CORRELATIONS BETWEEN DIFFERENT ITS AND UCS TEST PROTOCOLS FOR FOAMED BITUMEN TREATED MATERIALS

Transcription

1 CORRELATIONS BETWEEN DIFFERENT ITS AND UCS TEST PROTOCOLS FOR FOAMED BITUMEN TREATED MATERIALS M. Houston 1 and F. Long 2 1 Loudon International PO Box 543, Kloof, 4067, South Africa. matthew_houston@hotmail.com 2 CSIR Transportek PO Box 395, Pretoria, 0001, South Africa. flong@csir.co.za ABSTRACT This paper compares the strength parameters and optimum foamed bitumen contents of foamed bitumen treated materials. Correlations between ITS and UCS of 100 mm and 150 mm diameter briquettes are established for 23 samples treated with varying quantities of foamed bitumen and cement/lime. Optimum foamed bitumen contents derived from the different strength parameters are compared and the treated materials are classified based on the correlations established. Results indicate that the current use of UCS as a primary parameter for treated material classification and the use of the soaked 150 mm ITS as an indicator of treated material moisture sensitivity is not necessary. The results highlight inadequacies in the current material classification system. 1. INTRODUCTION In the past, 100 mm diameter (Marshall) briquettes were used for mix design and field quality control for foamed bitumen treated materials (Lewis et al, 1995). The Indirect Tensile Strength (ITS) was used as an indicator of relative strength in the dry state and as an indicator of moisture sensitivity in the soaked state. This method offered a simple and relatively inexpensive approach to indicate the relative quality of a foamed bitumen treated material and to optimise the foamed bitumen and cement/lime content; however, it could not be used directly in the structural design of the foamed bitumen treated pavement layer. Subsequent research led to the release of Interim Technical Guidelines: The design and use of foamed bitumen treated materials, TG2 (Asphalt Academy, 2002). The mix design procedure in TG2 is more extensive than was used previously, and 150 mm rather than 100 mm diameter specimens are used. Design engineers were reluctant to use the extensive mix design procedure. The correlation between the 100 and 150 mm specimens had not been established and consequently a database of parallel testing accumulated. The TG2 guideline classifies foamed bitumen treated materials according to Indirect Tensile Strength (ITS) and Unconfined Compressive Strength (UCS) on 150 mm diameter briquettes compacted at 100% Modified AASHTO compaction. Based on the treated material classification, structural design calculations could be made. The motivation for the use of both the UCS and ITS was to capture both the compressive strength and the flexibility of the mix. The UCS is a measure of compressive strength, whereas the ITS is used as a surrogate test for measuring flexibility. This paper presents a database of results from several foamed bitumen treated materials, tested with both 100 mm diameter briquettes and 150 mm briquettes. The objectives of the paper are to assess the correlation between properties of the briquettes with different diameters, and to assess the TG2 material classification system. Proceedings of the 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) September 2004 ISBN Number: Sun City, South Africa Proceedings produced by: Document Transformation Technologies cc

2 2. DESCRIPTION OF MATERIALS TESTED The materials tested were from various sites where foamed bitumen stabilisation was being used as the pavement rehabilitation method. A total of 23 samples were stabilised with varying quantities of foamed bitumen and cement/lime. The site locations, sample numbers, untreated blend components and approximate TRH14 (TRH14, 1988) classifications (CBR s were not tested) are included below: South Africa (MR439, Northern Zululand) 1. reclaimed calcareous sand: 40% dorbank (G8) 2. reclaimed calcareous sand: 40% -13.2mm crusher dust (G7) 3. reclaimed foamed bitumen stabilised calcareous sand: 33% -13.2mm crusher dust (G7) 4. reclaimed calcareous sand (G8) Zambia (Livingstone - Sesheke) 1. weathered basalt (G6) 2. weathered basalt: 12% reclaimed cement stabilised Kalahari red dune sand: 8% silty sand (G5) 3. weathered basalt: 20% silty sand (G5) 4. weathered basalt: 25% reclaimed Kalahari red dune sand: 15% reclaimed seal (G5) 5. weathered basalt: 25% reclaimed cement stabilised silty sand (G5) 6. weathered basalt: 33% reclaimed cement stabilised silty sand (G5) 7. weathered basalt: 25% reclaimed cement stabilised silty sand (G5) 8. weather basalt: 25% reclaimed cement stabilised Kalahari red dune sand (G5) Greece (Iliki - Athens - Korinthos) 1. RAP (G8) 2. RAP: 25% graded crushed limestone A (G6) 3. RAP: 50% graded crushed limestone A (G5) 4. RAP: 75% graded crushed limestone A (G6) 5. RAP: 25% reclaimed cement stabilised graded limestone (G6) 6. RAP: 50% reclaimed cement stabilised graded limestone (G5) 7. RAP: 75% reclaimed cement stabilised graded limestone (G5) 8. RAP: 25% graded crushed limestone B (G6) 9. RAP: 50% graded crushed limestone B (G6) 10. RAP: 25% graded crushed limestone C (G6) 11. RAP: 50% graded crushed limestone C (G6) The sample indicators (gradings, Atterberg limits), some MDD/OMC values and approximate untreated material classifications are contained in Table RANGES OF ADDITIVE CONTENTS USED The following ranges of additives were used: 1. Foamed bitumen content: 2-4.5%. 2. Cement content: 0-1.5%. 3. Lime content: 0-2%. 4. Materials with an untreated PI less then 14 and treated PI less than 6 (after the addition of lime and foamed bitumen). All results and conclusions apply to the listed conditions only. Cement or lime was used in combination with foamed bitumen i.e., never together. Conditions beyond those included above will probably seldom be encountered due to cost constraints and practicality. These would be extreme cases requiring careful consideration to evaluate the applicability of the results and conclusions presented in this paper.

3 Table 1. Sample indicators. Sieve size Sample number (mm) GM PL 32 LL NP NP NP NP 18 NP NP NP NP NP NP NP NP NP NP NP NP NP NP NP NP NP NP PI 14 MDD (kg/m3) OMC (%) TRH12 Classification G8 G7 G7 G8 G6 G5 G5 G5 G5 G5 G5 G5 G8 G6 G5 G5 G6 G5 G5 G6 G6 G6 G6 Percentage passing sieve by mass

4 4. BRIQUETTE PREPARATION mm Diameter Briquettes 100mm diameter briquettes were prepared in accordance with past practice (Lewis et al, 1995). This method was adapted from the Marshall method (for asphalt) with modifications to the compaction temperature and curing procedures. Soaked briquettes were subjected to 24 hours in water at 40 C as opposed to soaking under a vacuum for 1 hour. The properties and moisture regimes tested included: Dry ITS (ITS dry100 ). Briquettes were cured at 40 C for 72 hours. This procedure dried the briquette out to less than 1% moisture content without excessively softening the bitumen. Soaked ITS (ITS wet100 ). Dry briquettes were soaked in water for 24 hours at ambient temperature (approximately 25 C) mm Diameter Briquettes The 150mm diameter briquettes were prepared in accordance with TG2 (Asphalt Academy, 2002). The properties and moisture regimes tested included: ITS at equilibrium moisture content (ITS eq150 ). Briquettes were allowed to stand unsealed at ambient temperature for 24 hours and then 48 hours in a sealed plastic bag at 40 C. The moisture content using this method is aimed at simulating field equilibrium moisture. The cured moisture contents were a function of the compaction moisture content (which varied from sample to sample) and material type; they were found to be between 2.5 and 6%. Soaked ITS (ITS wet150 ). Briquettes cured to equilibrium moisture content were soaked in water for 24 hours at ambient temperature (approximately 25 C). The UCS tests were performed at equilibrium moisture content, i.e. the same curing procedures as the ITS eq150 briquettes were followed. The following modifications were made to the TG2 recommended procedures: 127 mm high briquettes were used for ITS determination. It was not possible to make 95 mm high briquettes using the laboratory equipment available. The briquettes were air dried at ambient temperature for 24 hours before placing in a bag for 48 hours at 40 C. This adjustment to the curing procedure was developed to simulate field curing conditions and equilibrium moisture content more accurately. This procedure was also more practical. On removal from the oven, the briquettes were cooled in the same bags and not in fresh, dry bags. This was a far more practical approach. A study showed that on average, cooling in the same bag retained 3 grams more water; a difference of approximately 0.1% moisture content, which was considered negligible. For the Greece briquettes, a pie-shaped rammer head was used. A comparison with a manual standard Modified AASHTO hammer revealed similar compaction effort. The mixes were prepared with a twin-shaft pugmill mixer. This type of mixer produces superior specimens than blender-type mixers. Duplicate briquettes instead of triplicate briquettes were used to help simplify the procedure.

5 5. ITS AND UCS RELATIONSHIPS A linear relationship generally existed between all properties. Data and the best-fit functions are illustrated, with the relevant figure numbers, in Table 2. The relationships investigated are illustrated in Figure 1, with the applicable correlation coefficients. The complete data set is included in Appendix A. 100mm Table 2. Best fit lines and correlation coefficients. 150mm ITS dry100 ITS wet100 ITS eq150 ITS wet150 UCS Best fit line Correlation coefficient Figure number x y y = 0.279x Figure 2 x y y = 0.709x Figure 3 x y y = 0.496x Figure 4 x y y = 3.970x Figure 5 x y y = 0.454x Figure 6 x y y = 0.670x Figure 7 x y y = 5.317x Figure 8 y x y = 0.117x Figure 9 Note: 1. The correlation coefficient is the covariance of the two data sets divided by the product of the standard deviations. 2. x and y in kpa. Figure 1. Relationships between variables investigated.

6 Figure 2. ITS dry100 versus ITS wet100. Figure 3. ITS dry100 versus ITS eq150.

7 Figure 4. ITS dry100 versus ITS wet150. Figure 5. ITS dry100 versus UCS.

8 Figure 6. ITS wet100 versus ITS wet150. Figure 7. ITS eq150 versus ITS wet150.

9 Figure 8. ITS eq150 versus UCS. Figure 9. UCS versus ITS wet150. Note that the correlation coefficient is the covariance of two data sets divided by the product of the standard deviations.

10 The following observations are made: A correlation of 0.47 exists between ITS dry100 and ITS wet100. The ITS dry100, therefore cannot accurately indicate the moisture sensitivity of a foamed bitumen treated material. This reinforces the need for testing ITS dry100 in parallel with ITS wet100. A correlation of 0.92 between ITS eq150 and ITS wet150 implies that the ITS wet150 test is redundant. A correlation of 0.86 between ITS dry100 and ITS eq150 and 0.83 between ITS dry100 and ITS wet150 implies that the ITS dry100 can be used to derive ITS eq150 and ITS wet150 with a corresponding level of confidence. A correlation of 0.94 between ITS eq150 and UCS implies that ITS eq150 can be used to derive the compressive strength of a material with a corresponding level of confidence. For the working range, the UCS parameter reveals little more of the treated materials properties than the ITS eq150. A correlation of 0.86 between ITS dry100 and UCS implies that the ITS dry100 can be used to derive the UCS, with a corresponding level of confidence. A correlation of 0.45 exists when comparing ITS wet100 with the ITS dry100. This, along with the low correlation between ITS wet100 and ITS wet150 (0.49) implies that the ITS wet100 values are independent of other parameters. Either they are misleading (due to over-sensitivity to variability) or they are the better indicator of moisture sensitivity. Due to factors such as compaction method (flat face impulse compaction versus the kneading action of 150mm compaction; field density control is specified relative to the Proctor density), compaction effort (Marshall compaction is higher than field compaction and 150mm briquette compaction), briquette size versus maximum particle size (the larger a briquette, the more it should be able to represent field conditions and the less influence a large aggregate has on the specimen) and the influence of plasticity, a lower level of confidence is given to the ITS wet100. The ITS eq150 and UCS results indicate that a FB3 material will seldom, if ever, be encountered. Encountering a FB3 material within the working range would probably indicate an error in the testing procedures. See Figure 8. This is In general, the correlation between 150 mm specimens is better than between 100 mm specimens and any other parameter. This suggests that there may be more variability in the results from the 100 mm specimens. This is probably due to the influence of the large aggregates in the smaller diameter specimen. 6. OPTIMUM BITUMEN CONTENT The optimum foamed bitumen and cement/lime content is generally based on the maximum ITS eq150. Both the ITS wet150 and UCS can be used (although seldom done simultaneously) to increase the level of confidence by minimizing the influence of human error in the mix design procedure, e.g. compaction differentials between briquettes caused by different compaction moisture contents and inaccurate foamed bitumen application. An exercise was performed to compare these optimum bitumen contents with those obtained by independently using the ITS dry100, ITS wet150 and UCS values. The ITS wet100 was not considered because of its relatively poor correlations with other parameters. Comparisons of the optimum foamed bitumen contents appear in Table 3.

11 Table 3. Optimum foamed bitumen contents (%) derived from different parameters. Sample ITS eq150 ITS dry100 UCS ITS wet150 1 (2% lime) (1% lime) (2% lime) (1% cement) (no filler) (2% lime) (1% lime) (1.5% lime) (1.5% lime) (1.5% lime) (1.5% lime) (1.5% lime) (1.5% lime) (1.5% lime) (1.5% lime) The following is observed: In all cases except one (Sample 2 (2% lime)), a difference in the optimum foamed bitumen content did not alter the TG2 material classification (Asphalt Academy, 2002). The ITS eq150 optimum foamed bitumen content can be derived from the ITS dry100 to: - within 0.5% foamed bitumen content (in 100% of the cases) - within 0.25% foamed bitumen content (in 80% of the cases ) - exactly (in 67% of the cases) The ITS eq150 optimum foamed bitumen content can be derived from the UCS to: - within 0.5% foamed bitumen content (in 100% of the cases) - within 0.25% foamed bitumen content (in 60% of the cases) - exactly (in 40% of the cases) The ITS eq150 optimum foamed bitumen content can be derived from ITS wet150 to: - within 0.5% foamed bitumen content (in 100% of the cases) - within 0.25% foamed bitumen content (in 87% of the cases) - exactly (in 53% of the cases) 7. TREATED MATERIAL CLASSIFICATION An exercise was performed to compare the treated material classification, based on actual ITS eq150 and UCS, to the derived ITS eq150 and UCS. The purpose of the exercise was to assess the reliability of the linear relationships (see Section 5) within a treated material class. Table 4 contains the percentages of samples correctly classified for each treated material class. The following cases were considered: Case A: ITS eq150 and UCS both derived from ITS dry. Case B: ITS eq150 and UCS derived from ITS eq150. Case C: ITS eq150 and UCS both derived from ITS wet150.

12 Table 4. % of correctly derived treated material classifications. Number of samples Percent classified in same category as actual ITS eq150 and UCS A B C FB0* FB FB FB all * Material properties fall below minimum criteria. The following comments can be made: ITS eq150 alone is generally a good indicator of the material class (average reliability of 97%). This indicator does not require the actual UCS to accurately classify the treated material. ITS dry100 alone is generally not a reliable indicator of the material class although it can be used with an acceptable level of confidence (91%) to classify FB1 materials. ITS wet150 alone is generally a good indicator of FB1, FB3 and FB4 materials. No FB3 materials were encountered. This implies that they will seldom be encountered within the working range (see Figure 8). For the materials from Greece (Samples 13 23), 45% were classified correctly for all Greece cases combined. Compared to 83% correctly classified for all cases combined, this implies that there were either problems with testing procedures or a high sensitivity with materials containing RAP. Indeed, samples containing high percentages of RAP have proved problematic on this project revealing a higher sensitivity to the time to compaction after the addition of foamed bitumen, the compaction temperature, the amount of time the briquette is allowed to stand in the mould and the compaction moisture content. For all the MR439 cases (Samples 1-4), the reliability of the ITS dry100 based classification was 50% whereas the ITS eq150 based classification was 100%. This implies that the ITS dry100 is an unreliable indicator for sandy materials (in this case, for materials with a grading modulus less than 2.0). 8. COMPARISON OF UCS AND ITS EQ150 It was found that by increasing the foamed bitumen content for a given cement/lime content, the UCS sometimes increased and sometimes decreased. The following comments can be made: For a constant cement/lime content and an increase in foamed bitumen content: For an average increase in ITS eq150 of 10%, the average increase in UCS was 0.5%. For an average decrease in ITS eq150 of 8%, the average decrease in UCS was 3%. For a constant foamed bitumen content and an increase in cement/lime content: For an average increase in ITS eq150 of 22%, the average increase in UCS was 11%. One decrease in ITS eq150 was encountered (15%) with a corresponding increase in UCS (113%); this result could be considered an outlier.

13 These results, along with the positive gradient of the ITS eq150 versus UCS best-fit line, imply that within the working range, UCS generally increases with increasing ITS and therefore both tests are measuring the same material property. 9. CONCLUSIONS The following conclusions and recommendations are made: A FB1 treated material can be classified with 100 mm diameter briquettes with a reasonable level of confidence. All other material classes can be derived from the ITS dry100, but with a low level of confidence. It is therefore recommended that 150 mm diameter briquettes be used for mix design and material classifications, and for field quality control. ITS eq150 can be used to derive UCS and ITS 150wet. It can also be used, on its own, to determine the optimum foamed bitumen content. The existing database of ITS results determined on 100 mm briquettes can be adjusted to the equivalent 150 mm ITS or UCS results with some confidence. This only applies to materials within the working range. The ITS wet150 tests show a good correlation with the ITS eq150. This may demonstrate either that the ITS wet150 is not a good indicator of the moisture sensitivity of a mix or that ITS eq150 is a satisfactory indicator of moisture sensitivity. It is not possible from these data to determine which statement is more correct. It is recommended that an alternative test for moisture sensitivity be investigated. The mix design procedures and classification system of foamed bitumen treated material contained in TG2 should be revised, as it is redundant to use both the UCS and ITS tests. A test that actually measures the flexibility of a material, such as the strain-at-break four-point beam test, should be investigated further for possible use in the material classification. Use of either the UCS or ITS in the mix design procedure should be sufficient for determining the optimum binder content. 10. REFERENCES Asphalt Academy. June Interim Technical Guideline: The design and use of foamed bitumen treated materials. Technical Guideline 2, Pretoria, South Africa. Lewis A.J.N.L., Barron M.G., Rutland G.P Foamed Bitumen Recent Experience in South Africa. International Road Federation (IRF) Regional Conference, Volume II. Johannesburg, South Africa. Pp Technical Recommendations for Highways, TRH14: Road construction materials. 1995, Pretoria, South Africa.

14 APPENDIX A: ITS AND UCS DATA Sample Lime (%) Cement (%) Foam (%) 100mm diameter briquettes 150mm diameter briquettes ITS dry100 ITS wet100 ITS retained ITS eq150 ITS wet150 ITS retained UCS (kpa) (kpa) (%) (kpa) (kpa) (%) (kpa)

15 Sample Lime (%) Cement (%) Foam (%) 100mm diameter briquettes 150mm diameter briquettes ITS dry100 ITS wet100 ITS retained ITS eq150 ITS wet150 ITS retained UCS (kpa) (kpa) (%) (kpa) (kpa) (%) (kpa)