Hollow Voids Behind a Formed Wall
You are installing 1/2-inch adhesive anchors into a 6-inch-thick concrete wall to mount a guardrail. During drilling, one hole breaks through into a void behind the wall at about 3-1/2 inches depth, and a second hole shows a large void starting at 4 inches. The specified embedment is 3-1/2 inches. The adhesive is a standard two-part epoxy intended for solid concrete; the manufacturer's instructions state it is for use in solid concrete and do not cover hollow or voided conditions. The crew has already injected adhesive into the first hole and inserted the rod before noticing the breakthrough. The guardrail is a safety component and must be reliable.
1. What is the problem with the breakthrough hole, and what should happen to the rod already installed?
Read the worked answer
A breakthrough into a void means the hole is not in solid concrete, so the adhesive has no bond surface beyond the void and can flow into the cavity, leaving an incomplete or unsupported anchor. The standard epoxy is not evaluated for hollow conditions, so the installation is outside the manufacturer's instructions and its capacity is unknown. The rod already installed in the breakthrough hole cannot be assumed adequate; it should be treated as a failed installation, not loaded, and referred to the engineer. Do not simply add more adhesive or torque it to 'test' it. The engineer must approve a remedy (e.g., relocation, a different anchor type rated for the condition, or a through-wall detail).
2. How do you handle the second hole and the remaining layout?
Read the worked answer
Stop drilling at the affected locations and probe/scan to map the void extent. For the second hole, do not inject adhesive into a known void; the location is unsuitable as specified. Options require the engineer: relocate anchors to solid concrete, change the anchor system to one evaluated for hollow or voided substrate, or use a through-bolt/backing-plate detail. For the remaining layout, verify each hole is in solid concrete before injecting (check depth, look for breakthrough, and confirm the hole holds air when blown out). Any hole that breaks through is a hold point. Document the void locations and the engineer's direction.
3. What verification would you require before the guardrail is placed in service?
Read the worked answer
The engineer should specify the acceptance criteria for the revised installation, including a proof load or torque check appropriate to a safety component. Verify cure time at the recorded temperature, then perform the specified field verification on the anchors and compare to the criteria. Any anchor that fails is corrected and re-verified. Keep records of the void mapping, the approved remedy, installation data, cure data, and verification results, and confirm in writing that the guardrail is not loaded until verification passes.