Strength

The microstructure model can also be applied to predicting the compressive strength development of ASTM C109 mortar cubes [14]. To relate compressive strength to degree of hydration, the gel-space ratio concept originally developed by Powers and Brownyard is employed [46]. The gel-space ratio, X, is defined by [46]:

$$X=\frac{0.68 \alpha}{0.32 \alpha + \frac{w}{c}}$$ (2)

where $\alpha$ is the degree of hydration. It has been shown that the compressive strength of ASTM C109 mortar cubes, $\sigma_c$ _c , can be related to the gel-space ratio via:

$$\sigma_c(t)=\sigma_A X(t)^n$$ (3)

where $\sigma_A$_A represents the intrinsic strength of the cement and n takes on values between 2.5 and 3. Since $\sigma_A$ _A is typically not known a priori, a 3-day compressive strength measurement is used to obtain its value. Then, the microstructure model is executed and based on the evolution of hydration with time, the compressive strength vs. age is predicted, and compared to the values measured at 7 and 28 days. Figure 5 provides an example of the results obtained for a typical OPC [7]. The model is seen to accurately predict the measured development of strength with time well within the standard deviation of the experimental testing program, and could provide a means for reducing the standard 28 day testing to 3 days, with the possibility of confirmatory testing at 7 days.

  

Figure 5: Model predicted and experimentally measured compressive strength development for an OPC paste. Crosses indicate standard deviation in experimentally measured values.