CHAPTER 7 REINFORCING STEEL

As a Steelworker, you must be able to cut, bend, place, and tie reinforcing steel. This chapter describes the purpose of reinforcing steel in concrete construction, the types and shapes of reinforcing steel commonly used, and the techniques and tools used by Steelworkers in rebar (reinforcing steel) work. This chapter begins with a presentation of fundamental information about concrete to help you understand rebar work fully.

REINFORCED CONCRETE

As a Steelworker you will be primarily concerned with reinforcing steel placement but you should to some extent, be concerned with concrete as well. Concrete with reinforcing steel added becomes reinforced concrete. Structures built of reinforced concrete, such as retaining walls, buildings, bridges, highway surfaces, and numerous other structures, are referred to as reinforced concrete structures or reinforced concrete construction.

CONCRETE MATERIALS

Concrete is a synthetic construction material made by mixing cement, fine aggregate (usually sand), coarse aggregate (usually gravel or crushed stone), and water in proper proportions. This mixture hardens into a rocklike mass as the result of a chemical reaction between the cement and water. Concrete will continue to harden and gain strength as long as it is kept moist and warm. This condition allows the chemical reaction to continue and the process is known as curing. Durable, strong concrete is made by the correct proportioning and mixing of the various materials and by proper curing after the concrete is placed.

The correct proportioning of the concrete ingredients is often referred to as the mix. The quality of the concrete is largely determined by the quality of the cement-water paste that bonds the aggregates together. The strength of concrete will be reduced if this paste has water added to it. The proportion of water to cement is referred as the water-cement ratio. The water-cement ratio is the number of gallons of water per pounds of cement. High-quality concrete is produced by using the lowest water-cement mixture possible without sacrificing workability.

Because concrete is plastic when it is placed forms are built to contain and form the concrete until it has hardened In short forms and formwork are described as molds that hold freshly placed concrete in the desired shape until it hardens. All the ingredients of the mix are placed in a concrete mixer, and after a thorough mixing, the concrete is transferred by numerous methods, such as by bucket, by wheelbarrow, and so forth, into the formwork in which the reinforcing steel has already been placed.

Concrete reaches its initial set in approximately 1 hour under normal conditions and hardens to its final set in approximately 6 to 12 hours. Before the initial set, concrete must be placed in the forms and vibrated to consolidate it into the formwork and ensure complete coverage of all reinforcing bars. Finish operations, such as smooth troweled finishes, must be performed between initial and final set. After the final set, concrete must be protected from shock, extreme temperature changes, and premature drying until it cures to sufficient hardness. Concrete will be self-supportive in a few days and attain most of its potential strength in 28 days of moist curing. For further information on concrete, refer to Builder 3 & 2, Volume 1, NAVEDTRA 12520.

CONCRETE STRENGTH

As stated previously, the strength of concrete is determined by the water-cement ratio. The strength of ready-mixed concrete ranges from 1,500 to about 5,000 pounds per square inch (psi); and, with further attention paid to proportioning, it can go even higher. Under usual construction processes, lower strength concrete will be used in footers and walls and higher strength in beams, columns, and floors. The required strength of concrete on a given project can be found in the project plans and specifications for a specific project.

NOTE: Quality control is important to ensure specific design requirements are met. If the design specifications do not meet minimum standards, structural integrity is compromised and the structure 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.)

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