Acceptance of a Mast Arm Installation: Cure Time, Clearance, and Grounding
A contractor has installed a 45 ft galvanized mast arm pole with a 35 ft arm at a new signal. The footing is 3 ft diameter by 8 ft deep, designed for f'c = 3,000 psi at 28 days. On day 7 the contractor's cylinders break at 2,050 psi, and the contractor wants to set the pole, hang signal heads, and energize on day 8 to meet an opening date. Field measurements: the bottom of the lowest signal head over the traveled way is 16 ft 6 in.; the contract requires 17 ft minimum, and a 2 in. mill-and-overlay of the arterial is scheduled for the next fiscal year. Grounding: one 8 ft copper-clad rod at the pole, measured 48 ohms by fall-of-potential. The contractor also bonded the pole to the service neutral and called the grounding complete.
1. The 7-day break is 2,050 psi. Do you allow the pole to be set and loaded on day 8?
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First, put the number in context. 2,050/3,000 = 68 percent of f'c at 7 days. For ordinary cement at normal temperatures that is entirely normal and is not evidence of a bad mix. The break is informational; it is not a rejection criterion.
The decision turns on the specification, not on the number. Most specifications permit loading only after a stated fraction of f'c is attained (commonly 75 percent) or after the 28-day break. At 68 percent the footing is below a 75 percent threshold, so the answer is no, not without the engineer's written approval based on the mix design, maturity data, or an approved accelerated mix.
Practical risk. Setting a pole on a marginally cured footing is where plumb and anchor bolt projection get damaged permanently. If the footing later fails, the remedy is demolition of the foundation and re-purchase of the pole.
What I would require now: keep the anchor bolt template in place, verify bolt projection and spacing, verify the conduit stub and weep hole, and grout the anchor bolts only after the pole is set and plumbed. If the date cannot move, request an engineer-approved mix and a revised break schedule.
2. The lowest signal head measures 16 ft 6 in. over the traveled way against a 17 ft requirement. How do you resolve this?
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Reject as built. 16 ft 6 in. is 6 in. below the contract minimum, and the scheduled 2 in. overlay will reduce it further to roughly 16 ft 4 in., so this deficiency compounds.
Verify the measurement before writing it up. Measure at the low point of the arm after the heads and conductors are hung and the arm is fully loaded, over the highest point of the traveled way, not over the shoulder or gore. Deflection under load and any arm sag belong in the number.
Options, in order of preference:
- Raise the arm attachment point or use the manufacturer's higher mounting bracket, then re-measure.
- Shorten the hanger or use a lower-profile head to recover the inches.
- Relocate the head laterally if it is over a shoulder rather than a through lane.
- Only as a last resort, pursue a documented design exception, and only if the overlay is removed or the clearance can be restored before paving.
Do not accept a waiver that ignores the future overlay; the post-overlay clearance is what the public will drive under.
3. Evaluate the grounding. Is it acceptable, and what would you require?
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Resistance. A single 8 ft rod at 48 ohms fails the usual 25 ohm criterion. Under NEC 250.56, a single rod that does not achieve 25 ohms or less requires a supplemental electrode, typically a second rod driven at least 6 ft away and bonded to the first. Require the second rod and a re-test.
Bonding the neutral. Bonding the pole to the service neutral is not a grounding electrode and is a code violation; it puts objectionable current on the neutral and the pole enclosure. The pole must be bonded to the equipment grounding conductor that runs with the circuit, and the grounding electrode conductor must connect the pole and the equipment grounding conductor to the electrode system. The neutral must remain isolated from ground except at the service.
Testing. Record the fall-of-potential reading with the probe spacing documented, note soil moisture conditions, and repeat the test after the second rod is installed. Add the grounding test to the energization checklist, along with insulation resistance and continuity of the circuit conductors, before the cabinet is powered.