When is additional precaution required to prevent plastic shrinkage cracking?

Additional precautions are required to prevent plastic shrinkage cracking primarily when bleed rates exceed evaporation rates. This situation indicates that while the concrete is still in its plastic (unset) state, it is losing moisture faster than it is able to bleed, which is crucial for maintaining its workability and preventing shrinkage stresses.

When bleed water is insufficient due to high evaporation, the surface of the concrete can dry out quickly, leading to shrinkage as the material hardens. This can create tensile stress that may result in cracks forming on the surface as the concrete continues to cure. Therefore, recognizing when the conditions allow for evaporation rates to surpass bleed rates is vital for deciding on additional measures such as using curing compounds, windbreaks, or even shading the concrete during hot weather.

In contrast, when evaporation rates are lower than the bleed rate, or during higher humidity levels, the risk of shrinkage cracking is considerably reduced, as there is enough moisture for the concrete to remain workable and for the bleed water to maintain surface hydration.