Grout Flow and Equipment Selection for a Deep, Narrow Void
A contractor must fill a void beneath a steel column base plate that is 1,200 mm × 1,200 mm and 150 mm deep, with a 40 mm gap between the plate and the concrete foundation. The grout must flow through a 40 mm gap and around four anchor bolts spaced 300 mm apart. The specified grout is a flowable, non-shrink, cementitious grout with a maximum aggregate size of 4 mm. The crew plans to use a gravity-fed hopper and a 25 mm hose, and intends to place the grout from one side only. The ambient temperature is 28 °C, and the grout's pot life at that temperature is 20 minutes. The crew mixes a full batch that takes 8 minutes to place, then stops to clean equipment before mixing the next batch.
1. Evaluate the equipment and placement plan. What problems are likely, and how should the plan be changed?
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Several problems are likely. First, a 25 mm hose is too small relative to the 40 mm gap and the 4 mm aggregate; the hose may clog or restrict flow, and the grout may not reach the far side. A larger hose (at least 38–50 mm) or a pump designed for the grout is preferable. Second, placing from one side only in a 1,200 mm square with four anchor bolts risks entrapping air and creating voids on the far side; grout should be placed continuously from one side with a vent or escape path on the opposite side, or from a central injection point, to push air ahead of the grout. Third, the 20-minute pot life and 8-minute placement leave little margin; stopping to clean equipment mid-placement risks a cold joint. The crew should mix batches sized to the placement rate, keep placement continuous, and have a second mixer or standby equipment ready. The gravity hopper may be adequate if the head is sufficient, but a positive-displacement pump is more reliable for this geometry.
2. How should the crew verify complete filling of the void, and what should be done if a void is detected?
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Verification methods include: observing grout escape from the opposite side or from vent holes; tapping the base plate and surrounding area for hollow sounds; and, for critical applications, using a borescope or drilling small inspection holes. The grout level should be checked at the plate edges. If a void is detected, the crew should not simply patch the surface. Options include pressure-injecting grout into the void through drilled ports, or, if the void is large, removing the grout and re-placing it. The cause should be identified - entrapped air, insufficient flow, or a cold joint - and the placement method corrected before re-grouting. Post-placement inspection should be documented.