Concrete vs. Cement Part I
One of building’s biggest banes 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. In construction, concrete slab moisture problems result from many possible mistakes. One is a common misunderstanding of the difference between concrete and cement.
Concrete: Cement’s Daughter
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 cements 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 cements 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) cements have 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 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.
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. Relative humidity 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 relative humidity in order to avoid concrete slab moisture problems.
Tune in to part II for more on the Rapid RH concrete moisture test.
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