Impact of deterioration on the safety of concrete structures – what can designers do to minimise risk?

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Impact of deterioration on the safety of concrete structures – what can designers do to minimise risks

This article discusses the impact of deterioration on the safety of concrete structures.  This is illustrated by examples where deterioration has led concrete structures to collapse.  The article suggests steps that designers can take to minimise the risks of deterioration to safety and discharge their duties under the CDM 2015 Regulations.

Introduction

Concrete structures deteriorate with time, typically as a result of the environment they are in.  The impact of deterioration on the safety of concrete structures would be felt by people using the structure, inspecting, maintaining or repairing it and passers-by.

Regulation 8 of the Construction (Design and Management) Regulations 2015[1] (CDM 2015) places duties on designers in Great Britain to eliminate, reduce or control foreseeable risks so far as is reasonably practicable.  This includes foreseeable risks to the health or safety of any person:

  • Carrying out or liable to be affected by construction work
  • Maintaining or cleaning a structure
  • Using a structure designed as a workplace

These duties to eliminate, reduce or control foreseeable risks therefore require designers to pay attention to durability and the potential for deterioration.

What is deterioration?

Deterioration can result from inadequate design or construction, operational accidents, aggressive environments, loading, fire, aging processes or any combination of these.

Deterioration typically results in:

  • Cracking in the structural member – from corrosion, fatigue, chemical attack, construction defects, loading or repeated movement
  • Loss of strength in the structural material – from corrosion and fatigue of reinforcement; chemical or frost attack in concrete and fire or explosion
  • Material coming loose and falling from the structure – from a range of causes including reinforcement corrosion causing the cover concrete to spall away; architectural attachments coming loose from the main structural members or delamination of rendered mortar finishes or tiling

What is the impact of deterioration on the safety of concrete structures?

The impact of deterioration is wider than we may initially think, and includes:

  • Collapse of structural members (or even whole structures) and the safety of the people using the structure or close by
  • Safety of the people who inspect the structures
  • Safety of the people who repair and maintain the structures

Deterioration of concrete structures

A range of mechanisms[2] can cause concrete structures to deteriorate.  These include:

  • Reinforcement corrosion – which reduces the strength of the reinforcement
  • Reactions that affect the concrete itself (alkali-silica reaction (ASR), frost attack, sulphate attack, thaumasite and delayed ettringite formation) – which can reduce the strength of the concrete

The most common form of deterioration in concrete structures is reinforcement corrosion.  This can result from:

  • Carbonation – carbon dioxide in the atmosphere reacts with calcium hydroxide in the cement paste to lower the pH of the concrete.  This will break down the protective oxide layer around the reinforcement. Carbonation typically occurs in environments with a relative humidity of 50 to 70% (i.e. neither too wet or too dry).
  • Chlorides – chlorides inhibit the mechanism by which the protective oxide layer around the reinforcement is maintained. Chlorides penetrate most rapidly when the concrete is primarily dry, but occasionally wetted.  Chlorides can be present in the mix (although this has not been permitted in the UK for 40 years) or they can be present in de-icing salts or marine environments.
  • Lack of cover – the time taken for corrosion to start is highly sensitive to even minor reductions in cover.

Reinforcement corrosion can cause:

  • Cracking of the concrete cover
  • Spalling of the cover concrete
  • Reductions in the reinforcement cross-section

Chloride-induced corrosion is the most significant source of corrosion as it can lead to local pitting where a large proportion of the reinforcing bar or prestressing tendon cross-section can be lost.

In many cases of corrosion in reinforced concrete structures, tell-tale signs of rust staining and cracking indicate that corrosion is proceeding.  This implies that remedial action is required.  In post-tensioned structures, however, there may be little or no visible sign if the tendons are corroding.

Collapse of the Ynys-y-Gwas post-tensioned bridge

The Ynys-y-Gwas bridge was a single-span segmental post-tensioned concrete bridge built in 1953[3].  There were no signs of distress, but on 4 December 1985, it collapsed with one vehicle on the bridge.  The bridge had longitudinal post-tensioned beams which were also post-tensioned transversely.  The failure was caused by severe corrosion of the tendons at both longitudinal and transverse joints.  In places, up to 90% of the tendon cross-section had been lost.  Chlorides from de-icing salts were the primary cause of the corrosion.

The chlorides were able to reach the tendons because de-icing salts were used on the road and:

  • The 25mm transverse mortar joints were considerably more permeable than the surrounding concrete
  • The longitudinal ducts that housed the tendons were unlined, but were designed to have metal sleeves where the ducts crossed the joints – cardboard was used instead
  • Some of the transverse dusts were well grouted, but others were almost empty
  • Grout leaked out at longitudinal and transverse joints
Collapse of the Piper’s Row reinforced concrete car park

Frost damage can be significant in colder climates, and is a hazard in the UK if poor quality concrete has been used.

Piper’s Row was a reinforced concrete multi-storey car park built in 1965 using the lift slab construction technique.  It failed catastrophically in 1997.

According to the HSE investigation[4] the concrete in the decks was of low quality and relatively porous in some locations which promoted frost damage.  These areas had been broken out and replaced with patch repairs at some point.

Early in the morning of 20 March 1997, a 15m by 15m section of the third floor collapsed suddenly; initiated by a punching shear failure at one column.  This then led to a progressive collapse as similar failures followed at eight adjacent columns.  Fortunately, the car park was empty at the time.

The investigation noted that:

  • Some of the repairs had poor compaction and had not bonded to the base concrete
  • In some areas, the deteriorated base concrete had failed
  • The bond and laps of the reinforcement were adversely affected by the deterioration in the surrounding concrete

What can designers do to eliminate, reduce or control the risks of deterioration?

Good design practice can be effective; in particular, designers should:

  • Eliminate design details that are more likely to deteriorate – so that potential defects are not in built (e.g. half-joints in bridges, as these are highly stressed and can be exposed to de-icing salts when movement joints leak; they are also difficult to inspect)
  • Design for the appropriate environment – so that the materials do not deteriorate prematurely (e.g. concrete structures in cold climates where they are exposed to de-icing salts will need a higher grade of concrete)
  • Design with inspection and maintenance in mind – so that critical areas that may degrade are ‘inspectable’ and are easily accessible without putting inspectors at risk
  • Use protective measures such as coatings – so that members in aggressive environments are protected from deterioration

Detailed guidance is available for designing bridges for durability[5],[6], maintenance[7],  and buildability[8].  Guidance is also available for designing car parks[9].  The information contained in these guides represent good practice that could be applied to other structures in severe environments where no equivalent guidance exists.

What can an engineer do to assess the impact of deterioration on the safety of concrete structures?

This is a specialist topic, as assessing the load-carrying capacity of deteriorated structures is not always as simple as putting deteriorated material strengths into design code equations.  Complexities arise as modern design codes assume:

  • The use of modern construction materials – those assumptions may give unsafe results for deteriorated structures (e.g. the Eurocodes assume that high tensile ribbed reinforcing bars are used in concrete, whereas many older concrete structures contain mild steel plain bars)
  • Ductile behaviour with warning of distress – whereas deterioration may cause brittle failure modes (e.g. shear or anchorage failures in corrosion-damaged concrete)
  • Some failure modes to be more likely than others – deterioration may cause other failure modes to become critical (e.g. corrosion near the supports of a reinforced concrete member may cause shear to become the critical load-carrying mode rather than bending)

Guidance is available on the appraisal of a range of existing structures[10].  Detailed guidance is available for the assessment of corrosion-damaged concrete structures[11],[12].  This detailed guidance contains proposals for amending UK codes to allow the load-carrying of corrosion-damaged structures to be assessed.

References

[1]  The Construction (Design and Management) Regulations 2015, SI 2015 No. 51

[2]  Concrete Society: Diagnosis of deterioration in concrete structures, Technical Report 54, 2000

[3]  Woodward, R J and Williams, F W: ‘Collapse of Ynys-y-Gwas bridge, West Glamorgan’, Proceedings of the Institution of Civil Engineers, Part 1, 1988, 84, pp 635-669

[4]  Health and Safety Executive: Pipers Row Car Park, Wolverhampton – Quantitative study of the causes of the partial collapse on 20 March 1997, Undated Report (http://www.hse.gov.uk/research/misc/pipersrow.htm)

[5]  The Design Manual for Roads and Bridges: Design for durability, Standard BD 57 (DMRB 1.3.7), August 2001 (http://www.standardsforhighways.co.uk/dmrb/vol1/section3/bd5701.pdf)

[6]  The Design Manual for Roads and Bridges: Design for durability, Advice Note BA 57 (DMRB 1.3.8), August 2001 (http://www.standardsforhighways.co.uk/dmrb/vol1/section3/ba5701.pdf)

[7]  Highways England: Designing for maintenance, Interim Advice Note IAN 69/15, April 2015 (http://www.standardsforhighways.co.uk/ians/pdfs/ian69_15.pdf)

[8]  Ray, S. S., Barr, J. and Clark, L. A.: Bridges – design for improved buildability, CIRIA Report 155, 1996.

[9]  The Institution of Structural Engineers: Design recommendations for multi-storey and underground car parks, Fourth edition, March 2011

[10]  The Institution of Structural Engineers: Appraisal of existing structures, Third edition, 2010

[11]  Design Manual for Roads and Bridges: The Assessment of Concrete Structures affected by Steel Corrosion, Advice Note BA 51/95 (DMRB 3.4.13), 1995 (http://www.standardsforhighways.co.uk/dmrb/vol3/section4/ba5195.pdf)

[12]  Webster, M P: The Assessment of Corrosion-Damaged Concrete Structures, PhD Thesis, University of Birmingham, 2000  (http://etheses.bham.ac.uk/259/3/Webster00PhD.pdf) (Appendix A contains the detailed amendments to UK codes required for the assessment of the load-carrying capacity of corrosion-damaged concrete structures)


About the author

Mike Webster is a chartered civil and structural engineer with over 30 years’ experience including ten years’ director-level experience.

Mike specialises in construction risk, in particular:

  • Construction safety and CDM
  • Safety and service life of concrete structures
  • Organisational risk and why things go wrong.

Over his career, Mike has:

  • Undertaken the design, appraisal and site supervision of building and bridge structures
  • Carried out research into the durability of concrete materials and the service life of concrete structures in order to develop national and international guidance
  • Led reviews into how construction organisations manage health and safety risks and apply the CDM Regulations
  • Prepared expert reports for civil and criminal cases, and given evidence in Crown Court

For more information, drop me a line at mike.webster@mpwrandr.co.uk

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