Concrete Material

Concrete

Concrete is a structural material which consists of Portland cement, aggregate (sand and rock), and water (to make the chemical reaction called hydration occur.) Concrete can sometimes contain other substances, such as fly ash from industrial smoke stacks, which can change its properties.

Concrete is a very strong material when it is placed in compression. It is, however, extremely weak in tension. It is for this reason that we use reinforcement in concrete structures. The reinforcement, which is usually steel, takes up the slack for the weakness of the concrete in tension.

There are many ways to test the strength of a batch of concrete. The tests used can be categorized as destructive and nondestructive tests. We will perform both types of tests in this class.

Usually when a batch of concrete is ordered on a job site it is specified to be of a specific compressive strength -- 4000 psi, for instance. When the concrete comes to the job site in a ready-mix truck, the contractor places some of the batch in cylinders which are 6 inches in diameter and 12 inches in height. These cylinders are cured for 28 days and tested by compression until they are crushed. This will give the contractor or the engineer the compressive strength for that batch of concrete. He or she can then compare that value to the design value used to make sure that the structure was constructed properly.

Once the concrete has been placed for a particular structure, there is a nondestructive test which can be performed to estimate the strength of the concrete. This method uses a Schmidt hammer (also called a Swiss hammer). This method of testing is based on the inertia of a ball inside the Schmidt hammer testing apparatus that is "bounced off" of the concrete.

Each group will perform two tests on a concrete cylinder. The first test will be the Schmidt hammer test. Each person in the group will take a turn using the Schmidt hammer and the results of the group members will be averaged. Each group will then take their cylinder into the structures lab where it will be tested to crushing in a compression testing machine.

Concrete and steel are the most widely used materials in engineering design. Concrete is very important material for the civil engineer designing in Florida because steel is not readily available and can be very expensive to bring to the site. Some advantages of using concrete in design are as follows: high fire and weather resistance, relatively low cost (most of the materials can be obtained locally), can be poured to fit odd shapes (good for unusual architectural designs). As you drive down I-75, I-4, or on the turnpike you will notice that almost all the bridges are constructed of concrete. As you walk around Weil Hall (the building you are in now) you will notice that the beams and columns are made of concrete. The new South End zone for the University of Florida's Football Stadium and the new addition to the commuter parking garage were constructed using concrete.These are just a few examples.

Concrete is composed of four ingredients: cement, sand, aggregate (stones, gravel, etc.), and water. The strength and other properties of concrete are dependent on how these four ingredients are proportioned and mixed. The most common type of cement is Portland.

Unlike steel, concrete is adequate in strength in only one direction. Concrete is very good in compression but useless in tension. Engineering design is based on concrete's compressive strength. Compressive strength, f'c, refers to what concrete is capable of resisting from loads when they are pushing on the concrete (compression). Compressive strengths for concrete are usually in the range of 3,000 to 5,000 psi (pounds per square inch). To correct for the lack of tension strength in concrete, high tensile strength steel is placed in the tension side of concrete. The steel used for reinforcement usually consists of round steel bars often called rebars. When this combination occurs it is called reinforced concrete.

When Civil Engineers design, they obviously need to know the strength of the material that they are using. By knowing the strength of the material that is being used and the loads (forces i.e. people, cars, furniture) that will be acting on the particular member (beam, column, arch, etc.) the engineer can pick the correct dimensions for the design.

In today's lab, two tests will be introduced to check the structural quality of concrete (find its strength). The first test involves loading the concrete cylinder shown in the drawing until failure. This test is useful for checking the strength of the concrete that is presently being used for a construction site. The American Concrete Institute's Code specifies that a pair of cylinders shall be tested for each 150 yd3 of concrete or for each 5000 ft2 of surface area actually placed. This is a quality control measure.

The other test that we will conduct in the lab today is called the Schmidt (or Swiss ) Hammer Test . This test gives an estimate of the concrete's compressive strength. This test is performed in situations where the concrete has already been poured and a cylinder test is not possible. If a structural engineer wanted to check a beam for its concrete strength 6 months after the building had been constructed the engineer might use this test. What are possible sources of error between the two tests and within the tests themselves?