1. A precast concrete beam is cast at 2:00 pm. It can only be stripped when the
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Question
1. A precast concrete beam is cast at 2:00 pm. It can only be stripped when the maturity exceeds 800 °C hrs. The concrete is cast at 20 °C and held at this temperature for 6 hours. Then steam heat is applied to increase the concrete temperature at a rate of 16 °C/hr until a temperature of 60 °C is reached. This temperature is held for 7 hours and then the heat is removed and the beam allowed to cool at a rate of 20 °Chr for 2 hours. The beam must be stripped at 7:30 AM. Using the simple maturity equation, determine if the beam will have adequate maturity for stripping at 7:30 am the next day (use the -10 °C datum temperature) What test should be done to check if the maturity estimate is accurate?Explanation / Answer
Maturity of concrete is defined as summation of product of age and curing temperature
Maturity =×( age x curing temperature)
Datum temperature = -10 degree
Thus curing temperature = day temperature - datum temperature
For the above problem when temperature of 20 degree Celsius curing temperature = 20-(-10) = 30 degree and for 6 hrs we get product = 180 C hrs
By applying steam temperature increased to 60 degree from 20 degree I.e. 40 degree increment @ 16 degree per hour therefore time required for increment is 40/16 = 2.5 hr
Thus average curing temperature for 2.5 hrs =( (20+60)/2) - (-10) = 50 degree and hence product = 50*2.5 = 125 C hrs
Temperature of 60 degree is maintained for 7 hours and so product = 70 * 7 =490 C hrs
Now temperature is lowered to 20 degree Celsius in 2 hrs thus average curing temperature = ((60+20)/2) - ( -10) = 50 and thus product = 50* 2 = 100 C hrs
Thus maturity = 180+ 125 + 490+ 100 = 895 C hrs which is greater than 800 C hrs and hence the beam could be stripped at 7 am as it is sufficiently mature.
Maturity estimate is generally made to develope strength maturity relationship and for checking the accuracy of maturity test a strength computed by using strength maturity relationship could be compared to the actual strength determined using strength test of concrete beam or cylinder specimens .
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