- Written by CP Staff
Ron Mulligan of Basalite Concrete Products, LLC represents a company that is one of a small (only 5 percent), yet growing number industrywide of precasters using accelerated curing to substantially lower operating and material costs, as well as increase quality to gain market share. Low-pressure steam curing Û an increasingly common practice in the precast industry Û is a form of accelerated curing that hastens the hydration process, he explains. Accordingly, it increases compressive strength, helps control shrinkage (especially when carbonation is added), and contributes to uniformity of appearance and performance.
Efficient and effective accelerated curing stands to benefit producers of precast, prestressed, pipe, block, hardscape units, and cast stone. Among those with the most to gain is the large number Û reportedly 90 percent Û unaware of the energy cost associated with product curing. Properly managed and controlled curing of concrete masonry units and precast concrete pipe and products is essential to achieving economical productivity with excellent finished product quality and appearance and superior installed performance, affirms John Blankenship, Hanson Building Products.
A thorough understanding of concrete curing includes procedures for the control of both temperature and moisture movement from and into the concrete to promote cement hydration for the development of such properties as compressive and flexural strength, durability, density, stability, resistance to corrosion and freeze-thaw cycles, and color stability. Accelerated curing refines that process by introducing acceleration of concrete hardening via controlled external means with the goal of reducing the duration required to obtain desired properties. By definition, accelerated concrete curing allows for significantly earlier product handling and mobility. Although acceleration of hardening can be achieved with chemical additives, emphasis here is placed on use of air heating and circulation, vapor, steam, and enclosure systems.
Not subject to alteration by manufacturer claims are empirical laws that dictate any process aiming to achieve efficient and, hence, economical concrete product curing. Twelve essential principles can be identified, which provide the foundation for corresponding practices.
Temperature affects concrete strength gain, which is accelerated by warm temperatures and retarded by cooler temperatures. Moreover, temperature affects concrete color, i.e., warm temperatures cause lighter colors, while cooler temperatures have a darkening effect. Consequently, variations greater than 5_F in the curing environment should be reduced or eliminated by proper heat distribution, air circulation, and enclosure insulation.
Due to latent heat of vaporization, heating concrete by means of steam or vapor is 10 times more efficient than using warm air. Thus, replacing hot air with steam or vapor as a curing medium will reduce heating costs by 90 percent. For most precast, prestress, pipe, block and cast stone products, the resulting additional moisture will promote better cement hydration on all surfaces exposed to the atmosphere during curing. Architectural precast and paver producers should take preventative steps, including air circulation and insulation, to safeguard against condensation issues, such as primary efflorescence, staining, hazing, and spotting.
Direct-fired vapor generators are 60 percent more efficient than steam boilers. Average costs using boiler steam and direct-fired vapor, respectively, to cure a) concrete block are $0.03 per block versus $0.01 per block; b) prestress concrete: $11.00 per yd. versus $4.00 per yd.; and, c) concrete pipe: $4.50 per ton versus $2.00 per ton. Replacing boilers with direct-fired vapor generators will result in annual energy savings between $40,000 and $200,000, depending upon production capacity. Additional annual savings of $15,000 to $100,000 can be gained from insurance, inspection, and maintenance expense reductions.
Insulated enclosures reduce energy costs associated with curing by 50 percent, compared to traditional methods, while preventing condensation and corrosion. Because the high humidity levels (+90% rH) needed to ensure proper cement hydration lead to saturation of conventional block walls, which act as a conductor (from warm to cool), more energy is required to heat traditional chambers. Additionally, condensation forms on interior surfaces. Designing curing chambers that incorporate nonabsorbent insulation, such as insulated metal-clad sandwich panels, minimizes radiant and conductive heat loss to save energy and reduces system size and cost.
We were losing heat and steam with our noninsulated masonry kilns, notes Chris Cook of New Albany, Ind.-based L. Thorn Co. By replacing them with insulated metal panels, we reduced energy costs, because all heat generated by the steam system, as well as the heat of hydration, was used for curing. No leaks, no wasted energy.
Replacement of portland cement with blast furnace slag, fly ash and other supplemental cementitious materials (SCM) at levels of 25 percent and greater in combination with accelerated curing will provide early strength results comparable to using cement only in the mix. For block, paver, precast, prestress, pipe or cast stone production, accelerated concrete curing systems can help achieve notable reduction in carbon-dioxide emissions by allowing significant SCM substitution for cement. Curing with heat and moisture is less expensive than overdosing a concrete mix with cement to achieve early strengths.
Stronger compressive strength from enhanced curing has allowed us to optimize our mix designs for cement usage and lower our cost, affirms Adam Benefiel of Goria Enterprises, an Oldcastle company. The increase in kiln temperatures also allowed us to replace a percentage of cement with fly ash, again reducing our product cost.
If concrete curing takes place in an enclosure at temperatures equal to 120_F or above, a system that removes heat from one enclosure during the exhaust phase at the end of the curing cycle and adds this heat to another chamber during ramp up at the beginning of a subsequent curing cycle will provide energy-saving benefits. Accordingly, a re-exhaust system can achieve savings of 30 percent of total energy costs associated with concrete curing. Further, a re-exhaust system provides a dry curing chamber at the conclusion of the curing cycle, eliminates staining issues, reduces concrete moisture content, minimizes chipping and breakage through the cuber, and allows for more even splits.
We were not expecting the addition of exhausting to make such a difference to our product, asserts L. Thorn's Chris Cook. With exhausting and the removal of residual moisture off the block, we realized less breakage and chipping during handling. The block also tested with better compressive strengths.
Direct-fired vapor may be controlled automatically for precise regulation of moisture levels to produce saturated or partially saturated vapor. Staining of concrete block due to high moisture content related to boiler steam is thereby eliminated. Thus, producers are advised to invest in systems that allow control of vapor temperature through addition or removal of water in the steam. Controlling water content also provides savings and resource conservation, as water consumption is based on product requirements, rather than system demands.
Curing tents, expandable during the production day, can prevent drafts from reaching freshly cast pipe, thereby avoiding cracks in components with cages or bending in those without rebar. To prevent drafts from damaging the pipe surface, therefore, the curing tent is immediately moved into place when fresh pipe is cast to effectively enclose the product. The tent provides convenient enclosure, due to its flexibility, ease of movement, and stowability.
Taller curing enclosures require more air exchanges in a given time period to maintain a constant temperature throughout the interior, i.e., number of air exchanges per hour varies proportionately with enclosure height in maintaining a less than 5_F temperature difference in the curing environment. A 10-ft.-tall enclosure, for example, requires five air exchanges per hour, while a 25-ft.-tall enclosure requires 12 air changes per hour in order to maintain a 5_F temperature difference.
A longer soak phase and higher curing temperature (up to 160_F) produce higher early strength. Because concrete maintained for a longer period of time at an elevated temperature (between 100_F and 160_F) develops a higher early strength, longer curing duration and/or higher curing temperature would suit concrete products that require high early strengths for secondary processing, shipping or installation.
Using hot water at 180_F to heat a dry-cast (zero-slump) mix is ineffective. While water is the easiest batch component to heat, drier mixes used in block, paver and pipe production require so little moisture that hot water may cause a rise in mix temperature of 1_F to 3_F at best. The following formula assists concrete producers in determining each mix-component temperature needed to achieve the desired temperature of fresh concrete.
T = temperature of freshly mixed concrete, _C (_F)
Ta, Tc, Tw, and Twa = temperature in _C (_F) of aggregates, cement, added mixing water, and free water on aggregates, respectively
Ma, Mc, Mw, and Mwa = mass, kg (lb.), of aggregates, cementing materials, added mixing water, and free water on aggregates, respectively [provided by Portland Cement Association]
- An aggregate heating system should maintain a constant temperature no less than 65_F year-round. Allowing for best asset utilization, an aggregate heating system can supply an additional four to six weeks of production every winter season, given proper monitoring and control of aggregate temperature for optimum mix batching. Running an aggregate temperature higher than 75_F is a potential cause of premature concrete hardening in mixer, feed screw, or hopper.
ACC SYSTEM COMPONENTS
When curing concrete, all fundamentals must be present [in appropriate measure]: time, temperature, moisture, insulation and circulation, emphasizes Ron Scherer, Oldcastle APG. Additionally, designing and engineering an accelerated concrete curing system entails consideration of storage and curing capacity, required number of molds or pallets, mix design, and product specification requirements.
Temperature Û For all products, increased temperatures accelerate the hydration process. The resulting reduction in curing duration facilitates faster turnaround times. In addition, higher curing temperatures provide higher early strengths, contributing to less breakage during precast/prestressed unit demoulding and detensioning. Yet, block and paver producers are advised to note that identifying the ideal temperature depends upon product specs, secondary processing and color requirements, plus rack or pallet space. Overall, accurate temperature control promotes product consistency as well as energy savings.
Humidity Û Improved cement hydration under humid conditions provides better ultimate strengths. Further, adequate humidity prevents premature moisture evaporation for reduced cracking. Harder edges and corners for less breakage during stripping and product handling are also promoted by proper humidity.
Duration Û Although longer curing duration provides greater strengths, inefficient times can waste space and pallet utilization. By contrast, proper times allow sufficient cement hydration, while ensuring efficient bed and form usage for precast, prestressed production. Duration determines when secondary processing applications can be performed on block and pavers, and correct timing in the addition of heat and humidity allows products to be handled only once.
Circulation Û Adequate circulation is essential to prevent condensation issues, e.g., drips on the product, and promote uniform colors and strengths, top to bottom and front to back in the curing chamber.
Insulation Û Benefits of proper insulation include prevention of heat and humidity loss from the chamber, energy cost reduction by at least 50 percent, and elimination of condensation issues.
Enclosure Û Surface cracking in pipe, manholes, and culverts is prevented by enclosure that protects them from drafts. Moreover, uniform temperatures are preserved to promote consistent strengths. Enclosing precast/prestressed during curing eliminates evaporation to deter cracking; and, tarps or other coverings prevent heat loss.
Citing numerous conversations with dry cast concrete producers regarding chipping, color consistency, dry side breakage and other assorted issues, NCMA Research and Development Laboratory's Mike Maroney concludes, It always gets back to two culprits: mix moisture and a lack of understanding the curing process. Û Adapted from a Kraft Energy Systems report, Accelerated Curing for Manufactured Concrete Products: An Economic Imperative; www.kraftenergy.com