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Rapid construction practices throughout the concrete industry have brought increasing pressure on specifying agencies to assess the quality of concrete at an earlier age than 7 or 28 days after placement. Currently, the later age is still specified for compression tests of 152 by 304-mm (6 by 12-in.) cylinders of concrete as delivered to the job site in order to determine quality. During the 28-day period, it is not unusual for a multistory building to rise several floors before the strength tests are conducted. This situation is considered by many to be too precarious for construction to proceed on a sound technical basis and with adequate assurance of safety. Furthermore, extensive and costly delays are encountered when 28-day test results are low, since a field investigation may be necessary to verify the load-carrying capacity of the structure. Further delay is certain if concrete must be reinforced or replaced. Conversely, economics dictate that work continue without knowing quality, thus setting the stage for a catastrophic collision between advancing construction technology and the price owners are willing to pay for their capital facilities. Surely, an earlier assessment of concrete quality is absolutely essential.
Supervisor, Cement Technical Center, Martin Marietta Cement, Baltimore, Md.