Precast Concrete Offers Achievable Solutions for Specifiers and Producers
By Sue McCraven
Sustainable development is that which meets the needs of the present without compromising the ability of future generations to meet their own needs.” United Nations Brundtland Commission Report
Green building, carbon footprint, energy conservation, recycling, greenhouse gas emission, waste reduction, LEED credits and environmental stewardship: These are all terms used in the current vernacular for sustainable design and development. In the precast concrete industry, as for all industries worldwide, sustainable practices are those that demonstrate the most efficient use of energy, water and materials based on currently available and economically feasible technologies.
But how can precast concrete producers and specifiers distinguish between the considerable popular rhetoric on sustainability and economically feasible practices that are also environmentally and socially responsible? Actually there are a number of ways.
Recycle concrete coring water
As an National Precast Concrete Association (NPCA)-certified and New York Department of Transportation (NYDOT)-approved precast concrete manufacturing facility, Jefferson Concrete Corp. of Watertown, N.Y., is a diversified manufacturer and installer of products ranging from wall systems, burial vaults and catch basins to retaining walls and agricultural silos. Jefferson Concrete addresses sustainability in the precast concrete industry by recycling drainage water from its concrete coring operations for NYDOT.
Mark Thompson, vice president, with more than 30 years of industry experience, claims that “the success and growth of the company depends not just on a quality workforce and quality products, but on trying to implement sustainable practices into production whenever possible.”
According to Mark Jareo, Jefferson Concrete’s operations manger, 95 percent of the water used in the concrete coring process is recycled. “Prior to shipping, many of our products are cored for drainage,” he says. In this process, the water used to cool the drill bit is reclaimed by sluicing water down a trench to a receiving pit. Water is then pumped into a decanting tower where fines settle out. Clarified coring water is reclaimed for future use, saving hundreds of gallons of water daily.
Disassemble and reassemble existing precast structures
What could symbolize sustainable construction practice more than that of reusing existing building components to expand or erect new structures? Is such a material- and energy-efficient concept even viable? In the precast concrete industry, the answer is “yes.”
In addition to providing owners with a durable, energy-efficient, aesthetically pleasing building enclosure, “precast concrete systems present important sustainable benefits that are often overlooked,” explains Don Zakariasen of Lafarge North America. Precast insulated wall panels can be easily uninstalled from a building that is in service to facilitate expansion. The robust nature of precast concrete elements permits disassembly with little or no damage to precast wall elements. “This was the case with the Tim Horton’s office and distribution center in Calgary, Alberta,” says Zakariasen.
The developer installed a temporary wall in the building to allow Lafarge’s Alberta Precast group to remove the entire east building wall and put it into temporary storage. New additional space was then constructed adjacent to the existing building, including new precast panels on the north and south elevations. The stored precast panels were then reinstalled into their new positions on the east elevation. The final step was to remove the temporary interior wall. The entire expansion of the center took place while the building remained in service. Reuse of the existing precast wall not only saved the owner money but also reduced adverse environmental impacts.
New concrete technology: 75 percent reduction in emissions
New concrete material technology has produced ultra-high performance concrete (UHPC) that offers superior strength, durability and aesthetics with design flexibility to produce much lighter and thinner components. Ductal, a registered brand by Lafarge, is one such product. The self-reinforcing concrete is manufactured without the use of conventional steel reinforcing bars but uses fiber reinforcement. Ductal projects can generate up to 75 percent fewer greenhouse gas emissions than conventional concrete and can be fabricated into remarkably thin, lightweight components. The significant reduction of the overall weight and mass of building beams and columns, whether for buildings or bridges, translates into energy and material efficiencies in production and fewer adverse environmental impacts. Precast concrete producers in North America and around the world are developing remarkable new precast products with UHPC.
Go green: don’t overdesign mix
Soil Retention Products (SRP), Romoland, Calif., designs, produces and installs retaining wall systems, articulated concrete block erosion control mattresses, flexible porous concrete mats and other soil stabilization systems. SRP is at the cutting edge in its use of supplementary cementitious materials (SCMs). Jim Blankenship, plant manager, explains how SRP’s use of maximum SCMs not only helps the environment by using recycled materials, but actually produces a better performing product. “By incorporating 25 percent to 40 percent Class F fly ash in mixes, we provide a product with increased sulfate resistance, decreased alkali-silica reaction (ASR, a serious concern in California soils) and decreased permeability.
“We can produce very cost-effective mixes by using elevated percentages of fly ash in our products,” says Blankenship. SRP takes advantage of the brain trust of its admixture, fly ash and material suppliers. “We get together to figure out how we can best use SCMs to produce the most economical, sustainable and resilient products for our customers.” SRP also operates its own materials lab where new mixes are tested for compressive strength and performance.
Blankenship explains how engineering specifiers often fall into the trap of overdesigning precast products by specifying the cement content and the water-to-cementitious materials ratio. It would serve the engineer and the industry better, reasons Blankenship, if engineers specified the end use and the design parameters and left the mix design up to the producer. “The precast producer has the experience to determine which concrete mix can best meet the service and strength needs of the project.” Specifying engineers often are not acquainted with how the precast concrete production industry effectively uses concrete technology to not only manufacture the most appropriate product for the job, but to also use maximum recycled materials in the process. Through collaboration, producers and specifiers can work together to use the attributes of precast concrete to achieve LEED (Leadership in Energy and Environmental Design Green Building Rating System) accreditation as part of the integrated design process.
Acknowledge resource conservation as good business
In Southern California, Arto Brick California Pavers, Gardena, manufactures products that architects and specifiers can use to secure LEED credits for projects in the following ways:
- Use high SRI (Solar Reflectance Index) colors in many products to reduce heat island effect.
- Use locally sourced raw materials to reduce transport costs and energy.
- Use recycled material that does not compromise product quality.
- Recycle gray water into concrete mixes.
- Establish 2009 Goal: zero wastewater stream through improved filtration and addition of recycled concrete to increase percentage of pre-consumer recycled material.
- Reduce material waste by producing to order.
- Donate excess materials to Habitat for Humanity.
- Add recycled concrete to increase percentage of pre-consumer recycled material.
- Use packaging made with recycled material; shred office paperwork for packing material.
- Work up-front with specifiers to conserve resources and maximize LEED credit potential.
“We seek to conserve resources and energy through efficient business practices that are both good for the environment and good for our bottom line,” says William Love, technical director at Arto Brick. Arto Brick products can help contribute up to 10 LEED credit points, including: 10 percent to 20 percent recycled materials; 10 percent to 20 percent regional materials; stormwater control with pavers or stepping stones installed on an aggregate base; heat island effect reduction with high SRI (more than 29) hardscape; and low-VOC materials in adhesives and sealants to improve indoor air quality.
Precast concrete’s thermal mass saves energy
The thermal mass of precast concrete is not related to wall insulating products. Thermal mass is defined by a material’s specific heat, density and thermal conductivity. Materials with high values of these properties can passively store and release significant quantities of heat from the sun and actively or passively transfer this energy to interior spaces. Precast concrete used in TermoDeck floor and roof slabs use hollow core slabs to store and transmit natural solar energy to indoor environs. TermoDeck, registered trademark, is a system that uses the thermal storage capacity of a building’s structural mass to regulate internal temperatures.
TermoDeck systems use smart floors in hollow core walls to ensure that hot and cold air can be stored in the slabs and released upon demand to save on building energy costs. During summer months, heat from the interior is absorbed by concrete panels through passive radiation. Ventilation air is then routed through the slab, absorbing heat from (warming) or releasing heat to (cooling) the exterior, depending on the temperature of the slab. During nighttime hours, cooler outdoor air is circulated to remove heat and pre-cool the slab for the following day. During winter heating seasons, slabs absorb interior heat from the occupants through radiation and store the energy in the slab. During the night, the air handling system provides warm air to heat the interior space.
For a modular classroom addition at the Hawthorne Village Public School in Milton, Ontario, for example, the TermoDeck system was installed to facilitate heating and cooling of the school. Heat is stored in the roof slabs and released as needed. The precast concrete system has proved highly efficient, requiring half the energy of a conventional school building. Air supply is made up of fresh outdoor air with no recirculation, while return air is passed through a heat recovery unit before being exhausted to the outside.
Collaborate up-front for competitive sustainable design
Without exception, project participants agree that fruitful initial collaboration between owner, designer, engineer, precaster and erector is the critical process that leads to unprecedented sustainable designs and green buildings. Whether the project is the construction of a DOT bridge, erection of a high-rise building or production of specialty precast products for landscaping, it is the initial teamwork and preliminary collaboration that garners and energizes the best ideas from design conception to finished product.
“It is only when the engineer, precast producer, material suppliers and designer sit down together to discuss sustainability goals in a project’s early phases that we find maximum potential for green construction and sustainability/LEED initiatives,” says Professor Tess Ahlborn, director of the Center for Structural Durability at Michigan Technological University. She adds that it is in taking advantage of the available brain trust that exists in the precast concrete industry’s professionals that “we can attain in practice what we espouse as the potential of the sustainability movement.”