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Civil MDC

Report on Behavior of Fresh Concrete During Vibration 2

Report on Behavior of Fresh Concrete During Vibration

Description

At the turn of the twentieth century, concrete was generally placed as very dry mixtures, and was deposited in thin lifts and rammed into place by heavy tampers, which involved extensive manual labor. Typical structures, such as foundations ,retaining walls, and dams, contained little or no reinforcement. The concept of rammed concrete in thin lifts can be traced back to the early Roman times, when the Pantheon was built .Many of these structures are still in service, proving that this type of construction produced strong, durable concrete. In the early twentieth century, the common use of reinforcing steel in concrete changed the consolidation requirements for concrete . Concrete sections were greatly reduced in thickness.

Constructors found that the dry mixtures could not betamped in the narrow forms filled with reinforcing steel and,consequently, water was added to facilitate placement intoforms without regard to effects on the mixture itself. Thechange from massive tamped concrete structures in the early1900s to relatively thin, reinforced concrete structures was amajor advance in engineering practice, but did not necessarilyresult in immediate improvements in concrete quality. Thedry, tamped concrete structures were somewhat less permeablethan the wet concrete placed into the first reinforced structures.

Methods other than tamping were tried to consolidate stifferconcrete. Compressed air was introduced into the freshconcrete through long jets. The practice of chuting concreteinto place resulted in excessively wet mixtures as the watercontent was increased (without increasing the cement) toallow the mixture to flow in chutes (Walter 1929). It becameapparent that these wet mixtures did not produce goodconcrete (Engineering News Record 1923). The result waslower strength, durability failures, and increased dryingshrinkage and cracking.

The poor durability of these firstreinforced concrete structures was of great concern to early practitioners. These mixtures would be described as“wetter,” though the slump test was yet to be standardized.The water-cement ratio concept, postulated by Abrams around 1920, demonstrated that the quality of concretedropped rapidly as more water was added to the mixture(Abrams 1922a). In addition, the development of the traditionalslump test around 1922 gave the first measurable parameterfor indicating concrete consistency suitable for placement and an indication of quality (Abrams 1922b). Abramsdocumented an increase in compressive strength by compactinglow-consistency concrete with mechanical jigging.


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