assumed as the concrete is uncracked and the reinforcement has sufficient length to develop its yield strength.  However, bond strength is reduced by deterioration mechanisms that reduce the concrete tensile strength and/or induce cracking around the reinforcement.  If bond is reduced, the reinforcement may not develop its full strength, which may lead to reduced load-carrying capacity, particularly in shear.

Accelerated laboratory tests have shown that the deleterious effects of ASR⁽⁵⁾, corrosion⁽⁶⁾ and frost⁽⁷⁾ on bond strength are offset by the presence of links (see Table 1).

Table 1:  Reductions in bond strength of ribbed bars for ASR, corrosion and frost

Other aspects of structural detailing which enhance the performance of deteriorated structures include:

• Details which are easy to construct (congested reinforcement can lead to poor compaction)
• Adequate anchorage and lap lengths
• Details which are largely protected from the deterioration (such as bent-up bars)
• Details which retain ductility
• Tie reinforcement to prevent progressive collapse
• Details which avoid or relieve stress concentrations

3  Interactions – Sensitivity of typical structural systems

The preceding sections indicate that the sensitivity of concrete structures to deterioration is a function of the components of a multi-layer system.

The layers were considered individually.  However, there is an element of interaction between them.  For instance, poor detailing and construction with concrete prone to bleeding can lead to plastic settlement cracking.  If this is not rectified, then the cracks can provide direct access to the reinforcement for water-borne chlorides.  Similarly, poor compaction could result from congested reinforcement, poor access for vibrators or an over-cohesive concrete mix.  These all lead to reduced protection to the reinforcement.

Table 2 contains a summary of the main points.  It also illustrates potential areas of interaction.

Table 2:  Contributions from the protective layers to the safety of deteriorating concrete structures

Figure 1 shows a typical bridge pier for a simply supported bridge, and illustrates this interaction.  In the Maunsell report, Wallbank⁽⁸⁾ identified piers and abutments as the bridge elements with the most severe exposure to salt water.  This was primarily due to leaking movement joints affecting the pier tops, but also due to salt spray at the base from passing vehicles.

There are a number of possible reasons for poor performance of members such as this.  These include plastic settlement cracking, cold joints, low quality kickers and lapped reinforcement that reduces bar spacing (which can lead to poor compaction).  In addition, the areas exposed to chloride  access are also the most highly stressed.  In particular, the bursting stresses under the bearings are particularly high, whilst the highest bending moments and shears will occur at the base.

Figure 1:  Possible access routes for chlorides into a bridge pier

Typical abutment gallery details are shown in Figure 4.1 of CIRIA C155 Bridges – design for improved buildability⁽⁹⁾.  Those details indicate that the sensitivity of the bridge abutment has been reduced by the inclusion of simple design details.  For instance, the assumption was made that the movement joint will fail.  As such, permanent formwork flashing is proposed on the exposed vertical faces both to protect those faces and to channel the water into a gutter.

Whilst most structures are unique, there are common features in typical structural systems.  If, for each system, the structural concept, critical features and likely protection system are considered, it provides a qualitative understanding of the robustness of those systems.  This analysis is shown in Table 3.  This approach can help in inspecting and appraising structures.

Table 3:  Sensitivity of typical structural systems

4  Quantitative assessment of the safety of deteriorating concrete structures

This article has focussed on the qualitative assessment of the safety of deteriorating concrete structures.  For those readers that require information on quantitative assessment, the topic is discussed in this post.  It summarises the effects of corrosion on cracking, bond strength, flexural strength, shear strength and column behaviour.  In addition, it contains proposals for simple quantitative models for assessing the safety and load-carrying capacity of corrosion-damaged concrete structures.

For readers requiring further details, the suggested modifications to UK code equations are contained in Appendix A of Reference 10 (click here to download).

5  Conclusions

In conclusion:

1. The response of concrete structures to deterioration can be categorised in terms of four components: design concept, design details, materials and structural detailing.
2. Whilst individual factors can be identified that contribute to the response of concrete structures to deterioration, there is an interaction between them.
3. Certain structural forms are likely to be inherently better than others in their response to deterioration.

6  References

1. HIGHWAYS AGENCY: The Design Manual for Roads and Bridges: Design for durability, Standard BD 57 (DMRB 1.3.7), August 2001 (Withdrawn in 2020 and replaced by CD 350 The design of highway structures (https://standardsforhighways.co.uk/dmrb/search/19858eae-6dd2-4669-90a7-38aa8c85a1dd))
2. WEBSTER, M. P.: ‘Multi-storey car parks – design and maintenance issues’, Structural Engineer, Volume 76, No 1, 6 January 1998, pp 15-16.
3. BRITISH CEMENT ASSOCIATION: Minimum requirements for durable concrete: Carbonation- and chloride-induced corrosion, freeze-thaw attack and chemical attack, Edited by D. W. Hobbs, BCA, 1998, 172 pages.
4. HOBBS, D. W.: ‘Chloride ingress and chloride-induced corrosion in reinforced concrete members’, Corrosion of reinforcement in concrete construction, Ed. C. L. Page, P. B. Bamforth and J. W. Figg, Special Publication 183, The Royal Society of Chemistry, 1996, pp 124-135.
5. CHANA, P.S.:  ‘Bond strength of reinforcement in concrete affected by alkali-silica reaction’, Transport and Road Research Laboratory, Contractor Report 141, Crowthorne, 1989.
6. 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, 1994, Vol. II, pp 315-326.
7. WEBSTER, M. P.: Structural implications of frost damage, BRITE/EURAM Project BREU-CT92-0591, Workshop, Imperial College, April 1995.
8. WALLBANK, E. J.: The performance of concrete in bridges – A survey of 200 highway bridges, HMSO, London, April 1989.
9. RAY, S. S., BARR, J. and CLARK, L. A.: Bridges – design for improved buildability, CIRIA Report 155, 1996.
10. WEBSTER, M. P.: The Assessment of Corrosion-Damaged Concrete Structures, PhD Thesis, University of Birmingham, July 2000.

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.