The Concrete Mix Substitution
Six weeks into a $31M mixed-use project, the concrete subcontractor proposes substituting a ternary-blend mix for the specified 30% fly-ash mix. The ready-mix plant says it cannot guarantee fly-ash supply for the next four months. The substitution, if approved, would recover 9 days on the structure. The structural engineer calls it "probably acceptable, but I'd want cylinder data." The owner's sustainability consultant says the change puts a LEED credit at risk. The specification has an "or equal" clause with a 10-day review period. The next structural pour is in three days and 180 CY are scheduled.
1. What is the correct process, and what do you do about the pour in three days?
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This is a substitution, not a field decision, and "probably acceptable" is not an approval.
Run it through the substitution clause: written request with mix design, trial batch results, 7- and 28-day cylinder breaks, durability and permeability data, and the plant's written supply commitment. Ten days of review cannot be met before a pour in three days, so you have two legitimate routes:
- Pour the specified mix for this placement. First confirm the plant can actually supply 180 CY of the specified mix - the supply problem may make this moot.
- Request a written extension of the review period from the owner and engineer, with a stated decision date.
Choose the route today and put it in writing to the sub, the engineer, and the owner simultaneously, with the deadline named. Silence is not agreement, and a pour made on a verbal "probably" is rework waiting to be discovered at the cylinder break.
2. How do you weigh 9 days against a LEED credit and the engineer's caution?
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Put a number on each of the three axes before forming a view.
Schedule. Are the 9 days on the critical path? Check the CPM, not the activity. If structure float is 15 days, recovering 9 changes the completion date by zero and the whole trade-off is illusory. If the structure drives completion, price 9 days against extended general conditions and any liquidated damages.
Sustainability. Which credit, how many points, and does losing it drop the project below a certification level the owner contracted for? If certification is a contractual deliverable, the substitution may be non-compliant regardless of schedule advantage.
Technical. Early-age strength and finishability drive the pour cycle; a mix that gains strength slowly can cost more form-cycle days than the 9 it recovers. Cylinder data answers this; the engineer's instinct does not.
Lay the three axes out for the owner as a decision matrix and let the accountable party choose. Your job is to make the trade-off visible and record it, not to absorb it silently.
3. How do you respond to the subcontractor?
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In writing, with a disposition and a path forward - never a corridor "fine, go ahead."
A workable response reads: "Not approved as proposed. Submit mix design, trial batch data, 28-day breaks, and the plant's supply letter by [date]. We will convene a review with the structural engineer within five business days of receipt. Proceed with the specified mix for the [date] placement of 180 CY unless you receive written direction otherwise."
Give the reasoning, not just the ruling: the owner has a sustainability commitment and the engineer requires data. Subs accept a clear process far better than a vague no, and a vague yes is worse than both.
Separately, note that a plant supply failure is generally the subcontractor's risk under its supply obligation, and reserve the owner's rights on time and cost - but state it neutrally, without turning a substitution request into a confrontation. Then diarise the review date so the process does not drift past the next pour.