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Report questions 90-minute discharge window in ASTM C94

Sources: Ready Mixed Concrete Research & Education Foundation, Alexandria, Va.; CP staff

Findings in a new RMC Research & Education Foundation report, "Impact of Extended Time to Discharge on Concrete Durability and Performance," support reexamination of the 90-minute delivery threshold in ASTM C94, Standard Specification for Ready Mixed Concrete. 

Investigators recorded negligible deterioration on specimens subjected to 343 freeze-thaw cycles. Photo: New Jersey Institute of Technology

The report covers a New Jersey Institute of Technology (NJIT) investigation of concrete specimen freeze-thaw performance and resistivity. Authors link the C94 discharge time limit to rejected loads that contribute—along with left over mixes returned to plants—to a waste stream estimated at 3 percent of the industry's annual volume. The 90-minute window, they contend, becomes more difficult to abide as traffic congestion increases. 

"As ready mixed concrete producers continue to look for ways to lower their environmental impacts, reduction of returned concrete waste is an important opportunity,” says 2020 RMC Research & Education Foundation Chairman Ted Chandler (Chandler Concrete).

“This research provides important validation that with technological advances in the production and evaluation of ready mixed concrete, we can and should reconsider arbitrary rules which increase waste and costs without any measurable impact on performance,” adds RMC Foundation Executive Director Julie Garbini. 

NJIT researchers observed acceptable results for all fresh concrete and durability indicator tests on specimens prepared from samples obtained at different times up to 150 minutes. Report authors conclude that the current C94 discharge time limits and specifications are conservative, and based on provisions dating to 1935. The RMC Foundation funded the investigation with the Portland Cement Association. The 28-page "Impact of Extended Time" is posted at www.rmc-foundation.org.