11.1 Curing Methods, Moisture Retention Materials & Duration Requirements (ACI 308R)
Key Takeaways
ACI 318-19 Section 26.5.3.2 sets 50°F and moist conditions for the first 7 days for ordinary concrete and 3 days for high-early-strength concrete, with a provision for accelerated curing.
Under ACI 301-20, check the 7-day or 3-day default, project modifications and the exact strength-test conditions if early termination is proposed.
ACI 301-20 ponding water is at least 50°F and no more than 35°F colder than the concrete surface at contact.
Under ACI 301-20, provide water throughout final curing of unformed silica-fume concrete. Fly ash or slag content alone does not establish a universal 14-day period.
Where membrane curing is specified, check final finishing, loss of water sheen, manufacturer application instructions and compatibility with later finishes.
11.1 Curing Methods, Materials & Duration
Curing maintains conditions for hydration and, where applicable, pozzolanic reactions. Temperature affects how quickly the concrete develops strength. Moisture control also matters near the exposed surface, where early drying can impair durability and increase cracking. Check the governing edition and project curing method for the member. Model IBC Table 1705.3 lists periodic verification of the specified curing temperature and techniques.
From placement through final curing
ACI 308R-16 explains why early protection is needed before final curing can be applied. ACI 308.1-23 is a separate external curing specification with its own contract provisions. ACI 301-20 Section 5.3.6 provides the example specification requirements here. Its initial protection for an unformed surface starts with fogging or an evaporation reducer when no bleed water sheen is visible after strike-off and initial bull floating. Follow the permitted method until final curing is in place. Apply the reducer according to its instructions and keep it out of the finishing process as a workability aid.
After placement and finishing, apply the specified final curing without marring or discoloring the surface. Under Section 5.3.6.4, time with forms tightly in place can count toward curing. If loosening or stripping exposes the concrete during the required period, switch to the applicable exposed-surface protection. If changing from one of the listed methods to another under Section 5.3.6.6(c), the default allows a change after the concrete is one day old if its surface remains moist through the change.
Add water or retain moisture
| Method | What to inspect under ACI 301-20 Section 5.3.6.5 |
|---|---|
| Continuous fogging | Coverage and moisture continuity. For initial fogging avoid accumulated standing water. The final-curing fogging provision permits it. Do not fog when air touching the concrete is at or below 32°F. |
| Ponding | Maintain water coverage of the entire surface. Use water at least 50°F and at most 35°F colder than the concrete surface when they come into contact. |
| Continuous sprinkling | Keep the surface wet throughout and prevent running water from eroding it. |
| Absorbent covers | Predampen the specified material and maintain continuous moisture. Add water without moving the cover. |
| Water-retention sheets | Cover exposed concrete, tape or lap seams, close holes and gaps, and secure the sheets so they remain in place. |
| Membrane-forming compound | After final finishing, apply as the water sheen disappears, following the manufacturer's recommendations for the specified finish. |
For ponding on a 90°F surface, the differential permits water at least 55°F. That is above the 50°F independent floor. A measured 52°F is 38°F colder and fails this ACI 301-20 criterion. ACI 308R-16 recommends a smaller 20°F differential to minimize thermal gradients. Apply a tighter project requirement if specified and use the thermal control plan for applicable mass concrete.
Section 5.2.1 distinguishes absorbent material under AASHTO M 182 from water-retention sheets under ASTM C171. The latter covers curing paper, clear or white opaque polyethylene film, and white burlap-polyethylene sheet. White reflective materials can reduce solar heating. Inspect contact, seams, edges and damage according to the specified product and method. For unformed silica-fume concrete under Section 5.3.6.3(a), use a final curing method that supplies additional water throughout the curing period. Retaining the water already in the concrete does not supply that additional water.
Membrane compound and finish compatibility
ASTM C309 covers liquid membrane-forming curing compounds. Type 1 is clear or translucent, Type 1-D adds fugitive dye and Type 2 is white pigmented. The dye helps track where product has been applied. ASTM C1315 covers membrane-forming compounds with specified properties for curing and sealing. Under ACI 301-20 Section 5.2.1.2 the default permits compounds conforming to C309 or C1315 and excludes silicate-based densifiers as curing compounds. Match the specified product and edition. The ASTM product designation alone does not determine application timing, coverage or suitability for a later bonded floor finish.
ACI 301-20 Section 5.3.6.5(f) calls for manufacturer-directed application for the specified finish as the water sheen disappears. The section calls for two applications at right angles for the stated rough finishes. Check rate and uniformity across the slab, including edges and areas shielded by equipment. If a product calls for 200 square feet per gallon total and the slab is 10,000 square feet, the calculation is gallons. If two equal passes together must deliver that total, each pass uses 25 gallons across the slab.
Read the finish schedule before the pour. For industrial floor slabs subject to ACI 301-20 Section 11.3.7, check its continuous moisture and duration requirements. Where later flooring or a densifier is planned, verify the compound is compatible or will be removed by a method the finish flooring or densifier manufacturer accepts. This also applies to a compound sold as self-dissipating. Document the product, intended finish and accepted plan for any incompatibility.
Duration and strength evidence
ACI 318-19 Section 26.5.3.2 generally calls for ordinary concrete to be kept moist and at least 50°F for the first 7 days, or 3 days for high-early-strength concrete, with separate conditions for an approved accelerated curing process. ACI 301-20 Section 5.3.6.6(a) defaults to 7 days, or 3 days for high-early-strength concrete, unless otherwise specified. Check the specific project curing requirements. Cementitious composition, exposure and performance can lead to a specified longer period. A fly ash or slag percentage alone does not set a 14-day ACI 301-20 minimum.
Where the governing documents and applicable code permit the Section 5.3.6.6(b) early-termination routes, check the type of result and its conditions. Do not exchange the thresholds among methods.
| Evidence under the reference specification | Strength condition |
|---|---|
| ASTM C31/C31M field-cured cylinders tested under ASTM C39/C39M | Test at least two 6-by-12-inch or three 4-by-8-inch cylinders, with the result at least 70% of . |
| Representative laboratory-cured cylinders | Strength exceeds 85% of , and the in-place concrete was maintained at least at 50°F during curing. |
| Accepted in-place testing under Section 2.3.4.2 | Strength reaches . |
For psi the first two thresholds are 3500 psi and strictly above 4250 psi, respectively. A laboratory result of 4250 psi does not meet the second route. Check the approved curing plan, applicable code requirements and evidence for any early change. A strength test used to decide when to end curing has a different purpose from the designated-age acceptance test for the concrete delivered. ACI 318-19 Section 26.5.3.2(e) separately evaluates protection and curing using companion specimens at the designated age. The field-cured average must be at least 85% of the companion standard-cured average or exceed by more than 500 psi. See Section 3.3 for specimen procedures.
Before the pour, verify the specified method, water or materials, protection and staffing. During the work, record the time, temperature and method, and confirm uninterrupted coverage at edges, corners and exposed surfaces. Report drying, interruption or a substituted material with the applicable requirement and follow the project's disposition process.
When should an approved ASTM C309 membrane compound be applied to finished flatwork under ACI 301-20 Section 5.3.6.5(f)?
After final finishing, as the water sheen disappears, following the product instructions
During initial screeding while bleed water is still present
Exactly 24 hours after placement
When the concrete first reaches 500 psi
For ordinary concrete containing 30% slag cement, what is the ACI 301-20 Section 5.3.6.6(a) default when no other period is specified and no permitted early termination is used?
3 days
7 days
14 days
28 days
Under ACI 301-20 Section 5.3.6.5(b), what is the minimum compliant ponding water temperature when the concrete surface is 90°F at contact and no stricter project rule applies?
50°F
52°F
55°F
70°F
Sections you finish are checked off in the contents.