Self-Desiccating Concrete

Moisture-related flooring problems have become a thorn in the sides of many  architects, contractors, and building owners. The failures are typically caused by left-over batch water (known as water of convenience) or moisture vapor from the subgrade. Groundwater is rarely a significant factor. The excess moisture vapor mixed with highly sensitive (that is, water-tight) floor coverings creates a situation where flooring materials blister, bubble and delaminate entirely.

Possible solutions now include a rapid-drying concrete that desiccates 3 times faster than standard concrete. Through a mixture of low w/cm ratio batching and sophisticated cement chemistry, less batch water is needed to mix and hydrate the cement.

Using less water has two effects: Firstly, less water in the beginning of batching means less left-over water after placement. Secondly, less water means a tighter capillary     matrix in the cement and, therefore, the concrete is less permeable; so less moisture vapor is able to migrate through the concrete from the subgrade.

In some cases, specially blended cements can hydrate more efficiently and thereby consume the available batch water more quickly, which provides a head-start to drying.

The rule-of-thumb for drying time of 4-inch thick, 0.45 w/cm ratio concrete sealed on the bottom is 90 days. Fast-track construction schedules frequently provide less than 90 days for concrete drying. Rapid drying concrete offers a product that can meet such aggressive schedules.

Regardless of the type of concrete, ACI 302.1R requires that slabs be placed directly onto a vapor retarding membrane that conforms to ASTM E1745.

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