An article in the ACI Materials Journal (Vol. 84, 1987, pp. 213-216) describes several experiments investigating the rodding of concrete to remove entrapped air. A 3-in diameter cylinder was used, and the number of times this rod was used is the design variable. The resulting compressive strength of the concrete specimen is the response. The data are shown in Table 4E.8.
a. Is there any difference in compressive strength due to rodding level? Answer the question by using analysis of variance with alpha = 0.05.
b. Construct box plots of compressive strength by rodding level. Provide a practical interpretation of the plots.
c. Construct a normal probability plot of the residuals from this experiment. Does the assumption of a normal distribution for compressive strength seem reasonable?