Work Conducted as part of Partnered Pavement Research Center Strategic Plan Element No. 4.17: HVS testing of pre-cast PCC panels in District 8

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1 September 2008 Research Report: Comparison of Pre-Cast and Cast-in-Place Concrete Pavements Under Heavy Vehicle Simulator Loads Author: Erwin Kohler Work Conducted as part of Partnered Pavement Research Center Strategic Plan Element No. 4.17: HVS testing of pre-cast PCC panels in District 8 PREPARED FOR: California Department of Transportation (Caltrans) Division of Research and Innovation PREPARED BY: University of California Pavement Research Center UC Davis and Berkeley

2 DOCUMENT RETRIEVAL PAGE Research Report No.: Title: Comparison of Pre-cast and Cast-in-place Concrete Pavements Under Heavy Vehicle Simulator Loads Authors: Erwin Kohler Prepared for: Caltrans Division of Research and Innovation and Caltrans District 8 FHWA No.: Work Submitted: April 28, 2009 Date: February 2008 Strategic Plan Element No.:4.17 Status: Draft Version No: Stage 2 Abstract: The California Department of Transportation (Caltrans) evaluated use of the Super-Slab System of pre-cast concrete pavement as a strategy for the long-life rehabilitation of concrete pavements by conducting accelerated pavement testing using the Heavy Vehicle Simulator (HVS) on a specially constructed experimental pavement in San Bernardino County. HVS testing of the pre-cast pavement occurred in Caltrans had conducted similar HVS tests on Jointed Plain Concrete Pavements (JPCP) on between 1998 and 2001 at SR-14 in Palmdale. This report compares the results of the accelerated tests on these two pavement types. A difference in the location of the loading made it necessary to apply conversion factors to the Palmdale JPCP data for the comparison. Tests on that pavement used edge loading, while testing on the San Bernardino pre-cast slabs used wheelpath loading. The comparison results suggest that similar service life ranges can be obtained from this pre-cast system and jointed plain concrete pavements. A rough estimate of about 200 million ESALs seem to represent this service life. There is insufficient data to judge the variability in the expected life of pre-cast pavements; however, significant variability was observed in the cast-in-place JPCP slabs tested. Keywords:pre-cast concrete slabs, Super-Slab, Heavy Vehicle Simulator, HVS, experimental pavement section, dry and wet pavement tests, accelerated pavement testing, instrumented slabs Proposals for implementation: None Related documents: Kohler et al. (2008). HVS Testing of Pre-Cast PCC Panels First-Level Report Test 597FD. (UCPRC-RR ). Kohler et al. (2008). HVS Testing of Pre-Cast PCC Panels First-Level Report Test 5987FD. (UCPRC-RR ). Du Plessis et al. (2002). HVS Test Results on Fast-Setting Hydraulic Cement Concrete, Palmdale, California Test Sections, South Tangent. ( UCPRC-RR ) Du Plessis et al (2005). HVS Testing of the Palmdale Test Site, North Tangent Sections: Evaluation of Long Life Pavement Rehabilitation Strategies-Rigid. (UCPRC-RR ) Signatures: Erwin Kohler First Author John Harvey Technical Review David Spinner Editor John Harvey Principal Investigator T. Joseph Holland Caltrans Contract Manager ii

3 DISCLAIMER The contents of this report reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the State of California or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. PROJECT OBJECTIVES This project is responsive to the objectives identified by Caltrans District 8 in its request to the Caltrans Pavement Standards Team (PST) for evaluation of the Super-Slab System: The short-term objectives are to answer a constructability question regarding opening to traffic in the ungrouted condition and to determine what the failure mechanisms of the Super-Slab panels are.. The long-term objective is to provide information to Caltrans to help determine how the performance of precast panels compares to that of current Long-Life Pavement Rehabilitation Strategies (LLPRS) designs for jointed plain concrete pavements. The project involves the evaluation of transverse joint behavior; measurement of load transfer efficiency; observations of possible joint deterioration; measurement of faulting, cracking, and settlement that may result from slab and cement-treated base (CTB) deterioration; estimation of expected service life of Super-Slab pavement based on HVS test results; and comparison of Super-Slab performance (expected service life) to Heavy Vehicle Simulator (HVS) tests conducted on cast-in-place jointed plain concrete slabs (SR-14 at Palmdale). iii

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5 EXECUTIVE SUMMARY In an effort to evaluate whether the pre-cast concrete pavement system known as the Super- Slab System can be used as a long-life pavement rehabilitation strategy, the California Department of Transportation (Caltrans) had the University of California Pavement Research Center (UCPRC) perform accelerated pavement tests (APT) using a Heavy Vehicle Simulator (HVS). This testing, which took place on a specially constructed experimental pavement in San Bernardino County, was similar to that on an earlier project that used cast-in-place jointed plain concrete pavements (JPCP) on State Road 14 (SR-14) in Palmdale. When the tests on the Super-Slab sections were completed, the results were compared with those of the Palmdale tests. This report presents the results of the comparison. In the San Bernardino pre-cast pavement two identical sections were evaluated. The sections were on two sides of a five-by two slab arrangement of 225-mm thick slabs constructed on a specially built 150-mm thick cement-treated base. For the cast-in-place, Palmdale JPCP slabs, a series of 14 HVS tests were conducted, all with one concrete mix, but with four different pavement structures. (Two additional cast-in-place sections are mentioned, but these were excluded from the analysis because the slab thicknesses were not comparable to the pre-cast slabs.) The four JPCP sections that were compared with the pre-cast ones consisted of sections with the following design characteristics: No dowel bars and no asphalt shoulder, paved on an aggregate base No dowels but an asphalt shoulder, paved on top of a cement treated base. Dowel bars and also a tied concrete shoulder, paved on a cement treated base Dowel bars and in a widened lane configuration, on a cement treated base One notable difference between the tests on the pre-cast slabs and those on the cast-in-place slabs was the placement of the HVS load: on the pre-cast slabs, the load was applied in the wheelpath, almost 2 ft (0.6 m) from the slab edge, and HVS loads on the cast-in-place slabs were applied at the slab edge (except for the widened lane, where the load was placed 2 ft [0.6 m] from the edge). Since the stresses that result from edge loading are much higher than those resulting from interior loading, the cast-in-place JPCP slabs failed under fewer load repetitions than the pre-cast slabs. A process was followed, based on earlier research, to compare edge loads to interior loads so that a meaningful comparison could be drawn. v

6 Another notable difference was that the JPCP slabs at Palmdale that are compared with the pre-cast in this project were allowed to cure for at least a year prior to HVS loading. Although the concrete mix used for the JPCP slabs consisted of a blend of fast-setting hydraulic cement and portland cement, measured values for strength from cores taken from the slabs showed that they had similar strength to JPCP slabs built by Caltrans at the time the Palmdale sections were subjected to HVS loading. Therefore the comparison presented in this report does not include comparison between pre-cast slabs and slabs placed in nighttime closures and opened to traffic the next morning. The results of the comparison between the two pavement types suggest that similar ranges of service life can be obtained from this pre-cast system and from jointed concrete pavements. Large variability was found in the expected life from cast-in-place JPCP pavements, both between pavements with different designs (i.e., base type, dowel/no dowels, tied concrete shoulder, widened lane) and within a given design. Given that only two sections of pre-cast pavements were tested, insufficient data are available to judge their variability. Assuming a fixed heavy traffic level of 6.6 million equivalent single axle loads (ESALs) per year, a service life for each was calculated. One pre-cast pavement section failed after the equivalent of 37 years, while the other was perfectly functional after 22 years of equivalent traffic levels and was not loaded to failure. The lives of cast-in-place JPCP pavements ranged between zero (slabs with lengths greater of 18 ft [5.5 m] cracked before any traffic loading in the hot, dry environment) and 80 years; and if only the designs on cement-treated bases and doweled joints are considered, this type of pavement would be expected to last on average the equivalent of approximately 23 years. vi

7 TABLE OF CONTENTS Project Objectives... iii Executive Summary... v List of Figures... viii List of Tables... viii Abbreviations Used in the Text... ix Abbreviations Used in the Text... ix 1 Introduction Background Types of Concrete Pavements Pre-cast Versus Rapid-Setting Concrete Process Used to Compare Performance of Cast-in-Place and Pre-cast Concrete Under HVS Loading HVS Experiments on Pre-cast and Cast-In-Place Concrete Experiments Pre-cast Concrete Pavement in San Bernardino Cast-in-Place JPCP Concrete Pavement in Palmdale Comparison of Structures and Traffic Loading to Failure Summary and Comparison References vii

8 LIST OF FIGURES Figure 1: Pre-cast pavement testing. (a) Placement of a slab, and (b) HVS applying traffic loads Figure 2: Dowel grouting. (a) Dowel bars are free immediately after slab placement, (b) dowel grouting completed in one of the two joints, and (c) dowel slot filled with grout... 8 Figure 3: Pavement condition at the end of the HVS test for Section 2 (not failed) and Section 1 (failed), plus map of cracking including location of MDD, excavation, and cores Figure 4: Typical cracks observed in slabs from Palmdale (14) Figure 5 Test sections characteristics for the cast-in-place study, thickness, materials, presence of dowel bars, and edge support conditions Figure 6: Comparison of equivalent single axle loads for the PCPS and the different JPCP pavements of the study LIST OF TABLES Table 1: Sequence of Test and Loading Conditions in Section Table 2: Sequence of Test and Loading Conditions in Section Table 3. Comparison of Structure, ESALs, and Expected Life for Precast and Cast-In-Place Pavements Loaded with HVS in California Table 4. Edge Load Factor, ELF (after Ref.[18]) viii

9 ABBREVIATIONS USED IN THE TEXT AASHTO Caltrans CTB DRI ESAL GPR HVS JDMD LTE LLPRS LVDT METS MDD PPRC PCC American Association of State Highway and Transportation Officials California Department of Transportation cement-treated base Division of Research and Innovation equivalent single-axle load ground-penetrating radar Heavy Vehicle Simulator joint deflection measurement device load-transfer efficiency Long-Life Pavement Rehabilitation Strategies linear variable differential transformer Materials Engineering and Testing Services multidepth deflectometer Partnered Pavement Research Center portland cement concrete ix

10 SI* (Modern Metric) Conversion Factors APPROXIMATE CONVERSIONS TO SI UNITS Symbol Convert From Multiply By Convert To Symbol LENGTH In. Inches 25.4 millimeters mm Ft Feet meters M AREA In.2 square inches square millimeters mm2 ft2 square feet square meters M2 VOLUME ft3 cubic feet cubic meters M3 MASS Lb Pounds kilograms kg TEMPERATURE (exact degrees) F Fahrenheit 5 (F-32)/9 Celsius C or (F-32)/1.8 FORCE and PRESSURE or STRESS Lbf Poundforce 4.45 newtons N Lbf/in.2 poundforce/square inch 6.89 kilopascals kpa APPROXIMATE CONVERSIONS FROM SI UNITS Symbol Convert From Multiply By Convert To Symbol LENGTH Mm Millimeters inches In. M Meters 3.28 feet Ft AREA mm2 square millimeters square inches In.2 M2 square meters square feet Ft2 VOLUME M3 cubic meters cubic feet Ft3 MASS Kg Kilograms pounds Lb TEMPERATURE (exact degrees) C Celsius 1.8C+32 Fahrenheit F FORCE and PRESSURE or STRESS N Newtons poundforce lbf kpa Kilopascals poundforce/square inch lbf/in.2 *SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. (Revised March 2003.) x

11 1 INTRODUCTION Use of precast slabs is being considered as a rehabilitation strategy for extending the service life of distressed concrete pavement. Benefits of the strategy include the long life expectancy of concrete slabs cast in factory-controlled conditions and the fact that fully cured pre-cast slabs can be put into use almost immediately after installation, which makes them attractive for use on heavily traveled highways where work windows for full-depth repairs are short. A Heavy Vehicle Simulator (HVS) research project to evaluate the expected performance of the Super-Slab system of pre-cast concrete pavement was conducted for the California Department of Transportation (Caltrans) by the University of California Pavement Research Center (UCRPC). This work was performed as Partnered Pavement Research Center Strategic Plan Element 4.17 (PPRC SPE 4.17), titled HVS testing precast PCC panels in District 8, and was approved by the Caltrans Pavement Standards Team in February The short-term objective of the project was to determine failure mechanisms of the Super- Slab panels and to answer a constructability question regarding opening to traffic in the ungrouted condition. This constructability question has to do with the effect on the pavement when the pre-cast slabs are opened to traffic shortly after they are placed and before grout is injected into the transverse joints and under the slab to activate their dowels. The project s long-term objective was to provide information to Caltrans to help determine how the performance of pre-cast panels compares to that of current Long-Life Pavement Rehabilitation Strategies (LLPRS) designs for jointed plain concrete pavements (JPCPs) constructed as part of lane replacement using extended traffic closures. The results in this report do not consider comparison of pre-cast panels with JPCP constructed in nighttime closures and opened to traffic the next morning with only several hours for strength gain prior to loading. This report is one of a series of documents generated by this study that cover specific aspects of this project. Among the other items published are a test plan (1), a report on construction of the test sections (2), a technical memorandum containing an interim assessment of structural life (3), and reports detailing the HVS tests and their results on each of the two sections (4, 5). This report provides comparative information regarding the structural capacity of the pre-cast concrete pavement evaluated in this project and the structural capacity of cast-in-place jointed plain concrete pavements (JPCP) previously evaluated by the UCPRC with an HVS. 1

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13 2 BACKGROUND 2.1. Types of Concrete Pavements The service life that can be obtained from alternative pavement systems is one of the most important factors that agencies take into account when deciding the pavement type for a specific project and it is a critical factor in performing a Life Cycle Cost Analysis to select the most cost-efficient design alternative for a project. Concrete pavements are in general considered to have longer lives than asphalt pavements, but even the performance of concrete pavements will vary depending upon design and type. Pavements across the nation contain an historical mix of different concrete pavement types. Jointed reinforced concrete pavements (JRCPs) are seldom used today, although they were used extensively in the 1960s. There are two problems with JRCP: One is the wide transverse joint openings caused by volumetric changes that are the result of long slabs, and the other is the appearance of transverse cracks caused by friction forces opposing the volumetric changes. Continuously reinforced concrete pavements (CRCPs) are generally considered the most durable type of pavement (6, 7, 8), and there are cases of CRCP serving heavy highway traffic for more than 60 years (9). Use of CRCP is increasing in the United States and in many European countries (10). However, by far the most common type of concrete pavement is jointed plain concrete pavement (JPCP). A great deal of field performance data exist for JPCP, and it shows a wide range of service lives. In cases where JPCPs have not performed as well as expected, problems have generally been attributed to design (specifically, to longer slab lengths and the absence of dowels in transverse joints) and construction issues (particularly built-in thermal curling which precludes good uniform support under the slabs). Pre-cast concrete pavement systems (PCPSs), a newer type of concrete pavement, has gained popularity for use in roadways in the U.S. during the last six to seven years. However, because its development and use are relatively recent, little performance data is available at this time. Although a market exists for PCPS pavements because of the short construction windows available during the rehabilitation of congested roadways, it is important for agencies charged with selecting the most cost-effective pavement solution to be able to compare the expected lives of PCPS against more traditional cast-in-place concrete pavements, particularly JPCP. 3

14 2.2. Pre-cast Versus Rapid-Setting Concrete To be effective, a conventionally produced replacement slab (of poured concrete) must gain flexural strength sufficient for opening to traffic within a short construction window. A study by Kumara et al. (11), which investigated the early strength requirements of concrete slab replacements by loading them with an HVS 6 hours after placement, confirmed that replacement slabs must possess greater flexural strength than the anticipated maximum induced stress at the time they are opened to traffic if they are to avoid premature failure. Although there are concrete slab-replacement products already in use that gain strength rapidly, it remains to be seen whether projects rehabilitated with rapid-setting concrete can last as long as projects rehabilitated with regular concrete. The issue here is consumption of fatigue life at early age, when some of the slabs have yet to gain enough strength to resist traffic. Evidence exists that shows that regular concrete made with Type I/II cement and rapid-setting concretes are comparable as long as the flexural strength is the same, if the beams are subjected to the same stress levels (12). Having same stress levels for pavement slabs means that they are subjected to similar traffic and to similar thermal curling and drying shrinkage conditions. Given the uncertainties regarding early strength gain and the associated risk of early cracking with opening to traffic that are entailed in using cast-in-place concrete without providing sufficient time for greater strength gain through use of weekend or extended traffic closures the concept of pre-cast pavements is an intelligent solution. However, impediments remain to the widespread use of pre-cast pavements. One of these impediments is contractors current preference for paving with rapid-setting concrete (which gains strength more quickly than conventional concrete precisely to allow earlier opening to traffic). Many contractors prefer this method because their crews are familiar with the required equipment and possess the skills to use it; using the installation technology for precast slabs would mean retraining and added risks for construction crews (or competition from a general contractor who normally would not bid for a paving job). A second impediment is posed by the logistics and increased attention to project coordination needed for use of pre-cast panels. Because panels of different sizes are used in a project, delivery of the right panel at the right time is essential, and without proper timing of delivery construction operations can come to a halt. Making this timing work places additional demands on the manufacturers when the panels are made, stacked, and loaded onto trucks, and on the contractor to saw cut and remove damaged pavement within a short work window. However, assuming that the pavement life yielded by each system is comparable, the decision to use pre-cast panels or rapid setting concrete is best taken in a case-by-case 4

15 basis. The remainder of this report focuses in finding out whether the assumption of comparable life is valid Process Used to Compare Performance of Cast-in-Place and Pre-cast Concrete Under HVS Loading Data from two sets of HVS tests were used to compare the estimated performance of precast and cast-in-place concrete pavements: Data from HVS testing of cast-in-place JPCP on the shoulder of State Route 14 (SR- 14) performed as part of PPRC SPE 4.2, titled Evaluation of Rigid Pavement Long- Life Pavement Rehabilitation Strategies (LLPRS-Rigid), from 1998 through 2001; and, Data from HVS testing of precast slabs as part of this project. The load repetitions for each set of sections were equated with each other using a Load Equivalence Factor (LEF) to convert repetitions of different loads into repetitions of Equivalent Single Axle Loads (ESALs). The paths of the wheel loads for the two sets of data relative to the edge of the slab, which were different, were then translated into equivalent repetitions based on previous analyses performed by others. With the assumptions inherent in these conversions, the performance in terms of load repetitions to failure of the two types of concrete pavement could be compared on an approximate basis, as if the loads and load locations had been the same. 5

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17 3 HVS EXPERIMENTS ON PRE-CAST AND CAST-IN-PLACE CONCRETE EXPERIMENTS 3.1. Pre-cast Concrete Pavement in San Bernardino A pre-cast test pavement was constructed near San Bernardino, an hour east of Los Angeles. It consisted of ten slabs in a two by-five arrangement. The details of the test slab installation were developed to mirror the pavement details of a potential specific project in nearby highway I-15. The main components of the installation included (2): Construction of a 150-mm (6 in.) cement-treated base; Placement of an 8-mm layer of fine bedding material (stone sand); Placement of precast Super-Slab sections, 225 mm thick (8-7/8 in.) on precisely graded sub-base; Application of grout through the slab surface at joints where slots for dowels and tie bars were present, followed by application of a second type of grout that reached under the slab (bedding grout); Diamond grinding of the surface of the slab to meet smoothness requirements of the project. Two test sections were evaluated between June 2005 and August 2006, one on each side of the two-by-five panel arrangement (Figure 1). The main test objectives were (1) to evaluate whether traffic could be safely allowed on the newly placed slabs before grouting (2) to identify how much traffic loading the system could carry (which relates to years of expected service), and (3)to determine failure mechanisms for this type of PCPS. a) b) Figure 1: Pre-cast pavement testing. (a) Placement of a slab, and (b) HVS applying traffic loads. 7

18 Traffic loading was applied to both sections to simulate the exposure to traffic from the time of placement of the slabs to the time of grouting, which would normally occur during the next nighttime closure. The grouting process completes installation of the pre-cast system. There are two types of grouting, one for dowel bars and tie bars, and the other for filling of voids under the slabs. The dowel grout activates the dowels bars, which would otherwise float in the open slots (Figure 2). a) b) c) Figure 2: Dowel grouting. (a) Dowel bars are free immediately after slab placement, (b) dowel grouting completed in one of the two joints, and (c) dowel slot filled with grout. After preliminary loading of the ungrouted pavement, the sections were grouted, and the grouted sections were then loaded for extended periods under different conditions in the sequence shown in Table 1 (note that testing on Section 1 resumed after completion of testing on Section 2). All the loading on these sections was performed in the wheelpath, 0.6 m (2 ft) from the slab edge. Section 1 was heavily overloaded to cause failure and allow identification of failure modes and Section 2 was utilized to determine performance under lower, more realistic load levels that were still heavy enough to potentially fail the pavement within a reasonable amount of time, such as a couple of months.. Wet pavement conditions were simulated by applying water directly onto the slabs at the joints. 8

19 Table 1: Sequence of Test and Loading Conditions in Section 1 Moisture Condition Slab Condition Traffic Mode Portion of Section Tested Traffic Load (kn) Date and Repetitions at the End of the Phase Date Each Repetitions Cumulative Dry Wet Uncracked Corner crack at cabin-end transverse joint Second corner crack Twoway Oneway Twoway Full Half Jun , , Jun , , Aug , , Aug , , Sep ,218 1,239, May-06 62,716 1,301, Jun-06 97,206 1,399, Jul-06 15,958 1,415, Aug-06 31,241 1,446, Aug ,689 1,605, Aug-06 77,332 1,682, Aug-06 2,018 1,684,422 Moisture Condition Table 2: Sequence of Test and Loading Conditions in Section 2 Slab Condition Traffic Mode Traffic Load (kn) Target Actual Date Date and Repetitions at the End of the Phase Repetitions Each Cumulative Oct , ,801 Dry Feb- 2,334,349 2,578, Twoway Uncracked Mar-06 2,456,357 5,034,507 Wet Mar-06 2,552,935 7,587, Mar-06 2,664,929 10,252, May-06 3,459,387 13,711,758 Only Section 1 reached the end of its serviceable life, and for that reason the results of Section 2 are presented first. No sign of any distress was observable at the end of the dry test. The dry test consisted of two loading conditions, 60 kn and later a 100 kn load level. Once the responses measured by the sensors indicated a stable condition at a 100 kn load, water application at the joints was initiated and loading continued. During the wet part of the test, loading was applied at levels between 60 kn and 100 kn (13.5 and 22.5 Kips). No distresses were observable at this stage either, despite the fact that considerable pumping of material from under the slab occurred during the wet trafficking. The pumping of fine sand, however, did not result in any significant rise in corner deflections. An investigation was carried out to evaluate the extent of the suspected voids under the slab caused by pumping and it revealed that the pumped material was comprised of the finer particles from the sand bedding layer and disintegrated bedding grout. There was no clearly observable void in the wheelpath under the joint, but there were widespread channels of washed fines. 9

20 Loading in Section 1 was applied with an aircraft tire able to apply load levels above 100 kn. Corner cracks appeared on both sides of one of the two loaded joints. HVS trafficking under dry conditions was stopped when the pavement responses were once again stable despite the cracks. After the wet traffic was initiated, the slabs were able to withstand nearly a half million more heavy load repetitions before traffic was stopped, which was a clear indication of a still sound structural condition of the cracked slabs. The cracks neither widened nor developed faulting or spalling. Failure of Section 1 was reached in the form of a localized collapse at one of the joints and a more extended corner crack on the other joint. Forensic investigation revealed that the localized failure happened between dowel bars, exactly under the wheelpath. The combined observations point toward concrete fatigue under channelized traffic and the loss of support caused by pumping. The other joint in Section 1 presented a failure that can be considered typical of cast-in-place, with large concrete cracks. Both failed joints in Section 1 are shown in Figure 3. Drill cores obtained from various locations in both test sections indicated very good performance of the dowel grout. There was no sign of looseness of the dowel, which means that the grout was strong enough to sustain the compressive forces of the dowel as load was transmitted across the joint. Likewise, there were no signs of debonding issues between the grout and the surrounding concrete of the slot. HVS wheel-path MDDs Slab excavation Cores Slab 2 Slab 3 Section 2 Section 1 Slab 4 Figure 3: Pavement condition at the end of the HVS test for Section 2 (not failed) and Section 1 (failed), plus map of cracking including location of MDD, excavation, and cores. 10

21 3.2. Cast-in-Place JPCP Concrete Pavement in Palmdale Approximately five years before this pre-cast experiment, an investigation was undertaken to evaluate the use of rapid-strength concrete for accelerating construction of new concrete pavements and for rapid rehabilitation of existing facilities (i.e., slab replacements). A series of HVS tests were conducted to evaluate the fatigue behavior of fast-setting hydraulic cement concrete pavements (13) and various design features, namely base type (granular and CTB), tied concrete shoulder, dowels, and widened truck lane. Six full-scale pavement test sections were constructed on State Route 14 (SR-14) near Palmdale, California, about an hour north of Los Angeles. Each section was 70 m long and consisted of approximately 15 slabs. The concrete used for the slabs was an 80/20 blend of Ultimax and Type I/II portland cement. The concrete mix met specifications for Fast-Setting Hydraulic Cement Concrete (FSHCC) at the time of construction. Joints were sawed at 90 degrees, and the joint spacing followed an alternating pattern of 3.7, 4.0, 5.5, 5.8 m (12, 13, 18, 19 ft). An important difference existed between the fastsetting concrete used in Palmdale and the concrete that is most regularly used in overnight slab replacements: The Palmdale sections used in this comparison were not trafficked until more than a year after construction, so their flexural strength at time of opening to traffic was not an issue (13). The point here is that in contrast to the typical case of overnight construction with traffic opening the same morning, the concrete at Palmdale had a chance to gain most of its final strength before it took any traffic loading. The early consumption of fatigue life in concrete that is opened to truck traffic within 2 to 3 hours of placement is likely one of the major causes of short life for slab replacements in the field.. Extensive laboratory testing (12) showed similar fatigue life for same stress ratio (ratio of tensile stress to flexural strength) for both types of material compared in this report, the blend used at Palmdale and the conventional Type I/II cement used at San Bernardino. For this reason, it is assumed that cement type is not a variable in the performance comparison. The pavements in Palmdale Sections 1, 3, and 5 consisted of plain jointed concrete slabs of various thicknesses, and were tested between July 1998 and May These three sections were placed on a compacted granular base. Sections 7, 9, and 11 were designed and constructed on 100 mm of cement-treated base on top of 150 mm of an aggregate base. The differences among these sections are as follows: Section 7 had no dowels and an asphalt concrete shoulder; Section 9 was constructed with dowels, normal lane width, and a concrete shoulder with tie bars (the tied shoulder was the adjacent lane); and Section 11 was constructed using a widened truck lane (4.26 m instead of 3.66 m wide) and doweled 11

22 joints. The testing on Sections 7, 9, and 11 took place between June 1999 and December The main mode of failure observed on all JPCP sections consisted of the development of a longitudinal crack parallel to the direction of traffic but outside the trafficking area. The majority of the cracks started as transverse cracks, but turned into longitudinal or, in some cases, corner cracks. Figure 4 shows typical examples of the cracking observed in the 23 tested locations within the six sections. Figure 4: Typical cracks observed in slabs from Palmdale (14). The reason for the longitudinal cracking was found to be associated with built-in curling of the slabs, and to a lesser extent, to the half axle loading. In addition to the use of fast-setting high early-strength concrete, these pavements were built in desert conditions and under daytime construction. These conditions resulted in top-down shrinkage and thermal cracks (15) and high estimated built-in temperature differential values (16). Regardless of the inclusion of dowels and tie bars on Sections 7, 9, and 11, the study showed that slab curling played a major role on the deflections observed under HVS loading, and therefore on the stresses that led to cracking of the concrete. The loss in support from the substructure, especially in the proximity of the longitudinal free edge, resulted in the development of high tensile stresses at the top of the concrete, which in turn caused the slabs to crack in the observed fashion. 12

23 All the test sections at Palmdale with 5.5 and 5.8 m (18 and 19 ft) slab lengths cracked before HVS loading was performed, and no other heavy loads were applied. These cracks were due to temperature curling and shrinkage only. All HVS tests were therefore performed on the 3.7 and 4.0 m (12 and 13 ft) long slabs. In general, deflection sensors placed just below the concrete in the base layer close to the edge registered very small deflections, even with the application of test loads greater than 90 kn (20.3 Kips). Deflections recorded in the base layer were typically less than 0.2 mm, while the surface-mounted sensors recorded deflections between 1.0 and 1.2 mm for the same test. This means that the slab was not in complete contact with the base, as less than 20 percent of the surface deflections were passed on to the base layer (17). Temperature also played a significant role in the structural behavior of the concrete slab. Daily variations in slab temperatures caused the slabs to go through cycles of expansion and contraction, which had a noticeable effect on the measured load transfer efficiency (LTE) (17). During the hot part of the day the slabs expanded, the joints locked up, and LTE values close to 100 percent were commonly calculated. At night, when slab contraction took place, LTE values dropped below 80 percent. Also, the concrete slabs went through curling cycles owing to temperature differentials (temperature at the surface of the slab with respect to the temperature at the bottom of the slab),. High deflection measurements were associated with lower surface temperatures. The advantages of dowels, tie bars, and a widened lane were clearly illustrated in the study. Even after the application of aggressive 150-kN loading (33.8 Kips), no obvious LTE deterioration could be detected on the sections constructed with dowels, tie bars, and widened lanes. Although significant cracks developed during the testing period, no significant drop in LTE values could be detected after the formation of the cracks, an indication of the effectiveness of the dowels to transfer load across joints even after extensive joint deterioration. The dowels had a significant influence in controlling slab edge movements. In contrast to this, the sections without dowels or tie bars experienced significant reductions in LTE due to trafficking and after the appearance of corner cracks. The damaging effect of repetitive loading caused a significant reduction in the life of the pavement in comparison with the doweled jointed sections. 13

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25 4 COMPARISON OF STRUCTURES AND TRAFFIC LOADING TO FAILURE Only one pre-cast pavement type was tested in San Bernardino, while various structures of jointed plain concrete pavement (JPCP) were studied in Palmdale. The test sections main structural characteristics, including slab thicknesses and edge support condition, are presented in Table 3. Note the slab thickness in JPCP Sections 1 and 3. They were constructed with reduced slab thickness of 100 mm and 150 mm, are not comparable to the pre-cast sections that were 225 mm thick, and therefore are not included in comparison with the pre-cast concrete pavement system (PCPS). The slab thickness of the other JPCPs was similar to the PCPS slab thickness (see Rows A and B. in Table 3), and even though the underlying materials do not exactly match (see Row C in Table 3), it was considered close enough to compare the performance of the two types of pavements. PCPS and JPCP sections of comparable structure are shown in Figure 5. The particular features of the JPCP pavements are listed in Row D, such as the use of dowels in the transverse joints and edge support conditions. The HVS used in these experiments was able to test up to 8 m (26.5 ft) of pavement, and therefore in most cases three slabs/two joints were evaluated per HVS test. As indicated previously, two HVS tests were conducted in the pre-cast pavement, one on each side of the two-by-five arrangement. On the JPCP there was room for up to five replicate tests in each section, as shown in Row E in Table 3. One important aspect of the comparison is the subgrade. Dynamic cone penetrometer (DCP) testing at the San Bernardino site indicated CBR values under the pre-cast sections between 45 and 80 percent, while CBR at Palmdale had been found to be close to 100 percent. This information is confirmed by the fact that backcalculated subgrade modulus at San Bernardino ranged mostly between 50 and 100 MPa, while at Palmdale the modulus was approximately 200 MPa. 15

26 Table 3. Comparison of Structure, ESALs, and Expected Life for Precast and Cast-In-Place Pavements Loaded with HVS in California Summary PCPS Section 1 PCPS Section 2 JPCP Section 1 JPCP Section 3 JPCP Section 5 JPCP Section 7 JPCP Section 9 JPCP Section 11 A. Design slab thickness, mm B. Mean measured thickness, mm C. Underlying structure, mm 150 CTB 150 CTB 150 AggB 150 AggB 150 AggB 100 CTB 150 AggB 100 CTB 150 AggB 100 CTB 150 AggB D. Design features Load transfer Shoulder Slab size Length, m Width, m Dowels, Asphalt Dowels, Asphalt No dowels None No dowels None No dowels None No dowels Asphalt Dowels, Tied PCC Dowels, Widened L E. Number of HVS tests F. Edge Load Factor G. Average ESALs, millions H. ESALs each test, millions - - 4, 4, 22 0, 4, 5, 11, 18 2, 8, 31, , 48, 115, , 162, , 205, 298 I. Years to failure* J. Range of years to failure* 37 > * Years to failure assumes 6.6 million ESALs per year. PCPS JPCP - 5 JPCP - 7 JPCP - 9 JPCP PCPS 8 Bed Sand 150 CTB (dowels & grout) 200 JPCP 100 CTB 150 AggB (no dowels) 200 JPCP 100 CTB 150 AggB (no dowels) 200 JPCP 100 CTB 150 AggB (dowels) 200 JPCP 100 CTB 150 AggB (dowels) AC Shoulder AC Shoulder AC Shoulder PCC Tied Shoulder Wide lane Figure 5 Test sections characteristics for the cast-in-place study, thickness, materials, presence of dowel bars, and edge support conditions. 16

27 The total number of Equivalent Single Axle Loads (ESALs) applied to a pavement takes into consideration the various load levels used during the course of an HVS test. It is a gross approximation at best, but provides a means to compare different loading sequences. The HVS passes are normalized to the passage of an 80 kn axle load through the ESALs calculation. The computation of ESALs in this study was done according to Equation 1, which used the Caltrans load equivalence exponent of 4.2, where L is the half-axle load level in kn and ELF is the Edge Load Factor. 4.2 L 1 ESALs = ELF (1) 40 An ELF accounts for the difference in pavement damage caused by a wheel load applied at the edge of the pavement with respect to the damage caused when the load is in the wheelpath. The ELFs for the HVS loads applied to the JPCP sections were determined from an Illinois DOT study (18). The ELFs are reported in Table 3 in Row F. For HVS tests on PCPS the ELF was taken as 1.0, as the wheel load was at the wheelpath, i.e., approximately at 460 mm from the edge, as can be seen in Figure 3. For the JPCP sections, the wheel loads were at the edge of the slab as seen in Figure 4 (except in the case of the widened truck lane, where the load was 0.6 m from the edge of wide slab, at the edge of the expected truck traffic wander pattern). The ELFs for edge loads were determined from Table 4, which is adopted (by interpolation) from work done at the University of Illinois (18). The edge load factors of 0.050, 0.161, and (shown in Table 3), taken as inverse numbers, imply that the numbers of repetitions for the same damage is respectively 20, 6.20, and 6.15, depending on the edge support condition of each section. Edge Support Asphalt Shoulder 3-m Concrete Shoulder (Medium load transfer efficiency) 3-m Concrete Shoulder (High load Table 4. Edge Load Factor, ELF (after Ref.[18]) Slab Wheelpath@0.46m* Wheelpath@0.3m thickness, mm k=100** k=200 k=100 k= transfer efficiency) Widened lane (0.6m) *Two cases of wheelpath distance from the edge, which depends on lane width and other factors. ** k-value is the modulus of subgrade reaction in pounds per cubic inch. 17

28 The total number of ESALs that each section was able to sustain before the end of the HVS loading is shown in Row G. of Table 3. Given that multiple replicate HVS tests were conducted in each JPCP section, the details of ESALs in each one of those replicate tests are shown in Row H. The average of ESALs in all the tests for a given section is the value reported in Row G. The individual test data exhibit a wide range of scatter, which is attributed to the variable field conditions. To make the results more easily understood, the total ESALs were converted to an equivalent number of years in service. This arbitrary conversion of the results used the current traffic on highway I-15 near the site of the pre-cast pavement experiment. The assumed total of ESALs per year at that location is 6.6 million. The results are presented in Rows I and J in Table 3, for average and range respectively, and reveal that these pavements should be able to remain in service between 1 and 37 years (JPCP sections 1 and 3 are excluded). These service lives are very conservative because the yearly traffic is high, despite of assuming constant ESALs for the computations (zero rate of increase over the years for traffic volume and axle loads). The 6.6 million ESALs per year used for estimation of life is the result of taking the AADT values for highway 15 in San Bernardino County (Caltrans District 8) near the junction with highway 10. For this segment the Caltrans reported traffic volumes in 2006 for trucks with 2, 3, 4, and 5 axels were respectively 6,401; 1,209; 526; and 13,000. The conversions for ESALs per truck were taken as 0.26, 0.42, 0.42, and This resulted in 18,027 ESALs a day, which translates to 6.58 million ESALs a year. Proper pavement design mandates that AADT has to be split equally into the two traffic directions and only a fraction of this traffic be assigned to a design lane through the use of a lane factor. Taking 60% as the lane factor and applying it to the one-direction traffic leads to an estimated 1.97 million ESALs per year, which in turn means pavement lives that are 3.33 times longer. This portends that the best estimate for service life is not up to 37 years, but up to 123 years. This is too long of a time span to be reported with confidence, so the very conservative approach that results from concentrating the two-way traffic count on a single design lane was adopted. Furthermore, the main reason to convert ESALs to years is to offer a simpler comparison between pavements, and should be seen more in relative rather than in absolute terms. A graphical representation of the ESALs to failure for the PCPS and JPCP sections is shown in Figure 6. These results are interpreted as a rough indication that the expected life of precast concrete pavement systems does not differ greatly from that expected from cast-in place jointed plain concrete pavements. However, these results should be used with caution due to the many assumptions used to permit comparison of the two sets of data. It should be 18

29 noted that HVS loading at PCPS Section 2 was halted at 142 million ESALs without pavement damage, and it is not known whether failure would have been achieved at a higher or lower number of ESALs than PCPS Section 1. From data shown in Table 3 and Figure 6 a rough estimate of about 200 million ESALs seem to represent the life expectancy for the type of PCPS that was evaluated, and also for the JPCP pavement types that were evaluated in this study ESALs (millions) Not loaded to failure 0 PCPS section 1 PCPS section 2 JPCP section 5 JPCP section 7 JPCP section 9 JPCP section 11 Figure 6: Comparison of equivalent single axle loads for the PCPS and the different JPCP pavements of the study. 19

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31 5 SUMMARY AND COMPARISON Use of pre-cast concrete pavement systems (PCPSs), which are relatively new and have not been widely deployed, is being considered as a strategic alternative to the use of conventional cast-in-place slab replacements for the rehabilitation of projects that carry large traffic loads. However, before pre-cast systems gain more widespread implementation, more information is required about how they compare to JPCP, especially data about their capacity to carry large amounts of traffic loads, which is a point of major controversy. Since long-term field performance data for pre-cast systems under real traffic conditions does not exist, and because this type of information is needed to provide a preliminary evaluation of their cost-efficiency, this study was undertaken. The study compared the number of ESALs that pre-cast and cast-in-place jointed pavement resisted under Heavy Vehicle Simulator (HVS) loads. The study provides the first, full-scale experimental comparison of load capacity between PCPS and JPCP and is intended to inform the discussion about which type of pavement lasts longer. It should be remembered that the JPCP slabs at Palmdale that are compared with the precast in this project were allowed to cure for at least a year prior to HVS loading. Although the concrete mix used for the JPCP slabs consisted of a blend of fast-setting hydraulic cement and portland cement, measured values for strength from cores taken from the slabs showed that they had similar strength to JPCP slabs built by Caltrans at the time the Palmdale sections were subjected to HVS loading. Therefore the comparison presented in this report does not include comparison between pre-cast slabs and slabs placed in nighttime closures and opened to traffic the next morning. The results of this study indicated that 200 millions ESALs is a valid rough approximation of the capacity of PCPS and JPCP of approximately 200- to 225-mm slab thickness. Unfortunately only two PCPS sections were tested and only one was trafficked until pavement failure was achieved. The two sections were loaded differently, which meant that there were no replicate tests, which therefore precluded an assessment of the results variability. One can speculate that slabs manufactured in a pre-cast plant would be more uniform than field-cast JPCP slabs in terms of thickness and strength, and that this would lead to them to have less field variability. The JPCP testing results agreed with field observations that there is considerable variability in JPCP performance, with some test sections failing (cracking) very early under live traffic, while others lasted considerably longer. 21

32 The variable nature of the performance of complex infrastructure systems makes it difficult to come up with test results that allow for prediction of the exact moment when either a precast pavement or a cast-in-place concrete pavement will fail, and in the pre-cast experiment it was unfortunately not possible to obtain replicate tests. The results showed that one section remained intact (no cracks or faulting or any other distress) after 142 millions ESALs, while another section failed after 242 million ESALs. Similarly, transverse or corner cracking was observed in one of the JPCP tests at as early as 160 thousand ESALs, while another withstood 536 million ESALs before cracking. Assuming a fixed heavy traffic level of 6.6 million ESALs per year, a service life for each type of pavement was calculated. One of the pre-cast pavement sections failed after what would be 37 years, assuming very conservative traffic levels, while the other was still perfectly functional after 22 years of equivalent traffic levels. The lives of cast-in-place JPCP pavements ranged between zero (long slab lengths) and 80 years, but if only the designs that include cement-treated bases and doweled joints are considered, this type of pavement would last on average around 23 years for the very heavy annual ESALs considered for this calculation (6.6 million ESALs per year). In sum, a preliminary assessment, based on the limited results in this project, is that doweled JPCP and PCPS pavements of similar thicknesses can be expected to have similar performance and failure mechanisms. 22