How Long a Ready-Mix Truck Can Sit Before It Has to Discharge
ASTM C94 dropped its 90-minute default in 2021, so the discharge window is set job by job. Who sets it, and when may water go in?
Use the Concrete Calculator — the cubic yards to order, worked out before the delivery limits are settled.
The default left C94 in 2021; the limit did not
The ready-mix specification in force on 26 September 2026 is ASTM C94/C94M-26c, which ASTM's store shows as Active, with a last update on 12 August 2026; the -26, -26a and -26b revisions are shown as historical.
The rule that matters here changed in C94/C94M-21. A quality bulletin from Atlantic Testing Laboratories reproduces the two clauses changed in that edition, a change it says took six years of balloting. Clause 6.1.10 puts the job on the purchaser: state a time limit running from the start of mixing to the moment discharge has to be finished. If the purchaser says nothing, the producer sets the limit and tells the purchaser before delivery, and the limit goes on the delivery ticket. Clause 12.9 then requires discharge to be finished inside that limit, and lets the purchaser waive it if the concrete can still be placed without adding water.
What disappeared was the printed default of one and a half hours. The aim, the bulletin says, was only to remove the default, not to lengthen or shorten the window; the parties can still settle on 90 minutes if they cannot find a more suitable limit. NRMCA's Guide to Improving Specifications for Ready Mixed Concrete (2PE004-26, March 2026) confirms the 90-minute limit is gone and recommends that project specifications neither set delivery time limits nor change them with the weather.
Clause numbers in this article are from C94/C94M-24a unless another edition is named. Check each against -26c before quoting it in a submittal.
Revolutions: from a fixed 300 to a limit someone must state
In C94/C94M-09, clause 12.7 required discharge to be complete within 1-1/2 hours or before the drum had turned 300 revolutions, whichever came first, counted from when the mixing water met the cement and aggregates, or the cement met the aggregates. The purchaser could waive both limits for concrete that could still be placed without extra water, and could specify a shorter time in hot weather.
C94/C94M-24a handles drum turns differently. Its clause 6.1.9 has the purchaser state any drum revolution limit on when discharge must begin; if none is stated, the producer determines one and tells the purchaser before delivery. Read with 6.1.10, that makes two limits: a revolution count by which discharge must start and a time by which it must end. Whichever one the purchaser leaves open, the producer has to set.
Older documents still carry the earlier rule. ASCC Position Statement #32, Concrete Discharge Time Requirements, marked 9/10 and published in Concrete International, gives C94's limit as 1-1/2 hours. Check the date on anything citing a 90-minute C94 limit.
Water at the jobsite: permitted, bounded, written down
Water can be added on site, within limits. In the -24a text, clause 12.7 bars water after batching except through the automated truck systems of 12.8 or when the slump on arrival has to be raised to meet the purchase order. NRMCA's CIP 26, Jobsite Addition of Water (revised 2023), summarizes the C94 procedure in six steps. Fix the highest slump and water content allowed for that load; check slump on a preliminary sample from the truck; add water only inside those limits; measure what went in and write it down; then turn the drum 30 revolutions at mixing speed.
The sixth step says when to stop: the maximum water-cementitious materials ratio is reached, the maximum slump is reached, or more than about a quarter of a cubic yard has been discharged. CIP 26 wants water in early, so the volume being retempered is still known.
Its rule of thumb is roughly a gallon of water per cubic yard for each inch of added slump. It also calls it good practice for the supplier to show on the ticket the trim water held back at batching, since that figure caps what can go in at the site.
All water added on site is to be measured and written on the delivery ticket, and CIP 26 asks a designated representative of the purchaser to sign or initial it. A purchaser who asks for water beyond the design mixing water, or slump above the permitted maximum, takes responsibility for the resulting concrete.
Hot and cold weather: what the specifications and guides set
Nothing in the -24a text of 6.1.10 shortens the window when it is hot. Instead, its Note 10 names ambient conditions among the things to weigh when choosing the limit, alongside the cementitious materials and admixtures, the placement procedure and the travel time from plant to site.
A 95 F limit comes from ACI. Its FAQ on hot-weather temperature limits gives 95 F (35 C) at discharge under ACI 301-20 and ACI 305.1-14 for general hot-weather work, mass concrete excluded. The limit is not absolute: under ACI 305.1-14 a mixture can be qualified above it on successful past field experience or preconstruction testing, submitted to the architect/engineer before placement, and computer modeling cannot stand in for that testing.
NRMCA's TIP 18, Managing Concrete Temperature for Specified Requirements, advised producers to aim below the specified maximum to allow for a rise of 1 to 5 F during delivery, driven by drum color, radiant heat and delivery time, and pointed to ACI 305R for a way to evaluate how fast a particular mixture loses slump.
ACI 306R-16, Guide to Cold Weather Concreting, adopted and published September 2016 and superseding 306R-10, treats it as cold weather when air temperature has dropped to 40 F, or is expected to drop below it, during the protection period. That definition is about protecting the placed concrete. The trip is handled separately: its table of contents lists a section 5.8 on temperature loss during delivery, and its notation defines Td, the drop expected over a one-hour delivery, which is added to the delivered temperature to give the recommended temperature at the plant.
ACI 306R-16 is a guide, and it prints a notice that it is not to be referenced in contract documents; if a project wants one of its recommendations, the architect/engineer has to restate it in mandatory language in the specification.
What to settle before the truck is batched
Once the truck arrives, -24a clause 7.2 has the concrete stay within the permitted slump range for 30 minutes, counted from arrival or from the first slump adjustment on site, whichever is later; the first and last quarter cubic yard are exempt. If the site is not ready to take the load, the producer stops being answerable for minimum slump 30 minutes after arrival or after the requested delivery time, whichever is later.
So agree the discharge time limit, the revolution limit and who on site may request water before the load is batched; CIP 26 suggests settling the last of these at a pre-placement conference, together with how added water will be recorded.
The concrete calculator gives the volume to order in cubic yards, so you know how much concrete those limits have to cover.
Sources
- ASTM C94/C94M-26c, Standard Specification for Ready-Mixed Concrete: store.astm.org listing, read 26 September 2026, shows 'Standard Active' and 'Last Updated: Aug 12, 2026'. The listings for -26 (Last Updated: Apr 15, 2026), -26a (Jul 08, 2026) and -26b (Aug 12, 2026) show 'Standard Historical'. Used for edition status only.
- ASTM C94/C94M-24a, Standard Specification for Ready-Mixed Concrete (superseded): specification text shown on the standards.iteh.ai page for this edition, dated 31-Mar-2024 on that page. Used for 6.1.9 (purchaser states any drum revolution limit on when discharge must begin; otherwise the producer determines and communicates it before delivery), 6.1.10 (time limit to completion of discharge; producer establishes it if the purchaser does not; stated on the delivery ticket), Note 10 (factors to consider in choosing the limit, including ambient conditions), 7.2 (30-minute slump availability; first and last 1/4 yd3 exempt; site-not-ready provision) and the opening of 12.7 (jobsite water only as permitted in 12.8 or to bring arrival slump up to the purchase order).
- Atlantic Testing Laboratories, Quality Bulletin: Review of Technical Changes to ASTM C94, Scott M. McCasland, PDF. Used for clauses 6.1.10 and 12.9, which it reproduces as the current text after the C94/C94M-21 change; the six years of balloting in Subcommittee C09.40; and the stated intent to remove the 1-1/2 hour default without extending or reducing the time limit, while still allowing 1-1/2 hours to be chosen.
- ASTM C 94/C 94M-09, Standard Specification for Ready-Mixed Concrete (historical edition), clause 12.7: discharge within 1-1/2 h or 300 drum revolutions, whichever first, counted from introduction of mixing water to cement and aggregates or of cement to aggregates, with purchaser waiver and a shorter time permitted in hot weather.
- NRMCA Publication 2PE004-26, Guide to Improving Specifications for Ready Mixed Concrete, March 2026, PDF: printed page 32, commentary under 3.15 C (90-minute limit removed from ASTM C94; limits on truck-mixer revolutions or time from batching set between contractor and producer; a default time or revolution limit not considered appropriate; recommendation that specifications not include delivery time limits or change them based on ambient conditions).
- NRMCA CIP 26, Jobsite Addition of Water, PDF with the revision line '1995, 2014, 2023': boxed summary 'ASTM C94 Jobsite Water Addition' items 1-6; items (c), (d) and (h) under 'HOW to Add Water at the Jobsite?' (add water before significant discharge so the retempered volume is known; 1 gallon of water per yd3 per inch of slump as a rule of thumb; measure and record on the ticket; purchaser's representative signs or initials; pre-placement conference to decide who may request water and how it is documented); 'WHY is Water Added to Concrete?' (purchaser assumes responsibility for water beyond the design mixing water or slump above the maximum permitted); closing paragraph (trim water on the ticket as good practice).
- NRMCA TIP 18, Managing Concrete Temperature for Specified Requirements: nrmca.org PDF (wp-content/uploads/2020/04/TIP18w.pdf), fetched 26 September 2026. Section 'HOW is Concrete Temperature Specified?': producer to target below the specified maximum for a 1 to 5 F rise during delivery; drum color, radiant heat and delivery time as factors; ACI 305R procedure for evaluating rate of slump loss.
- American Concrete Institute, FAQ 'Maximum temperature limits for hot-weather concreting' (concrete.org, faqid=15), read 26 September 2026: 95 F (35 C) at discharge under ACI 301-20 and ACI 305.1-14 for general hot-weather construction, not mass concrete; under ACI 305.1-14, qualification above 95 F by successful past field experience or preconstruction testing, submitted to the architect/engineer before placement; computer modeling not a substitute for preconstruction testing. ACI's store pages, read the same day, list ACI SPEC-301-20 and ACI SPEC-305.1-14(20) for sale.
- ACI 306R-16, Guide to Cold Weather Concreting (listed in ACI's store as ACI PRC-306-16), PDF: front-matter statement that it supersedes 306R-10 and was adopted and published September 2016, and the contract-documents notice; 1.1 and 2.2 (cold weather, 40 F / 4 C during the protection period); 2.1 notation (Td, drop expected during a 1-hour delivery time, added to the delivered temperature to give the recommended plant temperature); table of contents entry 5.8, Temperature loss during delivery.
- ASCC Position Statement #32, Concrete Discharge Time Requirements, PDF marked '9/10/ASCC' and 'As published in Concrete International'. Cited only as an example of the pre-2021 wording.
Guides
- How Long a Ready-Mix Truck Can Sit Before It Has to Discharge
- #57, #8, #411 and Crusher Run: Which Are Size Numbers and Which Are Something Else
- Checking Topsoil and Compost Before the Truck Tips It
- Reading the Delivery Ticket: Did You Get the Quantity You Paid For?
- What Strength Concrete to Order, and What Else to Tell the Plant
- Why the Yard Split Your Order Into Two Loads