weight concrete and an estimated 100 pcf for moisture-laden regular 

 lightweight concrete). If the application is for submerged structural 

 elements, such as beams, columns or shells (cold water pipe to OTEC) , then 

 the weight saving is 40% (saturated-in-seawater unit weights are 150 - 64 = 

 86 pcf for normal weight concrete and 115 - 64 = 51 pcf for lightweight 

 concrete). This is a significant weight saving. 



When comparing material strengths, the properties of normal weight 

 concrete can vary considerably, depending on the mix design and type of 

 aggregate. One mix design used recently by CEL and obtained from a 

 local transit mix company used 658 Ib/cu yd of portland type II cement, 

 water-to-cement ratio of 0.46, and river gravel of 1 inch maximum size. 

 The 28-day properties were: compressive strength, 6,060 psi; elastic 

 modulus, 3.2x10^ psi; and Poisson's ratio, 0.22. In comparison, the 

 high strength regular lightweight concrete and the high strength PFA 

 concrete had compressive strengths of 5,200 and 6,580 psi, respectively; 

 and the elastic moduli were one-third lower. In essence, the strengths 

 of PFA concrete and normal weight concrete were comparable. However, 

 normal weight concretes can be designed for strengths of 8,000 to 9,000 

 psi, which appears to be beyond the capability of PFA concrete. 



Cost is also important. Table 6 gives estimated costs for the 

 aggregate, concrete, and in-place concrete costs. The PFA cost is about 

 54 times that of normal weight aggregate and 13 times that of regular 

 lightweight aggregate. The added cost is that of polymer at about 

 $1.00/lb, plus 20% for manufacturing. PFA concrete costs about 9 times 

 as much as normal weight concrete and 6 times as much as regular light- 

 weight concrete. 



The most important cost parameter for comparison, however, is the 

 in-place concrete cost. This cost is obtained by dividing the total 

 structure cost by the total quantity of concrete. Typically, for an 

 offshore concrete structure the in-place cost is about $l,000/cu yd. 

 For simplicity, Table 6 shows the concrete material cost added to $1,000/ 

 cu yd to obtain the in-place concrete cost. For this case, PFA concrete 

 costs 1.30 times that of normal weight concrete and 1.27 times that of 

 regular lightweight concrete. 



For certain applications, the material selection can have a major 

 impact on life cycle cost through weight savings. For example, a struc- 

 ture such as OTEC would be a moored, floating platform which could 

 benefit by a lighter-weight construction material for the hull and cold 

 water pipe. The outside dimension of the hull is sized by the required 

 displacement to support the hull, internal hardware, and cold water 

 pipe. By reduction of the weight of the hull and cold water pipe, the 

 outside dimension of the hull can be reduced. A considerable volume of 

 material would be saved, which reduces first cost. In addition, the 

 smaller sized hull will produce lower drag forces that will reduce the 

 mooring forces. The mooring lines and anchors can be reduced in size 

 for a major cost savings. Over the life of the structure, several 

 mooring lines - and possibly anchors - will be required so the cost 

 savings accumulate. 



