Building With Concrete (Continued from p. 19 of the May 2009 issue of BD+C)
Dealing with lightweight concrete also demands special care, according to Dennis Ahal, chair and CEO of Ahal Contracting Company, St. Louis. “Because the porous coarse aggregate that contributes to the lower density also results in absorption of water as the concrete passes through the pump line, if steps aren’t taken to reduce this absorption, line blockages can cause placing delays,” says Ahal.
In the case of concrete containing mid-range water reducers, it falls on both the specifier and concrete producer to ensure that there is still enough unabsorbed mixing water to compensate for movement of water into the pores of the coarse aggregate during pumping. Otherwise, explains Ahal, “slump loss can be excessive and also result in line blockages.”
Ahal, who has been working as a concrete contractor and pumper for the past 40 years, stresses the importance of teamwork and collaboration. “A successful pumping experience is usually the result of teamwork among the specifier, concrete producer and lightweight aggregate supplier, the concrete pumper contractor, and the testing agency.”
One noteworthy timesaving advance in concrete pumping technology is the use of self-jacking placing booms. “Usually we have to pick the placing boom with the tower crane after every pour to set it for the next elevated pour, but with the self-jacking boom, it jacks itself into place without the assistance of the tower crane,” says George.
Once the concrete is placed in molds or forms, mechanical vibrators are then used to help the concrete spread to all corners of the mold and release trapped air pockets, according to Chusid, a Fellow of the Construction Specifications Institute and a member of several American Concrete Institute (ACI) committees. While water will help the concrete to flow, too much water weakens the concrete, so a delicate balance has to be achieved.
Air-entrained concrete. There are times when air is a desirable part of the mix, says Steven P. Osborn, PE, SE, president of CE Solutions, Carmel, Ind. “Air-entrained concrete is important for exterior applications that are exposed to freezing and thawing,” he says. “Adding air to the concrete creates microscopic voids that allow moisture to expand during freeze/thaw cycles, which helps avoid damage to the concrete.”
However, air-entrained concrete does require special finishing, according to George: “It is preferable to use a rough or broom finish to prevent blistering from the air in the concrete on the surface of the slab. When finishing an air-entrained slab, the surface water must be allowed to dissipate. An air-entrained surface can fool the finishers to get on the slab too early and harm the final finish.”
On the job site, explains Palmer, the construction team sets the concrete forms in place, places the plastic or metal rebar within, and sets up the window and door bucks. Then it’s time to fill the forms. According to the Insulating Concrete Form Association (ICFA), Glenview, Ill., placement usually begins through the opening in windowsill plates. As the concrete rises, the remaining pour comes through the top of the form, in lifts of approximately four feet.
However, when pumping, cautions Palmer, “The real concern is being aware how fast the concrete is being poured so that the forms aren’t blown out.” Similarly, the ICFA recommends not placing concrete too close to corners, openings, or thin columns, in order to not put too much stress on the forms.
In terms of cladding options, exterior finish systems such as synthetic stucco are most common, but brick can be used as well. For the interior, drywall is a popular choice, as it easily connects to attachment inserts in the ICF foam blocks.
The primary draw of insulating concrete forms, as their name implies, is the high their insulating value. “The stay-in-place concrete forms are made from an insulating material and are never removed, so they are very energy efficient and create a completely airtight wall,” says Palmer.
PRECAST CONCRETE
Unlike the ICF and tilt-up field techniques, precast concrete forms are prefabricated in an indoor, controlled environment, even though the largest structural pieces are sometimes cast outdoors. Ultimately, the quality of precast or prestressed depends on the rigor of the supplier’s operations. CE Solutions president Steven Osborn warns that “there should be effective quality control at the plant and consistency in fabrication of the pieces so that the precast stays within industry construction tolerances for fit-up and finish.”
Another advantage of precast systems, according to the NAHB’s http://www.toolbase.org/index.aspx, is that the foundations can be backfilled as soon as the slab and first floor are braced. This is because the concrete is pre-cured in the factory. Pre-curing also eliminates many problems associated with adverse weather concerns. In addition, some precast units are cast against foam insulation inside the form, much like ICFs, offering higher R-values for the structure.
According to the PCA, there are two basic types of precast products: standard products, such as beams, decks, and panels that can be reused, and specialty products, which are customized for a particular project. An important point with regard to precast construction is the fact that most of the work is done with self-consolidating concrete (SCC), which is a very workable and flowable type of concrete that can leave a smooth finished surface. With self-consolidating concrete, “You don’t have to worry about consolidating the concrete into the forms, as vibrating can actually segregate the coarse aggregate out of the concrete,” says Complete Construction Consultants’ Palmer.
Integral Waterproofing for Thirsty Concrete Although concrete is a very durable and widely used building material, its Achilles heel is water absorption. Applying external membranes to foundations and slabs has been the traditional approach to this problem, but the development of integral waterproofing technology at the admixture level is now seen as a viable alternative. According to concrete consultant Steve Crawford of Las Vegas, integral waterproofing offers a number of benefits: • Reduced construction cost – Integral waterproofing is typically up to 50% less expensive than other approaches in first-cost terms. • Speed of construction – Integral waterproofing eliminates the need for a membrane, allowing the Building Team to avoid this time-consuming step. The “pour-and-you’re-done” approach can shave weeks off a construction schedule, which translates to faster building occupation, lower risk, and money saved. • Safer working conditions – No hot rubber and less on-site labor is required. This means a lower risk of injury. • More durable structures – Integrals, which are physically embedded in the concrete, are inherently protected from damage. Some admixtures have also shown a double benefit of reducing corrosion. Yet another potential benefit to integral waterproofing is environmental. “Some membranes contain VOCs [volatile organic compounds], have high embedded-carbon footprints, and are often petroleum-based,” says Crawford. “At the end of a building’s life, those membranes are tightly adhered to the concrete, and at demolition, recycling concrete can be simply impractical.” As a result, many tons of concrete head to the landfills. On the other hand, “Newer admixtures are deemed safe for the environment, make recycling much easier, and can contribute to LEED credits on a building,” says Crawford. “Buildings can literally have a green foundation.” |
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One useful tool is a set of documents published by the NPCA to help designers write proper specifications for precast products. Osborn also stresses the importance of careful coordination among the trades when working with precast concrete: “In addition to coordination of premade openings, the team should strive for efficiency in the use of mechanical connections, as well as proper detailing to protect against corrosion.”
No matter which concrete technology you use, Osborn advises selecting a skilled, experienced concrete contractor, due to the unique construction variables inherent in the technology. For complex concrete structures, he recommends holding preconstruction meetings with the contractor, concrete supplier, quality control/testing firm, structural engineer, architect, shoring/formwork subcontractor, and pump supplier, to address potential issues early on, thus creating “a more seamless project process.” Further, he calls for implementation of an effective quality-assurance plan that includes frequent jobsite visits by the structural engineer of record.
INNOVATIVE CONCRETE PRODUCTS
As noted above, self-consolidating concrete enables users to reach spots where a concrete vibrator cannot reach. “SCC does this without losing strength as other concretes do when they are runny. The aggregate is smaller as well, allowing it to reach the hard-to-reach places usually found between large amounts of rebar,” notes Balfour Beatty Construction’s Jeff George.
Consultant Michael Chusid adds: “The key to SCC is the use of high-range water-reducing admixtures, also known as superplasticizers, plus other viscosity-modifying admixtures to create a concrete mixture that has high flowability without potential for segregation or the strength loss associated with adding water.”
Even though the product is still considered too expensive for many typical projects applications, some concrete contractors anticipate that this will change. “I have a feeling the cost will come down and the efficiency recognition will go up, making this a real option in the future,” says George.
Shrinkage-compensating concrete. Not to be confused with self-consolidating concrete, shrinkage-compensating concrete mixes expand to cancel out the shrinkage that commonly occurs during curing and which can result in cracking.
Concrete naturally shrinks an average of 0.04% after 28 days as a result of water evaporation; left unchecked, this can cause defects. “The cracks create channels through which liquids and gases can enter the concrete, leading to corrosion, freeze-thaw damage, or other deterioration,” warns Chusid. “Moreover, these small cracks become points of stress concentration and propagate larger cracks, and can be aesthetically undesirable.”
Shrinkage-compensating concrete is an alternative to cutting joints into the slabs to control cracking, as it expands to cancel out the shrinkage. What enables the product to expand is a special expansive cement, called Type K concrete, which replaces the 15% cement component in a standard concrete mix.
Ductile concrete. The defining property of this innovative concrete product is its high degree of flexibility, enabling it to bend without breaking. By manipulating its ingredients on a micro-level, performance and strength are maximized, allowing it to be used in very thin sections with less structural weight and fewer joints required, according to Chusid. Made from Portland cement, silica fume, fine aggregate, and superplasticizers, the mix is blended with either metallic or polyvinyl alcohol fibers to create a fluid, self-placing, high-performance concrete product.
Rapid-setting cements. Addressing the issue of protecting the concrete during the delicate curing process, rapid-setting cements speed up the process to ensure the concrete more quickly gains strength. In addition, its fast-drying properties make it a desired product for repairs and patchwork.
Aerated concrete. Although aerated concrete has been used in Europe since the early part of the 1900s, the blocks have only been manufactured in the United States since the mid-1990s. Offering thermal mass and acoustic insulation properties, the unique nature of aerated concrete stems from its ability to expand to five times its original volume. By adding aluminum powder or paste to the mix and then pouring it into a mold, a chemical reaction between the concrete and aluminum creates microscopic hydrogen bubbles, which cause the mixture to expand.
Concrete brick. Made from sand, crushed rock, water, and Portland cement, concrete brick is a popular alternative to fired clay brick, offering the same strength and density, according to Chusid. The concrete brick can also be made in a variety of colors through the use of pigmenting via mineral oxides.
Polished concrete. As a more sustainable and durable alternative to other floor coverings, polished concrete is becoming a popular choice for schools, retail, warehouses, and car dealerships. Offering abrasion resistance and a trendy sheen, the slab itself is used as the finished floor, eliminating the need for an added layer over the floor.
Photocatalytic cement. An environmentally geared product that protects exterior surfaces, photocatalytic cement actually “eats smog,” according to Wight & Company’s Womack. Through a chemical reaction with sunlight, the cement, which can either be added to the mix or applied as a thin layer on the surface, effectively neutralizes toxic particulates.
Translucent concrete with optical fibers. This class of materials (which includes a product called LiTraCon developed in 2001 by a Hungarian architect) is essentially a fine concrete embedded with bundled optical glass fibers. The resulting concrete blocks and panels transmit light and allow visibility through the structural mass. Because the optical fibers only account for 4% of the total product volume, they essentially become a structural component and do not compromise the concrete’s load-bearing properties.
Glass-fiber-reinforced concrete. Made from cement, sand, and special alkali-resistant glass fibers, glass-fiber-reinforced concrete or GFRC is a thin, high-strength product. Primarily for exterior use, GFRC is ideal for building façade panels, domes, columns, and other architectural details traditionally made from precast concrete, carved stone, or plaster. According to the group Glass Fibre Reinforced Concrete International (www.grca.org.uk), GFRC’s main benefits include:
• A higher strength-to-weight ratio than unreinforced precast concrete.
• Resistance to environmental degradation and corrosion.
• Easy workability, allowing flexibility in design.
About the authors
C.C. Sullivan is a communications consultant and author specializing in architecture and construction. Barbara Horwitz-Bennett is a writer and contributor to construction industry publications.
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