Hot-Weather Slab: Plastic Cracking, Late Saw-Cuts and a Dusty Surface
An exterior slab-on-grade, 150 mm thick, is placed as 5 m × 5 m panels with saw-cut contraction joints. Ambient temperature is 34 °C, relative humidity 28%, wind 24 km/h, and the concrete arrives at 31 °C with w/c 0.48 and 25% fly ash. The calculated surface evaporation rate is 1.1 kg/m²/h. The crew strikes off, bull-floats immediately, and power-trowels 25 minutes later while a sheen of bleed water is still visible; curing compound is sprayed 4 hours after finishing. Joints are saw-cut 22 hours after finishing to a depth of 25 mm. The next morning, fine random cracks 0.2–0.6 mm wide have opened across several panels, some crossing the panel centres rather than following the joints; short parallel cracks appear near the slab edges; and the trowelled surface is dusty, losing material under a fingernail to about 3 mm depth. No rain fell and no plastic sheeting was used.
1. Explain the edge and surface cracks: what does 1.1 kg/m²/h mean and how did the finishing sequence contribute?
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Interpreting the number. ACI 305.1 treats a surface evaporation rate above 1.0 kg/m²/h as the level at which special precautions are mandatory, and above about 0.5 kg/m²/h - roughly the rate at which bleed water can rise to the surface - as the level where plastic shrinkage cracking becomes possible. At 1.1 kg/m²/h the surface loses water faster than bleeding can replace it.
Mechanism. Menisci form in the surface pores, generating negative capillary pressure and shrinkage of a surface layer that has essentially no tensile strength. Restraint from the cooler, wetter interior and from the subgrade converts that shrinkage into tension, and short parallel cracks open perpendicular to the restraint - hence the edge cracks.
How the crew made it worse. Troweling 25 minutes after strike-off, over a visible bleed sheen, closes the surface while water is still rising. Water is trapped under a densified skin, raising the local w/c at the surface. The 3 mm dusty layer is the signature of that trapped water and of finishing too early; it also reduces the abrasion resistance and the effectiveness of any future coating.
2. Evaluate the saw-cut timing and depth, and explain the cracks that crossed the panel centres.
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Depth. A contraction joint has to reduce the section enough to guarantee the crack forms there. The usual rule is one-quarter of the slab thickness: 0.25 × 150 = 37.5 mm. The joint was cut to 25 mm, about one-sixth of the thickness, which often leaves enough uncut ligament for a random crack to form elsewhere.
Timing. The joint must be cut before the tensile stress built up by restrained contraction and cooling reaches the concrete's tensile strength. In hot, dry, windy weather with a 150 mm slab that window is often 4–12 hours; 22 hours is late, and the overnight temperature drop from 34 °C adds thermal contraction on top of drying shrinkage.
Panel size. The common guide for joint spacing is 24–36 times the thickness, i.e. 3.6–5.4 m here. At 5 m the panels sit at the top of that range with no margin for the severe conditions.
Interpretation. Random cracks crossing the panel centres mean the concrete found a weaker plane than the sawn joint. That is consistent with both the shallow cut and the late timing, and it is not evidence of a mix problem.
3. State what the correct curing and finishing regime should have been, and whether the dusty surface can simply be coated over.
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Finishing. Delay bull-floating and troweling until the bleed sheen has disappeared and the concrete will take the trowel without tearing; use fog spray or an evaporation retarder to hold the surface between operations. Do not power-trowel while water is still rising.
Curing. In these conditions, erect windbreaks, fog the slab immediately behind the screed, and start curing as soon as the free water film has gone - a spray-applied compound at the manufacturer's coverage rate, applied in two coats at right angles, or wet burlap kept wet for at least 7 days. A compound sprayed 4 hours after finishing onto a surface already drying at 1.1 kg/m²/h is applied too late and may be diluted by residual water.
The dusty layer. No. The 3 mm weak layer is a low-strength, high-porosity skin with poor tensile bond; any coating or topping applied over it will delaminate. The correct treatment is to evaluate the depth by core or by bond test, then mechanically remove the weak layer (grinding or scarifying) to sound, well-bonded concrete before any protective or wearing system is applied, and to verify the substrate by pull-off testing.