The Interim Certificate and the Day-Seven Strike
You are a graduate structural engineer with a mid-sized consultancy in Lahore, working as site engineer for a six-storey commercial building in Johar Town. The third-floor slab was cast nine days ago with M30 ready-mix concrete. The contractor struck the soffit props on day seven to reuse them on the floor above, although the specification allows striking only after 14 days or when cubes reach 70 percent of characteristic strength. Your site diary records the date. The contractor's seven-day cube results average 21 MPa. The client's tenant has a fit-out deadline in three weeks, and the client wants the third-floor slab shown as completed and approved on this month's interim payment certificate. Your project manager says the slab looks fine and that refusing to sign will delay the firm's fee and damage its chances of being shortlisted for the client's next project, which your firm would also supervise.
1. Identify the technical checks you must complete before any interim payment certificate is signed, and explain why each matters.
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Separate the two questions: is the slab safe, and is the work complete.
- Strength history. 21 MPa at 7 days against M30 is roughly the 70 percent normally expected, so the concrete itself may be acceptable. That does not justify striking at day 7, because the specification set both a time and a strength condition and the contractor met neither on the documented record.
- Structural check. With props removed, the slab carries self-weight plus construction load. Check whether the slab was designed for a 7-day strike, whether any backpropping remained underneath, and what loads (blockwork, stored material, plant) were applied after day 7. Compute immediate and long-term deflection; on a flat plate the punching check at the columns usually governs.
- Verify, do not assume. Order rebound-hammer and ultrasonic pulse-velocity tests on the suspect bays, and cores if the non-destructive results are inconclusive or fall below the acceptance criteria in the concrete code of record. Map soffit cracking and take levels.
- Certify measured fact only. An interim certificate is the client's basis for paying. Record the slab as cast, with the outstanding checks listed; do not record it as approved until they close. An inaccurate certificate is a misrepresentation whether or not the slab later fails.
Competence and public safety outrank the fee and the next job.
2. The project manager instructs you to sign. Describe what you do and what you commit to writing.
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Do not sign, and say so in writing the same day, with reasons rather than opinions.
- Reply to the project manager by email, copying the project file, setting out the facts: strike date, cube results, the specification clause, and the tests still pending. State that you will sign a certificate recording the slab as cast but not as approved, and that you will recommend approval as soon as the checks pass. This turns a personal argument into a documented technical position.
- Escalate. If the instruction is repeated, refer it to the firm's principal or quality manager and, if necessary, inform the client directly. Under the PEC Code of Ethics an engineer who certifies unverified work exposes himself to disciplinary action, and the firm's registration is at risk. A client deadline is not a defence.
- Do not sign under protest. A signature with a caveat in the margin is still a signature on a false statement, and it will be read that way in any later arbitration.
- Record everything. Site diary entries, dated photographs of the day-7 striking and of any props still in place, cube certificates, and every verbal instruction afterwards confirmed by email.
- Offer a path. Propose an expedited testing programme, an independent check, and a separate completion certificate for the unaffected floors so that legitimate payment is not held up.
3. If the slab later deflects and cracks, which records will decide where responsibility lies?
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Liability turns on contemporaneous records, not recollection.
- The site diary and dated photographs establish that striking happened on day 7. The supervisor's own record can be used against the firm, but being first to document the deviation is exactly what limits the damage.
- The inspection-request and approval chain shows whether the contractor asked permission or acted unilaterally. Unilateral action shifts primary responsibility to the contractor.
- Cube certificates, the mix design, batching-plant records and delivery tickets establish whether the concrete itself was deficient or the problem was purely premature loading.
- Your letters and emails show whether you warned the client and what you advised. Silence on file looks like acquiescence.
- Design calculations and site instructions show whether the slab was detailed for early striking and whether loads were imposed before it could carry them.
- A dated level and crack survey taken now gives the baseline against which any further movement can be measured, and is the single most useful thing you can create after the event.