ALTERNATIVE LABORATORY TEST METHOD AND CORRELATIONS TO ESTIMATE MODULUS OF RUPTURE OF CEMENT-TREATED BASE MATERIALS

Transcription

1 Lee, Faruk, Hu, Haggerty, Walubita ALTERNATIVE LABORATORY TEST METHOD AND CORRELATIONS TO ESTIMATE MODULUS OF RUPTURE OF CEMENT-TREATED BASE MATERIALS Sang Ick Lee (Corresponding Author) Texas A&M Transportation Institute TAMU, College Station, TX, USA Tel: --; Fax: --0; s-lee@tti.tamu.edu Abu N.M. Faruk Texas A&M Transportation Institute TAMU, College Station, TX, USA Tel: -0-; Fax: --0; a-faruk@tti.tamu.edu Xiaodi Hu Transportation Research Center, Wuhan Institute of Technology Wuhan, China Tel: () --0; huxiaodi@hotmail.com Brett Haggerty Texas Department of Transportation NW Loop 0, San Antonio, TX, USA Tel: 0--00; Cell: 0--; brett.haggerty@txdot.gov Lubinda F. Walubita Texas A&M Transportation Institute (TTI) The Texas A&M University System TAMU, College Station, TX, USA Tel: -- Fax: --0; lfwalubita@tamu.edu Word count:, words text + tables/figures x 0 =, Submission Data: July th 0

2 Lee, Faruk, Hu, Haggerty, Walubita 0 0 ABSTRACT Chemically stabilized base and soil layers including cement-treated aggregates are widely used to provide long-term support for pavement structures on the Texas highways. This is more prevalent especially with the recent surge in the heavy truck-traffic loading in the energy-sector impacted areas of south-central Texas. With the use of stabilized layers however, accurate characterization of the load-associated fatigue behavior in the pavement structure, quite often, becomes a complicated technical issue. The primary input required in the mechanistic-empirical (M-E) design procedures to predict the fatigue cracking in stabilized layers is the -day flexural strength (or Modulus of Rupture, MoR). The AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG) and the Portland Cement Association (PCA) recommend using the -day Unconfined Compressive Strength (UCS) to estimate the MoR for input Level analysis. This paper presents an experimental study to evaluate MoR-UCS relationships and develops a new predictive model to improve the estimation of MoR using three standard Texas base materials, treated with three cement contents (,, and %). Also, a new test method based on the indirect tensile (IDT) strength test was proposed to reduce test materials and resources required to run the MoR and UCS tests. New MoR-UCS relationship model developed in the study exhibited improved estimation of the MoR values. As well, the strong correlation between MoR and tensile strength obtained from newly formulated IDT test method indicated the possibility of estimating MoR required to predict the fatigue life of cement-treated base materials for the M-E design procedures. Key Words: Cement-Treated Base, CTB, Flexural Strength, Modulus of Rupture, MoR, Unconfined Compressive Strength, UCS, Indirect Tensile Test, IDT

3 Lee, Faruk, Hu, Haggerty, Walubita INTRODUCTION Chemically stabilized base and soil layers including cement-treated aggregates are widely used to provide long-term support for pavement structures on the Texas highways. The surge in heavy truck-traffic loading in energy-sector impacted areas is one of the major factors driving the demand for stabilized layers, where economical methods to strengthen the pavement quickly are critical. With the use of stabilized layers, however, accurate characterization of the loadassociated fatigue behavior in the pavement structure, quite often, becomes a complicated technical issue. For instance, if the stabilized layer lies directly underneath the hot-mix asphalt (HMA) layer, a portion of any fatigue cracking in the stabilized layer will contribute to cracking in the HMA. Thus, proper and adequate characterization of the load-associated fatigue behavior of stabilized base layers is pressingly imperative both at laboratory and field levels. The fatigue life of stabilized materials is related to the critical flexural stress induced within the stabilized layer of flexible pavement systems (). The fatigue model used in the AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG) is as follows: log N f = k β c ( σt M R ) k β c where N f is the number of repetitions to fatigue cracking, t is the tensile stress at the bottom of the stabilized layer, M R is the -day flexural strength (modulus of rupture or MoR) of the stabilized layer, k and k are the national calibration factors, and c and c are the local calibration factors which are set to.0 in the MEPDG (). As shown in Equation, the primary input required in the mechanistic-empirical (M-E) design procedure to model and predict fatigue cracking in stabilized layers is the -day flexural strength. The MEPDG provides three options to determine the flexural strength for M-E pavement design based on the input levels and the required degree of accuracy in the analysis. Input Level (highest hierarchal accuracy) stipulates to estimate the flexural strength directly from laboratory testing of beam specimens, while Level (lowest hierarchal accuracy) includes the default values for cement-treated aggregates. For Level analysis, however, the flexural strength can be estimated using the relationship with the -day unconfined compressive strength (UCS) of the cement-treated material as follows (): -day MoR (psi) = 0. UCS () where UCS is the UCS in pound per square inch (psi) after -day moist curing. The Portland Cement Association (PCA) also recommends a square root relationship with the -day UCS to determine the MoR for cement-treated base material as follows (): -day MoR (psi) =.0 UCS () STUDY OBJECTIVES Based on AASHTO T, MoR values can be measured from laboratory tests using a -day cured beam specimen for Level analysis (). However, this test is very time-consuming and presents difficulties for handling the relatively large test specimens. On the other hand, the use of a single default value under Level analysis is the simplest way to model and predict the fatigue life; however, this approach has latent inaccuracy to be used for different types of base materials. ()

4 Lee, Faruk, Hu, Haggerty, Walubita In practice, it is anticipated that the majority of M-E designs will prefer to follow the Level approach of estimating the flexural strength using the relationship models with the -day UCS. However, the UCS test still is not simple and requires a large amounts of materials (approximately, 0 lb. per sample), and more importantly, the failure mode of the UCS test is compressive, as opposed to the flexural beam test which has a tensile failure mode. Additionally, pavement engineers have to have an opportunity to better the state of the practice in a manner that will cost-effectively optimize and improve the reliability of M-E pavement designs and simplify testing requirements for practical and routine applications. Based on the above background, the main objective of this study was to propose an alternative simpler laboratory test method, which is more representative and simulative of the tensile behavior of beam samples, to estimate the flexural strength of cement-treated base materials. The second objective was to establish a new relationship with the flexural strength to replace reliance on the UCS test. To achieve these objectives, extensive laboratory testing using three types of base materials, commonly used on Texas highways and treated with different cement dosages, was performed to develop a new test method, as well as to establish correlations with the -day MoR values. In the subsequent sections, the laboratory test methods and experimental plan are discussed. Laboratory test results are then presented and analyzed, followed by a discussion and comparative synthesis of the test methods. The paper then concludes with a summary of the key findings and recommendations. LABORATORY TEST METHODS The laboratory test methods are presented and discussed in this section. These tests include the flexural beam, UCS, and indirect tensile (IDT) tests. A tabulated comparison of the test methods is also included. The Flexure Beam Test The flexure beam test was performed to measure the MoR of cement-treated base materials in this study. The specimens were fabricated in a 0 in. beam mold and compacted in two lifts, with a 0 lb. square-face drop hammer (). The drop hammer was applied at each lift until reaching maximum dry density obtained from the moisture-density (M-D) curve testing. After compaction, molded specimens were placed in the damp (moist) room for a period of -day before testing. Figure shows the compacting and the curing process of the beam specimens.

5 Lee, Faruk, Hu, Haggerty, Walubita 0 (a) Compaction (b) Curing FIGURE Preparation of flexural beam specimens. The flexural strength of the cement-treated base material was determined with the thirdpoint loading mode as shown in Figure and the monotonic loading rate was 0.0 in/min in accordance with ASTM D (). The MoR was calculated as follows: MoR = PL bd () where P is the maximum applied load, and L, b, and d are span length, width, and depth of beam specimen, respectively. Load (P) in. in. in. in. in. in. 0 Span length ( in.) FIGURE The flexure beam strength test setup. The Unconfined Compressive Strength (UCS) Test The UCS test is a simple performance test widely used to determine the compressive strengths of soil and aggregate materials. In this study, the UCS specimens were prepared in accordance with Tex-0-E; i.e., cylindrical specimens, molded to in. diameter and in. height with the Soil Compactor Analyzer (SCA) approved by Texas Department of Transportation (TxDOT) and thereafter, cured for a period of seven days in the damp (moist) room before determining the compressive strength (). The specimen was then loaded monotonically at a static loading rate of

6 Lee, Faruk, Hu, Haggerty, Walubita 0 % strain per minute (0. in/min) with no lateral confinement in accordance with the Tex-- E procedure (). The Indirect Tensile (IDT) Strength Test Since the IDT strength test is relatively simple and the specimen can be easily molded using a gyratory compactor in the laboratory, the test has been widely used to characterize the tensile strength and fatigue/fracture resistance properties of pavement materials (). Additionally, the IDT specimen requires less material (approximately lb.) than the MoR or UCS specimen. Using these technical and practical benefits of the IDT test, Scullion et al. developed a laboratory testing protocol to select the optimum stabilizer content for Full Depth Reclamation (FDR) basecourse mix designs (). The test specimens were molded in cylindrical molds of in. diameter and a in. height using the Texas Gyratory Compactor (TGC). Due to the size limitation, the specimens should not be molded with the aggregate size larger than 0. in. Figure presents the TGC and a sample in a mold, respectively. 0 0 (a) Texas Gyratory Compactor (b) Loose Sample in Mold FIGURE Texas gyratory compactor and sample mold setup. The molded specimens are placed in the damp (moist) room for a period of seven days for curing as shown in Figure. In accordance with ASTM D and Tex--F, the indirect tensile strength of cement-treated base material was measured under monotonic axial compressive loading at a loading rate of in./min (,). The compressive load was applied to the specimen diametrically and indirectly induces horizontal tensile stresses in the middle of the specimen, which causes indirect tensile failure as shown in Figure. The maximum load at sample failure is used as the parameter to characterize the IDT strength of the cement-treated materials as illustrated in Equation (): S t = P πtd where S t is the IDT strength and t and D are thickness and diameter of the IDT specimen, respectively. ()

7 Lee, Faruk, Hu, Haggerty, Walubita FIGURE Cured specimens in damp room and typical IDT test setup. Table comparatively describes and summarizes the three laboratory strength tests used in this study. It was found that the MoR test is the most time- and material-consuming test and that its specimen fabrication is more difficult compared to the UCS and IDT strength tests. The IDT strength test was found to be the most-simple method, requiring the least amount of material and time to perform the test, as well as being cost-effective and more practical for routine applications.

8 Lee, Faruk, Hu, Haggerty, Walubita TABLE Comparison of Laboratory Test Methods Item Flexural Beam UCS IDT Schematic P P P P Specimen dimensions Approximate material required Time to fabricate sample P P P P in. W in. H 0 in. L in. D in. H in. D in. T 0 lb. 0 lb. lb. hours hours hours Time for cure days days days Loading rate 0.0 in./min % strain/min in./min Test time min min min Output data/ Calculation Modulus of Rupture MoR = PL bd Compressive strength P UCS = π(d ) Tensile strength S t = P πtd Simulative of field condition High (Tensile) Low (Compressive) Medium (Indirect tensile) Reference ASTM D Tex-0-E, Tex--E Scullion et al. () Legend: W = width; H = height; L = length; D = diameter; T = thickness 0 EXPERIMENTAL PLAN In this study, extensive laboratory testing was performed to explore the correlations of the -day MoR with -day UCS and IDT strength values of cement-treated base materials. Three base materials were collected from highway construction fields in Texas, and Type I portland cement was selected as a stabilizer. The test samples were prepared with three cement contents of,, and %, respectively, by mass of the dry solid material. For all the base materials, a minimum of three replicate specimens were tested in each test method for accurate representation and reliable interpretation of the test results. Table lists the properties of each of the base materials used in this study.

9 Lee, Faruk, Hu, Haggerty, Walubita 0 0 TABLE Basic Properties of the Base Materials Material Properties Material Material Material Gradation (% passing per size) Atterberg Limit Moisture-Density Relationship (0)* ¾ in / in. / in. 0 0 # # 0 # 00 Liquid limit (LL) Plastic limit (PL) Plastic Index (PI) 0 Max. dry density (MDD) (pcf)... Optimum moisture content (OMC) (%)... Soil Classification (AASHTO/USCS**) A--a/SP A--a/GW A--/GW Used on Texas highway FM US IH (Corpus Christi) (Paris District) (Waco District) * Measured with samples containing % cement content ** Unified Soil Classification System The M-D curves were generated based on the standard TxDOT protocol including Tex- -E and Tex-0-E test specifications (,0). All the specimens were fabricated based on the moisture content and dry density obtained from the samples containing % cement. The molding water contents were altered as the percent cement was increased or decreased; that is, deducting and adding 0. % moisture for and % cement, respectively (). LABORATORY TEST RESULTS AND ANALYSIS This section presents the results obtained from all laboratory testing conducted for this study. Existing MoR-UCS relationships recommended by the MEPDG and PCA was also evaluated. Then, new relationship models to estimate MoR with UCS and IDT strength were proposed for cement-treated base required for M-E Level design procedure. Linear regression analyses were carried out on the laboratory test results to evaluate the existing models and develop new models. The adequacy of the regression models in this study was assessed using the coefficient of determination (R ) representing the proportion of variation in the dependent variables by the model and the root mean square error (RMSE) representing the standard error of the model. However, since all the test results pertain only to the base materials and laboratory test conditions used in this study, the overall findings may not be exhaustive. Comparative Analysis of the Results Table lists averaged results and coefficient of variation (CoV) obtained from all the laboratory testing conducted with three base materials treated with,, and % cement contents, respectively, including the modulus of rupture, compressive strength, and tensile strength. The results indicate that, as theoretically expected, base materials treated with higher cement contents exhibited higher flexural, compressive, and tensile strengths. When comparing the CoVs from all

10 Lee, Faruk, Hu, Haggerty, Walubita 0 0 test methods, the CoVs from UCS and IDT tests are relatively lower (within %) than that from the MoR test, which ranges from.0% to as high as.%. That is, both test methods are fairly repeatable. However, it is noted that the MoR test is still repeatable since its CoVs are within 0% in all cases (). Figure illustrates the regression analysis of test results, comparing -day MoR with - day UCS and -day IDT by cement contents and material type. From these analyses, it was found that no definitive relationship was found with respect to the cement contents; see Figure (a) and (b). However, good correlations were observed with respect to the material type; see Figure (c) and (d). TABLE Laboratory Test Results Material No. Cement Content -Day MoR -Day UCS -Day IDT Strength Average (psi) CoV (%) Average (psi) CoV (%) Average (psi) CoV (%) Ratio of IDT/UCS % % % % % % % % %

11 Modulus of Rupture (psi) Modulus of Rupture (psi) Modulus of Rupture (psi) Modulus of Rupture (psi) Lee, Faruk, Hu, Haggerty, Walubita Sqrt UCS (psi) 0 (a) MoR vs. UCS by cement contents IDT (psi) (b) MoR vs. IDT by cement contents Sqrt UCS (psi) IDT (psi) (c) MoR vs. UCS by material types (d) MoR vs. IDT by material types FIGURE Test result by cement contents and material types. Evaluation of the Existing MoR-UCS Relationships For model accuracy assessment, the MEPDG and PCA relationship models presented in Equation and were evaluated using the laboratory test results. The MoR values were predicted using the UCS test data and then compared with the measured MoR. Figure shows the linear correlation between the measured and predicted MoR values for all the cement-treated base materials. Although, there are good correlations with higher R values for both relationships, the MEPDG model tends to overestimate MoR while the PCA model underestimates. Those estimations might be due to a limited laboratory data set collected with only a single dosage of cement content, i.e., % ().

12 Measured MoR (psi) Measured MoR (psi) Lee, Faruk, Hu, Haggerty, Walubita 0 00 y =.0x -.00 R = 0.0 RMSE =. psi Predicted MoR (psi) (a) MEPDG (b) PCA FIGURE MoR comparisons measured versus the MEPDG and PCA models. Improving the MoR-UCS Relationship To provide more accurate MoR estimate of the cement-treated base material for the M-E design procedures, an improved relationship between the -day MoR and -day UCS was proposed. A new relationship model was recommended based on simple linear regression with square root of UCS, using the laboratory UCS and MoR test data, as follows: -day MoR (psi) =. UCS.0 () Figure (a) compares the measured MoR values with those predicted by the regression model described in Equation. The new relationship indicates that there is a good correlation (R > 0.) between -day UCS and -day MoR and lower RMSE values than the MEPDG and PCA models. Clearly, the RMSE values shown in Figure with the newly proposed models are lower than those shown in Figure for the existing models, illustrating the superiority of the new models for the materials evaluated in this study. 0 0 y =.x -.0 R = 0. RMSE =.0 psi Predicted MoR (psi)

13 Measured MoR (psi) Measured MoR (psi) Lee, Faruk, Hu, Haggerty, Walubita y = x R = 0. RMSE =. psi Predicted MoR (psi) (a) MoR by UCS (b) MoR by IDT FIGURE Comparison of measured and predicted MoR by new relationship models. MoR Prediction Model with the IDT Test As summarized in Table, the UCS test is still time-consuming and requires a massive amount of materials to fabricate specimens. Most of all, since the failure modes of the UCS test are totally different from the MoR test, it may be difficult to justify the correlation of the MoR with UCS to be used as a parameter for fatigue life prediction, although the two are highly correlated. Thus, a new relationship model to estimate the -day MoR was developed using the tensile strength values from the IDT test, which is an alternative simpler test method, as follows -day MoR (psi) =. IDT. () where IDT is the IDT strength after -day moist curing (psi). Figure (b) compared the measured MoR values with those predicted by the IDT models described in Equation. The regression model fits the data very well with an R value of 0. and RMSE value of. The strong correlation between MoR and tensile strengths obtained from the IDT test method indicated the possibility of estimating the MoR required to model and predict the fatigue life of cement-treated base materials for the M-E pavement design procedures. SYNTHESIS AND COMPARISON OF THE LAB TEST METHODS Table provides a subjective comparison of the test methods based solely on the materials evaluated in this study and on the authors laboratory experience with these test methods. Overall, Table suggests that the major challenges associated with these tests include laborious sample fabrication process and sample failure mode of test methods. 0 0 y = x R = 0. RMSE =.0 psi Predicted MoR (psi)

14 Lee, Faruk, Hu, Haggerty, Walubita TABLE Comparison of the Test Methods Test Advantages and Applications Limitations and Challenges 0 Flexural Beam UCS IDT - Actual -day MoR value of cementtreated-base material for Level M-E design input - Actual simulation of fatigue behavior (tensile stress) mode in a pavement structure - Reliable -day UCS value to estimate MoR for Level M-E design input - Required less amount of materials (0 lb.) than flexural beam test - Reasonable time for sample fabrication ( hours) and curing ( days) compared to flexural beam test - The most cost-effective and practical test method - Reliable -day IDT value to estimate MoR for Level M-E design input - Requires the least amount of materials ( lb. per specimen) among three test methods - Shortest time for sample fabrication ( hour) and test ( min.) - Comparable failure mode (indirect tensile) versus MoR (tensile) - Ideal for routine and screening testing - Most laborious and longest sample fabrication process - Requires a huge amount of material for each specimen (0 lb.) - Requires the longest time for curing ( days) - Difficult for handling test specimens - Not ideal for routine and screening test - Moderately laborious and long sample fabrication process - Requires a massive amount of material for each specimen (0 lb.) - Different failure mode (compressive) versus MoR (tensile) - Not ideal for routine and screening test - Requires more laboratory testing with a wide array of base materials and cement contests - Relatively newly proposed test. SUMMARY AND RECOMMENDATIONS This study presented a preliminary laboratory evaluation of a novel, simpler test method and relationship model for estimating the -day MoR values that are required to model and compute the fatigue life of cement-treated base layers for M-E pavement design procedures. Additionally, a new MOR-UCS relationship model was proposed to provide more accurate MoR estimates than the existing MEPDG and PCA models. In the study, an extensive laboratory testing using three base materials treated with three cement contents was accomplished to measure the flexural strength, compressive strength, and indirect tensile strengths of the cement-treated base materials with different cement dosages. On the basis of the results of this study, the following conclusions were drawn: The MoR test is time- and material-consuming and presents difficulties for handling the test specimens compared the UCS and IDT strength test. The laboratory test results indicated that, as theoretically expected, base materials treated with higher cement contents exhibited higher flexural, compressive, and tensile

15 Lee, Faruk, Hu, Haggerty, Walubita strengths. In other words, the material strength improved with an increase in the cement content. The existing MoR-UCS relationship models from the MEPDG and PCA poorly estimated the -day MoR. Based on the materials evaluated in the study, the models exhibited poor predictive accuracy with relatively higher errors. The new MoR-UCS relationship model developed in this study exhibited improved estimation of the MoR values; better than the MEPDG and PCA models. The IDT test proposed as an alternative to the UCS is the simplest test method and requires the least amount of material and resource (e.g., fabrication, manpower, etc.) efforts among the three test methods. It is cost-effective and practical for routine applications. The strong correlation between MoR and the tensile strength obtained from the new IDT test method indicated the promising potential of estimating the MoR required to model and predict the fatigue life of cement-treated base materials for the M-E pavement design procedures. The exhibited predictive accuracy in comparison to actual laboratory measure MoR values was satisfactory with a reasonably acceptable margin of error. Overall, the study findings suggest that for Level M-E pavement design input, the proposed model relationship between MoR and IDT strength could potentially be used as a surrogate method to estimate the MoR for the fatigue life modeling and prediction of cementtreated base layers. However, in order to reinforce these preliminary findings for wide usage and M-E applications, the authors recommend more laboratory testing with a wider array of base materials and cement contents. Additionally, validation with field data is also recommended in future studies. ACKNOWLEDGEMENTS AND DISCLAIMER The authors thank TxDOT and the Federal Highway Administration (FHWA) for their financial support and all those who helped during the course of this research work. Special thanks also go to all those who assisted with laboratory work and documentation during the course of this study. The contents of this paper reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein and do not necessarily reflect the official views or policies of any agency or institute. This paper does not constitute a standard, specification, nor is it intended for design, construction, bidding, contracting, tendering, certification, or permit purposes. Trade names were used solely for information purposes and not for product endorsement, advertisement, or certification.

16 Lee, Faruk, Hu, Haggerty, Walubita 0 0 REFERENCES. Mechanistic-Empirical Pavement Design Guide - A Manual of Practice (Interim Edition). AASHTO, Washington, D.C., 00.. Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures. AASHTO, Washington, D.C., 00.. Scullion, T., J. Uzan, S. Hilbrich, and P. Chen. Thickness Design Systems for Pavements Containing Soil-Cement Bases, SN, Portland Cement Association, Skokie, IL, 00.. Annual Book of ASTM Standards, Vol ASTM, West Conshohocken, Pa., 0.. Soil-Cement Testing. Tex-0-F. Texas Department of Transportation, Austin, TX, 0.. Triaxial Compression for Disturbed Soils and Base Materials. Tex--F. Texas Department of Transportation, Austin, TX, 00.. Lee, S., A. N. Faruk, and L. F. Walubita. Comparison of Fracture Cracking Parameters from Monotonic Loading Tests: IDT and Monotonic OT Tests. Presented at th Annual Meeting of the Transportation Research Board, Washington, D.C. 0.. Scullion, T., S.D. Sebesta, C.K. Estakhri, P.R. Harris, C. Shon, O.R. Harvey, K.R. Harvey. Full-Depth Reclamation: New Test Procedures and Recommended Updates to Specifications. Report FHWA/TX-/0--, Texas A&M Transportation Institute, College Station, 0.. Indirect Tensile Strength Test. Tex--F. Texas Department of Transportation, Austin, TX, Laboratory Compaction Characteristics and Moisture-Density Relationship of Base Materials. Tex--E. Texas Department of Transportation, Austin, TX, 0.. Brown, C. E., Applied Multivariate Statistics in Geohydrology and Related Sciences, Springer-Verlag, New York,.