Hot, Windy Slab With Early Troweling
A 600 m², 150 mm thick slab on grade is being placed in mid-afternoon. The air temperature is 35°C, the concrete temperature is 32°C, the relative humidity is 20%, and the wind is 25 km/h. The mix has a 100 mm slump and no evaporation retarder. The finishers strike off and wait for bleed water to disappear, but the surface crusts quickly. To keep up, they begin power troweling while a sheen is still visible and then apply a curing compound at about 1.5 hours after finishing. By late afternoon, numerous short, roughly parallel cracks appear, mostly perpendicular to the wind direction. The owner wants to know whether the cracks are structural and whether the slab can be accepted.
1. Assess the evaporation risk. What does the combination of conditions indicate, and how should the cracking pattern be interpreted?
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The conditions are a classic high-evaporation scenario. Air at 35°C, concrete at 32°C, relative humidity of 20%, and wind at 25 km/h produce an evaporation rate well above the 1.0 kg/m²/h level at which severe plastic shrinkage risk is expected. A nomograph or the ACI evaporation formula would place this pour in the high-risk range, probably around 1.2 kg/m²/h or more. The exact value depends on the calculation method, but the conclusion is the same: the surface can lose water faster than bleed water can replace it.
The crack pattern supports plastic shrinkage rather than structural overload. Short, parallel cracks perpendicular to the wind direction are typical of rapid surface drying and restrained shrinkage while the concrete is still plastic. Structural cracks from loading or settlement would generally be wider, less regular, and related to supports or construction joints. However, the pattern alone is not proof; crack depth, location, and the absence of load are part of the assessment. The cracks should be mapped and checked for depth. If they are shallow and surface-related, they are durability and appearance defects, not necessarily structural failure.
2. What should have been done before and during the pour to prevent the cracking?
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Before the pour:
- Consider a night or early-morning placement to avoid the peak temperature and wind.
- Cool the concrete: chilled water, ice, or shaded aggregates to lower the concrete temperature.
- Use a mix with adequate bleed capacity and consider an evaporation retarder or finishing aid.
- Set up windbreaks and shade, and plan fog spraying for the period between strike-off and finishing.
During the pour:
- Work in a sequence that keeps the exposed area small; do not strike off the entire 600 m² before finishing begins.
- Apply an evaporation retarder by fog mist as soon as the surface is placed, and reapply if needed.
- Use fog spray to maintain a humid layer, but do not add water to the surface to improve workability.
- Finish as soon as the concrete can support the trowel, and do not trowel over bleed water or a water sheen.
- Start curing immediately after finishing. A curing compound may not be enough in these conditions; wet burlap, plastic sheeting, or a combination may be needed.
The reasoning is to reduce evaporation, slow surface drying, and avoid finishing operations that draw water to the surface or trap it under a dense troweled skin.
3. The cracks are later found to be 2-5 mm deep and do not follow through the slab. What repair and acceptance position is reasonable?
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Shallow, plastic-shrinkage cracks of 2-5 mm depth are a surface durability and appearance issue, not a structural capacity issue, provided that the depth is confirmed by coring or crack-depth measurement at representative locations and that no cracking extends to the reinforcement or through the section. A reasonable position:
- Map and measure the cracks. Confirm they are limited to the surface zone.
- Clean the cracks by wire brushing or compressed air and fill with a low-viscosity epoxy or a suitable cementitious crack filler, depending on whether the slab will be coated or exposed.
- If the cracks are numerous and tight, a penetrating sealer or a thin polymer-modified overlay may be more appropriate than chasing each crack.
- Check the curing regime and the surface hardness. If the surface is dusting or weak, the repair may need to remove the weak layer.
- Acceptance should be based on the specification: crack width and depth limits, watertightness, and appearance. Document the repair and any reduced service life.
The reasoning is that the cracks affect durability and appearance, not load capacity, but they must be treated because they provide ingress paths. The owner should receive a documented assessment, not just a cosmetic patch.