kilonewtons per cubic meter (120 pounds per cubic foot) to 28.3 kilonewtons 

 per cubic meter (180 pounds per cubic foot). Unit weight of quarrystone 

 available from a particular quarry will likely vary over a narrow range of 

 values. The unit weight of concrete containing normal aggregates is usually 

 between 22.0 kilonewtons per cubic meter (140 pounds per cubic foot) and 24.3 

 kilonewtons per cubic meter (155 pounds per cubic foot). It can be made 

 higher or lower through use of special heavy or lightweight aggregates that 

 are usually available but are more costly than normal aggregates. The unit 

 weight obtainable from a given set of materials and mixture proportions can be 

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

 by the U.S. Army Engineer Waterways Experiment Station (1949). 



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

 following example problem. 



*************** EXAMPLE PROBLEM 39 ************** 



GIVEN : A 33.5-metric ton (36. 8 -short ton) concrete armor unit is required 

 for the protection of a rubble-mound structure against a given wave height 

 in salt water (w = 10.0 kilonewtons per cubic meter (64 pounds per cubic 

 foot)). This weight was determined using a unit weight of concrete w^ = 

 22.8 kilonewtons per cubic meter (145 pounds per cubic foot). 



FIND ; Determine the required weight of armor unit for concrete with 

 w =22.0 kilonewtons per cubic meter (140 pounds per cubic foot) and 

 w = 26.7 kilonewtons per cubic meter (170 pounds per cubic foot). 



SOLUTION ; Based on equation (7-116), the ratio between the unknown and known 

 armor weight is 



w 



3 



22.8 



22.8 

 10 



^-^f 



Thus, for w = 22.0 kilonewtons per cubic meter 



W(if§-) 



For w =26.7 kN/m 

 V 



10.9 10.9 



3 



= 33.5 X 1^ = 39.0 mt (42.9 tons) 



f!:lMii\ 



33.5 X -^^ = 17.5 mt (19.2 tons) 



10.9 10.9 



f. Concrete Armor Units . Many different concrete shapes have been 

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



7-214 



