quarta-feira, 14 de outubro de 2009

Curing and Sustainability

Curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. The result of this process is increased strength and decreased permeability. Curing is also a key player in mitigating cracks in the concrete, which severely impacts durability.

In addition to curing being a fundamental step in the concreting process, the drive for sustainability makes paying particular attention to curing all that more important. When smart, suitable, and practical curing is used, the amount of cement required to achieve a given strength and durability can be reduced by either omission or replacement with supplementary cementitious materials. Since the cement is the most expensive and energy intensive portion of a concrete mixture, this leads to a reduction in the cost as well as the absolute carbon footprint of the concrete mixture.

It is also necessary that the concrete be allowed to properly mature to provide the full service life. It is this service life, which can extend to hundreds or thousands of years, that provides concrete with the inherent ability to span the needs of many generations. More on curing concrete.

Focus on Floors (Click here or on individual topics.)

The following sections will help you through the various issues relating to concrete slabs and floors (click on topic). You will find that there is much more than you imagined to that hard surface you walk on every day.


Building Tips for Trouble-Free Concrete Slabs

Concrete is the material of choice for driveways, sidewalks, patios, steps, and for garages, basements, and industrial floors. It is relatively inexpensive to install and provides an attractive, durable surface that is easy to maintain. Proper attention to the standard practices and procedures for constructing exterior or interior concrete can yield a concrete surface that will provide long-lasting, superior performance.Click here for some building tips that will aid in the construction of quality concrete projects.

Placing Contraction/Control Joints in Concrete Flatwork

Placing contraction jointThe most widely used method to control random cracking in concrete slabs is to place contraction/control joints in the concrete surface. As concrete hardens, there is a reduction in volume, often resulting in cracking of concrete. Joints produce an aesthetically pleasing appearance since the crack takes place below the finished concrete surface. The concrete has still cracked, which is normal behavior, but the absence of random cracks at the concrete surface gives the appearance of an un-cracked section.

Contraction joints should be placed to produce panels that are as square as possible and never exceeding a length to width ratio of 1 ½ to 1. Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness and established to a depth of ¼ the slab thickness. Joints should be sawed as soon as the concrete will withstand the energy of sawing without raveling or dislodging aggregate particles. For most concrete mixtures, this means sawing should be completed within the first 6 to 18 hours and never delay more than 24 hours. More about contraction joints.

Isolation/Expansion Joints: Why and Where they are Used

Isolation joint around a columnIsolation joints are used to relieve flexural stresses due to vertical movement of slab-on-grade applications that adjoin fixed foundation elements such as columns, building or machinery foundations, bridge abutments, light standards, drop inlets, and so on. The isolation joint material allows the slab-on-grade to move up or down with the changes of soil support conditions. Heaving/settling of moist soils due to freeze/thaw cycles and long-term settlement are the primary causes of changes in soil support conditions. In addition, an isolation joint may be used in slabs that require a change in contraction joint layout, which would create T intersections. The isolation joint would be considered a free edge allowing the termination of a contraction joint at a T intersection.

Expansion joints are used primarily to relieve stress due to confinement of a slab. If the slab is placed adjacent to structures on more than one face of the slab, an expansion joint should be placed to relieve stress. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings. Confinement on three faces would normally be handled by placing expansion joints on all three faces. Confinement on four faces should be isolated on all faces. This allows for thermal expansion and contraction without inducing stress into the system.

Finishing Air-Entrained Concrete 

Micrograph of air-entrained concreteUsed in many applications, air-entrained concrete is concrete that uses air-entraining cement or an air-entraining admixture to produce a system of small voids within the hardened cement paste. These voids develop during the mixing process and stabilize through action by the air-entraining mechanism. The primary use of air-entraining concrete is for freeze-thaw resistance. The air voids provide pressure relief sites during a freeze event, allowing the water inside the concrete to freeze without inducing large internal stresses.

Hard troweling is a process by which a finisher uses a steel trowel to densify the surface of the concrete. This finish is optional and produces a hard, smooth surface. Hard-troweled surfaces are not recommended for exterior concrete slabs, because the smooth finish becomes slippery when wet. Hard troweling is also not recommended for air-entrained concrete for several reasons. More on finishing air-entrained concrete.

Safety Measures for Concrete Construction

Concrete construction is no exception to the importance of construction safety. Although claiming one of the lower jobsite-injury rates, dangers associated with both the material aspects and construction practices of concrete construction must be addressed to continue the industry's focus on safety. Heightened awareness, improved safety training programs, and diligent enforcement are the keys to improving safety on the jobsite. More.


Bugholes: Causes and Prevention of a Pesky Problem

One of the primary influences affecting the surface quality of concrete is bugholes. Bugholes, pinholes, blowholes, surface voids – they are recognized by various names, but all refer to a common problem that contractors want to avoid. Bugholes, are small, regular or irregular cavities (usually not exceeding 15 mm [9/16 in.]) resulting from entrapment of air bubbles on the surface of vertically formed concrete structures during placement and consolidation. More.

Conductive Concrete for Bridge Deck Deicing

Heated deck of Roca Spur Bridge in Nebraska is the world's first implementation using conductive concrete for deicing. To read full article by Christopher Y. Tuan, Ph.D., P.E., Associate Professor of Civil Engineering, University of Nebraska click here.


First Use of Ultra-High Performance Concrete for an Innovative Train Station Canopy

The Shawnessy Light Rail Transit (LRT) Station, constructed during fall 2003 and winter 2004, forms part of a southern expansion to Calgary's LRT system and is the world's first LRT system to be constructed with ultra-high performance concrete (UHPC). To read the full article by V. H. Perry and D. Zakariasen, Lafarge Canada Inc., click here.

Pervious Concrete and Freeze-Thaw

Pervious pavements have been used for years throughout the warmer climates of the United States with excellent results. However, in climates prone to severe freeze-thaw cycles, some are hesitant to use these pavements until it has been proven that pervious concrete can be made to resist freeze-thaw damage. Research on this topic is currently underway. More.

 

"Bendable Concrete" Replaces Bridge Expansion Joints


University researchers have collaborated with a state Department of Transportation to apply bendable concrete in a local bridge project. Engineered Cementitious Composites (ECC) have been shown to have all of the characteristics sought by highway designers and structural engineers for a highly durable concrete material. The distinctive property of ECC is the ability to bend while maintaining its compressive strength. These properties make the material a good fit for use in place of bridge expansion joints as demonstrated in this innovative project. More.

Concrete Shines as Solar Reflectance Material

Concrete does a very good job of reflecting solar energy. That is the finding from a recent PCA study which measured the solar reflectance of 135 concrete specimens from 45 mixes representing exterior concrete flatwork. In fact, all concretes tested in this study would qualify for LEED® credits for Heat Island Effect.

Solar reflectance index (SRI), a calculated value based on solar reflectance, SR, is one way to determine how much light energy a material reflects: stated another way, comparing SRI or SR of different materials tells which ones absorb less solar radiation. This is useful because darker materials absorb more heat, which is generally considered undesirable for its effect on the environment. This may have an immediate, local effect, like heat gain in urban areas (heat island). More.

Read the complete report describing test procedures, concrete mixes, materials, and other aspects of this study, Solar Reflectance of Concretes for LEED Sustainable Sites Credit: Heat Island Effect (SN2982).

Hot Weather Concreting

When the temperature of freshly mixed concrete approaches approximately 25°C (77°F) adverse site conditions can adversely impact the quality of concrete. Ambient temperatures above 32°C (90°F) and the lack of a protected environment for concrete placement and finishing (enclosed building) can contribute to difficulty in producing quality concrete.

Liquid nitrogen added into a truck mixer.The precautions required to ensure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general, if the temperature at the time of concrete placement will exceed 25°C (77°F) a plan should be developed to negate the effects of high temperatures. More on the suggested precautions for hot weather concreting.

Cold Weather Concreting

heated enclosureCold weather concreting is a common and necessary practice, and every cold weather application must be considered carefully to accommodate its unique requirements. The current American Concrete Institute definition of cold-weather concreting, as stated in ACI 306 is, "a period when for more than 3 successive days the average daily air temperature drops below 5°C (40°F) and stays below 10°C (50°F) for more than one-half of any 24 hour period." This definition can potentially lead to problems with freezing of the concrete at an early age. 

Rule number one is that ALL concrete must be protected from freezing until it has reached a minimum strength of 3.5 MPa (500 psi), which typically happens within the first 24 hours. In addition, whenever air temperature at the time of concrete placement is below 5°C (40°F) and freezing temperatures within the first 24 hours after placement are expected, the following general issues should be considered: (1) Initial concrete temperature as delivered; (2) Protection while the concrete is placed, consolidated, and finished, and (3) Curing temperatures to produce quality concrete. More about cold weather concreting.

Drying of Concrete

Unwanted moisture in concrete floors routinely causes millions of dollars in damage to buildings in the United States. Problems from excessive moisture include deterioration and de-bonding of floor coverings, trip-and-fall hazards, microbial growth leading to reduced indoor air quality, staining, and deterioration of building finishes.

The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs. More on the difference between curing and drying.

The term "concrete moisture" is understood to mean the total water used in the concrete batch, plus curing water, minus the water bound in hardened cement due to hydration. Drying begins when water is no longer available at the exposed surface. More on how long it takes concrete to dry.

The moisture content of concrete must be viewed from the context of total water content of the fresh concrete mixture and the available moisture content of the hardened concrete. The total water content of a fresh concrete mixture is a function of the total cementitious materials and water cement ratio (w/cm). Read an FAQ about the moisture content in concrete.

Identifying and Evaluating Concrete Defects 

Dusting of concrete surfaceConcrete structures are regularly constructed without complications. However, defects can occur that can be traced to problems related to environmental conditions during construction or with the concreting procedures used. In order to determine a repair method, it is necessary to identify what caused the defect . Evaluation of deficiencies helps ensure that repairs will be effective and the defect will not extend into the surrounding concrete.

Many concrete defects are immediately recognized and others are not. Concrete defects can be broken down into four broad groups based on visual observation: deformation of the surface, cracking of the surface, disintegration of the surface, and other defects.

Visual examination typically does not provide enough information to determine the cause or causes of the defect. In some cases, it may not provide evidence of a defect at all. In order to narrow the scope of an investigation to probable causes and suitable repair methods, the appropriate information factors and the proper evaluation methods need to be identified. More on concrete defects.

The Perils of Power Washing

Powerwashing graffiti covered wallIt is springtime and that means spring cleaning. Building owners and property managers will be looking at every exposed surface on their property and then deciding how to clean those surfaces. The cleaning method chosen for concrete requires careful consideration: Power washing would seem to be a good choice because, after all, it is simple, fast, and effective. Right? Not so fast…

Power washing concrete surfaces can cause real problems. Relatively inexpensive high-pressure power washing units are commonly available. Some of those units can deliver water at pressures well in excess of 6,000 psi! Moreover, it is not just high-pressure water that's the problem. The water exits the nozzle at both a high pressure and a high velocity. The resulting momentum is great enough to dislodge not only dirt and debris but also to create flakes, popouts, and even concrete spalls. Good quality concrete will also experience accelerated wear from high-pressure power washing. More on power washing concrete.

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