Plastic Shrinkage Cracking on an Exposed Slab
A crew is placing a 4,000 sq ft exterior slab on grade at 2:00 p.m. in July. Air temperature is 92°F, relative humidity is 28%, and there is a steady 15 mph wind across the pour. The mix is a 4,000 psi air-entrained mix with a 0.52 w/cm ratio, placed at a 5-inch slump. The contractor plans to bull-float, then wait for bleed water to disappear before troweling. By the time finishing begins, the crew notices short, parallel cracks running perpendicular to the wind direction, spaced roughly 12 to 18 inches apart. The cracks appeared before any control joints were sawed.
1. What is the most likely cause of the cracking, and what evidence in the scenario supports your diagnosis?
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The pattern - short, parallel cracks perpendicular to the wind, appearing before joints were cut - is the classic signature of plastic shrinkage cracking. The driver is rapid surface evaporation while the concrete is still plastic. Evaporation rate depends on air temperature, humidity, wind speed, and concrete surface temperature; at 92°F, 28% RH, and 15 mph wind, the estimated evaporation rate is well above the commonly used 0.2 lb/ft²/hr threshold at which precautions are recommended. The low humidity and wind pull moisture from the surface faster than bleed water can replace it, and surface tension in the capillary pores tears the weak plastic paste. The cracks are not from drying shrinkage (too early, wrong pattern), not from thermal cracking (would appear later and typically run through the section), and not from structural overload. The fact that they formed before sawing rules out joint-related random cracking.
2. List the specific actions the crew should have taken before and during placement to prevent this, and rank them by likely effectiveness for this pour.
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Preventive measures, ranked by impact for this hot, dry, windy pour:
- Evaporation control - windbreaks (to cut the 15 mph wind), fogging/misting above the surface, and/or an evaporation retarder applied between finishing passes. This directly attacks the mechanism.
- Timing - place earlier in the day or at night to avoid peak temperature and solar load. This is the single biggest lever but is a scheduling decision, not a field fix.
- Cool the concrete - chilled water, ice, or shaded aggregate to lower concrete temperature and slow evaporation and set.
- Cure immediately - apply curing compound or wet cover as soon as finishing allows; do not let the surface sit exposed.
- Reduce slump - a lower-slump mix bleeds less but also has less water to lose; use only in combination with the above, not as a substitute.
- Saw/score early - not a substitute for evaporation control, since these cracks form before joints can be cut. The most effective combination is windbreak + fogging + evaporation retarder, with immediate curing once finishing is complete.
3. The cracks are hairline and appear only at the surface. Should the slab be removed and replaced? Justify your answer with the criteria you would apply.
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Not automatically. Plastic shrinkage cracks are typically shallow and often heal or self-seal with curing and traffic, but the decision hinges on depth, width, and service conditions:
- Assess depth - chip or core a representative crack. If it is confined to the top 1/4 inch and does not reach reinforcement, structural capacity is not compromised.
- Assess width and activity - cracks that are tight and stable are cosmetic; wide or propagating cracks suggest a deeper problem.
- Service exposure - for an exterior slab exposed to freeze-thaw and deicing salts, even shallow cracks can admit water and accelerate deterioration; sealing is prudent. For a protected interior slab, cosmetic repair may suffice.
- Repair options - route and seal with a flexible sealant, or use a penetrating sealer/crystalline product for tight cracks; do not simply trowel over them, which can trap debris.
- Document and monitor - record locations and widths, and re-inspect after a season. Replacement is justified only if cracks are through-section, widespread, or compromise durability or structural function. The correct first step is investigation, not demolition.