Building With Concrete Continued

August 11, 2010

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.”

                    
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Reed Business Information is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. 
   

Other technical considerations, according to Steven Osborn, an active member of the American Council of Engineering Companies, ACI, and the American Society of Civil Engineers, include:
• Structural systems. Good detailing for proper location of joints is essential to allow for expansion and contraction from shrinkage and thermal change. This helps minimize cracking and improve overall performance.
• Pre-made openings. If openings are punched in wrong locations, the problem can only be rectified by core drilling or major excavation, especially in post-tensioned and prestressed concrete. Obviously, this is costly and wasteful, and points out why effective coordination between the trades is a must.
• Connections, bracing, and reinforcement. Mechanical connections need to be properly detailed and protected against corrosion.
• Concrete reinforcing. Again, good detailing is essential to facilitate proper vibration and avoid congestion of reinforcing steel, so that concrete easily flows between pieces of rebar.

Reinforcement alternatives. When considering such reinforcement options as mesh, fibers, and steel, Osborn suggests fiber micro-reinforcement to control shrinkage and cracking in slabs-on-grade. For hardened structures, he likes to go with steel fibers in combination with reinforcing steel for improved durability. For heavier load applications, structural engineer Osborn finds that steel plate dowels and load plate baskets provide better load transfer than welded wire fabric or traditional round dowels.

The use of wire mesh is common, but Osborn cautions that the placement of rolled welded wire mesh can be hard to control because it has a tendency to curl. He recommends using sheets of wire mesh that are properly spliced. He notes that laborers may have to walk on the mesh before and during concrete placement, which can make it difficult for the mesh to end up exactly where it is needed.

CAST-IN-PLACE METHODS
Cast-in-place concrete is often used for slab-on-ground and foundations, using ready-mixed concrete. Cast-in-place made its debut in Chicago skyscrapers in the late 1950s and early 1960s and has become a popular choice for beams and columns, floors, walls, and even roofs.

In the case of slab-on-ground, the first step is setting up the formwork with the minimal amount of reinforcement.  The PCA recommends that the speed of the concrete pour should be controlled to allow the concrete to be spread, struck off, and consolidated properly.

For setting up structural members, “One face of the forms will be set and then reinforcing will be positioned using different ties and spacers. Then the other form will be set prior to casting,” explains William D. Palmer, Jr., PE, president, Complete Construction Consultants, Lyons, Colo. Once the pour has been completed, the top is screed off to make sure it’s level, the surface is smoothed, and then the concrete is left to harden and gain strength. When the concrete has hardened to the point where only a ¼-inch indent can be made by the push of a finger, it’s time for trowelling to further smooth the surface for interior locations. For exteriors, a broom finish is applied, says Palmer. This is also the time to apply any decorative techniques, such as stamping or coloring.

The concrete then continues to dry and is set as the moisture more fully hydrates the cement, gaining strength and durability. During the curing process, the concrete must be treated carefully, says Palmer, formerly ACI’s director of education and an editor of Concrete Construction magazine. “It’s new and fresh and needs to be taken care of like a baby,” he says. “It can’t be too hot or too cold, and you don’t want it to dry out.”

This is especially the case when dealing with severe weather conditions. For example, in cold weather, says Palmer, hydration is very slow, which slows down the rate at which the concrete gains strength. “This is an issue with supporting loads as construction continues, so it’s important to keep the concrete warm enough so that it can gain strength,” warns Palmer, who also serves on ACI’s Cold Weather Committee, which is developing guidelines to help Building Teams get concrete to a point where it has the strength to resist freeze damage.

At the other end of the temperature scale, hot and dry weather poses the risk that the concrete may dry out before it properly hydrates and strengthens. “Contractors will try to protect it, put up barriers, or cover it with tents to try and prevent the sun from shining on it too much,” says Palmer.

   
 
Mass Concrete: Best Practices

One early fall day, after a construction team from L.L. Geans Construction Company, South Bend, Ind. (www.llgeans.com), placed a mass concrete foundation for a project, they discovered a large number of surface cracks in the foundation. After consulting with an expert at the American Society of Concrete Contractors, the team discovered that the problem stemmed from thermal cracking as a result of the thick foundation.

“The ASCC expert told us that many mass concrete foundations built today require careful control of maximum temperatures developed within the concrete, and maximum temperature differences between the core of the foundation and the surface. When cement hydrates, it gives off heat that increases the core temperature,” recalls company president Rocky Geans.

According to concrete experts, thermal cracking usually isn’t a problem in thin sections because the surface and core cool at about the same rate. In a mass concrete placement, however, the core doesn’t cool very rapidly and it takes a long time for the heat to come to the surface.

“The surface cools much more rapidly, especially if the air temperature is low,” explains Geans’s VP Mike Glenn. “That means the surface contracts due to the cooling, but the slower cooling concrete at the core restrains the contraction. Restrained contraction can cause tensile stresses that exceed the tensile strength of the concrete, and this causes surface thermal cracking.”

Based on the information gleaned from the ASCC, the contractors decided to make sure that the maximum temperature difference between the core and the surface did not exceed 50ºF.

“To do this we embedded thermocouples at the center of the foundation and near the surface so we could monitor temperature differences,” says Geans, who conducts technical seminars at the World of Concrete. “We then used several layers of our insulating blankets to control the rate at which the concrete surface cooled.” The approach proved successful as “no surface cracks were visible when we removed the blankets, and none appeared later,” according to Glenn.
   

Another helpful cast-in-place resource is a new book from the American Society of Concrete Contractors, Tolerances for Cast-in-Place Concrete Buildings. The book documents thousands of as-built measurements for buildings that are performing well but which don’t meet some of the commonly specified tolerances. According to co-author Bruce A. Suprenant, president of Concrete Engineering Specialists, Boulder, Colo., “It is particularly important to know which concrete construction tolerances are realistic when the cast-in-place concrete structural members will be interfacing with structural steel, precast concrete, cladding, and fenestrations.”

TILT-UP CONSTRUCTION AND FINISHING
Introduced in the mid-1940s, tilt-up construction has become a widely accepted building technique, with more than 10,000 such buildings being erected annually, according to the Tilt-Up Concrete Association (TCA), Mt. Vernon, Iowa.

A tilt-up project begins with a site evaluation and designing and engineering the structure. Next, the footings are installed, taking care to make sure they’re as level as possible, followed by double-checking the location, height, and dimensions of the tilt-up panels. When laying out the panels, the construction team commonly lines them up against a chalk line on the floor slab with bordering side forms, typically made from wood. After the panel perimeter is framed, door and window openings are formed. With greater market demand for floor slabs free of holes and patches, technology has been improving, enabling panel formation with minimal penetration of the floor slab, according to James R. Baty II, TCA’s technical director.

Baty says it is important to form the panels side by side, in an arrangement that maximizes the available floor area and creates a reliable panel joint design. If a patterned or textured surface is desired, the contractors will anchor reveal strips to the base slab, just before placing the reinforcement grid. Then, while the grid is being set up, embeds— pre-fabricated steel plates with lugs, which will eventually be used to connect the panel to other panels, the roof, or building accessories—and inserts, which provide attachment points for lifting hardware and braces, are installed.

After the concrete is poured and placed, the panels should be lifted using a carefully coordinated and planned lifting sequence, recommends TCA executive director Ed Sauter. Then, once a panel has been set but before it is released by the crane, braces must be installed to support the panel and hold it plumb. However, even though the braces will offer some support, Michael Knopoff, AIA, of Montalba Architects, Santa Monica, Calif., cautions that “the walls must be designed to withstand temporary load conditions during construction, even before they are in their final upright position.”

It is important, therefore, to wait until the roof and decking are installed before removing the braces, at which point the holes can be patched up along with other finishing work. Although painting, sandblasting, and exposed aggregate finishes are all common tilt-up finishing methods, Chusid claims that they all have their shortcomings. For example, sand-and-cement sacking will only get the surface so smooth, and epoxy has workability issues. As an alternative, Chusid, a member of ACI’s Concrete Aesthetics Committee, recommends a trowel-applied patching compound made from high-performance, rapid-setting cement. These products cure rapidly and have very low pH levels, so patching and painting can be done on the same day.

Another finishing technique is to leave the “accoutrements” of the tilt-up process in place, says Knopoff, pointing to Seattle University’s Chapel of St. Ignatius, designed by architect Steven Holl, as a prime example. Knopoff says he is not alone in admiring the way Holl left the panel lifting attachment points in place and highlighted them with a decorative boss, thus “expressing the tilt-up process.”

INSULATING CONCRETE FORMS
Yet another concrete construction approach, insulating concrete forms (ICFs), is commonly used for residential construction. However, these sandwiches of expanded polystyrene insulation and cast concrete also offer good project economics and energy efficiency for multifamily and nonresidential projects, notably hospitality and retail facilities. ICF construction has generally been limited to one- and two-story construction, but there have been successfully engineered freestanding, load-bearing structures as high as 48 feet.

   
 
  An example of a post-tensioned slab at edge beam condition.
   

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.”
   

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.

   
 
  The concrete floor at Sue Buel Elementary School in McMinnville, Ore., is diamond polished, providing a trendy sheen.
   

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.

   
 
  The main entrance gate to the Cella Septichora burial chamber in Pécs, Hungary, is made from fiber-optic embedded, light-transmitting LiTraCon concrete.
   

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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Reed Business Information is a Registered Provider with the American Institute of Architects Continuing Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA members. Certificates of Completion for non-AIA members are available on request.
       This program is registered with the AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

         
 

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