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2.5.1 Shrinkage

Shrinkage cracking falls in to two main categories:

• Drying shrinkage is a long-term phenomenon that will begin as soon as the concrete has hardened. The rate of drying shrinkage will decrease exponentially with time and a high proportion of the total possible drying shrinkage will occur within the first year after the application of the repair.

• Plastic shrinkage develops before the concrete hardens and whilst the concrete is still plastic.

Fine cracking and microcracking caused by drying shrinkage is one of the most common forms of cracking seen in the surfaces of concrete repairs. Cracking of this type is commonly most visible when the surface of the repair begins to dry out after being wetted. The potential for drying shrinkage is governed to a large extent by the cement and water contents of the repair materials and the high cement contents of many repair materials may contribute to the potential for shrinkage. Shrinkage-compensating

cements will help control cracking of this type but may not entirely eliminate shrinkage cracking.

Drying shrinkage cracking is normally very shallow and the cracks may only be a few micrometres wide. However, where reinforcement is present within the repair it is not uncommon for shrinkage cracks to continue for several centimetres and to intersect the surface of the reinforcement. Shrinkage microcracks that are very shallow may self-anneal with time in damp conditions. Shrinkage cracking that intersects the surface of reinforcement may be detrimental to the long-term durability of the repair if they allow chlorides, moisture or carbonation to reach the surface of the reinforcement. Such cracking is readily detected using petrographic techniques; the example in Figure 2.17 shows the localised penetration of carbonation along a vertical shrinkage microcrack which intersects the surface of the, as yet, uncorroded reinforcement.

Plastic shrinkage can develop in the surfaces of concrete repairs where the repair is exposed to warm windy conditions before it has cured. It occurs where the rate of evaporation exceeds the rate of bleeding. Very thin repairs may be prone to cracking of this type, particularly where there is potential for moisture from the repair material to be absorbed by the substrate. Plastic shrinkage cracks are generally restricted to the cement paste, tend to be non- parallel sided and commonly decrease in width rapidly with depth.

2.5.2 The effects of on-going deterioration in the concrete substrate

In some cases expansion of the concrete substrate due to on-going deterioration can lead to cracking along the interface between the repair

Figure 2.17 Photomicrograph showing a shrinkage fine crack developed in a sprayed concrete repair. The crack intersects the surface of the reinforcement and there is patchy carbonation either side of the crack shown by the very thin bright areas adjacent to the crack. Note the crack passes through the limestone aggregate particles as well as through the paste. Limestone aggregate particles occur on the left and right sides of the field of view.

and the substrate – especially where the bond is already weak. In other cases cracks may be traceable as originating within the concrete substrate and continuing into the concrete repair. Figure 2.4 illustrates an example of ongoing deterioration in the concrete substrate due to the reinforcement corrosion affecting a concrete repair.

Failure to correctly diagnose or to take account of on-going deterioration in the concrete substrate will in many cases greatly reduce the lifetime of a concrete repair. The petrographic examination of core samples that include concrete substrate as well as the repair can be used to detect features such as cracking that originate within the concrete substrate and continue into the concrete repair.

References

Allman, M. and Lawrence, D.F. Geological Laboratory Techniques. London, Blandford Press, 1972.

APG-SR2, 2010 Applied Petrography Group, ‘A code of practice for the petrographic examination of concrete.

ASTM C457/C457M – 10 ‘Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete’. American Society for Testing Materials, USA, 2010.

ASTM C856 ‘Standard practice for the petrographic examination of hardened concrete’. American Society for Testing Materials, USA, 2004.

British Cement Association. The Diagnosis of Alkali-Silica Reaction. Report of a working party, BCA, 1988.

Concrete Society, Technical Report No 30, ‘Alkali–Silica Reaction – Minimizing the risk of damage to concrete.’ 3rd Edition, The Concrete Society, UK, 1999. Concrete Society, Technical Report No 71, ‘Concrete Petrography.’ The Concrete

Society, UK, April 2010.

Eden, M.A. ‘Investigation Into the Effectiveness of the CP System in the Replaced RC Road Deck: West Tunnel, Dartford River Crossing’. Proceedings of Concrete Solutions, 1st International Conference on Concrete Repair, St. Malo, France. GR Technologie, Barnet, UK, 2003a.

Eden, M.A. ‘The laboratory investigation of concrete affected by TSA in the UK’, Cement and Concrete Composites, Vol 25 (2003b) 847–850.

Eden, M.A., White, P.S. and Wimpenny, D.E. ‘A petrographic investigation of concrete with suspected delayed ettringite formation – a case study from a bridge in Malaysia’. 11th Euroseminar on Microscopy Applied to Building Materials, 5–9 June 2007, Porto, Portugal.

French, W.J. ‘Concrete petrography: a review’. Quarterly Journal of Engineering Geology Vol. 24 (1), (1991) 17–48.

St John, D.A., Poole, A.B. and Sims, I. Concrete Petrography, A Handbook of Investigative Techniques, Edward Arnold, London, 1998.