Shrinkage repair

Although the acrylate adhesives are readily available and studies have shown that they can produce reasonable bonding properties, they have the disadvantages of having high shrinkage, high fluid absorption, and low service temperatures. Acrylate adhesive applications would be limited. The development of EB-curable epoxy adhesives would have applications in the aerospace and automotive industry and potential wider uses. The most immediate application for these resin systems is composite repair of commercial and military aircraft. 

Polymer resins were first introduced in the early 1940s as an aesthetic alternative to repair defects in anterior teeth. Some of the first resins were unfilled polymers of methyl methacrylate. Presently, these unfilled resins have been replaced by filled composite materials that limit the problems associated with polymerization volume shrinkage, abrasion or wear resistance, mechanical properties, water sorption, solubility, and thermal expansion. Polymeric composite materials generally consist of a monomer resin, a ceramic filler, a polymerization initiator or initiating system, and a coupling agent which binds the polymer... 

Since repairs are made on concrete which has already dried, additional shrinkage will not occur. The material used in repair should also be either shrink-free or have the capacity to shrink without losing bond. 

The surface of the concrete substrate has to be prepared to provide sufficient bond of the repair material. Factors that may affect the bond are the strength and integrity of the substrate, the cleanliness of the surface, and the roughness. The surface should be rough and dust or incoherent residues should be removed (for instance by sandblasting or waterblasting). This operation is usually not necessary if hydro-demohtion has been used. If the cementitious repair material is appHed directly on the surface of the concrete, the surface should be saturated by water in order to avoid absorption of water from the substrate and subsequent plastic shrinkage and incomplete hydration of the repair material, which will result in loss of bond. 

Polymer-modified mortars can be obtained by replacing part of the mixing water with a synthetic latex (e. g. styrene butadiene or acrylate) to the mix. Although the binder is still cementitious, and thus alkalinity is guaranteed, the latex may improve the workability, the waterproofness, the carbonation and chloride resistance, the tensile and flexural strength of the repair mortar [8]. It can also reduce the modulus of elasticity, increase the bond to the substrate, reduce the rate of drying out and thus the rate of shrinkage. 

Lambe, et al.l It l reported that repair mortars using redispersible polymer powders for concrete structures show high resistance to the diffusion of chloride ions, oxygen and carbon dioxide, and also low shrinkage. 

With RTSs, any loose resin and reinforcement should be removed and the affected area cleaned and dried. It may be useful to roughen surrounding areas, to obtain better adhesion. For superficial damage (to gelcoat only), apply catalyzed/activated resin to the damaged area and allow to set/solidify. Film, such as transparent ceUulose, tape or TP polyester, may be used to keep the resin in position and impart a smooth surface finish. Apply a thicker film of resin than usual, to allow for shrinkage. When the repair patch has fully hardened, dress the resin back to the correct contour of the molding. 

CIRIA Technical Note 141 (CIRIA, 1993) laid down basic requirements of shrinkage, expansion due to wetting and temperature, modulus of elasticity, creep, etc. These have been taken up in standards such as the European Standard on concrete repair BSEN1504 parts 1-10. This is discussed more fully at the end of this chapter. 

CIRIA (1993). Standard Tests for Repair Materials and Coatings for Concrete Part 3 Stability, Substrate Compatibility and Shrinkage Technical Note 141, CIRIA London UK. 

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