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Key points

  • Ready-mixed concrete is batched at a plant and brought to site in truck-mounted rotating drum mixers.
  • ASTM C94 required discharge on site within 90 minutes and before 300 drum revolutions after water was added to the cement.
  • Since ASTM C94/C94M-21, that fixed limit was replaced by a time limit agreed between the purchaser and the concrete producer.
  • Adding water on site to ease placing is a main cause of cracking; ask for a plasticiser in the mix instead.
Written for:OwnerContractor

"Ready-mixed concrete" means the mix is batched at a concrete plant in weighed quantities and brought to your site in truck-mounted rotating drum mixers1. It is now the usual way to pour footings, columns and slabs, instead of hand-mixing on site.

1) Why ready-mixed concrete?

  • The US National Ready Mixed Concrete Association notes that central mixing at the plant improves quality control and consistency1.
  • Weighed ingredients and a known mix design, instead of "measuring by eye" on site.
  • Fast placing of large volumes, especially with a concrete pump.

2) The 90-minute rule: what changed?

For many years the US standard ASTM C94 required concrete to be discharged on site within 90 minutes and before 300 revolutions of the drum after water was added to the cement1.

In the ASTM C94/C94M-21 edition, that fixed limit was replaced by a time limit established between the purchaser and the concrete producer2. NRMCA notes that admixtures such as set retarders may be added before discharge to compensate for long delivery times or high temperatures1. See concrete admixtures.

In practice: agree in advance with the plant on the maximum time from loading to end of discharge, especially in summer, for distant sites or expected traffic. Record each truck's arrival time.

3) No water on site

The association lists high-slump concrete and excessive water added on the job among the main causes of concrete cracking3. Extra water weakens concrete and makes it more porous, letting salts reach the steel. See rebar corrosion.

If the mix is too stiff for the pump, the answer is a plasticiser dosed by the plant, with the engineer's approval — not a water hose on site.

4) What to check on the delivery ticket

Common practical items on ready-mix delivery tickets; the association recommends stating the reference standard on the ticket where there are no detailed project specifications1.

  1. Strength class (e.g. C30/37) matching the drawings. See concrete basics.
  2. Cement type (ordinary or sulphate-resisting for footings). See cement types.
  3. Admixtures and their type.
  4. Batch time and arrival time.
  5. Volume in cubic metres and truck number.

5) Pour day in brief

  • Slump test and sampling (cubes or cylinders) in the supervisor's presence.
  • Compaction with a vibrator, then curing for at least 7 days. See curing.
  • In heat: pour in cooler hours and have evaporation control ready. See cracks and hot weather.

6) Pros and cons

Pros

  • Better quality and consistency than hand-mixing.
  • Fast placing of large volumes.
  • A documented mix with a delivery ticket.

Cautions

  • Limited time between loading and discharge; delays harm the mix.
  • Needs truck and pump access to the site.
  • Final quality also depends on compaction and curing on site.

Sources

The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.

  1. 1About Concrete — Ready Mixed Concrete (ASTM C94 delivery requirements) — National Ready Mixed Concrete Association (NRMCA)
  2. 2Concrete Q&A: "90-Minute Rule" for Ready Mixed Concrete (ASTM C94/C94M-21 change) — American Concrete Institute — Concrete International
  3. 3CIP 4 — Cracking Concrete Surfaces — National Ready Mixed Concrete Association (NRMCA)
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This guide is for general information and planning, based on standards and technical guides. Standards are revised periodically; in Saudi Arabia the binding reference is the Saudi Building Code and the instructions of the competent authorities. For design and execution, use a licensed engineer or technician. Found an error? Tell us.