[Edit] Properties
Main article: Properties of concrete
Concrete has relatively high compressive strength, but significantly lower tensile strength, and as such is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep.
Tests can be made to ensure the properties of concrete correspond to specifications for the application.
[edit] Environmental concerns
For the environmental impact of cement production see Cement
[edit] Worldwide CO2 emissions and global change
The cement industry is one of two primary producers of carbon dioxide (CO2), creating up to 5% of worldwide emissions of this gas, of which 50% is from the chemical process, and 40% from burning fuel.[25]. The embodied carbon dioxide (ECO2) of one tonne of concrete varies with mix design and is in the range of 75 – 175 kg CO2/tonne concrete.[26] The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix. Indeed, 900 kg of CO2 are emitted for the fabrication of every ton of cement.[27] Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide[28], and also through the use of energy, particularly from the combustion of fossil fuels.
[edit] Surface runoff
Surface runoff, when water runs off impervious surfaces, such as non-porous concrete, can cause heavy soil erosion. Urban runoff tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants from sidewalks, roadways and parking lots.[29][30] The impervious cover in a typical city sewer system prevents groundwater percolation five times than that of a typical woodland of the same size.[31] A 2008 report by the United States National Research Council identified urban runoff as a leading source of water quality problems.[32]
[edit] Urban heat
Both concrete and asphalt are the primary contributors to what is known as the urban heat island effect.
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature.[33] A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat and contributes to the warming of cities. By paving with light colored concrete, in addition to replacing asphalt with light-colored concrete, communities can lower their average temperature.[34]
Many U.S. cities show that pavement comprise approximately 30-40% of their surface area.[33] This directly impacts the temperature of the city, as demonstrated by the urban heat island effect. In addition to decreasing the overall temperature of parking lots and large paved areas by paving with light-colored concrete, there are supplemental benefits. One example is 10-30% improved nighttime visibility.[33] The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning decreases, saving vast amounts of energy.
Atlanta has tried to mitigate the heat-island effect. City officials noted that when using heat-reflecting concrete, their average city temperature decreased by 6 °F.[35] New York City offers another example. The Design Trust for Public Space in New York City found that by slightly raising the albedo value in their city, beneficial effects such as energy savings could be achieved. It was concluded that this could be accomplished by the replacement of black asphalt with light-colored concrete.[34]
[edit] Concrete dust
Building demolition, and natural disasters such as earthquakes often release a large amount of concrete dust into the local atmosphere. Concrete dust was concluded to be the major source of dangerous air pollution following the Great Hanshin earthquake.[36]
[edit] Health concerns
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns. Natural radioactive elements (K, U and Th) can be present in various concentration in concrete dwellings, depending on the source of the raw materials used.[37] Toxic substances may also be added to the mixture for making concrete by unscrupulous makers. Dust from rubble or broken concrete upon demolition or crumbling may cause serious health concerns depending also on what had been incorporated in the concrete.
[edit] Concrete handling/safety precautions
Handling of wet concrete must always be done with proper protective equipment. Contact with wet concrete can cause skin burns due to the caustic nature of the mixture of cement and water.
Secondary efflorescence: Water seeping through the concrete, often in cracks, having dissolved components of cement stone: like Osteoporosis of the concrete. This often happens in parking garages, as road salt comes off cars as a saline solution in the winter, then affecting the concrete floor the cars are parking on.
[edit] Damage modes
Main article: Concrete degradation
Concrete can be damaged by freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonation, chlorides, sulfates and distillate water).
Manufacturers of cement and concrete admixtures must keep on top of microbiological contamination in raw materials, intermediates and final products to prevent product spoilage. One method of keeping controlling contamination is through 2nd Generation ATP test.[38]
[edit] Concrete repair
Concrete pavement preservation (CPP) and concrete pavement restoration (CPR) are techniques used to manage the rate of pavement deterioration on concrete streets, highways and airports. Without changing concrete grade, this non-overlay method is used to repair isolated areas of distress. CPP and CPR techniques include slab stabilization, full- and partial-depth repair, dowel bar retrofit, cross stitching longitudinal cracks or joints, diamond grinding and joint and crack resealing. CPR methods, developed over the last 40 years, are utilized in lieu of short-lived asphalt overlays and bituminous patches to repair roads. These methods are often less expensive[citation needed]than an asphalt overlay but last three times longer and provide a greener solution.[39]
CPR techniques can be used to address specific problems or bring a pavement back to its original quality. When repairing a road, design data, construction data, traffic data, environmental data, previous CPR activities and pavement condition, must all be taken into account. Pavements repaired using CPR methods usually last 15 years. The methods are described below.
Slab stabilization restores support to concrete slabs by filling small voids that develop underneath the concrete slab at joints, cracks or the pavement edge.
Full-depth repairs fixes cracked slabs and joint deterioration by removing at least a portion of the existing slab and replacing it with new concrete.
Partial-depth repairs corrects surface distress and joint-crack deterioration in the upper third of the concrete slab. Placing a partial-depth repair involves removing the deteriorated concrete, cleaning the patch area and placing new concrete.
Dowel bar retrofit consists of cutting slots in the pavement across the joint or crack, cleaning the slots, placing the dowel bars and backfilling the slots with new concrete. Dowel bar retrofits link slabs together at transverse cracks and joints so that the load is evenly distributed across the crack or joint.
Cross-stitching longitudinal cracks or joints repairs low-severity longitudinal cracks. This method adds reinforcing steel to hold the crack together tightly.
Diamond grinding, by removing faulting, slab warping, studded tire wear and unevenness resulting from patches, diamond grinding, creates a smooth, uniform pavement profile. Diamond grinding reduces road noise by providing a longitudinal texture, which is quieter than transverse textures. The longitudinal texture also enhances surface texture and skid resistance in polished pavements.
Joint and crack sealing minimizes the infiltration of surface water and incompressible material into the joint system. Minimizing water entering the joint reduces sub-grade softening, slows pumping and erosion of the sub-base fines, and may limit dowel-bar corrosion caused by de-icing chemicals.[40]