Moisture in Concrete: Sources, Transport, and Effects
Why moisture matters
This domain tests the physics behind concrete moisture: where water comes from, how it moves through the pore system, and how it damages finished floor systems. The exam favors mechanism questions, not just definitions.
Sources and transport
- Mix water and curing water are the initial sources; water/cement ratio controls porosity and the amount of free water.
- External sources include rain, groundwater, slab-on-grade wicking, leaks, and wet construction activities. A broken or missing vapor retarder increases upward vapor flow.
- Water moves through capillary pores by capillary action, through diffusion as vapor, and by permeance under a vapor-pressure gradient. Evaporation dries the surface first, so the interior can remain wet long after the top feels dry.
- Drying is slow in thick slabs and in slabs with a vapor retarder below; one-sided drying is slower than two-sided drying.
Effects on floor systems
Moisture can re-emulsify water-based adhesive, blister coatings, promote efflorescence and mold, and cause dimensional movement or disbondment. The risk depends on the floor system's sensitivity, not only on the slab's total water content.
Worked example
A 150 mm slab-on-grade without a vapor retarder shows a dry surface at 28 days, but an in-situ test at 60 mm reads 90% RH. The dry surface is explained by evaporation; the interior is still wet. Because the slab dries mainly upward, a vapor retarder would not stop the internal water from affecting a moisture-sensitive adhesive; the floor covering should wait for documented drying and a passing test.
Traps
Do not equate dry to touch with low RH. Do not report moisture content as RH. Do not assume a thick slab dries faster. Do not ignore curing compounds or a missing vapor retarder.