1172
4596.002
Lewis, R.K.
Constructional Review
Article
1963
Feb. 1963; Oct. 1965
A)p.20; B)p.13
The two articles are closely related. Article B incorporates Article A material description by reference. Thus, Article A is required to completely follow Article B.ABSTRACT A. Steam curing at atmospheric pressure is frequently used to accelerate the development of strength of prestressed concrete and of concrete blocks. Many papers have been written on the effect of steam curing on the properties of concrete made with dense aggregates, but there appears to be little information available on concretes made with expanded shale or clay aggregates.
This paper covers an investigation into the effect of temperature-time gradient and delayed steaming on the physical properties of concrete made with a lightweight aggregate.
To study the effect of delay prior to steaming and of the temperature-time gradient, five delay periods (l/2, 2, 4, 6, 8 hr.) and three nominal temperature-time gradients (10 deg. C./hr., 20 deg. C./hr., and 40 deg. C./hr.) were chosen, these corresponding to ranges applicable under practical conditions. In each steam curing cycle the temperature of the chamber was increased until a maximum temperature of 85°C. was obtained, and the steam was then turned off and the chamber allowed to cool.
It is not usual in the steam curing of high strength concrete to cut off the steam supply on reaching the maximum temperature, but in this study this procedure was considered desirable, to avoid complications in interpreting the results. It will be necessary, therefore, to make further investigations into the effect of maintaining the maximum temperature for varying lengths of times, before optimum steam-curing cycles can be given.
ABSTRACT B. Previous papers have investigated the effect of the delay period prior to steaming, the temperature-time gradient, the maximum curing temperature, and the maturity on the steam curing of high strength lightweight concrete. These variables were studied to attain optimum steam curing treatment for a given concrete mix.
A past inquiry sought information on the steam curing of concrete that had previously hardened in moist air. In the particular case concerned the compressive strength after 7 days moist air curing was below that normally obtained, due to a cold spell of ambient temperature. The problem was to determine what steam curing treatment should be applied to increase the strength of the concrete to that desired. Unfortunately the existing maturity laws did not give satisfactory information. This series of tests was done to fill a gap in this knowledge.
Plowman has shown that for concrete cured at temperatures between 11 and 100°F., provided sufficient moisture is present for continued hydrating, the compressive strength is proportional to the product of the temperature and time. Thus instead of curing a given concrete at varying temperatures below 21°C. for varying times, namely 24, 41 and 65 hr. At these ages the compressive strength ranged from 26 to 77% of the 7-day continuously fog-cured strength. After these varying fog curing periods, some of the specimens were steam-cured.
The concrete was made from lightweight aggregate as used in the previous work, and had a cement factor of 3 bags per cu yd. The steam chamber temperature was increased at a rate of 10 dec.C./hr. The steam treatments chosen were the minimum and maximum steam maturities used in the previous work and were:
(a) Maximum temperature of
55°C. with no isothermal
period.
(b) Maximum temperature of
85°C. with an isothermal
period of 9 hr.