Method CRD- 3 of the Handbook for Concrete and Cement published by the 

 U.S. Army Engineers Waterways Experiment Station. Figure 7-87 illustrates 

 the effect of varying the value of the unit weight w^, on the weight of 

 the armor unit W in Equation 7-105. The weight factor of armor unit f 

 is the ratio of, 



The effect of varying the unit weight of concrete is illustrated by the 

 following example problem. 



************** EXAMPLE PROBLEM 



*************** 



GIVEN : A 36-ton concrete armor unit is required for the protection of a 

 rubble-mound structure against a given wave height. This weight was 

 determined using a imit weight of concrete w^ = 145 lbs. /ft? 



FIND : Determine the required weight of armor unit for w^ = 140 lbs. /ft? 

 and Wp = 170 lbs. /ft. concrete. 



SOLUTION : Using the lower curve in Figure 7-87, the weight factor for 

 f (w^ = 140 lbs./ft.3) = 1.38 , 

 f (w^ = 145 lbs./ft.3) = 1.18 , 

 f (w^= 170 lbs./ft.3j = 0.62 . 



Thus for, w^ = 140 lbs. /ft?, 



1 38 



W = 36 X - — = 42.1 tons , 



1.18 



say W = 42 tons. 



and for W^ = 170 lbs. /ft?, 



0.62 



W = 36 X = 18.9 tons , 



1.18 



say W = 19 tons. 

 ************************************* 



7.376 Conarete Armor Units . Many different concrete shapes have been 

 developed as armor units for rubble structures. The major advantage of 

 concrete armor units is that they usually have a higher stability coef- 

 ficient value, thus permitting the use of steeper structure side slopes 

 or a lighter weight of armor unit. This has particular value when 

 quarrystone of the required size is not available. 



7-180 



