Concrete vs. Cement
In construction, concrete slab moisture problems result from many possible mistakes. One is a common misunderstanding of the difference between concrete and cement.
Cement Is Simply an Ingredient of Concrete
The most basic concept to understand is that cement, by itself, will not produce concrete. Concrete is formed by mixing cement, aggregate, and water. The term “concrete technology” characterizes the myriad ingredient proportions which blend into a concrete mix. Each of the three ingredients performs specific roles.
Cement reacts chemically (called hydration) when mixed with water and forms a paste. Aggregates come in coarse or fine grains and can originate as gravel, stone or crushed concrete. Paste and aggregate draw together the thick liquid form of what will harden into a concrete slab. Concrete slab moisture problems are also formed this way.
Portland cement is considered one of the hydraulic types of cement because it will harden regardless of surrounding moisture conditions. (The binding chemical reactions will take place even under water.) There are eight different forms of Portland hydraulic cement, and each one has specific applications.
Type I and Type IA: general-purpose cement suitable for all uses where the special properties of other types are not required.
Type II and Type IIA: contain no more than 8% tricalcium aluminate (C3A) for moderate sulfate resistance.
Type II(MH) and Type II(MH)A: Type II(MH) cement has a moderate heat of hydration characteristic (the sum of C3S+4.75 C3A <100%), and moderate sulfate resistance (a maximum C3A content of 8%).
Type III and Type IIIA: chemically and physically similar to Type I, they are ground more finely to produce higher early strengths.
Type IV: used in massive concrete structures which require minimum rates and amounts of heat generated by hydration. It develops strength more slowly than other cement types.
Type V: contains no more than 5% C3A for high sulfate resistance.
Two key properties underlie concrete mixes that owners want: strength and durability. The engineered properties of a calcium-silicate-hydrate (C-S-H) gel are the most important influence on the setting, hardening, and strength of concrete. At the microscopic level, C-S-H structure determines the concrete’s durability over time.
Drying vs. Curing
One of building industry’s biggest problems is our expectation for concrete to hurry up and dry so we can get on with construction. It’s like telling a child to hurry up and learn. Problems ensue.
Because of the different types of cement mixtures possible to form concrete, concrete quality control is all about moisture measurement of the water vapor within the slab.
While the new floor is curing and drying (two different processes), contractors measure moisture content as relative humidity (RH) by placing probes inside drilled holes in the slab.
Each probe has a sensor which feeds RH data to a hand-held meter. RH is displayed by percentage, and RH can vary in different locations and depths of any concrete slab. Concrete slab moisture problems occur when the slab’s RH has not reached the levels necessary for the chosen flooring or finish application.
Wagner Meters has developed the Rapid RH®: an RH test method that accurately and cost-effectively assesses concrete moisture conditions. Once probes are inserted, they must equilibrate with the slab’s conditions before the testing begins.
Unlike its competitors, the proprietary Rapid RH Smart Sensor reaches equilibration in about 1 hour. Other such meters require up to 24 hours before testing can begin. With such immediacy, building professionals can accurately assess RH in order to avoid concrete slab moisture problems.
The concrete slab has been poured. Cement is chemically reacting with water, a process called hydration, and the resulting paste is bonding with aggregate in the mixture. Done well, concrete production goes a long way towards resolving many potential concrete slab moisture problems.
Water’s Okay… to a Point
The presence of water in concrete is as natural as the presence of kids in a playground. In fact, moisture remains in concrete slabs even after reaching their dry state. Concrete continues to exude water vapor in low relative humidity (RH) ambient conditions, and absorb it when ambient RH is high.
Moisture migration will be an ongoing reality until the slab is sealed. It’s after a slab is sealed that moisture mitigation can become problematic.
However, one must abandon the idea that water in concrete is inherently bad; its presence does not necessarily indicate an onset of concrete slab moisture problems.
Water is nature’s liquid influence on slabs. Concrete slab moisture problems result when the moisture conditions in the slab have not been measured accurately before the finish or flooring application is installed, or when moisture is allowed to enter a finished slab through groundwater wicking or seepage from another source.
Fortunately, concrete moisture measurement is a matter of simple metric assessment. Technology inspired Wagner Meters to develop their Rapid RH®: a cost-effective and reliable way to accurately measure slab moisture content during concrete installation. The Rapid RH® measures the RH of moisture at all levels of a slab.
Installers place in situ probes (containing Smart Sensors) into the slab. Within an hour, contractors can deploy moisture measurement at 40 percent depth of the slab to within a workable measurement for finishes and flooring.
(Note that while ASTM F2170 requires 24 hours equilibration for documentation, the accuracy of the Rapid RH® allows decisions to be made in a more timely manner for cost and materials optimization.) This ensures quality control.
The Preemption of “Hurry up!”
Construction is a tightly-timed and rigorously-budgeted endeavor. Time is seen as the ultimately unjustifiable building expense. However, Rapid RH® is one tool which offers options. If a Rapid RH® moisture measurement shows uneven levels of concrete readiness, installers can dehumidify the wetter sections of the slab surface or they can apply concrete surface membranes. However, savvy building professionals can preempt those options in two ways.
First, ensure proper mix specifications, starting with .5 w/c ratio and a LIGHTLY troweled surface. Second, get the building enclosed as quickly as possible to allow the concrete more consistent conditions.
If concrete slab moisture problems represent impending doom to successful installation, Rapid RH® moisture measurement represents their ability to identify those problems, before they happen.
Are you looking for a concrete calculator to estimate how many cubic feet and cubic yards of concrete you will need to fill your space? Try out our concrete calculator.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.
Last updated on September 26th, 2023