The 28-day compressive strength test tells you one thing: whether the concrete mix achieved its design strength in a controlled cylinder. It tells you almost nothing about surface durability, abrasion resistance or crack control on the actual slab. Those properties are decided in the first 24 hours after placement, before the cylinder has even been demoulded.
Poor curing is one of the most common causes of premature concrete surface failure in Australia, and it is also one of the most preventable. Understanding what happens to fresh concrete in the hours after finishing, and how different curing methods respond to it, is the foundation of specifying and applying curing compounds correctly.
What Happens in the First 24 Hours
Fresh concrete loses water through two mechanisms: bleed water rising to the surface and evaporation driven by ambient conditions. When evaporation outpaces bleed water supply, the surface dries faster than the bulk concrete beneath it. The surface paste shrinks; the concrete below resists. The result is plastic shrinkage cracking, a network of cracks that can appear within two to four hours of finishing on a hot, windy day.
The conditions that accelerate this are common on Australian construction sites: air temperature above 25°C, low relative humidity, wind across the slab surface, and direct sun. A combination of 30°C air temperature, 40% relative humidity and a 20 km/h breeze can produce an evaporation rate exceeding 1.0 kg/m²/h, which is the threshold at which AS 3600 and most state specifications require immediate protective action.
Beyond plastic shrinkage, inadequate curing in the first seven days limits the degree of cement hydration at the surface. The surface paste, which governs abrasion resistance, wear resistance and chemical resistance, is the thinnest and most exposed part of the slab. It is also the part most affected by early moisture loss. A slab cured for three days instead of seven can lose 30 to 50% of its potential surface hardness, even when the core strength meets specification.
Curing Methods Compared
Wet Hessian and Ponding
Wet hessian, wet burlap and ponding are the reference methods against which everything else is measured. When applied correctly and kept continuously wet, they maintain near-100% relative humidity at the surface and allow hydration to proceed at the rate the mix design intended. The problem is the word "continuously." Hessian that dries out, even briefly, can cause more damage than no curing at all because the dry fabric wicks moisture from the surface. On large slabs, in exposed locations or on vertical elements, maintaining continuous wetness is labour-intensive and often not achieved in practice.
Ponding works well on flat slabs with adequate edge bunding, but it requires water management and is impractical on slabs with falls, penetrations or complex geometry.
Membrane-Forming Curing Compounds
A membrane-forming curing compound is applied as a liquid immediately after finishing, or after the surface bleed water has evaporated but before the surface dries. It forms a continuous film that reduces moisture evaporation from the slab surface, retaining the mix water needed for continued hydration.
AS 3799 is the Australian standard that governs liquid membrane-forming curing compounds. It classifies compounds by their water retention efficiency, expressed as a percentage: the compound must retain a specified proportion of the water that an uncured reference specimen would lose over 72 hours. The standard also covers application methods, coverage rates and compatibility requirements. Any compound specified on a project in Australia should comply with AS 3799 and the certificate of compliance should be available from the supplier.
Membrane-forming compounds do not match the theoretical performance of continuous wet curing under ideal conditions. But in practice, on most commercial and industrial sites, they outperform wet hessian because they are applied once and do not rely on ongoing site labour to maintain effectiveness.
Resin-Based vs Wax-Based Compounds
The two main chemistries used in membrane-forming curing compounds are resin-based and wax-based (paraffin or hydrocarbon wax emulsions). The distinction matters for compatibility with subsequent work.
Resin-Based Compounds
Resin-based compounds, typically formulated from acrylic, styrene-acrylic or similar polymers, form a harder, more durable film. They generally achieve higher water retention efficiency ratings under AS 3799. Many resin-based compounds are designed to degrade gradually under UV exposure, which is useful on exterior slabs where the compound will eventually weather away. Coverage rates typically range from 4 to 6 m²/L depending on surface texture and porosity, though the manufacturer's datasheet is the governing reference for any specific product.
The key issue with most resin-based compounds: they leave a residual film that bonds to the concrete surface. If a coating, adhesive, topping screed or tile bed is to be applied, that film must be removed by mechanical means, typically diamond grinding, scarifying or shot blasting, before the next layer goes down. Failure to remove it is one of the most common causes of delamination of floor coatings and toppings on industrial slabs.
Wax-Based Compounds
Wax-based compounds form a softer film and generally have lower water retention efficiency than resin-based products. They are more commonly used on slabs that will receive a structural topping, tile adhesive or bonded screed, because the wax film is easier to remove by mechanical preparation. Some wax-based products are formulated to be compatible with certain adhesives without removal, but this must be confirmed against the adhesive manufacturer's data, not assumed.
Coverage rates for wax-based products are similar to resin-based compounds, typically 4 to 6 m²/L on a smooth-floated surface, with lower coverage on broom-finished or textured surfaces.
White-Pigmented vs Clear Compounds
Both resin-based and wax-based compounds are available in clear or white-pigmented formulations. White-pigmented compounds provide a visible application check, making it easier to identify missed areas or thin spots during application. On slabs exposed to direct sun, the white pigment also reflects solar radiation, reducing surface temperature and slowing moisture loss beyond the effect of the membrane alone. For most external slab work in Australian conditions, white-pigmented compounds are the better choice.
The Detail Most Jobs Get Wrong
The single most common error with curing compounds on commercial and industrial projects is applying a standard resin-based compound to a slab that will subsequently receive a coating, epoxy topping, polyurethane floor finish or bonded screed, and then failing to remove it before the next trade arrives.
The curing compound film is designed to resist moisture transmission. That same property means it resists adhesion. An epoxy coating applied over a resin-based curing compound without mechanical removal of the film will not achieve the bond strength required for long-term performance. Delamination may not appear immediately; it often shows up six to twelve months later under traffic loading, when the coating peels or blisters in sheets.
The correct approach depends on what comes next:
- Slab to receive a coating or topping: Specify a curing compound that is explicitly rated for removal by mechanical preparation, and confirm the preparation method with the coating manufacturer before work begins. Shot blasting to Sa 2.5 or diamond grinding to achieve a clean, open surface profile is the standard approach.
- Slab to receive a bonded screed or tile adhesive: Consider a wax-based compound or a purpose-formulated dissipating compound, and confirm compatibility with the screed or adhesive system before application.
- Slab with no subsequent coating: A standard resin-based compound is appropriate and the residual film is not a concern.
Some Fosroc products in the curing compound range are formulated with specific compatibility profiles for subsequent coatings and toppings. The Fosroc datasheets detail whether mechanical removal is required, what surface preparation standard is needed, and the minimum waiting period before overcoating. Those datasheets are the governing reference; do not rely on verbal advice from site.
Application: Where Compound Performance Is Lost
Even a correctly specified compound fails if application is poor. The most common application errors are:
- Applying too early: If bleed water is still visible on the surface, the compound will be diluted and the film will be discontinuous. Wait until the surface sheen disappears but the surface is still damp.
- Applying too late: If the surface has begun to dry, moisture loss is already occurring. The compound cannot recover water that has already evaporated.
- Insufficient coverage: Thin application leaves the film discontinuous. Apply at the manufacturer's specified coverage rate, and apply in two passes at right angles to each other on slabs where uniform coverage is required.
- Missing edges and formed faces: Edges and vertical formed faces lose moisture rapidly. They need compound applied immediately after formwork is stripped.
- Wind during application: Spray application in windy conditions causes uneven distribution and overspray loss. Use a low-pressure pump sprayer or roller in exposed conditions.
Specifying for Australian Conditions
For most commercial and industrial slab work in Australia, a white-pigmented, resin-based compound complying with AS 3799 is the baseline specification. Where subsequent coatings are planned, the compound selection and removal method must be confirmed with the coating supplier before the slab is poured, not after. Where slabs are in high-evaporation conditions during placement, a windbreak or shade structure during finishing and early curing is worth specifying alongside the compound.
Curing is not a finishing step. It is part of the concrete mix design performing as intended. The cylinder strength is a quality check on the mix. Surface durability is a quality check on the curing.
CSA supplies the Fosroc range of construction chemicals, including curing compounds, through our trade account desk and online at constructionsupplies.group/au. If you need product datasheets, coverage estimates for a specific pour or advice on compound compatibility with a planned coating system, contact the trade desk directly.