Cement and water form a paste that coats each stone particle and sandes the aggregates. Through a chemical reaction called hydration, cement paste hardens and gains strength. The quality of the paste determines the character of the concrete. Perhaps the most important thing to understand about concrete is the role of water.
First of all, it provides plasticity so that concrete can be poured into a form. However, its real importance lies in the hardening process. Wet concrete does not harden when it dries. Instead, water is a chemical component in a curing process.
Compounds that react with water are in portland cement. Grab a piece of string and pull in any direction. You just put the rope in tension. If you can pull hard enough, the rope will fail in tension when broken.
Concrete, although quite strong in compression, quickly fails in stress by cracking. The resistive strength of concrete for compression is around 4,000 pounds per square inch, while the resistive strength of concrete in tension is probably less than 400 pounds per square inch. In general, the tensile strength of concrete is less than 10% of its compressive strength. The amount of reinforcing bars used in typical structures is a small percentage of the amount of concrete.
Most beams, for example, use about 1% reinforcing bars to withstand tensile forces in bending. Columns can use up to 6% reinforcing bars, partly because the reinforcing bar withstands axial and tensile forces. As reinforcing bars cost much more than concrete, efficient engineering design minimizes the use of reinforcing bars. In the recent past, it was common for structural drawings to indicate a bar overlap of 40 mm in diameter for all splices.
Experience shows that the simple solution is too conservative in some cases and causes failure in other cases. Therefore, a significantly more complicated set of rules was adapted for the splicing of bars. It is important that the Construction Supervisor understand at least the terminology of the American Concrete Institute (ACI) rules for reinforcing bar splicing. An important element to understand in concrete work is the water-cement relationship.
A minimum amount of water, approximately 25% of the weight of the cement, should be included to chemically hydrate the concrete batch. However, in the actual mixing process, between 35% and 40% water is needed to complete the mixing process, reach the real cement and cause effective hydration. However, in practice, much more water is added to increase the workability of concrete. So why does it matter if there is a lot of water in the concrete mix? Any water above the theoretical ideal of 25% is not used in the chemical hydration process.
Therefore, excess water remains in the concrete while the concrete cures. Over time, this excess water evaporates from the concrete and voids remain. These gaps weaken the concrete, causing less resistance and more cracking. The crash test should be familiar to most workers on a construction site.
Wet concrete is placed in the shape of a steel cone and placed on a non-absorbent surface, with the widest part of the cone facing down. Then the steel conical shape is raised, allowing the wet concrete to sink a little, depending on the design of the mixture. A dry mix can only fall from 1 to 2.The normally specified settlement is around 4.Drops of 6 to 7 can be achieved through the use of high-end water reducing agents (superplasticizers). Special mixes for pumping concrete tend to have large sinking.
Another important test for concrete is the cylinder compression test. The strength of concrete is generally referred to as the 28-day compressive strength. Why 28 days? What's so magical about 28 days? Nothing. The 28-day period for testing the compressive strength of concrete is an arbitrary time selected to give consistency to the test procedures.
Therefore, the 28-day compressive strength of concrete has become the standard in the industry. Therefore, when specifying 4,000 psi concrete for a concrete beam, this means that the concrete actually placed must have a compressive strength greater than 4,000 psi after 28 days. Since the strength of concrete continues to increase over time, a standard period of time is necessary for the measurement of concrete strength. Concrete cylinders that are manufactured to determine the 28-day strength can also break earlier and provide useful information.
Cylinders commonly break within 7 days, which typically have developed about 75% of the 28-day strength. It's good to know 3 weeks before if there is a problem with a concrete batch. Breaking cylinders after 3 days can also provide useful data. If a supported slab has been laid, 3-day concrete breaks can be used to determine whether pickling or formwork and formwork supports will be safe.
Therefore, concrete cylinder breaks provide a lot of useful information. The basics of cylinder manufacturing should be understood by the Construction Supervisor. When wet concrete is laid, cylinders 6 in diameter and 12 in height are filled with concrete and carefully consolidated (see Manufacture of concrete cylinders for testing). These cylinders are then cured, hopefully under conditions similar to the curing conditions for the main concrete pour.
The concrete cylinders harden in a few hours and are stored for future testing. The Construction Supervisor must also pay attention to the care and storage of concrete test cylinders between the time they are manufactured and broken. A few years ago, during the construction of an addition to the building of a wastewater pumping station, the Construction Supervisor stored the concrete test cylinders inside the pumping station to protect them from inclement weather. When the cylinders broke within 28 days, the supposedly 4,000 psi concrete was reaching only 2,500 psi of compressive strength.
Immediately began the conversation about the demolition of the new concrete walls and began to point the finger for responsibility. A core drill was taken from the wall and the concrete was tested well above the 4,000 psi requirement. These problems do not mean that mixed concrete at the workplace is unacceptable, only that the quality of concrete will be much more variable than ready-mix concrete. Therefore, the Construction Supervisor must be careful to allow concrete mixing at the job site if the final attributes of the concrete are critical (i.e.
If 4500 psi concrete is needed for concrete columns or if any floor cracking would be a major problem). The United States Air Force Training Guide for Structural Concrete Work is a 39-page training guide that teaches some concrete basic skills. It is found in Air Force Qualification Training Package for Structural Concrete. A third problem is that water that has seeped into the concrete through the cracks can freeze in winter, which means that it will expand and cause more cracks through which even more water will penetrate, leading to a vicious circle of degeneration and decomposition.
So the walls could be made of concrete and stone, just like the foundations, since both of them mainly resisted the downward compressive loads. Additives are materials in powder or fluid form that are added to concrete to give it certain characteristics that cannot be obtained with simple concrete mixes. Another advantage is that it is possible to use less prestressed or post-stressed concrete or smaller and thinner parts to support the same loads, compared to common reinforced concrete. The U.S.
Department of Defense produced a 59-page training document that contains some good joint details and design guides for understanding concrete floors. We can solve that problem by casting wet concrete around strong steel reinforcing bars (tied together to form a cage). Another popular effect for floors and tables is polished concrete, where concrete is optically flat polished with diamond abrasives and sealed with polymers or other sealants. Similar systems are still used; depending on the volume of the pouring, the concrete mix used and the ambient air temperature, the cooling process can last many months after the concrete is laid.
Adding a pigment called titanium dioxide, for example, is a simple way to make concrete shiny and white within a million miles of the dull gray matter that gives concrete parking lots a bad name. Of course, all building materials have some degree of expansion and contraction, but with reinforced concrete, such forces can literally break concrete. Modern concrete fails through what is informally known as concrete cancer or concrete disease, which involves three interrelated problems. Dirty brown spots seen on concrete with cancer are often caused by rusty water that drains through cracks.
Workability can be measured by concrete slump test, a simple measure of plasticity of a new batch of concrete following test standards ASTM C 143 or EN 12350-2.This has a general environmental benefit, as it takes harmful waste from CO2 emissions from power plants and converts them into very useful concrete. Tests can be performed to ensure concrete properties match application specifications. . .
