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2013SprShotcreteEMag

Technical Tip Surface Preparation for Shotcrete Repair Wtance of the bond between the shotcrete not mandatory, to saw cut the perimeter of anysurfaces to be repaired. It is recommended, but-application, one cannot overstate the impor By Simon Reny hen placing shotcrete in a concrete repair mine the deteriorated concrete area and mark the and the concrete substrate. This bond is a critical concrete sections to be repaired, and feather factor in determining the overall performance and edging should be prohibited. The saw-cut perim- durability of a repair. Research has demonstrated eter separates the repair area from sound concrete. that bond strength between the concrete substrate The depth of the saw cut also determines the and the concrete repair, whether it is shot or cast minimum thickness of the repair. To prevent in place, is directly related to the quality of the further damage to the sound concrete and ensure surface preparation, as demonstrated by Talbot et long-term performance of the repair, hydro- al.1 Good surface preparation requires correct con- demolition (refer to Fig. 1) is the preferred con- crete demolition practices and properly cleaned crete demolition method, as it is most effective in surfaces. This Technical Tip briefly covers the best preventing the concrete substrate from further of these techniques for preparing the receiving damage, such as microcracking, that often results surface of a shotcrete repair. The first section will from using impact hammers. It is strongly recom- cover demolition of the deteriorated concrete. The mended to conduct a test on a concrete sample second section will treat the surface-cleaning that best represents the project conditions before requirements. The third segment will explain the project begins to calibrate the pressure of the quality control testing of the surface preparation. hydrodemolition equipment to obtain the desired results. It is also acceptable to use other methods, Concrete Demolition such as jackhammering, but the equipment used Concrete structure rehabilitation requires should be selected to minimize the potential proper removal of deteriorated concrete to a sound damage by microcracking of the substrate. concrete substrate before the surface preparation takes place. Qualified personnel must first deter- Surface Cleaning After the concrete removal process is completed, it is recommended that all exposed concrete surfaces be cleaned with a high-pressure water blast or with wet sandblasting, as dry sandblasting can be a safety hazard in some areas (refer to Fig. 2). This statement does not apply in the case of the hydrodemolition surface preparation method because this method provides the same result as water or sandblasting. It is important to differentiate high-pressure water blasting and normal high-pressure water washing. High-pressure water blasting character- istics are considered to be as effective as wet sandblasting and are capable of cutting into the concrete surface. Depending on the concrete substrate quality the required pressure can vary between 3000 and 7000 psi (21 and 48 MPa). Normal high-pressure water washing requirements can be defined as follows: pressure (2200 psi 15 MPa) and flow (5.3 gal./min 20 L/min). High-pressure water washing is mandatory for Fig. 1: A concrete surface after hydrodemolition the last cleaning procedure before shotcreting 28 Shotcrete • Spring 2013


2013SprShotcreteEMag
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