KSCE Journal of Civil Engineering (2011) 15(1):91-99 DOI 10.1007/s12205-011-0887-4
Highway Engineering
www.springer.com/12205
Improving the Performance of Full-Depth Repairs by Understanding the Failure Mechanisms Dar-Hao Chen*, Feng Hong**, Zhanyong Yao***, and John Bilyeu**** Received May 25, 2009/Revised 1st: September 16, 2009, 2nd: Decmeber 17, 2010/Accepted April 2, 2010
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Abstract In order to understand the reoccurring failures on the slabs or adjacent slabs that received Full-Depth Repair (FDR) treatments, efforts were made to investigate the causes of the failures on IH35W and US75. In addition, one successful FDR project on SH78 was studied to verify and validate the hypothesis of the failure mechanisms. Load Transfer Efficiencies (LTEs) were evaluated with a falling weight deflectometer, and the base and subgrade moduli were determined by Dynamic Cone Penetration (DCP). Cores were taken to examine the condition of the tie bars. Furthermore, design and construction practices were reviewed and evaluated. The common symptom for both IH35W and US75 projects was the poor LTE (less than 40%). When the LTE was lower, there were higher deflections, visible pumping, and settlement. Settlement is strongly related to the poor LTE. Tie bars were found to be ruptured in the US75 pavement, and they were not properly anchored in the IH35W pavement. Because of these ineffective tie bars, poor LTE and settlement was prevalent. On SH78, all LTEs remain above 90% even after 14 years of trafficking. The superior performance of the SH78 pavement suggests that proper procedures are critical when removing concrete and drilling holes for new tie bars, to avoid damage to, and subsequent failure of the adjacent slabs. The anchoring of the tie bar is critical to the performance of the FDR. Pull out tests should be performed to determine the type of epoxy, time before concrete pouring, embedment depth, and other construction parameters will provide adequate strength. Based on the DCP and field performance results, it is concluded that the most accurate indicator of joint performance is LTE, while the base and subgrade support are secondary. Keywords: concrete pavement, full depth repair, non-destructive testing, load transfer efficiencies ···································································································································································································································
1. Introduction Pavement engineers have been pursuing long-life pavements for years (Tayabji, 2008). It is believed that this can be realized through well designed and constructed Portland cement or concrete pavement. However, due to the sophisticated factors involving the interaction between material and traffic and environment, varying problems can occur on concrete pavements. To prevent premature deterioration of concrete pavements, transportation authorities have spent millions of dollars repairing distress with Full-Depth Repairs (FDRs). FDR is one of the most commonly used concrete pavement repair strategies as per Snyder and Darter (1990). When it is done properly, it can improve ride quality, enhance pavement structural integrity, and extend pavement service life (ACPA, 1995). As such, it consumes a significant portion of the rehabilitation budgets for most Departments of Transportation (DOTs). Although DOTs have utilized FDR for a long time, many premature failures continue to occur on the slabs that received FDR treatment, or the adjacent
slabs. For example, Fig. 1 shows additional cracks that developed near the boundary of FDR areas in less than 2 months. Fig. 1 also illustrates severe pumping at the FDR boundary along a longitudinal joint. Despite the efforts and expenditures by DOTs, sometimes the pavement conditions do not improve, as more distresses occur and the need for repairs continues. In other words, sometimes FDR does not prevent pumping, settlement, and cracking from reoccurring. Thus, there is a critical need to develop more cost-effective and reliable design and construction procedures for FDR of Portland Cement Concrete (PCC) pavements. This paper documents two premature failures encountered by the authors, and the reasons why such failures continue to occur. By understanding the types of situations that lead to persistent failures in practice, overall conclusions can be drawn and prevention methods can be developed and implemented to avoid reoccurrence in the future. One additional FDR project was studied to verify the hypothesis of the failure mechanisms. No visible distress can be observed on the slabs that received FDR treatments (or adjacent slabs) on the successful FDR project
*Professor, Changsha University of Science and Technology, Changsha, Hunan 410076, China and Texas Dept. of Transportation, Austin, TX 78731, USA (Coresponding Author, E-mail:
[email protected]) **Engineer, Materials Section and Pavements Section, Texas Dept. of Transportation, Austin, TX 78731, USA (E-mail:
[email protected]) ***Professor, School of Civil Engineering, Shandong University, Jinan, Shandong, 250061, PR China (E-mail:
[email protected]) ****Engineer, Materials Section and Pavements Section, Texas Dept.of Transportation, Austin, TX 78731, USA (E-mail:
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Dar-Hao Chen, Feng Hong, Zhanyong Yao, and John Bilyeu
Fig. 1. Premature Failures Occur on the Slabs or Adjacent Slabs that Received FDR Treatments
Fig. 3. Recurring Failure on the Slabs that Received FDR Treatments
completed 14 years ago. Note that all sites presented in the paper are located in northeast Texas and they experience similar environment To increase the probability of accurately identifying the distress mechanisms, all available testing techniques that could provide information on relevant factors have been utilized. It was hypothesized that the factors responsible for the recurring failure include (1) low Load Transfer Efficiency (LTE) at the transverse joint due to the absence or failure of tie bars, (2) lack of support and high deflections along the longitudinal construction joint due to the absence of a tied concrete shoulder, and (3) poor base and/ or subgrade support. FDR joints were evaluated for LTE using Non-Destructive Testing (NDT) methods such as Ground Penetration Radar (GPR) to locate tie location and Falling Weight Deflectometer (FWD) to measure LTE, as shown in Fig. 2. Dynamic Cone Penetration (DCP) was used to determine the base and subgrade support. Cores were taken to examine the tie bars for any corrosion or rupture, and the condition of the bond.
tie bars were found to be either ruptured or not properly anchored. Pull out tests should be enforced to make sure that (1) the epoxy type (2) embedment depth, and (3) curing time are all appropriate. Proper procedures when removing concrete and drilling holes for tie bars are critical for minimizing the damage and subsequent failures of the adjacent slabs.
2. Research Significance Many FDRs have suffered reoccurring failures. There is a critical need to understand the failure mechanism and to develop more reliable design and construction procedures. It was found that settlements are strongly related to the low LTE (80%) and no settlement. Thus, the condition of the tie bar is closely related to the amount of settlement. Tie bars play a critical role in maintaining JCP performance. When tie bars are damaged, corroded, or ruptured,
Fig. 9. Relationship between Longitudinal LTE and Normalized Maximum (Sensor #1) Deflection for US75
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Dar-Hao Chen, Feng Hong, Zhanyong Yao, and John Bilyeu
Fig. 11. Comparisons between Field Measurements (Obs) and Model Predictions (Cal) (Eqs. 4 and 5)
Fig. 10. Rupture/Necking of Tie Bar as a Result of Corrosion
LTE along the longitudinal joints will decrease substantially. Once this has occurred, edge deflections due to wheel loads will increase significantly, which in combination with moisture will cause pumping and erosion of the base, eventually resulting in slab settlement. Twelve DCP tests were conducted on this site. According to these tests, the base modulus ranged from 110 MPa (16 ksi) to 268 MPa (39 ksi), with an average of 194 MPa (28 ksi). Based on past experience, the modulus of aggregate base (by DCP tests) is normally above 345 MPa (50 ksi). The average base moduli of 194 MPa (28 ksi) here is significantly lower than normal. The average subgrade modulus was 80 MPa (11 ksi).
4. Impacts of Load Transfer on Deflection-Theoretical Aspects Based on the above two field performance results, one of the most critical factors that affects pavement performance is the LTE and the associated deflection. Efforts were made to seek a theoretical model that can describe the impact of joint characteristics on the difference in deflection across a joint. Yoder and Witczak (Yoder and Witczak 1975) proposed an equation to estimate this deflection (when the leading slab is loaded) as follows:
∆y = P(2 + β x)/(4β 3EI)
(4)
where, b = Steel bar diameter K = Winkler stiffness of the concrete around the steel bar E = Modulus of elasticity of the steel bar I = Moment of inertia of the steel bar P = Load carried by the steel bar in the leading slab x = Joint/crack width β = [Kb / 4EI]0.25 ∆y = Deflection at the joint
related to the condition of the steel bar. In addition, for a given pavement, the vertical deflection is proportional to the load P. Yoder and Witczak (Chen and Won, 2008) further established the relationship between the load carried by the steel bar (P) to the LTE as follows: 1 P = Peff -----------------1 + LTE
(5)
Where, Peff is the load transferred to the steel bar after considering number of effective steel bars. By applying Eqs. (4) and (5), and utilizing the variables obtained from field testing, the deflection at the joint and the associated LTE can be calculated. For comparison, the field measured deflections under a 40 kN (9,000 lb) load are grouped together with the calculated values using Eq. (4) and (5). Table 1 presents the variables employed in the calculation. In view of Fig. 11, Eqs. (4) and (5) generally follow the trend of increasing deflection with decreasing LTE. However, there is a consistent underestimation of the predicted deflection vs. the measured deflection. This may be related to calibration of the deflection measurement equipment, or assumptions in the model derivation. To address this issue, a shift factor of 1.30 (the mean of the shift factors at all test points) is applied. The “fit” line of the predicted deflection is also illustrated in the figure. The adjusted deflections for a given LTE are then well-matched to the measured deflections. As one of the objectives of this study, the effect of change in LTE on the change in deflection is of particular interest. Based on Eqs. (4) and (5), the relationship between change in LTE and the change in deflection for a given pavement can be derived as: ∆LTE ∆Def % = ------------------ × 100% 1 + LTE
(6)
Where, ∆Def % is the percentage change in deflection of the slab at the joint, and ∆LTE is the change in LTE. For example, when the LTE at a transverse joint is 50%, a 10% reduction of LTE will lead to a 6.7% increase in deflection.
It is clear from Eq. (4) that relative vertical movement is strongly
4.1 SH78 For comparison purposes, one FDR project that has performed
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KSCE Journal of Civil Engineering
Improving the Performance of Full-Depth Repairs by Understanding the Failure Mechanisms
Table 1. Material Parameters Used to Calculate the Relative Stiffness and Joint Deflection Winkler Stiffness of dowel bar support K, pci (N/m3)
9.0×105 (2.4×1011)
Dowel bar diameter b, in. (mm)
0.7 or (17.8) 2
Parameters for calculation of joint deflection (∆y)
Parameters for calculation of the relative stiffness (l)
Modulus of dowel bar E, psi (N/m )
2.9×107 (2.0×1011)
Moment of inertia of the dowel bar I, in.4 (cm4)
1.55×10-2 (6.45×10-2)
Coefficient of thermal volume change ε, in./in./0F (cm/cm/0C)
5.0×10-6 (2.8×10-6)
Total temperature drop ∆t, 0F (0C)
70 or (21)
Coefficient of shrinkage δ, in./in. (cm/cm)
5.0×10-5
Modulus of elasticity of slab E, psi (N/m2)
4.0×106 (2.8×1010)
Slab thickness h, in. (cm)
12 or (30.5)
Poisson's ratio of slab µ
0.15
Modulus of reaction below slab k, pci (N/m3)
2.3×103 (6.3×108)
Fig. 12. Pavement Condition 14 Years After Repair
well over 14 years (SH78) was investigated. SH78 was studied to verify and validate the hypothesis of the failure mechanisms and to determine the critical construction parameters that yield successful performance. Based on 2007 traffic data, the 20-year design traffic (20072027) for SH78 is 7.596 million ESALs with an AADT of 27,000 vehicles. The existing pavement consists of 229 mm (9 inches) of JCP and 150 mm (6 inches) of lime treated subgrade. Hundreds of FDRs were performed 14 years ago, and no visible distress can be found on the old slabs or adjacent slabs that received FDR treatment, as shown in Fig. 12. LTE tests on 40 transverse joints were performed. All LTEs exceed 90%. Three cores at different joints were taken and indicated that the tie bars are firmly attached to the existing concrete, as shown in Fig. 13. No evidence of rusting or corrosion can be observed, as most tie bar holes were completely filled with the epoxy. Minor corrosion was found where there was small void near a tie bar. Efforts were made to investigate the design and construction methods utilized on SH78. Apparently, the requirements outlined for SH78 were somewhat different from those in the TxDOT specifications. The diameter of the tie bar holes was only 3 mm (1/8 inch) larger than the nominal tie bar diameter. The current Vol. 15, No. 1 / January 2011
Fig. 13.Tie Bars Firmly Anchored to the Existing Concrete Pavement (SH78)
TxDOT specification has no requirement for the hole diameter. A tighter tolerance on the hole diameter appears to yield a better chance for higher LTE. As mentioned, the low LTE on IH35W was due to large diameter holes that significantly reduced the LTE. Pull out tests were performed on SH78 to determine (1) an appropriate type of epoxy (2) embedment depth, and (3) the minimum curing time for the epoxy. When tie bar passed the pull out tests, the right type of setup and epoxy were utilized. The pull out test was performed on the last inserted tie bar, to ensure that the epoxy was fully cured for all tie bars. The requirement for pull out test was to exceed 75% of the yield stress of the steel. Note that prefabricated epoxy cartridges were utilized on SH78. The cartridges were inserted first, and then punched by the sharpened end of the tie bar, which released the epoxy. Although TxDOT specification does mention pull out test, no specific purpose was outlined and thus not all FDR projects had enforced the pull out test. If the pull out test is enacted, the following situations can be prevented: (1) excessive dripping or flowing out of the epoxy due to inappropriate type of epoxy and
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Dar-Hao Chen, Feng Hong, Zhanyong Yao, and John Bilyeu
lack of a retention disk, and (2) concrete is poured and loads are applied before the epoxy is fully cured. It is recommended through field investigation that the minimum steel tress in pull out test be 75% of the yield stress of the steel. In other words, if the measured steel stress exceeds 75% of its yield strength, the pull out test is passed. If a tie bar passes the pull out test, it means that the tie bar has been successfully anchored. Note that the pull out test is conducted in the job site following the ASTM E 488 (ASTM E488-96, 2003) standard test method. Fig. 14 illustrates a typical pull out test to measure the ultimate tensile failure load. To meet the 75% of the yield strength of the #6 bar, the failure load should exceeds 88 kN (19,800lb) or the equivalent of 310 MPa (45 ksi). During tie bar installation, only core drills were allowed on SH78, to minimize damage to the existing concrete slab. Hammer drills were not allowed on SH78. Based on field observation, it is believed that many failures occur on adjacent slabs due to micro cracks induced during the installation of tie bars. Under truck traffic, the micro cracks increase in size and eventually lead to failures. In addition, only the lift-out method was permitted for removal of the deteriorated concrete. Lifting the old concrete imparts no damage to the adjacent slabs or subbase, is usually faster, and requires less labor than any method that breaks the concrete before removal. Furthermore, when a pavement crack is within 3ft of a transverse joint, the repair area was extended to a minimum of 1 m (3 feet) beyond the transverse joint, and the entire dowel basket assembly was replaced. This was to ensure that the repair boundary covered all necessary areas. No distress has been found on the slabs adjacent to the ones that received FDR. These three factors (the core drill, lift-out method, extend-
Fig. 15.Comparison of Base Layer Modulus under FDR Slabs and their Adjacent Non-FDR Slabs (SH78)
ed repair area) are believed to have contributed to the success of this project. Six DCP tests were conducted on slabs that received FDR and the adjacent non-repaired slabs at SH78. Fig. 15 presents the base layer moduli under the new slabs and their adjacent old slabs. Among the three pairs of randomly selected locations, the base moduli vary. Nevertheless, all of the slabs (new and old) performed very well over 14 years of service. Base moduli under the FDR and non-repaired slabs are similar at location #1 and different at locations #2 and #3. Moreover, the base moduli under FDR slabs are higher than under non-repaired slabs at locations #2 and #3. The maintenance personnel indicated that when weak and loose bases were encountered during the FDR operation, the top 150 mm (6 inches) of base was removed and replaced to ensure proper base support. Thus, higher base moduli in the FDR areas may be related to the base replacement. The average base and subgrade moduli (under both FDR and non-repaired slabs) from DCP testing is 112 MPa (16 ksi) and 53 MPa (8 ksi), respectively. The DCP results indicate that base and subgrade moduli are much lower than those on the IH35W and US75 projects. Of course the traffic on IH35W and US75 is much higher than on SH78. Integrating all of the underlying observations and the fact that FDR behaves well, provided the construction is appropriate and load transfer is satisfactory, it is reasonably believed that the most critical factor affecting the pavement performance is the LTE. The base and subgrade support is secondary. Since the FWD tests are conducted separately for each site, the results are presented individually for each site. For example, Figs. 4 and 6 are for IH35W and Figs. 8 and 9 are for US75. One of the main reasons is because temperatures have significant impacts on LTE. In addition, the pavement structures are different for the three sites presented. The main intent of the FWD tests is to demonstrate that FWD is a good diagnostic tool to explain the failure mechanism.
5. Conclusions Fig. 14. Schematic Shows the Typical Pull Out Test Conducted in the Job Site
Many Full Depth Repairs (FDRs) have suffered reoccurring failures on the slabs or adjacent slabs that received FDR treatment. There is a critical need to understand the failure
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KSCE Journal of Civil Engineering
Improving the Performance of Full-Depth Repairs by Understanding the Failure Mechanisms
mechanism and to develop more reliable design and construction procedures for FDR of PCC pavements. Efforts were made to investigate the causes of the failures on two such pavements, IH35W and US75. In addition, one successful project on SH78 was studied to verify and validate the hypothesis of the failure mechanism. LTEs were evaluated with FWD, and the base and subgrade moduli were determined by DCP. Cores were taken to examine the tie bar conditions. Design and construction practices were reviewed and evaluated. Based on the findings in this study, the following conclusions are made: The common symptom for both IH35W and US75 is the low LTEs (less than 40%). When there is lower LTE, there is higher deflection and visible pumping and settlement. Thus, settlement is strongly related to the low LTE. The tie bars were found to be ruptured in US75, and they were not properly anchored in IH35W, as the hole was much larger than the tie bar, and no evidence of epoxy was found. Based on the superior performance of the SH78 project, there is a need to revise the existing specification to include the size of the hole, type of drill, and repair area. Tighter holes increase the chance for higher LTE and thus better performance. Core drills minimize the damage to the adjacent slabs. Appropriate repair boundaries are critical to ensure hidden damages are remedied. Proper procedures when removing concrete and drilling holes for tie bars are critical for minimizing the damage and subsequent failures of the adjacent slabs. Anchoring the tie bar is critical to the performance of the FDR. Project inspectors should insist that the contractor follow the specifications strictly. Pull out tests should be included in the specifications and enforced to make sure that (1) the epoxy type (2) embedment depth, and (3) curing time are all appropriate. Based on the DCP and field performance results, it is suggested that the most critical indicator of performance is LTE, and the base and subgrade support is secondary.
Acknowledgements The support and assistance from David Wagner, Richard
Vol. 15, No. 1 / January 2011
Williammee, Ali Esmaili-Doki, Mykol Woodruff, Kevin Harris, Juan Gonzalez, and Maurice Pittman of Texas Department of Transportation, David Whitney of University of Texas at Austin, and Dr. Moon Won of Texas Tech University are much appreciated.
References AASHTO (1993). AASHTO Guide for design of pavement structures, American Association of State Highway and Transportation Officials, Washington, D.C. ACPA (1995). Guidelines for full-depth repair, TB002.02P, American Concrete Pavement Association, Skokie, Illinois. American Society for Testing and Materials (ASTM) E488 - 96 (2003). Standard test methods for strength of anchors in concrete and masonry elements, Annual Book of ASTM Standards, ASTM International,West Conshohocken, PA. Caltrans (2004). Slab replacement guidelines, California Department of Transportation, Sacramento, California. Chen, D.-H., Lin, D.-F, Liau, P.-H., and Bilyeu, J. (2005). “Developing A correlation between dynamic cone penetrometer data and pavement layer moduli.” Geotechnical Testing Journal, ASTM, Vol. 28, No. 1, pp. 42-49. Chen, D.-H. and Won, M. (2008). “Field performance monitoring of repair treatments on joint concrete pavements.” Journal of Testing and Evaluation, ASTM, Vol. 36, No. 2, pp. 119-127. ERES Consultants Division of ARA, Inc. (2004). Guide for mechanistic-empirical design of new and rehabilitated pavement structures, Final Report, Project 1-37A, National Cooperative Highway Research Program, Transportation Research Board, National Research Council, Washington, D.C. Snyder, M. B. and Darter, M. I. (1990). “Design procedure for dowel load-transfer systems for full-depth repair joints.” Transportation Research Record, No. 1272, pp. 50-64. Tayabji, S. (2008). “The quest for long-life concrete pavements: from theory to practice.” International Journal of Pavement Research and Technology, Vol. 1, No. 4, p. IV. TxDOT (2004). Standard specifications for construction and maintenance of highways, Streets, and Bridge, Texas Department of Transportation, Austin, Texas. Yoder, E. J. and Witczak, M. W. (1975). Principles of pavement design, John Wiley, New York, p. 711.
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