Plastic Shrinkage in Hot and Arid Environments by ProQuest


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									           Plastic Shrinkage
               in Hot and
           Arid Environments
     influence of dosage of water-reducing and retarding admixture and concrete
                         temperature on crack development

                            by AbdulrAhmAn m. AlhozAimy And AbdulAziz i. Al-negheimish

K    ing Saud University and its Center of Excellence for
     Concrete Research and Testing are located in Riyadh,
Saudi Arabia, in the center of the Arabian Peninsula. The
                                                                 Mixture T32-R04 had a WRR admixture dosage of 400 mL/
                                                                 100 kg (6 fl oz/100 lb) of cement.
                                                                    Three exposure conditions—outdoor, indoor with fan,
summer weather in Riyadh can be severe—the average               and indoor static—were investigated. For indoor-with-fan
daytime temperature ranges from 40 to 45 °C (104 to 113 °F),     tests, slabs were cast inside the laboratory and airflow
and the average relative humidity is about 15%—so it’s           from a fan was directed across their surfaces. For out-
fitting that faculty, staff, and students at the Center have     door-exposure tests, slabs were cast outdoors and
engaged in a comprehensive study of the effects of such          exposed to the sun and the summer environment. For
climate conditions on concrete construction.                     indoor-static exposure tests, slabs were cast inside the
    To date, these studies have verified that using water-       laboratory but were not exposed to moving air. For
reducing and retarding (WRR) admixtures and controlling          indoor-with-fan and outdoor tests, two slabs were cast
concrete temperature can mitigate harmful effects on the
production and delivery of concrete in hot weather.1 WRR
admixtures have been shown, however, to increase
plastic shrinkage, reduce tensile strength, and thus
increase the risk of plastic shrinkage cracking in hot,
dry environments.2 This article describes the initial
evaluation of the effects of WRR admixtures on the plastic
shrinkage cracking of ready mixed concrete (RMC)
produced and placed in this harsh summer climate.

ExPErimEntAl ProCEdurE
    Because no standard procedures are available, various
techniques have been used to study plastic shrinkage
cracking of concrete.3-5 We used 1 x 1 m (3.2 x 3.2 ft),
100 mm (4 in.) thick slab specimens. Each slab was
reinforced in both directions with four 8 mm (0.31 in.)
diameter steel bars spaced at 317 mm (12.5 in.) (Fig. 1).
    Five concrete mixtures were supplied by a local RMC
producer. Mixtures T25-R06, T32-R06, and T38-R06 had
respective concrete temperatures of 25, 32, and 38 °C (77, 90,
and 100 °F) and WRR admixture dosages of 600 mL/100 kg
(9 fl oz/100 lb) of cement, per the manufacturer’s recommen-
dations. Two additional mixtures had concrete temperatures       Fig. 1: Reinforcing bars and forms prior to casting of outdoor-
of 32 °C (90 °F). Mixture T32-NR had no WRR admixture, and       exposure test specimens

26    september 2009   / Concrete international
from each of the five mixtures. For          sand were 630, 420, 390, and
the indoor-static tests, two slabs           390 kg/m3 (1062, 708, 657, and
from each of Mixtures T38-R06,               657 lb/yd3), respectively.
T32-NR, and T32-R04 were cast. A                Casting was done at midday
total of 26 slabs were tested.               during August to represent extreme
                                             weather conditions. The casting was
mAtEriAl ProPErtiES                          carried out on separate days for each
   Coarse aggregates were typically a        mixture and appropriate measures
blend of 20 and 10 mm (3/4 and               were taken at the plant to achieve
3/8 in.) crushed limestone obtained          roughly the specified concrete
from quarries around Riyadh. The             temperature and slump. Concrete
fine aggregates were a blend of              quantity was kept constant for each
natural silica sand and manufactured         mixture. The trip to deliver the
sand obtained from crushed lime-             concrete to the concrete laboratory
stone with the blend ratio selected          at 
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