Concrete is artificial stone made from two main components: cement paste and aggregates. Aggregates usually consist of natural sand and gravel or crushed stone. The paste hardens as a result of the chemical reaction between cement and water and glues the aggregates into a rock-like mass. The concrete-manufacturing process is quite complex and includes a number of steps, such as proportioning, hatching, mixing, placing, compacting, finishing, and curing. The versatility of concrete construction, in terms of forms and shapes, is due to the fact that fresh concrete has no form of its own until it hardens. Concrete has a high compressive strength; however, its tensile strength is low (only about 10% of its compressive strength). For this reason, concrete is rarely used without the addition of steel reinforcement.
The four basic methods of concrete construction are
- cast-in-place construction
- precast construction
- tilt -up construction
- concrete masonry construction
1. Cast-in-place Construction
In cast-in-place concrete construction, each element is built at the construction site. Fresh concrete, obtained by mixing the ingredients (cement, sand, and aggregates) and adding water, is a shapeless slurry without any strength and has to be supported and shaped by the formwork. Before the concrete is poured into the formwork, reinforcing bars need to be placed in the position prescribed by the design. Once the concrete has hardened and developed sufficient strength, the formwork is removed (stripped) and the curing continues for a few weeks by keeping the new construction moist until it gains full strength. Concrete members under construction are often supported by a temporary structure called falsework or scaffolding. The cast-in-place construction process can be accelerated where required by the project schedule. The speed of curing can be increased by using chemical admixtures. The formwork system can be designed to increase the speed of the forming cycle. Also, much of the reinforcing steel can be prefabricated.
Formwork constitutes a significant portion of the overall cost in cast-in-place concrete construction. There are a number of proven systems available in the industry to facilitate concrete construction. Experienced concrete designers generally have a good knowledge of practical form work systems and are able to develop designs that are easily constructible. Cast-in-place construction is weather dependent. Special measures are often taken to maintain the required construction schedule under extreme temperatures.
Concrete is cast into permanent, reusable forms at an industrial plant. Fully cured structural units are then transported to the job site, where these units are hoisted into place and connected in a manner similar to structural steel shapes. Precast construction has certain advantages over cast-in-place construction: it offers improved quality control as well as uniform and fast construction on site since the elements are manufactured away from the construction site. In general, precast concrete elements are usually prestressed and hence lighter weight than those that are cast in place. However, precasting also has some disadvantages: it involves the shipping cost of precast members from the plant to the construction site and it requires the use of special equipment (large cranes) to hoist and place precast members in position. Precast construction can be very cost effective if there is much repetition on a project. In Canada, precast concrete elements are commonly used for bridge construction.
Tilt-up wall construction is a form of precast construction. Tilt-up wall panels are cast horizontally on the floor slab. Once the panels have attained sufficient strength, a mobile crane sets them on footings. The panels are temporarily braced while the floor and roof structures are erected. Finally, the panels are connected to other structural members by welding embedded steel plates. Tilt-up construction is mainly used in low-rise warehouse or office buildings.
Concrete Masonry Construction
Concrete masonry is used for warehouse, industrial, and institutional construction. Masonry load-bearing walls are assembled by laying masonry units in mortar and providing vertical and horizontal steel reinforcement.