Proper curing is a critical component of manufacturing quality precast concrete.
By Debbie Sniderman
Editor’s Note: This is the second article in a four-part series on materials that enhance the performance of concrete. This segment covers liquid curing compounds. Future segments will cover fly ash and accelerators.
Click here to read Material Matters: Fly Ash.
An important factor in achieving the ultimate strength, durability, watertightness and surface hardness of concrete is proper curing. While there are several ways to cure concrete, methods that involve complete submersion or curing chambers with humidity, temperature or moisture control aren’t feasible for large precast structures.
Liquid curing compounds are the most common way to cure precast concrete structures stored outside. They seal the surface of freshly produced concrete as soon as it’s stable to prevent the rapid loss of moisture during the early hardening period. As an easy-to-apply method, liquid curing compounds offer the precast concrete industry real benefits.
Liquid curing compounds are applied to fresh concrete as soon as the final finish is completed, when the most water is present. They retain the mix water so the cement can more fully react with it by slowing the rate at which water leaves the concrete. This allows more time for the cement to stay fully hydrated and react, providing a higher ultimate strength.
There are two types of curing compounds. Permanent, impervious curing compounds are used on finished products that won’t have any future coatings. They fill all of the pores on the concrete’s surface and form membranes that don’t allow anything – such as paint or other coatings – to bond to the surface.
Temporary curing compounds chemically react or oxidize when exposed to the sun and gradually wear away over time without staining or discoloring the concrete surface. Most dissipating compounds are chemically formulated to wear away after 28 days by design. According to Dianne Carey, director of technical services at W. R. Meadows, the longevity of a temporary membrane depends on the amount of exposure to foot or wheel traffic.
Although there are water-based and solvent-based curing compounds, Carey explained that the majority are water-based due to recent VOC and solvent restrictions. Water-based compounds are emulsified resins or waxes and contain proprietary additives that make them compatible with water. These compounds are sprayable and have a consistency like paint.
How they work
According to Kenneth Kruse, admixture systems industry director at BASF Corp., liquid curing compounds provide the maximum degree of water retention in the most economical and foolproof method. Liquid curing compounds are applied by rolling, brushing or spraying, and can be applied indoors or outdoors.
The coatings work by allowing water vapor to pass through them at a controlled rate determined by the chemistry and thickness of the application. Kruse said that when properly sprayed on, liquid curing compounds form a protective membrane that is about .002 of an inch thick and are capable of maintaining a minimum of 95% of the original moisture content in a concrete mix, which he said is an American Concrete Institute standard.
Curing compounds can be white pigmented or clear, and the clear compounds can be wax-resin or all-resin types. White pigmented compounds reflect more ultraviolet rays than non-pigmented curing compounds and reflect more heat to maintain temperature. This is important during initial curing, when the strength of the concrete is not sufficient to resist thermal stresses produced by extreme temperatures.
For non-pigmented curing compounds, wax–resin is suitable for concrete that will not subsequently be painted, tiled or treated in any manner. All-resin compounds can be used when subsequent treatments will be applied, since they don’t leave any adverse residues on the concrete surface.
Why they’re important
The proper curing of concrete prevents rapid loss of moisture and ensures the cement has a complete hydration reaction to achieve desired strength. For precast stored outdoors, where high temperatures, direct sunlight or wind evaporate water faster, it is especially important to take curing methods into consideration.
Proper curing increases strength and improves many other properties. Since the concrete is allowed to continue reacting before the water evaporates, cement particles have more time to touch and fill the air voids inside the concrete. More hardening takes place, producing concrete with decreased permeability and fewer voids. This leads to better freeze-thaw resistance, reduced scaling, and better chemical and abrasion resistance.
Advancing precasters’ work in the plant
Liquid curing compounds provide precasters a fast and convenient way to cure for jobs that have to meet the requirements of ASTM C3091, the specification that sets the amount of moisture loss that comes out of concrete.
Jeff Bishop of Nox-Crete Products Group said that precasters typically use liquid curing agents on products or panels that have large concrete surface areas exposed to air, such as the smooth bottom surface of a septic tank. They also use them on extremely thin sections that may be susceptible to moisture loss.
“Small shrinkage cracks – detectible signs that the surface has dried out too fast – would also lead them to cure with liquid agents,” he said. “The higher strength benefits precasters could gain by using these relatively inexpensive products that are quick to spray on are often overlooked and could reduce other costs. Indoors or outdoors, proper curing is encouraged, and the higher strengths are better for the concrete.”
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company.