Prediction of Strength Development

To predict compressive strength development using the CEMHYD3D model, the gel-space ratio theory of Powers is utilized [29]. This theory states that the compressive strength, $\sigma_c$, is related to the gel-space ratio, X, as shown in the following equation:

where $\sigma_0$ ₀ is an intrinsic strength that depends on the cement composition and PSD, and n generally assumes a value between 2.6 and 3.0. The gel-space ratio, X is defined by [29]:

$$X=\frac{0.68 \alpha} {0.32 \alpha\ +\ \frac{w}{c}}$$ (7)

where $\alpha$ is the mass-based degree of hydration of the cement. For strength predictions using CEMHYD3D, generally, a value of n=2.6 is used [1, 2] and $\sigma_0$ ₀ is calibrated based on one early-age (1 day or 3 day) strength determination.

For cement 133, the compressive strengths of standard ASTM C109 [30] mortar cubes (w/c=0.485) have been determined by a number of testing laboratories (as part of the Cement and Concrete Reference Laboratory Proficiency Sample Program [9]) after 3 days, 7 days, and 28 days. The mean 3-day value, along with the model-predicted 3-day degree of hydration, was used to determine the value of $\sigma_0$ ₀ in equation 6 as 83.55 MPa (12,120 psi). This value and the CEMHYD3D-predicted hydration development with time were used to predict the strength development of cement 133 (and specifically the strengths that would be achieved after 7 days and 28 days of hydration). The results shown in Fig. 17 clearly illustrate the ability of CEMHYD3D to predict longer term (7 day and/or 28 day) strengths from one early time assessment, in agreement with previous results [1, 2]. The predicted values clearly fall within the standard deviations determined in the CCRL testing program [9].

  

Figure 17: Experimentally measured (circles) and predicted (line) compressive strength development for mortar cubes prepared with cement 133 (w/c=0.485). Crosses indicate +/- one standard deviation from the mean, as determined in the CCRL testing program [9].