Hole Depth Shortfall on a Wet-Formed Slab
You are inspecting anchors for a new guardrail base on an interior slab poured 40 days ago. The approved submittal calls for 5/8-inch diameter threaded rod, adhesive anchors, with a specified effective embedment of 4-1/2 inches and a hole drilled to the adhesive manufacturer's published maximum hole depth. The installer drilled 20 holes with a rotary hammer, blew them out with compressed air once, and set the rods. During your spot check you measure several holes and find drilled depths from 3-7/8 to 4-1/8 inches, all shallower than the specified effective embedment. The installer says the rods bottomed out and that 'the nut is tight, so it must be fine.' The ambient temperature is 68 F and the slab surface is dry. The manufacturer's published data states that the anchor's tension capacity is based on the specified effective embedment and that reducing embedment requires engineering evaluation.
1. How should you disposition the anchors already installed at the short embedment, and what is the controlling reason?
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Reject the short-embedment anchors; do not accept on the basis of a tight nut.
Reasoning:
- The published capacity is tied to the specified effective embedment (4-1/2 in.). Holes at 3-7/8 to 4-1/8 in. cannot develop that embedment, so the installed condition is outside the basis of the approved submittal.
- A tight nut only indicates the rod is restrained; it does not demonstrate the adhesive bond length or the steel embedment. Capacity is a function of bonded length and concrete condition, not torque feel.
- The correct path is to stop, document the measured depths, and route the deviation to the engineer of record for evaluation. Options are typically: remove and re-drill deeper if slab thickness and reinforcement permit, or obtain a written engineered acceptance (e.g., reduced capacity, added anchors). The inspector does not self-approve a reduced embedment.
- Record the deviation in the inspection report and hold further installation until the drilling depth issue is corrected (drill to the manufacturer's maximum hole depth, verify with a depth gauge or marked bit).
2. The installer proposes to 'just add more anchors' between the short ones to make up the difference. What must happen before that is acceptable, and what inspection evidence would you require?
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Adding anchors is a design change, not a field fix - it requires engineering approval first.
- The added-anchor scheme changes the load path and spacing/edge-distance assumptions. Spacing and edge distance must still satisfy the manufacturer's published tables and the project's engineered design; crowding anchors closer can reduce capacity per anchor and risk overlapping bond zones.
- Before accepting, obtain a written engineered detail from the EOR (or manufacturer's technical services with EOR concurrence) showing the number, location, embedment, and spacing of added anchors and the basis of capacity.
- Inspection evidence to require: as-built layout with measured spacing and edge distances; drilled depth verification for every new hole (not spot checks) since depth was already the failure mode; documented hole cleaning per the manufacturer's instructions; adhesive cartridge identification, batch/expiration, and mixed-nozzle use; cure time record consistent with the product data at 68 F; and proof/verification testing if the specification or EOR requires it.
- Do not let 'more anchors' substitute for correcting the short-embedment anchors unless the engineer explicitly accepts them in the revised scheme.