It is clear, however, that the combination of cement type and material under expected construction conditions will affect the density and strength of the material obtained in the field. CSIR Transportek, in collaboration with VKE, Soillab and Frank Netterberg, contracted the Cement and Concrete Institute to investigate the impact of the new SABS EN 197-1 cement specification on the use of cement as a road material stabilizer.
INTRODUCTION
Background
Terms of Reference and Objectives
The information obtained from Task 1, together with the new specifications for cement, had to be evaluated in terms of the theoretical chemical reactions affecting the soil stabilization process. This report summarizes the first phase of testing (Task 2) based on the findings and recommendations of the above report.
THE MATERIALS
- Introduction
- Weathered dolerite (CCI 1)
- Weathered norite (CCI 2)
- Test results
- Discussion
A number of material sources were tested, but none had plasticity indices of less than 6 percent. There appears to be nothing unusual about the properties in Tables 2.1 and 2.2, except for the somewhat high pH of 9.0 of the norite and the amorphous silica content of 1.6 percent of both materials.
THE CEMENTS
- General
- Details of cements
- Testing carried out on clinkers and cements by PPC
- Setting Times at Various Temperatures
- Discussion
The results are summarized in Table 3.5 and presented graphically in Appendix C. Table 3.5: Setting times of cement at different temperatures. The setting times of CEM III cement were closest to the proposed European standard.
LABORATORY TESTING OF SOIL-CEMENT MIXES
General
Initial Consumption of Lime (ICL) and Cement (ICC)
However, the interpretation of ICC, which is a more recent development, is uncertain and probably requires reaction (and hydration) times longer than the one hour used for ICL. It should be noted that the definition of the end point in ICL is not always clear, as small pH changes at high pH are ongoing.
Density
In all cases, the addition of cement reduced the MDD of the dolerite and increased the OMC, as would be expected from cement stabilization of gravelly sand material. Cements, on the other hand, had a much smaller effect on the MDD of norite, but increased the OMC in all cases. CEM III A and fly cement (CEM II B-V 32.5) had the least effect on the MDD of dolerite, but had the greatest effect of all cements (albeit minimal) on norite.
No particular relationship is evident, although trends show that as initial curing times increase, MDD of lunatics decreases, contrary to expectations. However, it appears that there may be a tendency for the MDD of dolerite to increase with increasing initial curing time. Graphs of initial and final setting time and MDD at 22°C against type of cement (number) for dolerite and norite are given in Figures 4.2 to 4.4.
The maximum decrease in compacted density compared to the MDD of the natural material (regardless of temperature) in the dolerite was 3.8 percent after 2 hours of conditioning, compared to 4.7 percent after 4 hours of conditioning. The average density of the norite after 2 hours of conditioning (at 23°C) was higher than that at 10°C but after 4 hours the density at 23°C was generally less. The difference between the density of the norite at 23°C and 40°C was much greater than the differences for the dolerite.
Unconfined compressive strength (UCS)
A plot (Figure 4.11) of cement type versus average UCS (average of duplicate samples at all temperatures and conditioning times) showed that for the dolerite, the CEM I cements (numbers 1 and 2) produced lower strengths than the CEM II 42.5 cements (numbers 3 and 4) and strength generally decreases thereafter as the cement type changes through the CEM II 32.5 (5 and 6) to the CEM III. The strengths obtained from all cements, except the CEM II B-V 32.5, exceeded the upper limit for a C3 material on the other hand, with the CEM III producing the highest strength of all. It should be noted that the inability of the CEM III to produce the minimum strength of 1500 kPa after 7 days is not necessarily a disadvantage, provided it has other advantages and reaches comparable strengths to the other cements after, for example, 28 days.
To normalize the density, these data (Figure 4.13) were replotted using the ratio of density at testing (only the data from the samples conditioned at 23°C and for 4 hours were used) to the MDD for each cement, i.e. Except for cements 6 and 7, all samples were within one percent of MDD. Similar plots were produced for norite and the UCS density ratio plot is shown in Figure 4.14.
Conditioning the sample material (soil, cement and water) for 4 hours after mixing and before compaction resulted in a reduction in strength compared to conditioning for 2 hours in each case (Figure 4.15). Laboratory conditioning temperatures are assumed to represent ground cement temperatures during mixing and compaction on the roadway. It is clear that the strength produced by some cements is more affected by conditioning time and material than others, e.g.
Indirect tensile strength (ITS)
However, there are no established trends and, for example, a cement grade (cement 7 - CEM III A) that was strongly influenced by conditioning time and temperature reacted poorly with dolerite, but produced the highest overall strength for norite. It is therefore believed that the ITS is more closely related to the density of the test sample than the UCS. Conditioning the sample material (soil, cement and water) for 4 hours after mixing the cement and water and before compaction resulted in a reduction in the average indirect tensile strength compared to conditioning for 2 hours in almost all cases (Figure 4.26) .
The significant decrease in average strength as conditioning temperature (assumed to be that of mixing and compaction) increases is clearly illustrated and follows a similar trend to that for the UCS (Figure 4.16). This is further illustrated in Figure 4.28 for dolerite in relation to the conditioning time and cement types. The norite results were similar except that less cement increased in tensile strength after 4 hours of conditioning.
It is clear that the tensile strength produced by some cements is affected to a greater degree by conditioning time and material than others. However, there are no fixed trends and one cement (CEM III A) that was strongly affected by conditioning time and temperature reacted poorly with dolerite for example, but gave some of the best results for norite. Again the contribution of lower densities to the reduction in indirect tensile strength cannot be quantified from the available data.
Discussion
The effect of temperature on CEM III A with norite after 4 h conditioning was negligible and conditioning time had little overall effect on ITS. Little correlation between cement type, material type, conditioning time or temperature was evident from the testing performed. Graphs of the relationship between unconfined compressive strength and indirect tensile strength with respect to cement types are shown in Figures 4.32 and 4.33 for dolerite and norite, respectively.
This comparison ignores the effect of variations in compacted density of the specimens on the strength data. Apart from one "outlier" for the norite (cement number 3), the trends between the two strength measurements are very comparable. The sulfate content of the cements varied between 0.8 (this value, determined by PPC Group Laboratory Services, appears low - the Lafarge analysis on this cement gave a value of 1.19 percent) and 2.77 percent SO3, all well within the upper limit of 3.5 or 4.0 percent specified for the various strength classes in EN 197.
As discussed earlier, there was little correlation between the setting rates and the sulfate content of the cements. The alumina content of the cements ranges from 3.9 to 11.4 percent, with only cements containing fly ash and slag being higher than 6 percent (alumina content is meaningless in these cements). These results are comparable to typical analyzes of a range of European CEM I cements, which have an alumina content of 4.1 to 6.2 percent11.
ALLOWABLE CONSTRUCTION TIMES
This working time depends on the materials and also on the environmental conditions and temperature and humidity must be taken into account.”. Although insufficient information is available to determine the working time according to Australian practice, it does provide a means of relative assessment of the workability of the soil cements tested (Tables 5.1 and 5.2). The results after 2 hours have therefore been taken as standard instead of one hour and 98 percent of the density and 90 percent of the UCS after 2 hours has been used to take some account of this relaxation.
The comparison between the MDD (determined at 4 hours) and the dry density at 23°C at 4 hours provides a rough check on the quality of the work: these figures should ideally all be close to 100 percent. In the case of the dolerite, only two cements at 10°C gave working times of more than 2 hours. At 23°C, only one cement had a working time of more than 2 hours, while that for two of the cements was much less than 2 hours.
In the case of norite, at 10°C only one cement gave a working time greater than 2 hours. However, the same cement was the worst or one of the worst with dolerite. 6 There appears to be a relationship between the initial and final setting time of the cement at the temperature closest to its working time (the slower the setting, the better in the case of dolerite, but not with norite).
CONCLUSIONS
On this basis, all possible construction materials to be stabilized should be tested with cement that can be used in the proposed project and in the expected ambient conditions to identify the expected allowable construction time and the combination that ensures workability. longer/.
RECOMMENDATIONS
APPENDIX D: SUMMARY OF STABILIZATION TEST RESULTS Summary of density and UCS results for sample CCI 1.
