In Part 2 of this series, the concrete construction industry’s standard methods of curing were described. As a reminder, they are:

• Water Curing by ponding, sprinkling, or immersion
• Curing By Covering with wet burlap, plastic film, or reinforced paper
• The application of Membrane-Forming Curing Compounds or Cure & Seals

Each method has its benefits and drawbacks, and may or may not be appropriate for a specific project. But they all perform very well when implemented properly, as proven by decades of durable concrete in service having been cured by these methods.

When analyzing these standard curing methods, two commonalities are noticed: all three are applied externally to the concrete surface, and all involve the application of a material (water, coverings, or membrane-forming liquids) that lay on the concrete surface and slow the evaporation of water from the bulk concrete. This next part of the Demystifying Curing series will present three “non-standard” methods for curing concrete. One is done from the inside out, another with penetrating, chemically reactive materials – and one method that doesn’t involve any curing at all.

Internal Curing

The first thoughts on internal curing were published in the 1957 Proceedings of the World Conference on Prestressed Concrete. Paul Klieger, who was an outstanding concrete technologist with the Portland Cement Association, wrote that “lightweight aggregates absorb considerable water during mixing which apparently can transfer to the paste during hydration.” Today the American Concrete Institute (ACI) defines internal curing as a “process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water.”¹ This is accomplished by replacing some of the fine or intermediate aggregate with pre-wetted lightweight aggregate. Some call this curing from the inside out.

Cementitious materials in concrete consume water within the slab or element as they chemically react and the concrete hardens. If the concrete has a lower w/c ratio (think high performance mixes), this reaction requires more water than is available. When this happens, the concrete dries out, strength gain ceases, and the surface begins to develop shrinkage cracks.

This situation can be avoided by using pre-wetted lightweight aggregate to supply the needed additional water.  The mechanics that enable this transfer of water are fascinating! As the cementitious material hydrates, open pores develop in the paste. (These tiny voids are what cause the internal stresses that result in chemical shrinkage.) Because the water-filled pores within lightweight aggregate are generally larger than the empty pores within the surrounding paste, capillary pressure develops in the paste pores that enable moisture to be drawn out of the wetted lightweight aggregate and into the pores, keeping them saturated. This allows the cementitious paste to continue to hydrate, gain strength, and fight off shrinkage and other deleterious effects of premature drying. Internal curing can yield a more robust concrete, especially for low w/c designs.

However…

Internal curing does not replace conventional surface curing! The requirement for water retention at the concrete surface still exists when internal curing is used. Along with its use in high-performance mixes, internal curing used in conjunction with surface curing can also help compensate for less than ideal environmental conditions during placement, and is especially beneficial in concrete containing supplementary cement materials, since fly ash and slag require more water to react vs. portland cement.

Reactive Silicate Solutions as Cures

The practice of treating concrete with a silicate solution has been performed for many years; silicates were used to quickly strengthen the surface of new concrete runways on military bases during World War II. Silicates reduce concrete surface porosity, help prevent dusting, and increase wear resistance. When applied to already cured, sufficiently aged concrete, a chemical reaction occurs between the silicate and excess calcium hydroxide present within the concrete surface to produce calcium silicate hydrate (C-S-H), the same binder that results from adding water to cement and gives concrete much of its strength and durability. This newly created C-S-H is deposited primarily in the pores within the surface of a slab, leading to the improved surface toughness.

Silicate solutions are water based, typically have zero VOCs or odor, leave no residue when applied correctly and dry quickly. You can see why silicate solutions are attractive to use as curing materials! Proceed with caution, however, as the current concrete industry specifications and guidelines on curing do not accept silicates as curing materials. Although subcommittees within ASTM and ACI have formed with the intent of studying this subject in depth, the current documents still state:

“Silicate solutions are chemically reactive rather than membrane-forming, therefore, they do not meet the intent of this specification.” – ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete

Also:

“Products in this group are not specifically formulated for curing applications and do not meet the requirements of either ASTM C309 or ASTM C1315 for liquid membrane-forming compounds. Whereas their use may offer some desirable benefits when applied after curing, they should not be applied on fresh concrete for the purpose of curing.”  - ACI 302.1R-15, Guide to Concrete Floor and Slab Construction

That being said, there are many silicate solutions marketed today as suitable curing materials for fresh concrete, and many concrete placements have been treated with a silicate solution in lieu of a standard curing method, and these floors are strong and durable, having been in service for years. When deciding whether to use a liquid curing material that doesn’t comply with industry standards, consider the environmental conditions expected during placement; hot and/or windy conditions prescribe an extremely effective, industry-proven curing method. Also, the success of high-performance concrete mixes with a low w/c ratio relies on good, dependable curing. Finally, keep in mind that curing concrete with materials tested and proven to comply with industry standards provides a bit of insurance and “backup” should problems with the concrete arise later.

Passive Curing

All of the curing methods discussed in this series have been deliberate actions taken, or methods put into place. There is a concept called “passive” curing as well, sometimes called “natural curing” or “field curing” when it pertains to cylinders or other test specimens prepared on the job site. These are all different ways of saying that no deliberate curing is being done at all. ACI’s Guide to External Curing of Concrete (ACI 308R-16) provides some advice on this subject:

“Under conditions that prevent excessive moisture loss from the concrete, or when the required performance criteria for the concrete are not compromised by early moisture loss, it is entirely possible that no deliberate action needs to be taken to protect the concrete. The best source for guidance on the impact of ambient conditions on hardened concrete properties would be field experience with environmental conditions and the concrete mixture in question. Note that in most environments it is unlikely that favorable, natural conditions will exist for the duration of the curing period. The contractor should therefore be prepared to initiate curing measures as soon as favorable ambient conditions change.” 

It’s also been proposed that hard-troweled, dense concrete floor finishes act as a curing layer and retain slab moisture sufficiently. The article “Self-Curing Warehouse Floors?” written by Joe Nasvik and published in Concrete Construction August 2010 is an interesting read on this subject. In the article the author asks, Are curing compounds necessary for interior warehouse floors? Does a hard-troweled, “self-curing” slab surface provide a serviceable finish with longevity equal to a traditionally cured surface? The article reports on a study of one warehouse floor where concrete was placed inside a building under controlled, favorable ambient conditions. Permeability studies of this floor indicated the surface of the concrete met the ASTM requirements for water vapor transmission of a curing membrane. And based on other floor samples studied, less densely troweled floor surfaces are not as effective at retaining moisture. More work and research is needed on this subject to come to a conclusion – but the concept is very thought-provoking, indeed.

In the fourth and final installment in the Demystifying Concrete Curing & Sealing series, we’ll focus specifically on curing compounds and cure & seals, how to choose the right product for a job and how to troubleshoot and solve problems that sometimes occur when they are used.

Reference

1. ACI 308-213R-13, Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate