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|Title:||Effects of accelerated curing on hydration products of cement and cement-fly ash pastes|
Burks, J. P.
Wong, G. Sam.
Reinhold, Ronald E.
Boiling water curing
Normal consistency tests
Scanning electron microscopy
Warm water curing
|Publisher:||Concrete Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; C-78-11.|
Abstract: Accelerated curing procedures are being used more and more frequently to obtain an estimate of concrete strength by testing accelerated specimens at ages ranging from 24 hr ± 15 min, 28-1/2 hr ± 15 min, or 48 to 96 hr, depending on whether the warm water, the boiling water, or the autogenous curing procedure standardized in ASTM C 684 is followed. Modifications of these procedures also exist. Good correlations between strength of that of 28-day specimens offer the advantage early accelerated specimens and of evaluating the strength level of the concrete within 1 to 4 days from the time of placement which provides economies in construction by giving early warning of unsatisfactory strength. We believe that in the future design strengths will be specified as accelerated strengths. Our investigation was intended to discover, by using X-ray diffraction, scanning electron microscopy, and strength measurements, all done on parts of 1-in. by 1-in. by 11-1/4-in. (25- by 25- by 286-nnn) bars of normal consistency cement paste, or of cement paste with 30 percent replacement by fly ash, whether the accelerated curing methods altered the hydration products to a degree that made accelerated-cured specimens different in ways that produce specimens substantially different from those produced by curing in a moist room meeting ASTM C 511 or by immersion in saturated limewater at a temperature of 23° ± l.7°C (73.4° + 3°F). Our conclusions are: the warm-water method does not substantially affect the hydration products or crystallinity or texture of the accelerated specimens; the boiling-water method substantially degrades the crystallinity of ettringite, a normal early hydration product of portland cement, and accordingly produces specimens not accurately representative of those obtained by standard curing; autogenous curing produces specimens accelerated only by the heat generated by each individual mixture, and consequently any two cements or combinations of cementitious materials even at the same water-cement ratio will not see the same time-temperature history. The same circumstances exist when two presumably identical structures are built with different cements, particularly different cements of two types. We did not find recognizable differences in the nature of the hydration products or texture in the autogenous cured specimens as compared to their standard cured controls. The scanning electron microscope showed us the microarchitecture of concrete in three dimensions. At early ages even with low water-cement ratios the structure is more open than expected'; with time it densifies and becomes much more homogeneous, interrupted only by occasional voids and occasional crevices that probably represent incomplete consolidation or localized carbonation. The scanning electron microscope provides new micro orders of magnitude that make the development of hydrated paste more understandable. It is likely that any or all of the three methods of accelerating the strength development in test specimens for compressive strength may be used without serious difficulty in evaluating strength of concrete at early ages. The warm-water method appears best since it yields specimens most reproducibly and with least difference from those cured normally. The boiling-water method while essentially of similar reproducibility does cause differences in hydration products of the cement as compared with normal curing. The autogenous method does not give a uniform degree of acceleration.
United States. Assistant Secretary of the Army (R & D).
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