PART 2: DESIGN AND CONSTRUCTION OF SLAB-ON-GROUND: APPLYING ACI 318

By Eamonn Ryan, derived from an ACI podcast

…continued from Part 1.

Understanding exposure categories and moisture levels

ACI 318 outlines several exposure categories (W, S, C) based on the presence of water and water-soluble sulfates, each with varying classes from dry (Class 0) to severe exposure (Classes 1, 2, 3). These classifications dictate the permissible moisture levels in contact with concrete slabs-on-ground. Effective waterproofing solutions become crucial when dealing with rising water tables or hydrostatic pressures, ensuring that moisture-sensitive concrete remains protected from detrimental effects.

Moisture transmission through concrete slabs involves a complex interplay of vapor movement from the ground upwards, potentially condensing beneath the slab due to dew point conditions. This transmission can introduce water-soluble sulfates to the slab’s surface, highlighting the need for effective vapor retarders. These materials slow down vapour transmission, reducing the likelihood of moisture ingress and subsequent damage to the concrete. Current standards suggest a permeance of 0.01 perms or less for materials to be classified as vapor barriers, underscoring their importance in moisture-sensitive applications.

Vapour retarders play a crucial role in impeding the transmission of moisture vapor through concrete slabs. By effectively slowing down vapor movement, they create a barrier that prevents moisture from reaching critical levels beneath the slab. As vapour reaches the underside of the retarder, it can condense into liquid due to dew point conditions. This condensation occurs beneath the retarder, maintaining a sulfate solution away from direct contact with the concrete slab.

A key consideration in concrete durability is managing sulphate exposure, particularly in soils where sulphates are water soluble. These compounds do not travel through vapour; they only pose a risk when dissolved in liquid water. Effective vapour retarders ensure that any sulphate-rich water beneath the slab remains isolated, thereby achieving a desirable sulphate exposure class, such as S0, as per ACI standards.

For decades, a common misconception suggested that placing concrete directly on vapour retarders could lead to curling or warping. Tarr dispels this myth based on extensive observations spanning two decades. Contrary to belief, concrete slabs placed on vapour retarders consistently exhibit less curling compared to those without. This phenomenon occurs because vapour retarders prevent the bottom of the slab from constant rehydration and saturation. Without a vapour retarder, moisture from the ground continues to permeate upwards, maintaining the underside of the slab at high humidity levels.

The primary benefit of using vapour retarders lies in stabilising moisture differentials within concrete slabs. By allowing the underside of the slab to remain drier, vapor retarders minimise the differential moisture content between the top and bottom surfaces. This reduction in moisture differential directly correlates with reduced curling and warping tendencies in concrete slabs, particularly crucial in industrial and commercial applications.

Recommendations and conclusion

Tarr concludes with a strong recommendation for incorporating vapour retarders in industrial floors, warehouse settings, and office environments. These applications benefit significantly from the controlled moisture environment provided by vapour retarders, ensuring long-term durability and performance of concrete slabs.

In essence, vapour retarders are indispensable tools in modern concrete construction, safeguarding against moisture-related issues and enhancing the overall longevity of concrete structures. These insights underscore the importance of informed construction practices and adherence to ACI guidelines for achieving resilient, high-performance concrete slabs.