THE SCREED SCIENTIST
CRACK CONTROL
Whilst screed cracking is a natural consequence of the drying process, it can also be a sign of more worrying, structural problems that can lead to screed failure. This month the Screed Scientist focuses on the most common causes and proactive prevention of screed cracking.
Causes of cracking fall into five main categories:
Moisture evaporation Moisture evaporates from the surface of the screed faster than from the deeper layers. As a result, the surface shrinks more than the interior of the screed, leading to surface cracks. Differential drying shrinkage is enhanced by hot, dry weather and excessive ventilation. To prevent cracking, screeders use polyethylene sheets to cover freshly laid screeds and control moisture evaporation, or opt for fast-drying or anhydrite screeds instead of traditional screeds. More severe cracking, called reflective cracking, can occur if the screed is laid over a damp
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concrete slab. To reduce the risk of severe structural problems and surface cracking, any cracks observed in the slab must be repaired using appropriate materials and techniques before installing the screed.
Water content in the screed mixture Incorrect water-to-cement ratios not only make the screed more prone to cracking but also affect its performance in service. That is because screeds with excessive or insufficient amounts of water or cement may fail to develop the strength required to withstand the stresses and loads specified.
Stress Cracks can occur anywhere throughout a screeded area, but one place they
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