Steps in Construction of Reinforced Concrete Structures

Once the design has been completed, a team of experienced and knowledgeable construction workers and supervisors is required to transform the structural design into the finished construction. Concrete construction is a complex process that involves several activities, from hatching and mixing the ingredients to pouring fresh concrete into forms and curing the new construction. There are several factors that might influence the quality of the final product in this process, that is, the built structure. The critical activities associated with the construction of cast-in-place reinforced concrete structures are described below.

Development of Structural Drawings

Structural design should be based on the sound application of fundamental principles of reinforced concrete design and a knowledgeable use of building codes and standards. However, structural design is ineffective if it is not possible to ensure its proper implementation. The structural engineer has a critical role in communicating the design information to personnel involved in the construction process. In the world of structural engineering, structural drawings and specifications are a critical means of communication between the structural engineer and the contractor. Consequently, the importance of an accurate set of structural drawings that communicate clear and concise information cannot be underestimated.

In reality, even a well-designed structure may result in a variety of problems, both financial and legal, when structural drawings do not properly communicate the design intent. A poor set of structural drawings may expose the engineer to various problems, such as extra costs, delays in the construction schedule, disputes in the field, construction errors, unhappy contractor/owner, and even legal disputes.

Construction Estimating and Tendering

Once the structural drawings have been completed, the contractor produces an estimate of the concrete and reinforcing steel quantities, as well as the amount of formwork and shoring. This estimate is based on structural drawings and specifications that are issued for construction. Based on these drawings, the estimator should be able to estimate the quantity of materials and also the extent of complexity in the construction procedures, any unusual complications associated with the erection, formwork, and shoring, etc. The drawings must be clear and concise and without ambiguities. When drawings are ambiguous, there could be large variations in price between competitive bids submitted by different contractors in the bidding process. In general, a variation in the bid price of over 10% is considerable, as it leaves the owner confused about the proper market price for the project. This is an unfavourable situation for the owner, who ultimately pays for the construction. On the one hand, the owner may feel that (s)he is overpaying if the highest bid is accepted. On the other hand, a bid below fair market value may lead to problems during construction, usually in the form of numerous requirements for extra payments by the contractor in trying to recover losses caused by the low bid.

Detailing of Reinforcement

The reinforcing contractor interprets the information on structural drawings and produces reinforcing steel detail sheets. These sheets are primarily used by the shop to cut and bend the reinforcing bars required for the project. Each bar has a unique code that is referred to by the person responsible for rebar placement in the field.

Placing of Reinforcement

Once the reinforcement has been detailed and cut in the shop, it is delivered to the site in bundles. In some cases, rebars are cut and bent at the construction site. Placement of the reinforcing steel is carried out by rebar placers. Their role is to interpret structural drawings along with the detail sheets and then place the reinforcing steel as precisely as possible. Rebar clearances must be properly shown on the structural drawings in order for the rebar placer to accurately place bar supports to ensure proper bar position.

Supplying the Concrete

The concrete ready-mix company will supply concrete based on the concrete strengths specified on the structural drawings and the mix designs that meet the specific climate and workability requirements of the site. Special concrete placement procedures, such as placement by pumps or wheelbarrows; site batching; or cylinder testing by an independent testing agency, must be carried out in accordance with the structural drawings and specifications.

Installation of Concrete Formwork and Shoring

Formwork and shoring support the weight of the wet concrete and other construction loads. Form work is a temporary structure made of wood, metal, or plastic, and it is constructed to form the final shape of a concrete member. The concrete formwork contractor must hire an engineer who is responsible for designing and producing drawings showing the formwork and shoring supports for concrete beams, slabs, walls, columns, and foundations. The formwork must be built precisely in accordance with the structural drawings and must allow for the proper placement of reinforcing steel and wet concrete without any major deviations in shape and stability.

Placing and Curing the Concrete

The proper amount of concrete must be placed such that the slab thickness, beam sizes, and wall and column dimensions are in accordance with the structural drawings. In some cases, special procedures and construction sequences are required to minimize the development of cracks in the concrete due to structural or architectural restraints or temperature changes. For example, pour strips are often used in large floor structures to separate different floor sections to be placed in the same pour. In some cases, a continuous gap of minimum 300 mm width is left between two adjacent floor sections. A delay period (usually 28 days) is specified to allow for shrinkage to take place between these pours. Subsequently, pour strips between floor sections are filled with concrete. In some cases, allowance for various types of joints (such as control joints and expansion joints) needs to be made during construction.

The curing process is critical for fresh concrete to gain the required design strength. Improperly cured concrete tends to exhibit extensive cracking and creep-induced deflections. Structural drawings need to specify the curing procedures in accordance with the design requirements. It is commonly required to provide curing for fresh concrete over a 28-day period.

Reshoring and Special Construction Procedures

By and large, construction projects require an accelerated concrete placement schedule. In some cases, concrete formwork may need to be stripped (removed) as quickly as 3 days after the placement. Green concrete (concrete that has not attained design strength) must develop sufficient strength to support its self-weight plus the construction load. However, concrete stripped shortly after casting may develop larger creep deflections than concrete stripped after proper curing. Hence, a proper re shoring sequence must be specified by the engineer responsible for construction to prevent sustained loads from being prematurely applied to green concrete.

Procedures that are relevant to the overall design objective need to be outlined on the structural drawings. Such special procedures are generally related to the mitigation of the effects of shrinkage and cracking, as well as creep and long-term deflections. These procedures usually bear certain cost implications and must be clearly communicated on the structural drawings before construction starts. When the structural engineer requests a special procedure after construction has begun that was not specified on the drawings, there is a chance that the contractor will charge extra to carry it out.