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The paper starts with a brief overview of the concreting operation, presents the research method and data analysis, provides a discussion of the results obtained, and concludes with practical recommendations geared toward further enhancing the buildability level, and thus the operation efficiency of the concreting trade.
Concreting Operation Overview
The concreting operation includes the following four basic processes: (1) transporting; (2) placing; (3) compaction and consolidation; and (4) surface leveling and finishing to the required specifications of the contract (Mindess and Young 1981). Concrete can be transported to job sites in several ways, but the most common is by ready-mixed trucks. The placement process requires proper handling of the fresh concrete to prevent segregation of the coarse aggregates, which can result in the formation of pockets, commonly referred to as “honeycombing”. Concrete can be placed by different methods; however, the most universal two are the skipped and pumped. Skipped concrete requires the use of mobile crane, hoist, or overhead cable way. Concrete skips, also referred to as buckets, are available in different standard volumes ranging from one-third to 2 m 3 . Concrete pumps on the other hand, are powerful tools, which are most suitable for large volume placement.
Whether pumped or skipped, concrete should fall vertically, however, should not be allowed to free fall for long distances as segregation is enhanced when the material strikes the reinforcing steel bars or bounces off from form faces. Therefore, a drop chute or pipe is strongly recommended to protect the concrete during its fall (Mindess and Young 1981).
Although other methods of concrete placement are used, e.g., slip forming, shotcreteing, and tremie concrete, this research focuses on the pumped and skipped methods.
Compaction and consolidation are important in eliminating voids and entrapped air, and in effectively distributing concrete around the reinforcement bars and into corners of formwork molds. Concrete workability is defined as “the amount of mechanical work or energy required to produce full compaction and consolidation of fresh concrete without segregation” (Mindess and Young 1981). Consolidation of fresh concrete is achieved by vibration. Vibrators, often called spud or poker vibrators, apply periodic forces with an eccentric rotating mass, which allows the fresh concrete to flow under the shear forces accompanying the vibration, and thus compacted away from the vibrator.
The efficiency of the compaction and consolidation process is largely affected by (1) the degree of reinforcement congestion within the element concreted, and (2) the workability of the fresh concrete. The higher the reinforcement congestion and lower the workability, the lower the compaction and consolidation efficiency (Jarkas 2005).
The final process of the concreting operation is surface leveling and finishing. The main objective of this process is to produce a dense, compacted, well graded, and free of defects concrete surface which is suitable for the conditions of service. Although the observation and data collection phase of this study was limited to surface leveling of the horizontal members monitored, and thus beyond its scope, several forms of concrete surface finishes are available, e.g., screeding, floating, trowelling, texturing, and hardening (Mindess and Young 1981).
The concreting operation, on the other hand, is faced with several challenges which require extensive collaboration and coordination among site management, concrete suppliers, plant operators, foremen and crew members. Collaboration between site management and concrete suppliers is essential to ensure timely supply, especially in large volume pours, and hence avoiding extensive delays to the placement process. Close coordination between plant operators and crew members, under the direct supervision of foremen, is further required to optimize the selection of locations on which plants, e.g., mobile cranes or pumps, should be placed, in the first instance, to provide maximum reach to the various elements within the concreted area; consequently, to assist in minimizing possible disruptions to the process as a result of plants movements and relocations.