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Concrete In Australia : December 2011
24 Concrete in Australia Vol 37 No 4 Stage Key Requirements Selection of material • Appropriate material for application method and service environment taking sugar exposure, traffic, skid resistance, temperature, maintenance and any other relevant conditions into account. Surface preparation • Concrete substrate needs to be structurally sound, clean, dry and of sufficient profile to optimise bond. Surface preparation methods include dry or wet abrasive blasting, scabbling or hydroblasting. Further guidance on surface preparation for coatings and overlays is provided in ACI 548.5-94, SSPC-SP13/ NACE 6 and ASTM C 811. • Existing cracks should be repaired to prevent reflective cracking in the overlay. • Concrete substrate should have a minimum tensile bond strength of 1.4 MPa (light service) or 2.1 MPa (severe service) when measured in accordance with ACI 503R or ASTM C 1583. • Prior to application of the overlay the moisture condition of the concrete substrate should be checked using the plastic sheet method described in ASTM D 4263. ere should be no visible moisture when determined by this method unless otherwise accepted in writing by the manufacturer. Prior to application • Application should not proceed if either the substrate or ambient temperature is less than 10 °C or greater than 30 °C (or as specified by the manufacturer) or expected to exceed this within the application time. • Overlays should be applied when temperature is constant or decreasing to prevent outgassing. • Application should not proceed if rain is imminent or if the relative humidity is greater than 85%. • All materials should be stored at temperatures between 20 °C and 25 °C for at least 24 hours prior to application. e substrate, materials and equipment should be protected from direct sunlight. Joints • Expansion joints should be installed in the overlay corresponding with all existing construction and expansion joints in the concrete substrate. Mixing of materials • All materials should be mixed in accordance with the manufacturer s recommendations. Mixing should be performed immediately prior to application. Materials that have gelled or otherwise deteriorated should not be mixed or used. • When the pot life limit is reached, the remaining material should be discarded, the equipment cleaned and new material mixed. Primer coat • e primer coat should be applied over the prepared concrete substrate at the manufacturer s recommended application rate and within a time window such that the overlay can be applied. For example, epoxy polymer concretes are typically required to be applied while the primer is still tacky (not cured) and the timeframe for this is usually less than 30 minutes, or lower at elevated temperatures, depending on the material. Application of overlay • e overlay should be applied over the primed surface so it can be placed and finished within the required timeframe. Any overlay which begins to gel prior to complete placement and finishing should be discarded. • e overlay should be placed and finished at the specified thickness and surface texture. Any overlay which does not meet the specifications or shows any other defects such as delamination, undulations, improper profiling, voids or insufficient drainage should be completely removed and replaced. Quality control • Quality control testing should be performed during and after overlay placement to ensure that the overlay meets specifications. ese tests include: o Viscosity and gel time of binder. o Initial set time. o Bond strength (minimum typically 1.7 MPa). o ickness. o Identification of discontinuities. o Compressive strength. o Density. Table 1. Requirements for installation of protective overlays.