Concrete PSI and mix choice (planning note)

What suppliers ask

• Strength (PSI/MPa) and aggregate size.
• Slump (workability) and whether you need fiber.
• Admixtures (accelerator/retarder) based on weather and timing.

Delivery timing and slump

Your mix choice should match placement timing and finish needs. Slump affects workability and finishing time.

• Ask the supplier about allowable water addition on site.
• Plan crew size so placement and finishing happen on time.

Strength selection (PSI/MPa) — what to clarify

Strength is not just a number: it affects cost, finishing window, and sometimes minimum cement content. When you hear a strength target, confirm whether it is a code minimum, a contractor preference, or a requirement for a specific use case.

If you are unsure what to order, your supplier can usually recommend a standard mix for your application—but you should still confirm any local requirements for structural work.

• Confirm whether the job is structural or non-structural (slabs, footings, piers, etc.).
• Ask if air-entrainment is recommended for freeze/thaw exposure in your area.
• Confirm the maximum aggregate size if you have tight reinforcement spacing or small forms.

Slump (workability) — why it changes placement

Slump is a quick way to describe how workable the concrete is. Higher slump can make placement easier in some situations, but too much water can reduce performance and cause finishing issues.

If you need easier placement, it is often better to use the right admixture plan than to add water on site. Discuss this before the truck arrives.

Air entrainment (freeze/thaw durability)

In freeze/thaw climates, air-entrained concrete is often used to reduce surface scaling and damage. If your area sees freezing conditions, confirm whether air entrainment is required or recommended.

• Air entrainment affects finish and strength; coordinate with the mix design.
• Do not add water on site to “fix” workability in air-entrained mixes.

Fiber, reinforcement, and cracking (what to expect)

Fiber can help with plastic shrinkage cracking and handling, but it is not automatically a replacement for reinforcement. Crack control depends on joints, thickness, base condition, curing, and reinforcement strategy.

The most common planning miss is ignoring joints and curing. Even a great mix can crack if joints are wrong or if the surface dries too fast.

• Plan control joints and spacing before you pour (layout matters).
• Plan curing method and timing (especially in hot, windy, or dry conditions).
• Confirm reinforcement requirements for the project; local practice varies.

Admixture checklist (common options)

• Accelerators for cold weather or tight schedules (project dependent).
• Retarders for hot weather or large pours that need more working time.
• Water reducers for workability without adding water.
• Fibers for plastic shrinkage control (not a rebar replacement).

Reminder

Mix selection affects cost and placement. For critical work, follow code and supplier guidance rather than generic assumptions.

Typical strength ranges (planning examples only)

Exact requirements vary by local code, exposure conditions, and engineer/contractor specs. Use these as conversation starters, not as a substitute for requirements.

Weather planning (hot vs cold)

• Hot/windy weather can dry the surface fast: plan curing and finishing timing to reduce cracking risk.
• Cold weather slows strength gain: you may need an accelerator, protection, or schedule changes (project dependent).
• If you need a longer working time (large pour), discuss retarder and delivery timing before the truck is dispatched.

Delivery checklist (avoid day-of surprises)

• Confirm access: can the truck reach the forms, or do you need a pump/wheelbarrow plan?
• Confirm discharge time limits and washout location.
• Have labor/tools ready before arrival: rakes, screed, float, edger, and curing plan.
• Plan a small buffer: short loads can stop the pour at the worst time.

Common mistakes that cause weak concrete

• Adding water on site to increase slump (often reduces strength and increases shrinkage).
• Skipping curing because the surface “looks dry.”
• Poor base prep and drainage: slabs fail from below more often than from mix design alone.

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