洛阳理工学院毕业设计论文
weighed. 5. Results
5.1. Pastes containing ash The compressive strength of paste mixtures containing fly ash is plotted as a function of time in Fig. 2. Although the a/c ratio of these mixtures varies from 1 to 3, the water/binder ratio was kept constant at approximately 0.3 and the strength development of the paste mixture with the
Table 2
Composition of mixtures
same w/c ratio but containing no ash is shown for compar-ison. From this graph, it is clear that the compressive strength of the ash mixtures increases over a much longer period of time than that of mixtures containing no ash. The gain in strength between 28 and 365 days is 37 and 44 MPa for the mixtures with the a/c ratios of 1 and 2, respectively,and after 270 days they have reached strengths of approxi-mately 80 MPa, which is similar to that of the cement paste(containing no ash) with the w/c of 0.3 at the same age.After a period of 1 year, the paste with an a/c ratio of 3 has achieved a strength of only 58 MPa compared with 80 MPa for the other two mixes. This difference in strength remains approximately the same for all ages of testing. The com-pressive strength of mixtures containing Pozz-fill is plotted as a function of time in Fig. 3. These results show a similar trend to that observed for the mixtures containing fly ash.As far as ultimate compressive strength is concerned, there seems to be no significant difference between the mixtures
27
洛阳理工学院毕业设计论文
containing fly ash and those containing Pozz-fill. These results seem to indicate that the classification of the ash does not improve its effectiveness as far as contribution towards compressive strength is concerned.
The mathematical analysis used to determine the con-tribution of the ash towards the compressive strength of the cement paste was simplified by assuming that the w/c and age of the paste are the only factors affecting the compres-sive strength. A fraction of the ash was taken to be active, and this fraction was added to the actual cement content when the w/c ratio was calculated. The compressive strength was used to determine what the actual size of the active fraction at any given time should be. The effective w/c ratio of the cement paste at any given age can be calculated
Fig. 2. Compressive strength of pastes containing fly ash (pfa).
28
洛阳理工学院毕业设计论文
Fig. 3. Compressive strength of pastes containing Pozz-fill.
(based on the work conducted by Smith [11]) using the following equation [Eq. (1)]:
where: W/C = effective water/cement ratio; w/c = actual water/cement ratio; a/c = ash/cement ratio; k = cement-ing efficiency.The cementing efficiency is dependent not only on the source and quality of the ash but also on the age, the w/c ratio and the a/c ratio of the paste. Mathematical modelling was simplified by assuming that there is no marked difference between the cementing efficiency of fly ash and that of Pozz-fill. The cementing efficiency for the mixtures containing ash was determined using a multiple linear regression model and the following equation was derived [Eq. (2)]:
where: k = cementing efficiency; t = time since casting (days); a/c = ash/cement ratio (by weight).The R2 statistic indicates that the model as fitted
29
洛阳理工学院毕业设计论文
explains 85.6% of the variability in k . The k value increases from approximately 0.21 after 7 days to between 0.55 and 1.1 after 365 days. Smith [11] recommended a k value of 0.25 based on 7- and 28-day concrete strengths and these results seem to confirm his recommendation. These results do however suggest that his assumption of a constant k value might be conservative as the efficiency definitely increases with time up to ages of at least 1 year.A new relationship between compressive strength,effective w/c ratio and time can now be established using the calculated k value. The k value is used to calculate an effective w/c ratio that can be used as an independent variable in a multiple regression analysis. The relationship between compressive strength and time since casting as well as effective w/c ratio can be expressed using the following equation:
Where: fc= cube compressive strength (MPa); t = time since casting (days); W/C = effective water/cement ratio.
The R2 statistic indicates that the model as fitted explains 96.3% of the variability in strength, while the adjusted R2 statistic is 96.2%. The standard error of the estimate shows the standard deviation of the residuals to be 4.2MPa.Stepwise regression indicates that 83.5% of the variation in compressive strength can be explained by the variation in effective w/c ratio. Adding the time since casting as a second independent variable increases the percentage varia-tion in compressive strength that can be explained by the fitted equation to 96.3%. The calculated k values were used to adjust the w/c ratios of the mixtures containing ash, and the effective w/c ratios were used in Eq. (3) to calculate the compressive strengths as indicated with the solid lines on the graphs in Figs. 2 and 3. 5.2. Strength of foamed concrete
The compressive strengths of foamed concrete mixtures with different ash contents and casting densities of 1500 and 1000 kg/m3 are plotted as function of time in Figs. 4 and 5, respectively. The top graph in each of these figures is
30
洛阳理工学院毕业设计论文
for mixtures containing fly ash while the bottom graph is for mixtures containing Pozz-fill. The results for casting density of 1250 kg/m3 have been omitted for brevity but show a similar trend to that of 1500 kg/m3. From Fig. 4 it can be seen that after 1 year (365 days), the compressive strengths of all six mixtures with casting densities of 1500 kg/m3 is approximately 40 MPa. Neither the a/c ratio nor the type of ash used (fly ash or Pozz-fill) seems to have a significant effect on the long-term strength of these foamed concrete mixtures. After 9 months, the compressive strengths of the
31