Essential Monitoring: Relative Humidity and Moisture in Concrete Construction
Throughout the construction of a concrete building, one pivotal phase in flooring application is the meticulous monitoring of relative humidity (RH), moisture levels, and the slab’s moisture evaporation rate. Neglecting this aspect could lead to catastrophic flooring failures due to excess moisture and heightened RH levels, ultimately resulting in substantial repair expenses and potential liabilities. But what delineates relative humidity from moisture, and why is it imperative to grasp this distinction?
Relative Humidity (RH) is often mistaken for moisture
Relative humidity (RH) is frequently conflated with moisture, yet these terms assess distinct attributes of concrete. Moisture content denotes the quantity of water in its liquid state, while humidity levels gauge the volume of water in vapor form. In essence, relative humidity delineates the proportion of water vapor present in the air relative to its capacity for saturation.
What is the difference between moisture and relative humidity?
To elucidate the concept of relative humidity, envision a sealed bottle of water:
- If the bottle is full to the brims with water, both the moisture and relative humidity levels register at 100%.
- Upon removing half of the water, the moisture content diminishes to 50%, while RH remains at 100% since the air remains saturated.
- Subsequently, as most of the water is extracted from the bottle, the diminished moisture content precipitates a decline in RH, as the air is no longer fully saturated.
Moisture = 100%
Relative Humidity = 100%
Moisture = 50%
Relative Humidity = 100%
Moisture = 5%
Relative Humidity = 100%
Understanding Relative Humidity in Concrete
Despite appearances, the dryness of your concrete’s surface doesn’t guarantee uniform dryness throughout. Water can persist within the concrete in the form of relative humidity. ASTM F2170 notes that during the drying process, concrete slabs typically have lower moisture levels near the surface and higher levels towards the slab’s bottom. Hence, measuring relative humidity (RH) levels at 40% of the slab’s depth (or 20% if drying on two sides) provides a more accurate representation of the concrete’s overall humidity conditions.
Several factors can impact the moisture content within a concrete slab, resulting in elevated relative humidity (RH) levels that can persist for six months or longer post-pouring.
The sources of moisture in concrete include:
- Wet curing methods.
- Initial water content in the concrete, particularly from a high water-to-cement ratio.
- Ambient conditions, such as high relative humidity, cold temperatures, limited air movement, and additional water sources.
- Utilisation of pozzolans like silica fume, which tend to retain water and prolong drying time.
- Absence or insufficient moisture barriers in slab-on-grade constructions, enabling external moisture infiltration into the slab.
Premature installation and sealing of flooring without adequate drying time can trap excess moisture within the concrete. Consequently, this trapped moisture endeavours to evaporate, resulting in pressure accumulation beneath the flooring, thereby jeopardising material integrity.
Common concrete failures attributable to relative humidity include:
- Delamination of floor adhesives.
- Blistering of epoxy coatings.
- Re-emulsification of adhesives (stain).
- Curling, cracking, or bubbling of flooring materials.
Prevent Flooring Failures with Concrete Relative Humidity Meters
To prevent flooring failures, various test methods are employed to assess concrete moisture levels prior to installation. Since the 1940s, contractors have predominantly utilised surface-based techniques to gauge relative humidity (RH) and moisture content. These methods include anhydrous calcium chloride (CaCl) testing, the hood method, the plastic sheet method, and handheld moisture meters. However, these approaches solely measure surface or near-surface moisture, neglecting RH levels within the slab, leading to unreliable assessments.
Consequently, in-situ RH testing has emerged as the preferred standard for flooring applications. By analysing moisture levels within the slab, more precise RH readings can be obtained, facilitating the determination of the optimal flooring placement time. While probes and wired sensors exemplify in-situ methods, they necessitate drilling multiple holes in the slab and ongoing monitoring, consuming time, and resources. Moreover, drilling alters moisture flow in the concrete, impacting RH measurement accuracy. Consequently, an increasing number of contractors and project managers are adopting wireless relative humidity and temperature sensors, to address these challenges.
Contact Precision Structure Health Sensors and arrange a call with on of our civil engineering and monitoring specialists who can advise you on the right sensor for your project.
