Get the most out of concrete’s many excellent benefits with strict attention to proper curing.
By Evan Gurley
To cure or not to cure? This should never be the question for anyone who has an acting role in producing concrete, whether precast or cast-in-place. Shakespeare isn’t the only analogy that works here; medicine is another. Curing concrete may not involve white coats and waiting rooms; it does, however, involve sound treatment for good health. Without proper treatment (sound curing practice to retain adequate internal moisture), the health of concrete will suffer.
Maturity matters in concrete
Precasters pay so much attention to ensuring raw ingredients meet quality standards, calculating the best mix design and training personnel exhaustively on how to batch, place and consolidate. While these are all essential steps in manufacturing quality concrete, it is all in vain without proper curing.
The world’s project specifiers rely on precast concrete because of its exceptional hardened properties, such as its durability, strength and resistance to harsh environments. Curing defines those properties.
Why is curing often neglected? Our desire for rapid production is usually the No. 1 culprit. Time is money, and very few precasters can afford to leave product in forms for seven days or more. Also, it’s mostly a hands-off process: waiting. Humans often lack patience, especially when time is money, and curing demands serious patience and attention to detail.
Curing is much more than simple hardening due to water and cementitious ingredients undergoing a chemical reaction. Fully cured concrete results from hydration between water and cement (see Figure 1).
During hydration, calcium silicate hydrate (CSH) gel forms and makes the “glue” that enables concrete to harden. CSH binds all of the concrete ingredients together, greatly increasing its strength, watertightness and service life. No glue: no concrete.
Concrete cures to maturity over time, but the rate of curing depends on the mix design and the curing environment (hot, cold, windy, rainy). When we think of curing, we usually think of making sure we keep moisture on the concrete’s surface. But a wet surface isn’t enough. For full design strength and all the service it delivers, concrete must be adequately cured at its surface and deep within its matrix.
Hydration depends on the availability of water and the curing environment. If there is not enough internal water, cement particles remain unhydrated and will not crystallize to form the strong bonds we need (see Figure 2).1
A key to concrete’s strength and durability is not so much the degree of cement particle hydration as it is the degree to which the pores between the particles have been filled with hydration products. In addition, the initial water-cementitious ratio (w/c) plays a major role. Mixes with a lower w/c are better at filling pores, because low w/c mixtures start out with low porosity. Conversely, high w/c mixtures need to work harder to hydrate.
Hydration, therefore, is a direct function of both the w/c and available water. Designers know a low w/c means an inherently stronger, more watertight and durable product. On the other hand, the w/c must be sufficient to provide the cement with enough water to promote a high degree of hydration (see Figure 3).
For the specified mix design strength, the majority of the cementitious materials needs to be hydrated to form the glue required to bind cement with the aggregate. Mixtures with a low w/c (less than 0.40) may require special curing conditions. Low ambient humidity could cause a low w/c mix to dry out to a point where hydration can actually cease. That’s why dry weather requires external water for proper curing. If the internal humidity (relative to the air) falls below 80% within the first seven days, strength and service life may be jeopardized (see Figure 4).
Along with the right w/c, a controlled environment facilitates curing of freshly cast concrete, because hydration is always a thermally dependent process. High temperatures accelerate hydration, and low temperatures slow it down. Ambient temperatures below 50 F (10 C) are bad news, because when concrete falls below 40 F (4.5 C), hydration virtually stops.
Hydration is so dependent on temperature that an increase of just 18 F (10 C) effectively doubles the hydration rate. The higher the curing temperature, the faster the hydration, the greater the strength gain and, in general, the shorter the cure time. Excessive loss of water by evaporation can delay or prevent adequate hydration. Poorly hydrated (cured) concrete results in strength-loss permeability, and vulnerability to freezing and thawing conditions (see Figure 5).
Three stages of wet-cast curing
Curing has typically been viewed as a single-step process. Adequate moisture control, however, is never a simple procedure (see Figure 6).
1. Initial curing: Initial curing occurs between concrete placement and final finishing to reduce moisture loss from the finished surface. In this stage, curing measures should commence immediately after the bleed water sheen disappears, as the surface is protected against drying as long as it is covered with bleed water. If finishing begins immediately after the disappearance of the bleed water, initial curing measures are unnecessary. Initial curing measures are usually needed for concretes that exhibit low or negligible bleeding, such as mixtures with silica fume, fine cements (or other fine cementitious materials), low w/c, high air contents, or water-reducing admixtures. Initial curing reduces the chance of plastic-shrinkage cracking and often includes evaporation reducers (burlap, straw, tarps) and fogging (misting the surface).
2. Intermediate curing: Intermediate curing includes procedures after finishing but before concrete’s final set. This is the period when surface evaporation may need to be reduced but when the concrete is not ready for a plastic or fabric covering (as coverings may damage the surface). If specifications allow, liquid-curing membranes may be used.
3. Final curing: Final curing refers to the procedures executed after final finishing and after final set has been achieved. At this point, final curing applications such as the use of saturated burlap coverings/additional wet coverings or liquid-membrane-forming curing compounds are permitted.
Wet-cast curing methods and materials
Increased cement contents, admixtures and other means accelerate normal curing in precast plants. In addition to these curing accelerators, good physical practices can also accelerate and enhance curing and, therefore, production rates.
Effective curing methods depend on materials used, intended use of hardened concrete, construction methods and, especially, hot or cold weather conditions. Conventional curing entails the continuous saturation of the freshly placed concrete’s exposed surface for a predetermined time. Curing measures should begin when concrete is no longer susceptible to damage at ambient temperatures and the surface begins to dry (as accumulated bleed water evaporates faster than water rising to the surface). 2
Curing methods usually involve:
- Maintaining mixing water in the concrete during early hardening and after final set (fogging or spraying; use of natural, saturated coverings)
- Fogging. Fogging or spraying involves the use of a relatively inexpensive nozzle that atomizes the water into a fog-like mist, increasing the humidity of the air above the concrete surface and reducing the rate of evaporation of the concrete surface. Concrete must be kept continually moist. Fogging is very effective when the ambient temperature is well above freezing and humidity is relatively low. Water from fogging should not be worked into the surface and should be removed or allowed to evaporate before finishing. Fogging or spraying is an effective way to minimize plastic shrinkage cracking but obviously requires a good source of water and close attention.
- Natural coverings. When burlap, cotton, straw or other moisture-retaining fabrics are used, they should be kept saturated at all times and free of deleterious (oil, excessive dirt, abrasive metals) substances. As soon as the freshly placed concrete has hardened adequately, wet coverings may be applied. Coverings should be kept consistently moist to avoid cycles of wetting and drying that can lead to surface crazing or cracking. While a seemingly simple procedure, coverings are said to be one of the most effective curing methods.
- Reducing the loss of mixing water from the freshly placed exposed concrete surfaces (plastic or polyethylene sheeting, membrane-forming curing compounds, internal moist curing with forms left in place)
- Plastic sheeting. Polyethylene sheeting is an effective way to cure concrete by retaining moisture and heat on freshly cast concrete. Plastic film used as a moisture barrier for curing concrete should comply with ASTM C171,3 which specifies the minimum thickness of the film to be 0.004 in. (0.10 mm). Polyethylene sheeting should also be applied to the concrete as soon as it has hardened sufficiently. The sheeting should be overlapped approximately 18 in. (455 mm) and weighted as needed to prevent moisture loss. All exposed surfaces, including exposed edges and joints, must be covered.
- Membrane-forming compounds. Membrane-forming curing compounds should conform to ASTM C309 or ASTM C1315. 4They are used to reduce the moisture loss from the surface of freshly placed concrete. Common membrane-forming curing compounds consist of resins, waxes, chlorinated rubber and other materials, and are clear or translucent. Membrane-forming curing compounds should be:
- Uniformly sprayed on concrete surfaces as soon as the water sheen disappears following final finishing (one to three hours after placement)
- Stirred or agitated before use
- Applied in two perpendicular directions
- Left unmodified and never thinned
- Forms left in place. Keeping steel or wooden forms on freshly cast concrete for as long as possible is an effective way to protect against moisture loss. Forms help seal the exposed concrete surfaces and retain the moisture.
- Accelerating strength gain by supplying outside heat and moisture (electrical heating, oil heating, microwave and infrared curing, and steam curing)
- Electrical, oil, microwave and infrared curing. Alternative curing methods include electrical, oil, microwave and infrared applications. Circulating hot oil through steel forms to heat the concrete, or the use of infrared rays under a covering or enclosed in steel forms are used by some precasters in place of traditional curing methods. Other examples include reinforcing steel as the heating element, electric blankets, electrically heated steel forms, using wire as the heating element, and the use of concrete itself as the electrical conductor.
- Steam curing. Steam curing significantly accelerates the rate of hydration by adding moisture and elevating temperature. Steam is especially important when early strength gain is necessary or when additional heat is required for hydration (cold weather). See the sidebar “Essentials of Steam Curing” for more detailed information.
The curing process is particularly important with dry-cast mixes. As with wet-cast mixes, adequate curing of dry-cast concrete greatly enhances strength, impermeability, surface hardness and crack resistance. Early curing is most critical to ensure protection from extreme temperatures and dry, windy conditions that can cause cracking.
Dry-cast concrete cannot hydrate properly without adequate water in the mix. As forms are removed immediately, dry-cast products have a tendency to desiccate too quickly without protection. Dry-cast concrete, therefore, benefits from insulated curing enclosures with misters or steam. In many cases, full design strengths can be achieved in one day.
Curing, indeed, is the question
The importance of proper curing cannot be overstated. It can literally make or break a precast concrete product, in spite of the most technologically advanced batching systems, highly trained personnel or top-quality raw ingredients. A closer look at curing procedures and educating plant personnel on the critical phenomenon of adequate cement hydration will, without doubt, bring precast production one step closer to the successful final act it deserves.
Autoclave, dry heat and live steam systems have all been utilized for accelerated heat curing. Since precasters rarely use costly autoclave furnaces, dry heat and live steam are typical methods of choice.
- Dry heat: As long as it is humid enough for moisture on fresh concrete not to readily evaporate, dry heat systems can be effective. The concern with using dry heat, of course, is insufficient ambient humidity, which can lead to surface cracking or crazing. If using dry heat, it is critical that no mix water is lost during curing. Examples of dry heating systems include electric heat, radiant heaters, and the use of heat blankets (in addition to the use of a misting system to ensure adequate humidity). Blower heaters are not recommended, as dry, hot air will tend to desiccate fresh concrete, leaving a weak and chalky surface.
- Moist heat: Live steam curing provides an additional advantage over other curing methods, as this method supplies both the necessary heat and moisture required for successful accelerated curing. Live steam curing enclosures can be constructed using canvas canopies where the steam is circulated through holes in distribution piping or hoses. If using steam curing, remember not to use other means of accelerated curing until after the concrete has attained its initial set. Initial set can be determined in accordance with ASTM C403 5. Early application of heat can cause permanent damage.
A typical steam-curing cycle consists of:
- An initial delay prior to steaming or a “preset time.” Concrete should sit for a predetermined period of time (minimum of 30 minutes, recommended two to three hours) or until initial set.
- A period for increasing the temperature or “ramp time.” Duration of time required to increase curing-cell temperature from initial temperature to target temperature. The ramping temperature should be limited to a minimum rise of 20 F (11 C)/hour and maximum rise of 40 F (22 C)/hour. Any rise in temperature exceeding 40 F/hour may cause thermal shock, resulting in cracking; anything below 20 F/hour will stunt rapid curing. NPCA’s QC Manual states: Ambient curing temperature shall not exceed 150 F (65 C) unless measures to prevent delayed ettringite formation (DEF) are employed.
- A period of time holding the maximum temperature constant or “holding time.” Duration of time the concrete is maintained at the predetermined target temperature, which should be held until concrete reaches desired strength. Holding time will depend on the concrete mixture and steam temperature in the enclosure.
- A period of time for decreasing the temperature or “soak time.” Duration of time that concrete is allowed to cool after the steam has been shut off and prior to removing the enclosure.
These curing cycles will vary with the type of product being cured. Precasters using steam-curing methods should take proper measures to monitor temperature and curing cycles as well as ensure there are no leaks in the enclosure. Casting temperature sensors can be used to monitor time.
Evan Gurley is a staff engineer with NPCA.