Report on Behavior of Fresh Concrete During Vibration
|Publication Date:||1 August 2008|
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 be tamped in the narrow forms filled with reinforcing steel and, consequently, water was added to facilitate placement into forms without regard to effects on the mixture itself. The change from massive tamped concrete structures in the early 1900s to relatively thin, reinforced concrete structures was a major advance in engineering practice, but did not necessarily result in immediate improvements in concrete quality. The dry, tamped concrete structures were somewhat less permeable than the wet concrete placed into the first reinforced structures. Methods other than tamping were tried to consolidate stiffer concrete. Compressed air was introduced into the fresh concrete through long jets. The practice of chuting concrete into place resulted in excessively wet mixtures as the water content was increased (without increasing the cement) to allow the mixture to flow in chutes (Walter 1929). It became apparent that these wet mixtures did not produce good concrete (Engineering News Record 1923). The result was lower strength, durability failures, and increased drying shrinkage and cracking. The poor durability of these first reinforced 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.