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Why lightweight concrete can lose useful moisture quickly
Fresh concrete loses water from its surface by evaporation. Lightweight concrete can also experience self-desiccation: hydration consumes pore water, lowering internal relative humidity even when the surface is protected. The risk is especially important in high-performance mixtures with high cementitious-material content and low water-to-cementitious-material ratios, and where external curing is difficult, such as vertical surfaces and thick sections. Early moisture loss can increase plastic or autogenous shrinkage, reduce hydration, and leave a surface that is weaker, dustier, or more prone to cracking.
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Lightweight aggregate is not automatically an internal-curing system. Only absorptive aggregate that has been selected, tested, and prewetted as part of the mixture design should be credited with supplying curing water. Ordinary lightweight aggregate may hold water, but its absorption, desorption, grading, and moisture condition determine whether that water is released when the cement paste needs it.
Immediate field procedure
- Read the project specification first. Confirm the required curing method, duration, finish restrictions, and any standard named by the specification. If the specification and a product label differ, obtain the engineer’s direction rather than choosing the less demanding requirement.
- Prepare the curing materials before placement. Have correctly sized film or paper, tape or edge seals, weights, sprayers, and any required wet coverings at the work area. Delays while materials are fetched allow the surface to dry.
- Finish without trapping bleed water. Complete the specified finishing operation, but do not apply a film or compound while finishing would mar the surface. Do not use a curing compound until the surface water sheen has disappeared.
- Start external curing as soon as practical. Place the selected cover in direct contact with the surface when it can be done without damage. For a liquid membrane, spray at the manufacturer’s recommended rate immediately after the sheen disappears and before the surface dries.
- Seal the system. Overlap and tape or otherwise seal sheet edges, close gaps around penetrations, and weigh the cover down so wind cannot lift it. Check corners, form lines, joints, and vertical transitions first; these are common leakage paths.
- Maintain protection for the specified period. Keep the covering intact and prevent premature traffic, wind exposure, heating, or wet-dry cycling. Where no more demanding requirement applies, NRMCA guidance calls for the specified duration or at least 3 to 7 days.
- End curing gradually. Remove sheets or wet coverings so the concrete does not move abruptly from a saturated condition into hot, dry air. Follow the project’s requirements for subsequent drying, coating, or opening to service.
External curing methods
The practical choice depends on application timing, labor, weather, appearance, and the next finish. No single method is universally best; the specification and the surface’s intended use control the decision.
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| Method | How to use it | Advantages | Risks and finish checks |
|---|---|---|---|
| Plastic sheet | Use film conforming to ASTM C171, at least 4 mil (0.10 mm) thick. Lay it directly on the surface as soon as practical, overlap and seal joints, and weigh it down. | Fast to deploy and highly effective at limiting evaporation. Clear, white/reflective, and black films are available; reflective film reduces heat gain in warm, sunny conditions. | Wrinkles and uneven contact can produce mottling or streaks from different moisture and temperature conditions. Confirm that the appearance is acceptable before use. |
| Waterproof curing paper | Use the two-layer, fiber-reinforced kraft paper described by NRMCA and install it like sheeting, with sealed overlaps and restrained edges. It should conform to ASTM C171. | Retains moisture while being less likely than plastic to mar the surface. | Any torn, displaced, or unsealed area can dry. Inspect overlaps and perimeter restraints throughout curing. |
| Membrane-forming curing compound | Use a product meeting ASTM C309 or ASTM C1315. Apply at the stated coverage rate after the sheen disappears and before drying; two coats at right angles improve uniform coverage. | Useful where sheets are impractical, including many slabs with complex geometry and vertical work. | Wax or resin films may interfere with paint, overlays, or bonded finishes. A wax-free compound is recommended where a coating or bonded covering will follow, subject to a compatibility check. |
| Wet covering retained by plastic | Keep burlap or another approved wet covering continuously moist, then place plastic over it to reduce evaporation. | Combines a moist contact layer with a vapor-retaining cover. | The covering must remain wet without washing paste from the surface. Plastic alone over a dry surface is not equivalent to wet curing. |
Plastic film: installation details
Plastic works only when it remains in close contact with the concrete and the enclosure is substantially continuous. Cut pieces large enough to reach beyond the work area, overlap joints, and tape or seal the overlaps. Use battens, sandbags, or other non-staining weights to hold edges and seams; do not drag weights across a green surface. A clear film allows visual inspection, while white or reflective film is preferable where solar heating could create a damaging temperature differential. Black film absorbs heat and should be selected only when the thermal conditions and specification permit it.
Wrinkles create alternating zones of contact and air space. Those zones can differ in temperature and drying rate, leaving a mottled appearance even though the concrete is adequately hydrated. On appearance-critical work, make a small trial area or use a method less likely to imprint the surface.
Waterproof curing paper
Waterproof paper is installed with the same urgency and edge discipline as plastic. Its fiber reinforcement helps it resist handling damage, and it is generally less likely to leave plastic-related marks. Keep it tight enough to prevent wind movement without pressing debris into the surface. Replace sections that become saturated, torn, or detached.
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Liquid membrane-forming compounds
Do not spray while bleed water is standing: dilution and poor film formation can leave pinholes. Do not wait until the surface has visibly dried, because the first moisture loss has already occurred. Follow the product’s stated application rate and equipment requirements. Two passes applied at right angles are desirable for even coverage, but the product instructions control whether a second coat is permitted.
Before using a compound on concrete that will be painted, overlaid, tiled, or otherwise bonded, verify compatibility with the finish manufacturer and project specification. Wax-free chemistry is the usual starting point for a later bonded finish, but “wax-free” is not a substitute for a documented compatibility check or any required removal procedure.
Wet coverings and evaporative retardants
Wet burlap or another approved absorbent covering must stay wet for the entire curing interval. Plastic placed over a wet covering helps retain that water; plastic placed over a dry covering does not create the same reservoir. Evaporative retardants can reduce evaporation before setting and help prevent plastic-shrinkage cracking, but NRMCA identifies them as temporary aids, not final curing. They must be followed by the specified curing system.
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Internal curing with lightweight aggregate
Internal curing is a mixture-design method, not a last-minute surface treatment. NIST reproduces the American Concrete Institute’s definition as “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation.” The reservoirs are absorptive lightweight aggregate particles distributed through the concrete.
For this approach, the aggregate is prewetted to a defined moisture condition and proportioned so it can release water without creating unacceptable workability, strength, density, or durability changes. ACI’s 2024 TechNote preview identifies prewetting the lightweight aggregate to its 72-hour absorption as the relevant preparation condition. That is a mixture-design instruction, not permission to add an arbitrary amount of water at the jobsite.
How much lightweight aggregate is involved?
The ACI PRC-213.2-24 preview describes fine lightweight aggregate in conventional internally cured mixtures as varying between 8 and 12% of concrete volume. The required quantity depends on cementitious-material volume, the internal moisture needed, and the aggregate’s absorption and desorption behavior. Treat the range as technical context, not a field proportioning recipe. The approved mixture must be designed and verified for the actual aggregate source and project performance requirements.
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Test and control aggregate moisture
ASTM C1761/C1761M-23 covers lightweight aggregate intended to provide internal-curing water and includes procedures for determining absorption and desorption. FHWA emphasizes that moisture control is more critical in internally cured mixtures because fine lightweight aggregate has higher absorption. Record the aggregate’s stockpile condition, prewetting method, drainage time, and batch corrections. If the aggregate is drier or wetter than the design condition, the concrete may receive too little internal water or an unintended increase in mixing water.
Internal curing is particularly useful where external curing is constrained, including high-performance concrete with a low water-to-cementitious-material ratio, vertical surfaces, and thick sections. It does not make exposed concrete immune to evaporation. Continue to use the specified surface-curing method to protect the outer zone.
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Choosing a method for the actual placement
| Project condition | Practical emphasis | Questions to resolve before placement |
|---|---|---|
| Flat, appearance-sensitive slab | Use a method that will not imprint or streak the surface. Waterproof paper may be preferable to wrinkled plastic; a trial area can reveal appearance effects. | Will the finished surface be visible? Is sheet contact uniform? Is a later coating planned? |
| Warm, sunny weather | Use reflective film where permitted, seal the enclosure, and control temperature differences. Avoid leaving dark film exposed to intense sun without a thermal review. | Can the selected cover cause excessive heat gain? Are edges secure against wind? |
| Windy or irregular geometry | Prioritize a continuous membrane or a mechanically restrained sheet system. Detail penetrations, corners, and vertical transitions. | How will every edge be sealed? Can the crew maintain contact without damaging the surface? |
| Surface will receive paint or a bonded overlay | Check chemical compatibility before curing. A wax-free compound may be suitable, but the finish system may still require testing or removal of the membrane. | What does the coating or adhesive manufacturer require? Is a curing residue acceptable? |
| High-performance internally cured mixture | Use the approved prewetted aggregate condition and still apply external curing. | What are the documented absorption, desorption, and batch water corrections? Is the internal-curing mixture approved for this member? |
Curing duration and safe termination
Curing time is specification-dependent. NRMCA guidance says to follow the specified duration or continue for at least 3 to 7 days when that guidance governs. The correct period can be longer when the cementitious system, temperature, member geometry, exposure, or required strength development demands it. Do not shorten curing because the surface looks hard; early hydration and shrinkage control continue after finishing.
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At the end of curing, avoid a sudden change from a sealed, moist condition to drying air. Remove covers in a sequence that limits thermal and moisture shock, and protect the concrete if the next operation does not begin immediately. Coordinate the transition with any coating, overlay, or moisture-content requirement.
Common failure modes and corrections
| Symptom or event | Likely cause | Corrective action for the next placement |
|---|---|---|
| Light and dark bands under sheet joints | Unsealed overlaps or different contact conditions. | Increase overlap, tape or seal joints, use adequate weights, and check the layout before the concrete sets. |
| Mottled or streaked appearance | Wrinkled plastic, trapped air, or differential temperature and moisture beneath the film. | Use flatter, uniformly contacting sheets, reflective film in hot sun where allowed, or a less imprint-prone curing paper. |
| Dry edges, corners, or around penetrations | Gaps in the curing enclosure or wind lifting the cover. | Detail edges separately, seal penetrations, and inspect restraints after weather changes. |
| Powdering or a dry, weak skin | Compound applied after drying, poor coverage, or delayed sheet installation. | Stage materials in advance, apply compound after sheen loss but before drying, and verify spray coverage. |
| Cracking before final curing begins | Rapid evaporation, often with wind, heat, or low humidity; an evaporative retardant was mistaken for final curing. | Reduce exposure promptly, use the specified cover, and retain the full curing duration. Review placement timing and weather controls. |
| Coating or overlay will not bond | Incompatible wax or resin residue, or insufficiently prepared surface. | Confirm compatibility before placement; use an approved wax-free system where appropriate and follow the finish manufacturer’s preparation requirements. |
| Internal-curing results vary between batches | Lightweight aggregate moisture differs from the design condition. | Measure absorption and batch moisture, prewet consistently, allow the specified conditioning time, and correct batch water. |
Field quality-control checklist
- Specification and curing duration are available to the placing crew.
- Plastic film, when selected, conforms to ASTM C171 and is at least 4 mil (0.10 mm).
- Sheet edges, laps, penetrations, and corners are sealed and weighted.
- Reflective film has been considered for warm, sunny exposure.
- Waterproof paper is intact, correctly overlapped, and protected from displacement.
- Curing compound meets the specified ASTM C309 or C1315 requirement.
- The compound is applied at the stated rate after the sheen disappears and before drying.
- Compatibility with paint, overlays, tile, or bonded coverings is documented.
- Wet coverings remain continuously wet; evaporative retardant is not being treated as final curing.
- For an internally cured mixture, aggregate absorption, desorption, prewetting condition, and batch water corrections are recorded.
- Covers remain in place for the specified period, with a minimum of 3 to 7 days only where that NRMCA guidance is the governing requirement.
- Removal is gradual and coordinated with the next construction or finishing operation.
What to specify and document
A reliable curing plan states the material standard, installation timing, coverage rate or sheet thickness, edge-sealing method, inspection responsibility, duration, and termination procedure. For internally cured concrete, it also identifies the lightweight aggregate source, absorption and desorption data, prewetting target, conditioning process, and batch-water adjustment. Documenting these items turns “keep it damp” into an instruction that different crews can execute consistently.
When conditions fall outside the approved method—unexpected heat, wind, rain, a delayed finish, a changed aggregate source, or a planned bonded finish—pause and obtain the responsible engineer’s or concrete supplier’s direction. The right response depends on the mixture, member, exposure, and specification; adding unplanned water or switching products without review can create a second problem while trying to solve the first.
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