Moisture in Concrete - Part 1

This post focuses on the moisture phenomenon in concrete and the difference between lightweight and normal-weight structural concrete. Curing versus drying and the standards used to determine relative humidity levels are also addressed. Part II will address design recommendations and roof assembly selection.

Moisture in Newly Poured Structural Concrete Roof Decks
When investigating roofs for leaks, invariably, moisture is found beneath the roof membrane. However, the source of moisture is not always a roof leak. Newly poured structural concrete could be a contributor to the presence of moisture beneath a new or a replacement roof.

Concrete is a mixture of several components that reaches its optimum strength through a chemical reaction induced by water. Concrete needs water to allow for flowability and workability, however, water also has adverse effects. Once the concrete has cured, the remaining water is considered “free water”, or moisture which is no longer consumed by the curing process. Rain and snow add moisture to exposed concrete roof decks and further prolong the drying. As an example, a 4” slab of structural concrete contains as much as 200 gallons of free water per 1,000 square feet.

Structural Concrete Mix Ratio
The ratio for both normal-weight and lightweight structural concrete (LWSC) is generally the same:
• 10-15% cement
• 60-75% aggregate (fine and coarse)
• 15-20% water

The difference is in the aggregate; the lightweight aggregate is pre-saturated prior to mixing. The lightweight aggregate, which is made up of shale, slate slag, or clay, can absorb 5-25% of its mass. Normal-weight structural concrete, however, utilizes aggregates such as sand and stone, which are not as porous and do not need to be wetted before adding to the mix.

The popularity of LWSC is increasing due to:
• Lower building structural cost;
• Lesser density for reduced dead loads; and
• Environmental and sustainability claims.

Drying Time
To reach a 75% relative humidity for normal-weight structural concrete, it will take approximately three months. However, achieving the same 75% relative humidity for LWSC will take twice as long. According to the Portland Cement Association, the dry-down time for LWSC is more than normal-weight structural concrete.


Standards for Moisture Testing
For many years, the roofing industry has used a curing time of 28 days after the concrete is poured. However, there are test methods published by ASTM for determining the moisture content in concrete.

Qualitative tests, such as the plastic sheet test and electrical resistance and/or impedance are good indicators of the presence of moisture in a given area but are not as accurate as quantitative tests.

Quantitative tests, such as the moisture vapor emission rate test, surface humidity, or in-situ relative humidity tests demonstrate levels of moisture present in the concrete.

The recommended quantitative test is the in-situ relative humidity test (ASTM F2170), in which a sleeved probe is placed in a drilled hole in the concrete and left in place for 24 hours. After the 24 hours, an electronic reader is attached, and the information is read directly from the sensor. The relative humidity reading should be less than 80% at a depth of approximately 40% of the thickness of the slab.

The moisture values and test duration stated above have been slightly modified to better suit outdoor roof conditions. 

Site Considerations
The concrete pour schedule can affect moisture testing and provide inaccurate moisture values. Therefore, in phased construction, the field testing and the roofing installation should be aligned with the concrete pour schedule and ICRI-Certified Concrete Inspectors should be commissioned.

For in-depth information, the International Concrete Repair Institute (ICRI) offers various resources that can aid with the proper steps required for testing and evaluation. Stay tuned for part II for recommendations on design and the selection of an appropriate roofing assembly.