is considered unsafe. For this reason, the compressive strength of concrete is checked on all projects.
The strength of the concrete is checked by the use of cylindrical molds that are 6 inches in diameter and 12 inches in height. Concrete samples must be taken on the jobsite from the concrete that is being placed. After being cured for a time period that ranges between 7 to 28 days, the cylinders are "broken to failure" by a laboratory crushing machine that measures the force required for the concrete to fail. For further information on concrete strength and testing, refer to Engineering Aid 3, NAVEDTRA 10696, and NAVFAC MO 330. (The MO 330 should be maintained in a battalion's tech library.)
Reinforced concrete was designed on the principle that steel and concrete act together in resisting force.
Concrete is strong in compression but weak in tension. The tensile strength is generally rated about 10 percent of the compression strength. For this reason, concrete works well for columns and posts that are compression members in a structure. But, when it is used for tension members, such as beams, girders, foundation walls, or floors, concrete must be reinforced to attain the necessary tension strength. Steel is the best material for reinforcing concrete because the properties of expansion for both steel and concrete are considered to be approximate] y the same; that is, under normal conditions, they will expand and contract at an almost equal rate.
NOTE: At very high temperatures, steel expands more rapidly than concrete and the two materials will separate.
Another reason steel works well as a reinforcement for concrete is because it bonds well with concrete. This bond strength is proportional to the contact surface of the steel to the concrete. In other words, the greater the surface of steel exposed to the adherence of concrete, the stronger the bond. A deformed reinforcing bar adheres better than a plain, round, or square one because it has a greater bearing surface. In fact, when plain bars of the same diameter are used instead of deformed bars, approximately 40 percent more bars must be used.
The rougher the surface of the steel, the better it adheres to concrete. Thus steel with a light, firm layer of rust is superior to clean steel; however, steel with loose or scaly rust is inferior. Loose or scaly rust can be removed from the steel by rubbing the steel with burlap or similar material. This action leaves only the firm layer of rust on the steel to adhere to the concrete.
NOTE: Reinforcing steel must be strong in tension and, at the same time, be ductile enough to be shaped or bent cold.
Reinforcing steel can be used in the form of bars or rods that are either plain or deformed or in the form of expanded metal, wire, wire fabric, or sheet metal. Each type is useful for different purposes, and engineers design structures with those purposes in mind.
Plain bars are round in cross section. They are used in concrete for special purposes, such as dowels at expansion joints, where bars must slide in a metal or paper sleeve, for contraction joints in roads and runways, and for column spirals. They are the least used of the rod type of reinforcement because they offer only smooth, even surfaces for bonding with concrete.
Deformed bars differ from the plain bars in that they have either indentations in them or ridges on them, or both, in a regular pattern. The twisted bar, for example, is made by twisting a plain, square bar cold. The spiral ridges, along the surface of the deformed bar, increase its bond strength with concrete. Other forms used are the round and square corrugated bars. These bars are formed with projections around the surface that extend into the surrounding concrete and prevent slippage. Another type is formed with longitudinal fins projecting from the surface to prevent twisting. Figure 7-1 shows a few of the types of deformed bars available. In the United States, deformed bars are used almost exclusively; while in Europe, both deformed and plain bars are used.
Figure 7-1. - Various types of deformed bars.Continue Reading