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Crane accidents are preventable. Most are caused by inexperience, neglecting to preplan a lift, failure to inspect the crane and rigging before a lift, not setting up the crane properly, improperly rigging the load, and other similar problems. Knowing the basics of safe crane operation not only...
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Craftsmanship, quality control, and teamwork were basic elements in producing an attractive, durable concrete floor at a competitive price for a warehouse store in Westland, Mich. The 122,000-square-foot, 8-inch-thick floor was completed in 10 working days. The floor profile quality exceeded...
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Simply said, shotcrete is sprayed concrete or mortar. There are two different types of shotcrete: wet mix and dry mix. With dry mix, the dry ingredients are mixed and blown in a stream of compressed air through the hose to the nozzle where water is added immediately before the material exits the...
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Higher prices are fine if you can deliver quality workmanship. What happens when you are charging more than other contractors who offer the same services? If you find that's the case, it's time to realize that you aren't competitive and that you can do one of two things: Improve your competitive...
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Cooperation seems to be the watchword for the 1993 winners and runners-up in the fourth annual National Awards Program for Excellence in Concrete Pavement. The awards recognize contractors, engineers, and owners who completed projects in 1992 that exemplify state-of-the-art construction and...
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Deciding to invest in a power trowel or to continue using traditional methods of hand finishing is not always an easy choice to make. Although there isn't a mathematical formula contractors can use to arrive at a decision, factors such as job size, time, and desired quality should be considered...
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Skill in handling finishing tools only comes with training and practice. But the first step to successful finishing is choosing the right tool for the job. Here are the basic tools needed for most flatwork finishing operations, and the uses for each:
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Whitetopping, placing a concrete overlay on deteriorated asphalt pavements, is Iowa's solution to higher maintenance costs. Since 1960, Iowa counties have whitetopped more than 300 miles of asphalt pavement, eliminating potholes, cracks, raveling and rutting surfaces, and loose gravel.
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The first shotcrete homes were built shortly after the shotcrete process was invented in 1910. In the 1960's a new shotcrete construction panel system was developed. Referred to as welded wire sandwich panels (WWSP), the system has a core of polyurethane or polystyrene insulation sandwiched between...
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Because so much of construction work is bid oriented, many contractors see little benefit in marketing their services. Remember, wherever there is a restricted bid list, there have been several successful marketing efforts. The reason is that the contractors on the list convinced a decision maker...
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Better safe than sorry - the old adage applies nowhere better than to formwork for concrete. Educate workers to construction site safety so they don't take unnecessary risks. Discussing chains of events that cause forms to fails is one way to help the crew erect systems that work well. Organize...
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Common materials for column forms include plywood, paperboard, plastic, and steel. Most systems produce simple round or rectangular columns. However, almost any column design is possible by building custom wood forms, modifying existing forms, or using precision-cut polystyrene forms or form...
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Composites, also known as fiber-reinforced plastics (FRP), have found a niche in the aerospace, automotive, and sporting equipment industries. More recent developments show they have promise for use as structural materials, including reinforcement for concrete.
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The proof of concrete quality is field performance under actual loading and environmental conditions. But before concrete experiences much of this exposure, it's necessary to determine its acceptability and make projections about its long-term performance. These judgements are based, in part, on...
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When it was completed in 1903 in Cincinnati, the Ingalls Building became the tallest reinforced concrete building in the United States. Although its overall dimensions were only 50 x 100 feet, the 16-story structure was twice as tall as any previous reinforced concrete building. It offered economy...
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Concrete has a natural tendency to crack because it's usually relatively weak in tension. When tension resulting from applied loads, restrained shrinkage, or temperature drops exceeds the strength of the concrete, a crack forms. Concrete can crack while plastic and after hardening. Cracking can be...
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In June 1993, students gathered in Sacramento, Calif., to find out who had the best-designed and fastest concrete canoe. They represented 20 universities that had claimed regional championships and been invited to take part in the Sixth Annual American Society of Engineers National Concrete Canoe...
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Scheduled to open in December 1993, Denver's new $2.7-billion International Airport will be the nation's second largest public works project.
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The purpose of isolation and expansion joints is to prevent damage from differential movements.
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A small percentage of foundations fail or become distressed, and the deficiency usually puts much more than the foundation at risk.
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Dependability is one of the hallmarks of modern boom trucks. They work hard and run long. Breakdowns are rare. In addition, they place concrete quickly and efficiently. Some pumps are capable of delivering more than 200 cubic yards of concrete per hour. Versatility is another attribute of today's...
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During the summer of 1992, a group of 21 U.S. highway and pavement experts including contractors, engineers, and representatives of government and trade organizations observed the condition and construction of highways in western Europe. Called the U.S. Tour of European Concrete Highways (U.S...
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Planning ahead and communicating with the supplier are fundamental aspects of ordering ready mix. Though every contractor has preferences on how to do things, here are some important considerations.
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Durable concrete resists weathering, chemical attack, and other types of deterioration. An important factor in concrete durability is concrete quality. Quality concrete has properties that match the application and expected exposure conditions.
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ASTM standards define precise procedures for performing field tests to determine the quality of freshly mixed concrete. Usual field tests measure consistency, strength, unit weight, air content, and temperature. Many test details seem trivial, even arbitrary. However, ASTM standards establish...
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Tilt-up concrete is a flexible building product that can provide superior results if managed and specified properly. But to achieve success in the tilt-up market, concrete contractors must do more than just master tilt-up construction procedures. They must establish a strong customer base by...
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Tilt-up concrete housing makes more sense today than ever before. Victor Greimann, a tilt-up pioneer from Kettering, Ohio, certainly thinks so. Greimann still remembers his first building endeavor with nostalgia. It was 1946, just after World War II. The homes were relatively small, like many...
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A sculptor and art instructor for more than 10 years, Rick True has used a variety of materials. He has created art from cast bronze, welded steel, and latex, and is best known for his large-scale wood sculptures. But True's ecological concerns about using lumber motivated him to explore the use of...
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Chemical admixtures help concrete contractors gain more control over their work and meet customer demands for year-round construction, shorter project completion times, and improved concrete performance. For example, accelerators can be used to shorten set times in cold weather, superplasticizers...
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SOLUTIONS FOR: Blistering, bugholes, crazing, curling, dusting, honeycombing, low test results, plastic shrinkage cracking, scaling, uncontrolled shrinkage cracking, uneven color, and wavy surface
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Several major bridge structures and buildings in San Francisco were damaged during the 1989 Loma Prieta Earthquake. The review of some structures revealed damage that could not be repaired economically.
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Machine-mounted hydraulic impact hammers and concrete crushers both demolish concrete methodically and efficiently. Using them effectively and economically, however, requires knowing which type of demolition tool is best for the job, what size tool is needed, and how to properly match tool to...
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Before beginning a demolition project, OSHA mandates that an engineering survey of the structure be undertaken by a competent person. The survey determines the condition of the framing, floors, and walls, and also the possibility of the unplanned collapse of any portion of the structure. Safe floor...
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Recently the Children's Hospital in Detroit experienced a unique construction occurrence - quiet demolition. This interesting method will have further applications for contractors working with tight restrictions on demolition.
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Breaking up an old concrete pavement and reusing it as aggregate is a cost-effective option for reconstructing deteriorated pavements. Recycling eliminates disposal problems and tipping fees. In urban areas where landfill space is scarce, dumping concrete is difficult and costly. The cost of...
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In most building construction, concrete effectively protects reinforcement from damage caused by environmental factors or exposure to fire. In severe environments found in some industrial plants, marine structures, bridge decks and parking structures, destructive effects of reinforcement corrosion...
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The accelerating effects of hot weather not only cause concrete to lose workability and finishability faster, they also can reduce long-term strength. There are many simple and effective solutions for avoiding hot-weather concreting problems, such as breaking the job into smaller placements or...
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Knowing the types of material hoists available and their operations can help you choose the hoist that is right for you.
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When Milwaukee's General Mitchell Airport Authority decided to build a 600,000-square-foot, 2,300-car parking structure, one of the bid requirements called for separate proposals for silica-fume concrete and a traffic membrane. In a bid submitted by Milwaukee contractor C.G. Schmidt Co....
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Architects specify patterned concrete surfaces for the aesthetic interest generated by a play of light and shadow, or to simulate economically in concrete the traditional patterns of brick, stone, and wood. A patterned concrete surface does not reveal so plainly minor imperfections that can mar a...
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Tilt-up is one of the fastest growing industries in the United States. This is due, in part, to the economics of tilt-up, which combine reasonable cost with low maintenance, durability, speed of construction, and minimal capital investment. A successful tilt-up project begins long before concrete...
The many ways to color concrete range from simply covering the surface with paint to coloring the entire placement (integral coloring) using pigments and special aggregates. Color effects can work two ways: they can mask surface blemishes or accent special features. Coloring materials and methods...
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In 1938, Dr. Ludd Spivey, president of Florida Southern College, commissioned Frank Lloyd Wright to produce a master plan for the school's lakeside campus. Situated on 100 acres in Lakeland, Fla., Wright set out to design a truly American campus.
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Embedded in concrete, across and along the joint, flexible waterstops prevent leaks in the joints of concrete structures where one side is subject to hydrostatic pressure. Flexible waterstops come in a variety of profiles and material types. To choose a waterstop suitable for your joint sealing...
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Safety has to be brought to a personal level. You've got to show your employees what's in it for them. For example:
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Epoxy injection is a proven technique for bonding cracked concrete sections. A successful epoxy injection requires evaluation, preparation, and planning. Before beginning epoxy injection repairs, identify the cause, or causes, of the problem. Also, be sure that epoxy injection is a proper repair...
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Vibrating screeds not only make a job less labor-intensive, they also let you do it faster and more efficiently. Some manufacturers claim that using a vibrating screed can cut finishing costs up to 50%. Savings like that can allow you to do more jobs at greater profit.
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Small and mid-size contractors should be aware of floor flatness concepts and some inexpensive ways to improve flatness. Improved flatness can be a marketing advantage, attracting new and repeat customers and providing opportunities for more work.
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Contractors who build circular tanks can choose from a variety of forms - wood or steel, job-built or manufactured, ganged or handset, custom or off-the-shelf. Some forms are adjustable for a wide range of radius dimensions; others can be used for only a single radius. All the methods, properly...
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Current methods can measure only the total air content of fresh concrete, not the gradation of air bubbles. Even so, it's important to measure the total air content in fresh concrete for two practical reasons. First, specifications may require a minimum or maximum air content. Testing shows if the...
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Because of finishing techniques for shotcrete construction, some tolerances must be larger than those achievable with formed construction. This allowance for increased tolerances need not diminish the serviceability, durability, or integrity of a structure. Tolerances provide an indication of the...
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Using tunnel forms for the first time, Blake Construction Co. Inc., San Diego, built an 11-story, concrete-frame residence hall in 16 months. The hall, completed in September 1992, is located at San Diego State University. Blake completed the structure on schedule, building a 30-room floor every 6...
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Recently, more and more U.S. businesses have turned to Total Quality Management (TQM) to give them a competitive advantage. Gradually, construction firms also have begun to adopt TQM philosophies.
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An accident's economic costs are far greater than most contractors realize. The reason is that many accident costs are hard to recognize or given little thought. However, like the underwater part of an iceberg, they loom large and dangerous.
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Costs of forming account for about half the value of typical cast-in-place concrete construction. Planning for maximum form reuse can reduce forming costs and provide an opportunity to win bids in competitive markets.
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Because appearance is important, patching and repair of architectural concrete needs to be done carefully. Attempts to patch some minor defects may make the flaws more noticeable. Since experts recommend that repairs be inconspicuous from about 20 feet away, flaws visible only at very close...
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Certification programs fit into two main categories: plant certification and personal certification. Prestressed, precast, and ready mixed concrete producer associations have established programs requiring thorough inspections of production facilities. The inspections are conducted by impartial...
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The Dunks Ferry Road Bridge over Poquessing Creek at the boundary between Philadelphia and Bucks counties in Pennsylvania demonstrates that small bridges need not be mundane. This single 60-foot-span bridge, a combination of cast-in-place and precast concrete, was deigned and is owned by the City...
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Panels with a 4-mil-thick, hard polymeric resin coating applied to exterior-grade plywood require no release agent. These are 4x8-foot panels bonded by waterproof glue and sealed at the edges to prevent moisture penetration.
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Reduce cold joints, honeycombing, and bugholes by training workers how to place and vibrate concrete in walls. The basic principles of placement and vibration are the same for any placing equipment used. If problems occur, however, corrective action can minimize their recurrence.
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A joint English and French project involved boring three tunnels through 31 miles of a chalk marl formation to link the cities of Dover and Calais. Two tunnels will accommodate high-speed trains; the third will be a service tunnel. The tunnels are lined with precast concrete ring segments. The...
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Sheet membranes have many benefits
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Though concrete has a tendency to crack and does crack in most cases, cracks can be controlled by joints, or planned separations in concrete. Cast-in place concrete walls use three types of joints.
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Air bubbles added to fresh concrete improve workability and frost resistance. Frost-resistant concrete requires an adequate number and volume of closely spaced, microscopic air voids. This is best accomplished with a minimum total air content and minimum impact on strength provided by a fine...
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In the dry-mix shotcrete process, dry blended mix materials are delivered through a line or hose to a nozzle, where water is added. There are two distinct types of dry-mix shotcrete equipment: single- and double-chamber pumps, and continuous-feed or rotary pumps which come in two types: barrel and...
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The Vegas World Stratosphere Tower, a reinforced concrete observation tower soaring 1,102 feet over Las Vegas is the tallest structure of its kind in the US.
For every crack, a trained investigator should record, with photographs and sketches, the following information:
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Las Vegas hotel construction is booming - good news for area concrete contractors and producers. Two of those hotels make extensive use of precast and cast-in-place construction.
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Building a 243,000-square-foot reinforced concrete warehouse floor to meet specified pour widths of 56 feet and floor flatness and levelness requirements of FF35 and FL25 is a challenge for any concrete contractor. Meidling Concrete Inc. had to build such a floor for a Shopko Stores Inc...
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For the past 23 years, Frank Rusk of Rusk Concrete Textures, Las Vegas, has been transforming conventional concrete flatwork into works of art. Using coloring agents and texture-stamping tools, he creates tile, slate, stone, and other custom-designed patterns in concrete. Applications range from...
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We're bidding a job that requires a white concrete floor. Another bidder has proposed mixing white latex paint in with the concrete. Can this work?
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We're repairing a bridge deck that requires several partial-depth patches. The engineer recently handed me a computer printout of the bridge deck that showed the delaminated areas. I asked him how the delaminated areas were detected, and he said that one method they use is called infrared...
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The late winter and early spring concreting seasons are coming up. Are there any special precautions we should take?
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We are bidding a job that has a slab-on-grade gymnasium floor with specified floor flatness of FF 100 / FL 50. Is this reasonable?
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Is it OK to shear steel rods when fabricating dowel bars for concrete slabs?
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Can you explain the difference between a sealer and a sealant?
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Where can we find information on systems for reinforcing building slabs built in the 1930s, and earlier?
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We sometimes patch existing asphalt parking lots with concrete. On a couple projects, we sawcut the area to be patched, removed the bad material, and replaced it with air-entrained concrete. This seems to be economical. The one drawback is the patches are so noticeable. Has anyone else done this...
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For the first time in years we have a job calling for lightweight concrete, but we haven't kept up-to-date on specifications for testing. The aggregate supplier told us to air dry the test cylinders after a seven-day moist cure. Is this correct?
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The Problem Clinic in the June 1993 issue of Concrete Construction, described the use of pull-out tests to determine the in-place strength of concrete. What about the break-off test?
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I once saw some rules of thumb about the effects of adding water to a concrete mix, but I can't find them.
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Bagged cement labelled "Type I/II" is being sold in our area. Is this a new type of cement in ASTM C 150?
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We are placing 4-inch-thick slabs. Can we use dowels or keyways for load transfer?
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Will immersion-type vibrators damage epoxy-coated rebars? I've heard that they do. If so, how do you consolidate concrete that contains epoxy-coated bars?
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We read your recent article on cleaning concrete (November 1992, page 791) thoroughly but failed to find any recommendations on the removal of straw stains from concrete. In the past year we've had several requests for help with yellowish brown stains cau
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We build residential basements and tried to work later in the season this year, but were concerned about the concrete coming up to strength in cold weather. Is there a field test that we can use to measure in-place strength?
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How can we give an existing concrete surface an antiqued finish?
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We supply ready mixed concrete for many jobs that use the American Concrete Institute "Specifications for Structural Concrete for Buildings," ACI 301. We have many field-strength test results for all our standard mixes and we use these to show that proposed concrete proportions will produce the...
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We've heard of the mat test for moisture in concrete. Where can we find out more?
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We're building a floor for which the plans call for keyed construction joints. The plans also call for wire mesh that runs continuous through the joint. We don't think this is a good detail either for constructibility or for floor performance. How can we convince the engineer that there are better...
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We're going to be placing and finishing the concrete for a post-tensioned concrete slab on grade in a warehouse. It's the first time we've done a post-tensioned floor. How does a post-tensioned floor differ from a conventional floor, and where can we get
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We're doing some remodeling of a hospital building that includes installing new floor coverings. The floor slab is a lightweight concrete placed over metal deck forms. Evidently when the concrete was placed, the top surface froze and was patched. Then it was covered with a rubber flooring. We're...
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Does an air-entraining agent serve any purpose in wet-process shotcrete? I believe shotcrete is placed with such force that any air-entraining effect is negated. Should we use air-entraining agents in concrete that will be placed by wet-mix shotcreting?
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Our standard flatwork mix is six-bag, 3/4-inch limestone, with 6% air and 1 1/2 pounds of polypropylene fibers. It is placed at a slump of 4 inches. When ambient temperatures dictate, we use a maximum of 1% calcium chloride. Adding the chloride at the ready mix plant ensures adequate mixing, but...
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We need to remove hardened construction adhesive from a new white concrete surface. We've tried grinding, but the material melts. Any other suggestions?
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We need to submit a concrete mix to be placed by pump. Slump and air are specified at the point of placement. Can we increase the initial mix water content to compensate for water lost in pumping and also satisfy the limit on water-cement ratio at the point of placement?
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What is the tolerance for elevations on cast-in-place concrete footings?
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A ready mix truck arrives at the jobsite with 8 cubic yards of concrete. Travel time was 10 minutes. We discharge a yard, then take a slump and air test. The air content is 6% and slump is 2 1/2 inches. We add 10 gallons of water to increase the slump, then discharge 5 more yards in about 15...
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A subcontractor built 350 lineal feet of 10-inch-thick foundation wall that's 8 feet high. When the forms were stripped, there were streaks that looked like veins on the surface. The subcontractor says they were caused by his using new aluminum forms. The
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We're going to be placing and finishing the concrete for a post-tensioned concrete slab on grade in a warehouse. It's the first time we've done a post-tensioned floor. How does a post-tensioned floor differ from a conventional floor, and where can I get information about construction methods for...