The ultimate strength of solidified/stabilized waste is an important physical property that frequently must be determined for each batch of treated waste. The usual test for this purpose is the unconfined compressive strength (UCS) taken after 28 days of cure. An alternative test method, the resistance to penetration or cone index (CI), as measured using a cone penetrometer, can also give meaningful information on the setting and strength development properties of cementitious materials after as little as one, five, and ten days of cure. This paper presents an analysis of the possible correlation of the 28-day UCS with the one-, five-, and ten-day CI for a variety of low-strength, solidified/stabilized waste materials.
The data used in this analysis were developed in an evaluation of the effects of ten materials that have the potential to interfere with the contaminant containment and strength development properties of a solidified/stabilized metal hydroxide sludge. Three binder systems (cement, lime/fly ash, and cement/fly ash) were used to solidify/stabilize a specially prepared metal hydroxide sludge containing substantial concentrations of four metal contaminants (cadmium, chromium, nickel, and mercury). Ten interfering materials were individually added to the sludge. The effects of these materials on the physical properties of the treated sludge were evaluated using UCS and CI testing methods. Substantial differences in the setting and strength development characteristics of the different binder systems precluded pooling of the total data set from all binder systems. Even variations within the populations from each binder system are quite large.
The analysis of data generated during the study indicated a significant correlation (99% confidence level) between the one-, five-, and ten-day CI and the 28-day UCS for specimens from all three binder systems except the five-day specimens prepared with cement and the one-day specimens prepared with lime/fly ash. However, variability in the test values between the different batches used in the experiment resulted in high standard deviations and relatively low correlation coefficients (around 0.5). This variability reduces the confidence in the ability to predict 28-day UCS from the CI in quantitative terms.
The data set and analysis procedure is a stringent test of the predictive relationship between CI and UCS. In general, the interfering materials used in the underlying study had greater effect on the early setting and strength development properties (one to ten days) than on the longer term (28 days) strength. Materials that cause retarded set of cementitious materials, and thus strongly affect the one-to five-day CI, often resulted in long-term strengths greater than the control specimens. Based on the data collected during this study, a one-to five-day measurement of the CI is shown to provide a good indication of the 28-day strength. However, it seems unlikely that a short-term test of any type can be used as a reliable quantitative predictor of long-term strength in the highly variable waste streams and low-strength products typically encountered in the treatment of hazardous wastes.