Concrete discharge time requirements are crucial in ensuring that the hardened concrete meets the design specifications. These requirements determine the maximum amount of time available for placing the concrete in formworks after it has been mixed. When water is added to the dry mixture, chemical reactions occur, leading to a decrease in the water content. As a result, the workability of the concrete diminishes over time. Therefore, it is essential to discharge, place, and compact the concrete before it loses its workability. This article provides a discussion on concrete discharge time requirements and the potential consequences of exceeding the specified time limit.
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What are the Time Requirements for Concrete Discharge?
Concrete discharge time limits are established in codes of practice such as ACI 301 and ASTM C94. These codes specify that concrete should be discharged within a range of 1 hour to 1.5 hours after water is added to the mixture. However, it is possible to exceed these limits if the concrete remains workable and suitable for placement after 1.5 hours. It should be noted that the reasons allowing for the violation of these time limits are not well understood or strong. According to the American Society of Concrete Contractors and Lobo and Gaynor, the 1 to 1.5-hour time limit is conservative and can generally be exceeded without causing any detrimental effects on the concrete. If it becomes necessary to increase concrete workability, high range water reducing admixtures can be used. However, if the concrete loses its workability and is unsuitable for placement upon arrival, it should be rejected. ASTM C94 recommends checking the air content, slump, and specified temperature of the concrete if the 1.5-hour limit is exceeded and the drum has revolved 300 revolutions. It is advised to discuss concrete discharge time limitations in preconstruction meetings to ensure that all parties involved in the construction understand the conditions under which these time limits can be violated and when such violations are permitted.
Possible Consequences of Concrete Discharge Time Limit Violations
The honeycomb pattern in concrete refers to the presence of voids or cavities that resemble the cells of a honeycomb. These voids can weaken the structural integrity of the concrete and reduce its strength. High permeability in concrete refers to its ability to allow fluids or gases to pass through it easily. This can result in moisture penetration and reduced durability of the concrete. Cold joints are areas in concrete where fresh concrete is poured against already hardened concrete. These joints are prone to reduced bond strength and can result in weak points in the structure. It is likely that the concrete in question has low strength, possibly due to the presence of honeycomb voids, high permeability, and the formation of cold joints.
References:
The American Concrete Institute (ACI) released the Committee 301 specifications for Structural Concrete in 2011. These specifications provide guidelines and requirements for the use of concrete in structural applications. They cover various aspects such as materials, design, construction, and quality control.
The American Society of Concrete Contractors (ASCC), located in St. Louis, published a document on Concrete Discharge Time Requirements. This document outlines the recommended time limits for the placement and finishing of concrete. It provides guidance on how long concrete can remain in the delivery truck before it needs to be discharged and placed at the construction site.
The ASTM Committee developed a standard specification for Ready-Mixed Concrete in 2000. This specification defines the requirements for ready-mixed concrete, including its composition, testing methods, and delivery. It serves as a reference for contractors and suppliers involved in the production and use of ready-mixed concrete.