洛阳理工学院毕业设计论文
Fig. 4. Compressive strength of 1500 kg/m3 mixtures as a function of time.
mixtures containing fly ash is slightly lower than that for mixtures containing Pozz-fill but this difference is not significant. The 1-year compressive strengths of the mix-tures is approximately double the 28-day strengths, indicat-ing a similar trend in long-term strength gain to that observed for the mortars containing ash (see Figs. 2 and 3).
From Fig. 5 it can be seen that after 1 year, the compressive strengths of the mixes with casting densities of 1000 kg/m3 varies between 6 and 10 MPa. Although the a/c ratio does not seem to have a significant effect on the compressive strength, the type of ash used (fly ash or Pozz-fill) does; the compressive strength of mixtures containing Pozz-fill are significantly lower than those of the mixtures containing fly ash. It is interesting to note that, unlike those mixtures with higher casting densities, there seems to be virtually no increase in compressive strength after more than 180 days. The contribution to the long-term gain in strength of the ash seems to be reduced at these low densities.
The compressive strength of concrete is not only a function of the w/c ratio but also a function of the density ratio of the concrete [12]. The density
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洛阳理工学院毕业设计论文
ratio is defined as the ratio between the actual density of the concrete and the density of the concrete when fully compacted. The com-pressive strength of the paste in the foamed concrete can be expressed in terms of the age and the effective w/c of the paste. If one assumes that the paste in the foamed concrete has the same strength as the paste on its own, the only factor that should cause a reduction in strength should be the volume of air added to the mixture. As even the mixtures containing no foam might contain significant volumes of entrapped air, the theoretical volumes of binder were used as a base for comparison. The binder contents (by volume) of the foamed concrete mixtures were expressed as a fraction of the binder content of the pastes and the compressive strengths as calculated with [Eq. (3)] for pastes containing ash were adjusted taking the binder ratios into account. The effect of binder ratio on the compressive strength of foamed concrete can be expressed as follows:
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洛阳理工学院毕业设计论文
Fig. 5. Compressive strength of 1000 kg/m3
mixtures as a function of time.
where: fcc= compressive strength of foamed concrete;fc= compressive strength of paste as calculated using Eq.(3); ab= binder ratio.
Regression analysis was conducted on all the compres-sive strengths obtained for foamed concrete mixtures and it was concluded that the combination of equations Eqs. (3) and (4) explains 97.9% of the variation in compressive strength of the foamed concrete. Since Eq. (4) does not contain a constant, the correlation percentage as calculated cannot be compared to that of models containing a con-stant, and the value of 97.9% might be misleadingly high.The solid lines plotted on the graphs in Figs. 4 and 5 were calculated using Eqs. (3) and (4). On each graph, three lines are shown with the top line (the highest compressive strength at any given age) representing an a/c ratios of 1, while the bottom line (the lowest compressive strength at any given age) represents a/c ratios of 3. Relatively large differences occurred between some of the calculated strengths and the actual measured values, suggesting that some factor other than age, effective w/c ratio and binder ratio may be having an effect on the compressive strength of the foamed concrete mixtures. The difference between the predicted and the actual behaviour of the
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洛阳理工学院毕业设计论文
foamed concrete seems to increase with increased air content,indicating that the nature as well as the volume of air voids might have an effect on the compressive strength. In the volumetric approach used here, only the volumes of binder were taken into account and it was assumed that the water/binder ratio remains constant and therefore, does not influence the results. 5.3. Effect of dry density on compressive strength
The 28-day and 1-year compressive strengths are plotted as a function of dry density in Fig. 6 where the top graph shows the results obtained for fly ash and the bottom the results obtained for Pozz-fill. The dry density was only determined from cubes that were cured for 28 days before drying and it has been assumed that the dry density will not change significantly with longer periods of curing.From Fig. 6, it can be seen that there is an exponential relationship between dry density and the 28-day compresive
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洛阳理工学院毕业设计论文
Fig. 6. Compressive strength at 28 days and 1 year as a function of dry density.
strength and there seems to be little difference between the strengths obtained from mixtures containing fly ash and those containing Pozz-fill. The mixtures containing no ash seem to have slightly higher strengths than the mixtures containing ash, confirming the fact that the early strength (up to 28 days) is reduced with high ash contents. The 28-day strengths of all the foamed concrete mixtures are,however, significantly higher than those obtained by other researchers. The strengths shown in Fig. 6 (15 MPa for a dry density of ? 1250 kg/m3) are approximately double those published previously for other protein foams (7 MPa for a dry density of ? 1200 kg/m3) [13].
From Fig. 6, it can also be seen that after 1 year, for the same dry density, the mixes containing ash have marginally higher compressive strengths than those without ash. As before there is virtually no difference between the fly ash and the Pozz-fill mixtures, with the Pozz-fill mixture strengths being
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