concrete slab curing
Concrete is designed to ‘cure’ which is essentially a chemical process called ‘hydration’ where the water reacts with the cement and solidifies, hardens, and binds the paste and aggregates together. Depending on its intended use and the resulting associated engineering principles, concrete can be designed to cure to strengths as high as 10,000psi or more, however the most common strength ranges for exposed architectural concrete are between 3,000 – 4,500psi. Such curing is typically facilitated in one of 2 ways: water curing or by way of a membrane-forming curing compound.
Water curing refers to installing a layer of water over the concrete surface followed by a visqueen or burlap cover, which traps in moisture that is consistently and readily available for use in hydration during the curing stage. Some engineers feel water curing and cover produces a stronger finished product, due to more plentiful water availability for hydration. However the more common means of modern concrete curing is by way of a membrane-forming curing compound due to its effectiveness, economy, and ease of use. Also, if the moisture and cover cannot be maintained for more than 7 days consistently, membrane-forming curing compounds are typically used/specified. If a concrete floor is to receive a hardener, epoxy or urethane sealer, or a water repellant – water curing is typically done, unless the curing compound is intended to be stripped after curing.
A membrane-forming curing compound can be rolled on but it is most commonly sprayed on with a hand held pressurized sprayer, often called a ‘Chapin’ sprayer. Over application of membrane curing compound can cause puddling which leads to long term streaking and surface discoloration. Under application can lead to water escaping the concrete and in turn concrete that doesn’t reach its intended strength, making it more susceptible to deterioration and uncontrolled cracking.
Membrane-forming curing is the utilization of a membrane (typically water based or solvent based) brushed or sprayed onto the finished concrete surface which locks in the moisture from escaping during the hydration period. Some membrane-forming curing compounds are white pigmented to help the installer identify which areas have been covered and also to reflect sunlight exposure. Also, some resin based compounds are often available which will typically oxidize and wear off if exposed to sunlight and heavy traffic. Non-resin curing compounds will often require stripping by way of light sand-blasting, chemical application, abrasive brushes, or light pressure washing. A rule of thumb to determine if the membrane curing compound has wore off is to place a few drops of water on to the surface and observe if the water absorbs into the concrete. If absorption occurs, the curing compound is typically sufficiently stripped.
Wet burlap cloth is another form of curing concrete slabs and structures. The cloth is kept continuously damp providing a good environment for hydration.
Lastly, visqueen over a water flooded slab is another way to cure or promote good slab hydration.