Australian Society for Concrete Pavements 4 th Concrete Pavements Conference. Factors Influencing CRCP Performance in Texas

Transcription

1 Australian Society for Concrete Pavements 4 th Concrete Pavements Conference Factors Influencing CRCP Performance in Texas Moon Won 1, P.E., Ph.D. and Pangil Choi 2, Ph.D. 1 Professor and 2 Senior Research Associate Texas Tech University, USA ABSTRACT As of 2016, there are 21,799 lane kilometres of continuously reinforced concrete pavement (CRCP) managed by the Texas Department of Transportation (TxDOT). CRCP is a premium pavement type, with a higher initial construction cost than jointed plain concrete pavement (JPCP), primarily due to the material and installation cost of steel reinforcement. In Texas, the long-term performance of CRCP has been far superior to that of JPCP. Based on the performance histories of the two Portland cement concrete (PCC) pavement types in Texas, in 2001, TxDOT made it a policy to utilize CRCP when a rigid pavement is selected for projects. Over the years, CRCPs designed and built in accordance with TxDOT s design standards and specifications have provided overall excellent performance; however, some premature distresses have been observed in CRCP. Since CRCP is more expensive than JPCP, and is placed where heavy truck traffic volume is high, minimizing premature distresses and thus eliminating the need for lane closures for any maintenance work is of prime importance to TxDOT. To achieve the goal of developing near zero-maintenance CRCP, TxDOT has initiated a number of research studies and forensic evaluations, with a primary goal of identifying the mechanisms of premature distresses and developing design standards and construction/materials specifications that would result in minimizing premature distresses and extending CRCP pavement life.

2 The major findings from those studies and investigations included; (1) structural distresses in CRCP are rare in Texas where tied-concrete shoulders and stabilized bases were utilized, (2) most, if not all, of the distresses are due to deficiencies in quality control for materials and construction, (3) many of what appear to be full-depth failures are actually partial-depth failures, (4) transverse crack spacing does not appear to have significant effects on longterm performance, (5) concrete properties, especially the coefficient of thermal expansion (CTE), have significant effects on major spalling development, and (6) the use of lower amounts of longitudinal steel than the generally accepted 0.6 % resulted in poor performance. Based on these findings, TxDOT revised its CRCP design standards in 2013 and construction/material specifications in The implementation of those design standards and specifications is expected to further enhance long-term CRCP performance in Texas. 2

3 Introduction In Texas, various types of Portland cement concrete (PCC) pavement have been used. They include jointed plain concrete pavement (JPCP), jointed reinforced concrete pavement (JRCP), continuously reinforced concrete pavement (CRCP), precast concrete pavement and cast-in-place prestressed concrete pavement. The last two types were built as test sections for research and implementation projects. In general, the performance of JRCP was not as good as JPCP or CRCP, therefore TxDOT discontinued the use of this pavement type. Accordingly, only JPCP and CRCP are now utilized in Texas. Figure 1 illustrates the lane kilometres of JPCP and CRCP managed by TxDOT from 2006 to It shows that the usage of CRCP has increased while that of JPCP has steadily decreased. Figure 1 Lane kilometres of PCC pavement managed by TxDOT The reason for this trend is that, in 2001 TxDOT made it a policy to utilize CRCP when a rigid pavement is selected for projects, and since then most of the PCC pavements built in Texas, if not all, were CRCP. TxDOT districts are allowed to use JPCP in certain locations or 3

4 with the approval from the administration. The TxDOT policy of exclusive use of CRCP was based on the overall poor performance of JPCP and the excellent performance of CRCP. Figure 2 shows the percentage of lane miles with distress scores of 80 or above for various pavement types for the last 4 years (TxDOT PMIS, 2016). CRCP has the highest percentage, while JPCP has the lowest. Figure 2 Percentage of lane miles with distress score of 80 or above Figure 3 Percentage of lane miles with ride score of 3.0 or above Figure 3 illustrates the percentage of lane miles with a ride score of 3.0 or above for various pavement types for the last 4 years, which clearly indicates the superior ride quality of CRCP 4

5 to other pavement types, better than that of asphalt concrete pavement. It also indicates the poor ride quality of JCPC. The superior performance of CRCP over other pavement types in Texas is the result of continuous efforts made by TxDOT. Over the years, TxDOT has sponsored a number of research studies on CRCP behaviour and performance, with the objectives of obtaining optimum CRCP performance by improving design standards and specifications. This also resulted in a better understanding of CRCP behaviour under various loading conditions, as well as the mechanisms of various distress types in CRCP. In addition, a long-term CRCP performance study was conducted, which spanned from 2005 to 2013 (Choi, et al, 2014). This study evaluated the field performance of CRCP throughout Texas, and investigated detailed structural responses of CRCP. The findings from this study indicated that most of the CRCP distresses in Texas were not related to structural deficiency of the pavement system; rather, they were due to poor quality control issues in materials selection and/or construction. Also discovered were (1) punchout distress, which is considered the only distress type caused by a structural deficiency of CRCP, is not always a full-depth distress, (2) there was a poor correlation between transverse crack spacing and punchout distress, (3) most of the severe spalling problems were due to the use of coarse aggregate with a high coefficient of thermal expansion (CTE), and (4) the amount of longitudinal steel plays a role in CRCP performance. To improve long-term CRCP performance, it is of a great importance to understand correct mechanisms of CRCP distresses, which requires an accurate understanding of CRCP behaviour and interactions between steel and concrete under environmental and wheel loading. This paper presents the findings made from various research studies conducted in Texas on CRCP behaviour and important factors on CRCP performance. This paper also discusses technical implications in terms of what needs to be done in design, materials selection and construction practices to further enhance CRCP performance. 5

6 CRCP Behaviour under Environmental and Wheel Loading Portland cement concrete undergoes volume changes when subjected to temperature and moisture variations. In PCC pavement, these volume changes are accommodated in two different ways. In JPCP, means are provided so that concrete volume changes are least restrained. Those means include smooth dowels and a short joint spacing. Accordingly, curling stresses in concrete are minimized. On the other hand, in CRCP, concrete volume changes are severely restrained by longitudinal reinforcement, which causes larger curling and warping stresses and results in numerous transverse cracks. In JPCP, transverse cracks are considered as a distress, while in CRCP they are not. Currently, punchout is considered the only distress type caused by a structural deficiency of CRCP systems. All the other distresses, such as spalling, are considered functional distresses. Accordingly, the objectives of structural designs of CRCP are to limit the number of punchout distress at the end of the design period to an acceptable level. In most CRCP design procedures, prediction of punchout is made by estimating concrete stresses in the transverse direction at the top of the slab and resulting fatigue life of concrete due to wheel loading applications and environmental loading (ARA, 2003). In the development of those design procedures, the following assumptions were made: (1) Crack widths vary almost linearly with crack spacing, i.e., the larger the crack spacing, the greater the crack width. (2) Transverse cracks go through the slab depth. (3) Crack width increases over time due to continued drying shrinkage. (4) LTE at transverse cracks vary with crack widths (and crack spacing), i.e., the larger the crack widths or crack spacing, the lower the LTE, and vice versa. With the above assumptions, an optimum transverse crack spacing concept was developed. To minimize the punchout distress, the AASHTO Guide for Design of Pavement 6

7 Structures (AASHTO, 1993) recommends crack spacing between 1.1-m and 2.4-m. The minimum value of 1.1-m was derived from the assumption that, if a transverse crack spacing is smaller than this value, wheel load stress in the transverse direction will be large and critical, which could cause longitudinal cracks between two closely spaced transverse cracks, resulting in punchout. The maximum value of 2.4-m was set to minimize the potential for spalling, which is derived from the above assumption (1). In other words, larger crack widths could increase the potential for spalling. The relationship between crack spacing and crack width was investigated, which is illustrated in Figure 4 (Suh et al, 1992 and Nam, 2005). Figure 4-(a) shows a somewhat inverse relationship between crack spacing and crack width, which is contradictory to the above assumption (1). In Figure 4-(b), there is no correlation in Austin (04) or Cleveland (04) projects, while a loose correlation is observed in the Baytown (03) project. A number of reasons present themselves for the discrepancy between the above assumption (1) and the field observations as shown in Figures 4-(a) and 4-(b). One reason is that not all transverse cracks occur at the same time. Those that occur at early ages will experience more drying shrinkage with time, resulting in larger crack widths. On the other hand, those cracks that occur at later ages will have quite small crack widths because the drying shrinkage of concrete that occurred up to that point of cracking must have been absorbed by the creep of concrete. Another reason for the poor correlation between crack spacing and width might be that restraints on concrete volume changes by longitudinal steel are limited to about 305 mm from transverse cracks, even though this distance might vary depending on the environmental loading. Steel strains were evaluated in longitudinal steel as shown in Figure 5-(a) (Nam, 2005). 7

8 Figure 4-(a) Crack spacing vs crack width in Houston Figure 4-(b) Crack spacing vs crack width in Baytown, Cleveland and Austin 8

9 Figure 5-(a) Testing plan for steel strain measurements Steel strain gages were installed at 0-mm, 152 mm, 305 mm, 457 mm, and 610 mm from the induced transverse crack. Figure 5-(b) illustrates the steel strains at different distances from a transverse crack. It shows that steel strains remain almost zero beyond 305 mm from the transverse crack, which means that concrete stresses beyond 305-mm from a transverse crack are nearly uniform. This indicates that concrete volume changes contributing to crack widths at the steel depth are limited to about 305 mm from a transverse crack by bond slippage. The contribution of the concrete beyond this point to crack width is minimal. In other words, as long as crack spacing is larger than 61 cm, the effect of crack spacing on crack width will be negligible. Based on the poor correlations observed in the field between crack spacing and crack width and the restraints by longitudinal steel on concrete volume changes being limited to about 305 mm from transverse cracks, the validity of the assumption (1) is questionable. It should be noted that spalling has not been observed in 9

10 CRCP with limestone coarse aggregate regardless of how large a crack spacing is. It appears unreasonable to place a limit on a maximum crack spacing Steel Strain (microstrain) S-C2 S-6 S-12 S-18 S Age (Days) Figure 5-(b) Steel strain variations at different distances from a transverse crack In CRCP, temperature and moisture variations in concrete slabs are largest near the concrete surface, and decrease further down the slab depth. Since transverse cracks in CRCP occur in order to relieve excessive concrete stress due to temperature and moisture variations and due to the existence of longitudinal reinforcement, transverse cracks normally do not go through the slab depth and are quite often limited to the top half of the slab. Figure 6-(a) shows a transverse crack with a large crack width on the surface. The concrete slab was cut longitudinally for widening, and Figure 6-(b) illustrates that the crack did not go through the whole slab; rather it stopped midway between the concrete surface and the longitudinal steel. This pavement on US 290 in Houston was placed in 1977, and this crack maintained its integrity for over 30 years. It is true that transverse cracks could go through the slab depth if slab support is not adequate, or if the concrete slab thickness and/or longitudinal reinforcement is deficient. However, in Texas, base layers are required to be stabilized with either cement or asphalt, and slab thickness and longitudinal steel amount are quite adequate. Accordingly, most of the transverse cracks in CRCP in Texas are partial- 10

11 depth cracks, and as will be discussed later, LTE values in all the transverse cracks evaluated were quite high. Figure 6-(a) Large crack width on the surface Figure 6-(b) Crack stopped above steel It has been assumed that crack width in CRCP increases over time, resulting in reduced LTE at transverse cracks and ultimately contributing to punchout. However, as discussed above, even in CRCP sections as old as 33 years, transverse cracks were kept tight. To evaluate variations in crack widths over time, vibrating wire strain gages (VWSG) were installed in a CRCP construction project. The pavement structure consisted of 279-mm CRCP over 102- mm asphalt stabilized base on top of a flexible base. VWSGs were installed at the top, middle, and bottom of the concrete slab. A transverse crack was induced at the location of the vibrating wire gages. Crack widths were calculated by the product of strains recorded in VWSG and gage length. In this calculation, tensile strains in concrete due to stresses within the gage length of VWSG were ignored. In other words, the crack widths estimated by this method might slightly over-predict actual crack widths. Figure 7 shows the crack widths values at mid-depth over about a three-year-period. It illustrates that crack widths vary almost linearly with concrete temperatures. It also illustrates that crack widths decreased over time, which is contradictory to the above assumption (3). Redistribution of concrete stresses and displacements in CRCP as more cracks form as well as continued creep and 11

12 stress relaxation in concrete are considered the mechanisms of the reduction of crack widths over time. Figure 7 Variations in crack width over time Figure 8 compares actual crack width measurements with predicted crack width values from a theoretical model (Kohler, 2005). Dr. Kohler used accurate input values for the mechanistic-empirical model developed for MEPDG. It shows that actual crack widths are much smaller than predicted values. In general, the difference in terms of ratio of actual to predicted crack width values gets larger with time, which appears to support the information in Figure 7. This also indicates that the theoretical model in the MEPDG for the estimation of crack widths severely over-predicts crack width. Deterioration in LTE has been cited as the most critical process of punchout development in CRCP (ARA, 2003). To evaluate LTE conditions in Texas, two test sections with 305-m long in each section were selected at 27 CRCP projects. In each test section, a total of 24 slab segments 4 each of small, medium, and large crack spacing were selected for each test section. The crack spacing for small spaced cracks is 60 to 90-cm, for medium spaced cracks 120 to 180-cm and for large spaced cracks 210 to 300-cm. 12

13 Figure 8 Comparisons of crack widths from mechanistic model with actual values To evaluate the behaviour of a transverse crack with a specific crack spacing, two slab segments with comparable spacing at both sides of the crack were selected. Falling weight deflectometer (FWD) drops were made at the mid-slab of the upstream section, at the upstream of the crack, at the downstream of the crack and at the mid-slab of the downstream section for each crack. In order to determine the effect of temperature on the average slab deflection as well as the LTE of the transverse cracks, FWD testing was conducted twice a year for all the sections, once in the summer and once in the winter for all the test sections. The age of the pavements varied from 7 years to 43 years when the evaluations were completed, and the slab thicknesses were from 15.2-cm to 38.0-cm. Accordingly, the inference space for the LTE evaluations was quite large, encompassing CRCP projects with small to large thicknesses and from relatively young and quite old. Figure 9 illustrates the effects of crack spacing and concrete temperature on LTE in all the sections evaluated in this study. Blue columns show LTE values for small crack spacing, red for medium, green for large crack spacing and pink at transverse construction joints. Quite high LTE values were obtained regardless of time of testing (temperature effect) and crack spacing. These high levels of LTE values are supported by the information in Figures 6 13

14 through 8, which is that crack widths in CRCP are maintained quite small if CRCP was built with adequate longitudinal steel, slab thickness and base support. Figure 9 Effect of concrete temperature and crack spacing on LTE Figure 10 shows the results of LTE evaluations conducted for LTPP (Tayabji, et al, 1999). It shows 96 percent of LTE values were larger than 90 % LTE. In Figure 10, larger crack spacing resulted in greater LTE values, which is contradictory to the above assumption (4). It is clear that if CRCP is built with adequate designs (slab thickness, amount of longitudinal steel and base support), the punchout mechanism advocated in the concrete pavement research community (increased crack width over time leading to lower LTE and punchout) is not applicable in Texas. It appears that researchers over the years adopted distress mechanisms in JPCP in modelling punchout distress in CRCP. It should be recognized, however, that punchout does take place in Texas, and the next section discusses the mechanisms and factors involved. 14

15 Figure 10 Effect of crack spacing on LTE Punchout Mechanisms in CRCP and Factors Involved Punchout distresses observed in Texas have common denominators. They are (1) CRCP with asphalt shoulder, (2) evidence of pumping, (3) heavy truck traffic, and (4) the use of a non-stabilized base. Figures 11-(a) and 11-(b) illustrate typical punchouts observed. The distress in Figure 11-(a) shows slab segmentation under the wheel path. This pavement of 20.3-cm CRCP, which was built in the early 1960s, is a connector between Loop 610 to IH- 10 west in Houston. It is interesting to note that the edge of the pavement in the distressed area preserves its shape, which strongly indicates the cause of this distress is poor slab support at a localized area. It is also observed that there is a longitudinal crack in the middle of the outside lane, and transverse cracks in the inside half of the outside lane appear to be quite tight. Accordingly, this distress does not appear to be related to deteriorations of transverse cracks. Rather, repeated wheel loading applications for 50 years on the small location of deteriorated base support resulted in this distress. Figure 11-(b) shows a distress type that resembles a traditional punchout. 15

16 Figure 11-(a) Slab segmentation Figure 11-(b) Typical punchout This section of 20.3-cm CRCP is in IH-35 W in Dallas, which was completed in Evidence of minor pumping is observed. Also, deteriorated transverse cracks are observed within the distressed area. However, transverse cracks in the inside half of the outside lane are in good condition. The deteriorated transverse cracks within the distressed area are not the cause of the distress; rather, they are the result of the distress, which was caused by other factors than lower LTE at transverse cracks. The half-moon shape of the concrete crack indicates that the slab has been pushed down by repeated wheel loading applications, and during that process, transverse cracks deteriorated and two longitudinal cracks developed between two adjacent transverse cracks. The two distresses shown in Figure 11 appear different; however, the underlying cause appears to be identical, which is a poor slab support. Figure 12-(a) shows the initial stage of punchout distress, and in order to identify the cause of this distress, deflection testing with FWD was conducted at a 3-m interval. This pavement section, 20.3-cm CRCP on asphalt stabilized base on IH-45 in Dallas was completed in When the evaluation was made, the pavement was already 35 years old. The testing results are shown in Figure 12-(b), which illustrates poor slab support at the location of the distress. LTE values were evaluated at two transverse cracks (LTE 3-1 and LTE 3-2 in the Figure 12-(a)) and the values were near 100 percent. Dynamic cone 16

17 penetrometer testing was also conducted, and average back-calculated modulus values at locations of no distress was 1,730 MPa, while the value at the middle of initial punchout area was 1,261 MPa, which clearly explains the mechanism of this distress. Figure 12-(a) Initial stage of punchout Figure 12-(b) Deflections along PO area Figure 13 shows a different distress type in CRCP in Texas. This pavement, consisting of 22.9-cm CRCP slab on 10.2-cm asphalt concrete pavement cm reworked base cm lime treated subgrade, was placed in 2002 on IH-35 in Cotulla. Figure 13 CRCP distress due to heavy wheel loading applications 17

18 The distress shown in Figure 13 is different from a traditional punchout, since no evidence of edge pumping or slab depression is observed. Forensic investigations were conducted and the concrete segmentation was confined at the top half of the slab. The bottom concrete was solid. Since there were no distresses in the inside lane, it was postulated that this distress was related to heavy wheel loading applications. Traffic data on this section was obtained from the weigh-in-motion (WIM) station located in this section. Figures 14-(a) and 14-(b) illustrate single and tandem axle truck traffic applications in 2007 and 2011, respectively. In Texas, the legal weight limit of single axle load is 20-kips (89 kilo-newton) and that for tandem axle load is 34-kips (151 kilo-newton). Figure 14-(a) Single axle load distribution Figure 14-(b) Tandem axle load distribution Figures 14-(a) and 14-(b) show the applications of weights that far exceeded legal weight limits. Structural responses of CRCP on the application of overweight axles were investigated by analysing CRCP systems with a 3-dimensional finite element program as a part of the development of the mechanistic-empirical CRCP design program for TxDOT (Ha, et al, 2012). Figure 15 illustrates the analysis results for 40 kilo-newton wheel loading application near a transverse crack. Maximum principal stresses at the top or bottom of concrete slabs near transverse cracks are much smaller than those in the concrete surrounding longitudinal steel. It is also interesting to note that crack stiffness does not have substantial effects of wheel load stress at the top or bottom of the slab, while principal stresses in concrete around longitudinal steel are affected. The analysis results imply that 18

19 the interactions between longitudinal steel and surrounding concrete are significant, resulting in large concrete stresses near longitudinal steel, and if excessive, the large concrete stresses could cause cracks near longitudinal steel in the form of horizontal cracking and ultimately segmentation of concrete slab as shown in Figure 13. Lately, CRCP distresses as shown in Figure 13 were more frequently observed in Texas than in the past. Applications of illegally overweight trucks are considered as a cause for those distresses. A new technology called fracking has been implemented in Texas to extract oil and natural gas that previously were locked away in shale and other tight-rock formations. This technology requires enormous amounts of water and other materials, and hauling them to the job site economically appears to be responsible for the applications of illegal overweight trucks. The distress type shown in Figure 13 is relatively new, and increasing slab thickness/changing the depth of the longitudinal steel could provide a solution to this problem. Maximum Principal Stress (psi) Around the longitudinal steel At the top of the slab At the bottom of the slab Crack Stiffness (psi/in.) Figure 15 Concrete stresses at various depths near a transverse crack Another type of distress observed in CRCP is shown in Figure 16. These distresses occur at the transverse construction joints (TCJ). The frequency of this distress type is not high; however, since distresses due to structural deficiency of CRCP system are quite rare in 19

20 Texas, other than those due to poor slab support or illegal overweight truck applications, and distresses in TCJ could occur at relatively early ages of pavement, TxDOT sponsored a research study to identify the mechanisms of this distress type. Figure 16 Distresses at transverse construction joints Extensive evaluations of CRCP behaviour at TCJ, including stresses at longitudinal steel and concrete strains and displacements at TCJ, indicated that the structural responses of longitudinal steel and concrete were not excessive. Instead, the construction practices, which includes dumping excess concrete mortar collected on the sides of the pavement into TCJ or improper vibration in the morning side of the TCJ, were considered as a potential cause for the distress. Another significant distress type has been major spalling as shown in Figure 17. Major problems with severe spalling include the degradation of ride quality, and potential safety hazard. Severe spalling in CRCP was a major issue in Texas since the early 1980s. A number of research studies were conducted over 30 years, and the last study was conducted in 2011 and 2012, which revealed an excellent correlation between CTE of coarse aggregate used and the potential for severe spalling (Ryu, et al, 2012). Sections with severe spalling and no spalling were identified, and concrete material properties were evaluated from cores taken from those sections. The concrete material properties evaluated 20

21 included modulus of elasticity and CTE. The correlation between concrete modulus and severe spalling was not as good as that between CTE and severe spalling. Figure 17 Severe spalling in CRCP in Texas CTE values of concrete from sections with no spalling problems were lower than 9.9 microstrains/ C, while those from sections with severe spalling problems were larger than 9.9 microstrains/ C. Based on the research finding, TxDOT included CTE as a requirement for coarse aggregate property that should be met if the coarse aggregate would be used for CRCP. In Texas, crushed limestone (LS) and siliceous river gravel (SRG) are the two major coarse aggregate types used in PCC pavement construction. Crushed limestone is of angular shape and has a rough surface texture, which is believed to provide better bonding between the coarse aggregate and surrounding mortar. On the other hand, SRG is of round shape and has quite a slick surface, which does not promote good interfacial bonding between coarse aggregate and the surrounding mortar. Measurements of mechanical properties of concretes made with those two coarse aggregate types reveal that concrete with LS has lower modulus and CTE values than concrete containing SRG (Won, 2005). As discussed earlier, in CRCP, concrete volume changes due to temperature and moisture variations are severely restrained by longitudinal reinforcement, and larger CTE and modulus values of concrete with SRG, along with lower interfacial bond strength, are believed to be responsible for severe spalling distresses for CRCP containing SRG as a 21

22 coarse aggregate. It is interesting to note that spalling is not an issue in JPCP containing SRG. It is because in JPCP, concrete volume changes due to temperature and moisture variations are not restrained as much as in CRCP, and the resulting bond stresses at the interfaces between coarse aggregates and surrounding mortar are kept to a minimum. The role of longitudinal reinforcement in CRCP is to restrain concrete volume changes due to temperature and moisture variations, which cause transverse cracks, and to keep transverse cracks tight so that the continuity of the slab is provided. Accordingly, the amount of longitudinal steel has effects on CRCP behaviour. Traditionally, the ratio of the steel cross-sectional area to concrete area of 0.6 % to 0.7 % has been used. In 1989 and 1990, TxDOT constructed test sections in 3 different projects to investigate the effects of longitudinal steel amounts, coarse aggregate types, and concrete placement temperatures on CRCP behaviour and performance (Suh, 1993). In Figure 18, the inside 2 lanes were built with SRG as the coarse aggregate, while the concrete in the outside lane has LS as the coarse aggregate. The marked difference in performance in terms of spalling is observed. Figure 18 Effect of coarse aggregate type on spalling Also, three different steel amounts (0.38 %, 0.50 % and 0.62 % with 19.1-mm bars) were used. Additional section was built at 0.48 % with 22.2-mm bars. Figure 19 shows the 22

23 percentage of transverse cracks that experienced severe spalling. There is a reasonable correlation between the amount of longitudinal steel and spalling ratio the more longitudinal steel, the lower the percentage of cracks that developed into spalling. It is postulated that larger amounts of longitudinal steel restrain concrete volume changes more effectively, resulting in higher concrete stresses, more cracks and less concrete strains at transverse cracks, which caused fewer spalling. On the other hand, concrete strains become larger at transverse cracks when smaller amounts of longitudinal steel are used, resulting in a higher frequency of spalling. Figure 19 Longitudinal steel amount and spalling 23

24 Acknowledgements: The technical discussions made in this paper were possible thanks to a number of researchers as well as TxDOT staff engineers. Special thanks are made to the late Dr. B. Frank McCullough, my mentor and supervising professor. This study was conducted under a research project (Development of Eco-Friendly Pavements to Minimize Greenhouse Gas Emissions) funded by the Ministry of Land, Infrastructure and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KAIA). The author would like to thank the members of the research team, MOLIT and KAIA for their guidance and support throughout the project. References: 1. American Association of State Highway and Transportation Officials, AASHTO Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials, Washington, D.C., ARA, Inc., ERES Division, Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Final Report, Appendix LL, Champaign, Illinois, Choi, P.G., Ryu, S.W., W. Zhou, S. Saraf, Yeon, J.H., and Won, M., Project Level Performance Database for Rigid Pavements in Texas, II, Research Report FHWA/TX , Texas Tech University, Lubbock, Texas, Ha, S.J, Yeon, J.H., Cho, B.H., Jung, Y.S., Zollinger, D.G., Wimsatt, A., and Won, M. Develop Mechanistic-Empirical Design for CRCP, Research Report FHWA/TX , Texas Tech University, Lubbock, Texas,

25 5. Kohler, E.R., Experimental Mechanics of Crack Width in Full-Scale Sections of Continuously Reinforced Concrete Pavements, Ph.D. Dissertation, University of Illinois, Urbana Champaign, Illinois, Nam, J.H., Early-Age Behaviour of CRCP and Its Implications for Long-Term Performance, Ph.D. Dissertation, The University of Texas at Austin, Austin, Texas, Ryu, S.W., Choi, P.G., Zhou, W., Saraf, S., Senadheera, S., Hu, J., Siddiqui, S., Fowler, D., and Won, M. Optimizing Concrete Pavement Type Selection Based on Aggregate Availability, Research Report FHWA/TX , Texas Tech University, Lubbock, Texas, Suh, Y.C., Hankins, K. & McCullough, B.F., Early-Age Behaviour of Continuously Reinforced Concrete Pavement and Calibration of the Failure Prediction Model in the CRCP-7 Program, Research Report Centre for Transportation Research, The University of Texas at Austin, Austin, Texas, Tayabji, S.D., Selezneva, O, and Jiang Y.J, Preliminary Evaluation of LTPP Continuously Reinforced Concrete (CRC) Pavement Test Section, Publication No FHWA-RD , Federal Highway Administration, July Texas Department of Transportation, Condition of Texas Pavements PMIS Annual Report FY , Austin Texas, July Won, M. Improvements of Testing Procedures for Concrete Coefficient of Thermal Expansion, Transportation Research Record 1919, Transportation Research Board, National Research Council, Washington D.C., pp ,