ACI 308 Curing Guide, Summarized

When a spec says “cure per ACI 308,” it can mean two documents, and one of them is not allowed in a contract. This page sorts out which is which, what changed in 2023 and 2026, and which numbers you can quote. The full ACI texts are paid; everything here comes from ACI’s free previews and FHWA’s summary, each linked.

What is ACI 308?

ACI 308 is the curing work of ACI Committee 308. It produces two documents: a guide that explains curing and a specification you can cite. The guide is now ACI PRC-308-26, Curing of Concrete Guide, 44 pages, published in 2026.

The PRC-308-26 preview gives the history: first published as a standard in 1971, revised in 1992, turned into a guide in 2001, and revised in 2016. If your spec still says 308R-16 or 308.1-11, ask whether the newer edition is acceptable.

What is the difference between ACI 308 and ACI 308.1?

The guide explains; the spec requires. PRC-308-26 states that “Reference to this document shall not be made in contract documents” and that curing measures, in general, are specified in ACI SPEC-308.1. So the guide is for learning and planning, and 308.1 is what goes in the spec book.

The SPEC-308.1-23 preview calls it a reference specification, titled External Curing of Cast-in-Place Concrete. It was approved in March 2023, published in June 2023, and supersedes 308.1-11. The project spec cites it and fills in the choices it leaves to the designer.

One trap: the 308.1-23 preview says not to use it alongside ACI 301 or ACI 350.5 unless the contract documents state that 308.1 governs. ACI 301 carries its own curing rules, and two sets of rules on one slab invite a dispute.

What does the ACI 308 curing guide cover?

The 2026 guide covers why concrete needs curing, the methods and materials, curing for different kinds of work, and how to check that curing worked. Per its contents page, the main chapters are:

• Understanding curing: hydration, the curing-affected zone, properties curing changes, elevated temperatures, sustainability
• Methods and materials: water curing, initial curing, final curing, termination, cold and hot weather, accelerated curing, minimum requirements, temperature limits
• Types of construction: pavements and slabs on ground, buildings and bridges, mass concrete, colored floors and slabs
• Monitoring: site conditions, verifying the curing went on, maturity method, assessing effectiveness

The scope note lists topics added or expanded in this edition: internal curing, curing at elevated temperatures, sustainability, curing of moisture-sensitive flooring, sensors for mass concrete, and new monitoring methods.

It also flags low-carbon cements. The guide says Type IL cements are in current use, their curing needs are being studied, and “Early indications are that initial and intermediate curing are critical to the concrete performance.” If you pour a lot of Type IL mixes, do not skip the early steps.

What are initial, intermediate and final curing?

ACI splits curing into stages by what happens on the slab. The stage definitions are from the PRC-308-26 preview. The measures column draws on FHWA’s summary of 308.1-98 and Dayton’s curing guide.

The point of the split is timing. A curing compound is a final curing measure, so it goes on after finishing, not during it. For how each method compares on the job, see our concrete curing methods guide.

The guide defines curing as keeping moisture and temperature conditions right so the cement can hydrate and the mix can develop its potential properties. Its objectives are to prevent moisture loss and keep a favorable temperature for long enough.

How long does ACI require concrete to be cured?

The current duration clauses sit in the paid SPEC-308.1-23 text and are not in the free preview. The best public numbers are from older editions, as summarized by FHWA’s guide for curing concrete pavements. Treat them as history and check the current edition.

The 7 day figure appears in each of these documents for ordinary cement, which is why it is the number crews remember. For how cure time changes with weather and mix, see how long concrete takes to cure.

When does ACI call for a curing compound?

ACI does not require a compound over other methods. Dayton Superior’s Guide to Curing says the ACI 308 guide addresses water, plastic film, reinforced paper and liquid membrane-forming compounds. Dayton adds that wet methods work only if the concrete is kept constantly damp for 7 days.

FHWA’s summary pulls out the older ACI points that touch compounds:

• Application rate: ACI 308-92 recommended 0.20 to 0.25 L/m², about 165 to 205 ft²/gal (4.0 to 5.0 m²/L).
• Timing: ACI 301-96 says apply after the water sheen has disappeared; 308-92 warns against applying while bleeding is still going on.
• Hot weather: ACI 305R recommends a white pigmented compound to reflect sunlight.
• Low water ratio: ACI 305R calls for water curing rather than a membrane when the w/c ratio is below 0.40.

The product itself is specified by ASTM, not ACI. A plain curing compound is tested to ASTM C309. A cure and seal is tested to ASTM C1315, and Dayton Superior’s Guide to Cure and Seals says ACI 308 recommends a C1315 Type I Class A product for colored floors and slabs.

If the slab surface is dusting or soft after curing

Look at the curing record first. The guide’s “curing-affected zone” is the top layer most influenced by surface curing, and that is the layer that dusts. Late compound, a thin film, or curing pulled early all show up there first.

The ACI 308R-16 preview cites research that exposure to 90% relative humidity or less is enough to suspend hydration, and that cement at 80% hydrated at about a tenth of the rate. A surface that dries in the first days stops gaining strength.

Questions to answer before you call it a material problem:

1. When did the compound go on, compared with the water sheen disappearing?
2. Was the application rate on the data sheet met across the whole slab?
3. Was the curing film or wet cure left in place for the specified period?
4. Was there wind, sun or low humidity during finishing with no initial curing?

For repairs and how to tell dusting from crazing and curling, see dusting, crazing and curling.