1. REPAIRS USING EPOXY-BONDED STEEL PLATES
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| Repair using epoxy bonded steel plates (Image courtesy: http://www.sh-horse.com) |
1.1 Introduction
It appears that the use of steel plates bonded to the tension face of reinforced concrete beams was developed in France and South Africa in the 1960s. In the UK the first recorded use of resin-bonded steel plates to strengthen an existing building was in about 1966; for the strengthening of road bridges the first use was in 1975 for the Quinton Interchange on the M5.
The use of steel plates bonded to the sides of beams to increase shear resistance would appear to be a possibility, but I have not found any record of such use. However, it has been used to stiffen reinforced concrete floor slabs by bonding the steel plates to the top surface of the slabs, and to strengthen the connections between beams and columns.
1.2 Information on the technique
This technique depends on composite action between the steel plates and the concrete, and therefore requires maximum bond between steel epoxy resin-concrete. This in turn requires very careful preparation of the contact surfaces. The steel must also be adequately protected against corrosion on the exposed surfaces. When the work is properly carried out and the bond strength has fully developed, the strength of the epoxy joint should exceed that of the concrete which will fail in horizontal shear.
The strength of epoxy resins deteriorates rather rapidly at temperatures in excess of about 65°C and this constitutes a fire hazard when used in building structures, but is of little significance in bridges.
A considerable amount of work has been carried out on the development of this technique at the Transport and Road Research Laboratory at Crowthorne, and at Sheffield University, and the universities of Dundee and Warwick.
The technique is discussed in detail in the Report by the Standing Committee on Structural Safety, set up by the Institution of Civil Engineers and the Institution of Structural Engineers. The Report recommends caution in the use of this method and emphasizes that it should not be used to strengthen structures in which the concrete is poor quality, or the reinforcement has suffered chloride attack, or the concrete has been damaged by alkali-silica reaction, unless these basic defects are first corrected.
2. THE USE OF FIBER-REINFORCED PLASTICS
According to Research Focus, No. 21, April 1995, the use of fiber reinforced plastic (FRP) as a substitute for steel has been investigated in Switzerland, Germany and the USA, and at Oxford Brookes University under an EPSRC (Engineering and Physical Sciences Research Council) grant.
2.1 Carbon fibre composites
An alternative to this method of strengthening reinforced concrete beams by epoxy-bonded steel plates by using carbon-fibre reinforced composites was reported by Dave Parker. It is stated in the article that to provide the additional strength required, the epoxybonded steel plates would have had to be 7mm thick, but these were replaced by a 1mm thickness of carbon fibre composite (a Sika Carbodur System). The work was part of a major refurbishment.