tests in accordance with ASTM C-496. The modulus of elasticity and 

 Poisson's ratios were obtained using the standard procedure in ASTM 

 C-469. 



In an attempt to obtain air-dry unit weights for the concrete, two 

 specimens of PFA and regular lightweight concrete were placed in a 

 30% RH environment. Unit weights for saturated concrete were also 

 desired, so two specimens of each were placed in a pressure vessel at 

 500 psi for periods of 14 to 17 days. 



TEST RESULTS 

 Strength Results 



Table 5 presents the results from the compressive and split tensile 

 tests. In compression, the concrete mixtures increased in strength as 

 the cement contents increased from 460 to 710 Ib/cu yd (Mix no. 1 through 

 3). The cement content was the same for Mix no. 3 and 4 at 710 Ib/cu yd, 

 and the compressive strengths are essentially equal. 



For the regular lightweight concrete, the maximum compressive 

 strength f averaged 5,200 psi. Failure was caused by rupture of the 

 aggregate particles. For the PFA concrete, the maximum f was 6,530 

 psi - an increase of 26% over that of regular concrete; failure in this 

 case was caused by failure of the bond between the aggregate and cement 

 matrix. Thus, the strength of the PFA concrete will increase with 

 continued fog-curing while the regular concrete had attained its maximum 

 strength limit. Figure 5 shows a photograph of the different types of 

 failure modes. Even though the specimens in Figure 5 are from split 

 tension tests, the same appearance was found for compression specimens. 

 Thus, it is important to state that the strength difference between 

 regular and PFA concrete will increase with age beyond the present 26%. 



For split tensile strengths, the PFA concrete showed an average 

 increase of only 4% over that of regular lightweight concrete. This was 

 surprising because the failure modes are different (Figure 5); however, 

 the test results are quite consistent. A split tensile strength of 500 

 psi appeared to be the limit for the concretes. 



The elastic modulus for the PFA concrete was also 4% greater than 

 that of the regular concrete. The stress-strain behavior for the con- 

 cretes was quite similar (Figures 6 through 9). For the higher strengths, 

 both types of concrete showed little nonlinear behavior before failure. 

 The specimens showed predominantly vertical cracking behavior at failure. 



The stronger aggregate particles of the PFA concrete probably 

 contributed to a greater ultimate strain, which was an average 

 3,300 |Jin./in., compared to 2,750 |Jin/in. for the regular concrete. The 

 ultimate strain values were also quite consistent. 



Poisson's ratio varied considerably from test to test, which is 

 ti^pical for concrete. However, the overall average for the PFA and the 

 regular concrete was the same at 0.25. 



