Concrete properties, uses, and applications

Concrete

HISTORY of CONCRETE

The history of concrete goes back about twelve million years according to the Department of Materials Science and Engineering at the University of Illinois at Urbana-Champaign (www.matse1.mse.uiuc.edu).What happened back then was that limestone and oil shale mixed to create a "spontaneous combustion" in what is now the country of Israel, and the result of that combustion was the formation of cement compounds. In 3,000 BC the Egyptians mixed mud and straw to "bind" dried bricks. They also used gypsum mortars and mortars of lime in the building of the pyramids. In 800 BC the Greeks on Crete and Cyprus used lime mortars that "were much harder" than the mortars used later in history by the mortars.

The Romans used "pozzolana cement" from the Mt. Vesuvius region to build their Roman baths, the Coliseum and the Pantheon. There was a period in the Middle Ages when the use of pozzolan materials was "lost," according to the University of Illinois data; then in the 17th and 18th Centuries experiments with limestone, hydraulic cement (stucco), calcareous cements produced various adaptations of what we now know as concrete (cement is an ingredient of concrete). In 1824 Joseph Aspdin of England invented "Portland cement" by burning "finely ground chalk with finely divided clay in a limekiln until carbon dioxide as driven off."

In 1887 Henri Le Chatelier of France was first to establish "oxide ratios" in order to prepare the proper amount of lime to produce Portland cement. His components (he named them) include: alite, Belite, and Celite. In 1889 the first concrete reinforced bridge was built, and in 1903 the first concrete high rise building was built in Cincinnati, Ohio. In 1936, the first "major concrete dams, Hoover Dam and Grand Coulee Dam," were constructed. The Federal Interstate Highway Act was signed into law in 1956.

Another of the forerunners to today's concrete was "grout"; and according to the Milton House Museum in Milton, Wisconsin, the Milton House (built in 1844) is the "oldest poured grout (concrete) structure in the United States" (www.miltonhouse.org).The builder, Joseph Goodrich, reportedly made architectural and structural history with his hexagon-shaped hotel and tavern. He used "slaked (burnt) lime, sand broken stone, gravel and water, all materials were native to southern Wisconsin. He used one bushel of lime to every seven or eight bushels of gravel. What was truly unique about this grout building was that Goodrich was a stopover for runaway slaves as part of the Underground Railroad. He built a tunnel under the Milton House which led to a cabin out back, where the runaways could have safe hiding until being spirited away into northern Wisconsin or Minnesota; there, they were given jobs on farms, safe from bounty hunters.

DIFFERENT KINDS of CONCRETE

First of all, what is concrete? According to the Physics Factbook (a project of Brooklyn College's Advanced Placement Physics teacher Glenn Elert and his students) concrete is the "artificial material similar to stone" used in a variety of structural environments. It is made by mixing several different "coarse aggregates such as sand and pebbles with water and cement" - followed by a process of letting it harden by hydration. What does hydration do? It causes crystals to form and the crystals "interlock and bind together." The Physics Factbook quotes the Brooklyn Public Library as claiming the first concrete was made in 500 BC, and that concrete can last "up to 50,000 years."

There are several kinds of concrete; reinforced concrete is one that is very important, and is strengthened by steel; to make this product, concrete is cast around steel rods or bars for reinforcement. Bridges and large buildings use this strategy for extra strong applications of concrete. Then there is "pressed concrete" which is produced by casting concrete around steel cables that are stretched by hydraulic jacks. Once the concrete hardens, according to the Physics Factbook, the jacks are released and the cables compress the concrete. After all, concrete is at its strongest and hardest when it is compressed. Roofs, floors, and other surfaces are where compressed concrete is most appropriately used. Precast concrete is used in building materials when there is a need for "a mass number of concrete building materials" - usually as blocks. About two-thirds of masonry walls in the United States are composed of precast concrete blocks.

Another kind of concrete is called "air-entrained" which is used for areas with very harsh, cold weather; airport runways and roads in cold climates utilize air-entrained concrete. High-early-strength concrete hardens quickly and is lighter than ordinary concrete because it is made with pumice. It is used in "hurried jobs and cold weather," the Physics Factbook explains. The normal density of concrete is 2,400 kg/m3; but the density of lightweight concrete is 1,750 kg/m3. Typically, the average density of concrete is 2,300 kg/m3.

The most common kinds of concrete that are available in homebuilding and commercially, as explained by the Robinson-Vitale Companies in New York State, are "interior" concrete and "exterior" concrete. When freezing will occur, exterior concrete is vital, but where there will never be any freezing, interior concrete is appropriate. As explained in the previous material in this paper, the air-entrainment approach is used in concrete that will be outdoors in harsh weather. Robinson-Vitale's Web site: (http://www.robinsonrolloff.com) explains that air entrainment - when "mixed properly in the batch" - will produced "countless numbers of microscopic air pockets."

Those tiny air pockets then allow space for the moisture that will be drawn into the concrete to expand during the time when moisture freezes. Why the air pockets? The analogy is this: if you fill a glass jar up to the top, cap it and put it into the freezer, it will shatter. There is no room for expansion. Fill it only part way up, and it won't shatter because there is room to expand. Cold expands and heat contracts, and it is true even with a very hard substance like concrete. For building purposes, concrete is broken down into "footing mix" and "slab mix," according to Robinson-Vitale companies. A mix of 2,500 Psi is a basic footing mix, and there are higher mixtures, up to 3,000, 3,500, and 4,000 and beyond.

DIFFERENT APPLICATIONS of CONCRETE

Recycling and sustainability issues: The company "Cement Americas" - in a section called "Cement, Concrete and the Environment" - explains that the use of concrete "...minimizes the depletion of our natural resources." The ingredients come from "water, aggregate (sand and gravel or crushed stone), and cement." And since cement is composed of 75% limestone (the most common mineral on the planet), and supplies of limestone are "virtually inexhaustible" - and the extracting raw materials for concrete has a lower impact on the environment than many other construction materials - concrete is relatively resource efficient. There is always a price to pay when digging in the earth, extracting minerals for building, but Cement Americas claims that quarries, which are the primary source of raw materials, can readily be "reclaimed for recreational, residential, or commercial development." They can also be restored fairly easily "to their natural state."

Concrete is nearly inert, so it is "quite suitable" as a medium for recycling waste or other industrial byproducts, Cement Americas asserts. Materials that would normally be dumped into landfills are today useful in the making of concrete. Aggregate can contain blast furnace slag, which is a byproduct of steel making. Just about all concrete contains "fly ash," which is a byproduct of coal-fired electric generating plants, Cement American explains on their Web site. There are about twenty million tons of fly ash that are produced annually, and seven million of those twenty million are used in making concrete. Making cement is a useful way to utilize certain waste materials, like scrap tires, which have high energy content and they "supplement coal as a fuel," the Web site explains. As from coal combustion, fly ash from power stations, and mill scale and foundry sand from steel casting provide "the silica, calcium, alumina and iron" that are needed as products for the making of cement. "Even kiln dust, a solid waste" that is generated by the manufacturing of cement, can be recycled back into the kiln as a raw material.

Even old abandoned chunks of concrete can be recycled and used as aggregate for new concrete mixtures, Cement Americas continues. Concrete yields "45% to 80% usable coarse aggregate" and it can be reused by crushing it with other materials that go into the making of new concrete. The energy efficiency of concrete is well-known; the only energy intensive ingredient that goes into concrete is Portland cement, and cement is only 10% to 15% of concrete. The other ingredients previously mentioned of course are aggregate and water, which have los energy requirements.

And what are the costs in terms of transporting concrete? Cement Americas says those costs are generally low because most concrete "is produced locally." Fuel requirements for the transportation and handling of concrete are "minimized" because "at least 60% of all concrete" is produced within about one hundred miles of the job site where it is being used. In homes and buildings, the thermal mass of concrete plays a big role in energy efficiency. The high thermal mass of concrete offers this benefit: it stores and releases the energy required for heating or cooling and hence, reduces "temperature swings in homes and buildings."

Interestingly, concrete also helps big rig trucks and "over-the-road trucks" use less fuel; that is because concrete's rigid pavement design is better than asphalt pavement in terms of fuel consumption. And concrete pavement is "light-reflective" and so it requires "less energy than other materials to illuminate." The Cement Americas narrative goes on to report that members of the Portland Cement Association have voted to adopt a goal of reducing carbon dioxide emissions "per ton of product by 10% (from 1990 levels) by the year 2020."

The Portland Cement Association (PCA), meantime, has its own informational Web site (www.cement.org) and the PCA claims that concrete is not only durable and easy to use in construction, but it "often is the most economical choice." That is because load-bearing concrete exterior walls in buildings "...serve not only to enclose the buildings" and of course keep the elements out; but also concrete walls "carry roof and wind loads, eliminating the need to erect separate cladding and structural systems."

Of course the manufacturers of concrete are in competition with steel manufacturers, so one would expect to read positive comparisons in favor of concrete; the PCA narrative does say that steel construction "can be advantageous" in certain areas of the U.S. "where local market conditions and traditions favor it." But in the south and western regions of the U.S. (that have traditionally strong "masonry" architecture), concrete is the most cost-effective choice.

Hurricanes are a reality in Florida, so concrete is frequently the material of choice PCA says because concrete can withstand high winds (tornadoes as well as hurricanes) in most instances. It is also resistant to insects (unlike wood), which are a huge pest issue in Florida. In California, where fires are a frequent yet tragic event, concrete is used because it doesn't burn.

The PCA explains that there are four methods of concrete construction used to build "load-bearing walls for low-rise construction;" Tilt-up, precast, concrete masonry and cast-in-place. Two of those will be described on the following page.

Tilt-up construction: this is suited to shopping center and warehouse construction because contractors "can form the windowless, unarticulated wall panels quickly and economically," PCA points out. Tilt-up can also be used for buildings that do have windows. The way tilt-up construction is done, the concrete is poured in a horizontal position, then lifted with a crane into place to actually construct the building.

Precast construction: this was discussed earlier, but suffice it to say this kind of concrete can be appropriate for buildings in which the concrete patter "can be repeated"; and the "...more times a concrete shape or panel can be repeated, the greater economy can be achieved." There is an advantage for contractors with precast concrete and that is "factory control"; the strength, appearance and quality is able to be very carefully monitored and regulated in a factory environment where supervision and oversight are part of the normal daily operations.

Earthquakes are commonplace in California, Japan, and elsewhere along the various tectonic plates, and as to concrete and its benefits during a big earthquake the PCA explains that depends on whether the structure was "properly designed, detailed, and constructed to resist the lateral side-to-side loading created by the shaking of the earth." And for a concrete association to admit that their product isn't necessarily the most effective is instructive to the reader. To wit, the likelihood of a given structure surviving a big earthquake depends "more on how the structure is engineered than on what type of material is used to build it." big earthquake struck Kobe, Japan in 1995, and the follow-up engineering data shows that only 4.9% of concrete buildings and 5.3% of steel constructed buildings collapsed. There were 5,000 deaths, and 34,000 injuries that resulted from the Kobe earthquake, and most of those, the PCA narrative points out, were caused by "the widespread collapse of traditional one-and-two-story, wood post-and-beam houses." The problem with these structures is they relied on interlocking pieces of wood, "rather than with nails or other positive connectors."

Earthquakes and concrete: An article in the publication of the American Society of Civil Engineers (Cardno, 2006) (Civil Engineering News) explains that "fiber-reinforced concrete can markedly increase the ability of slab-to-column connections" in terms of sustaining its integrity during an earthquake - without substantial damage resulting. Researcher from Michigan and Minnesota were searching for cost-effective ways to make slab-to-column framing systems more secure in seismically active locations. The amount of lateral movement that is possible in a slab-to-column connection before there is "punching shear damage" depends on the gravity load that is present, the article explains. It was a matter of trying reinforcing fibers of varying strength until the right combination would solve the problem, the writer explains.

In order to do the testing at various levels of earthquake-like shaking, wireless sensors are used as monitors to record the behavior of the fibers that are built into the concrete. And so, besides the advantage of stronger concrete, the sensors would, in the "immediate aftermath of a catastrophic event," record data that would then be instantaneously routed to emergency response centers "via satellite-based communications devices" which would very probably be able to sustain integrity during a big temblor. That way the damage done could be assessed electronically and digitally prior to a "catastrophic failure" would occur. Lives could be saved and damage minimized, all because small fibers (and sensors) would be planted inside concrete.

New West Virginia Concrete Bridge: The longest concrete box girder span in the United States is under construction across the Kanawha River in West Virginia, according to Civil Engineering News. The bridge will be 2,975 feet long and will link the cities of South Charleston and Dunbar. The main span will be 760 feet, a record in the U.S. according to the article, which explains that the bridge is being built using the "balanced-cantilever" method so no temporary structures will be needed during construction (Brown, 2007). Any temporary structures could have impacted commerce on the rive, because coal barges move up and down the river frequently. The bridge will be less expensive than a steel bridge would have been; the low bid for a steel constructed bridge was $113 million, while the low bid (winning bid) for a concrete bridge was $83 million.