Forensic Investigation of Ultrathin Whitetopping Pavements in Florida

Developed in the early 1990s, ultrathin whitetopping (UTW) is a relatively new technique for asphalt pavement rehabilitation. To evaluate the applicability of UTW pavement in Florida, in 1997, the Florida Department of Transportation (FDOT) constructed an experimental UTW pavement in a weigh station along I-10, located in north Florida. The performance of these test sections, however, was less than ideal, with the observation of some early cracking on the concrete surface, which developed into severe cracking with time. Therefore, a forensic investigation was conducted to determine the causes of the problems in these UTW sections, so that lessons could be learned from this experimental project, the use of UTW under Florida’s conditions could be adequately assessed, and UTW technology could be properly applied in the future. The scope of work consisted of field evaluation, laboratory testing, and pavement design evaluation. Field evaluation included a pavement condition survey, pavement temperature measurement, nondestructive load testing using a falling weight deflectometer, and slab thickness determination. Laboratory tests were performed to determine concrete and asphalt material properties. Other design and traffic data were also acquired from FDOT. Data collected from the field evaluation and laboratory testing were used in conjunction with a mechanistic UTW pavement design/evaluation procedure to determine the possible causes for premature failure. From this comprehensive evaluation, the primary cause for the failure was found to be inadequate UTW pavement design. The inadequacy of the combination of thickness and slab dimensions contributed to the early cracking of the UTW pavement.     

Evaluation of Thermal Cracking at Elmendorf AFB Using SHRP Technology

An evaluation of runway and taxiway pavements was conducted using technology developed or utilized during the Strategic Highway Research Program (SHRP) to determine the effectiveness for identifying thermal cracking propensity of asphalt pavements. SHRP performance grades (PG) of PG52-28 and PG58-28 were measured for the 3 and 6% (weight-to-weight ratio) styrene-butadiene-styrene copolymer-modified asphalt binders employed in taxiway and runway construction. The high temperature SHRP performance grades were above that required by SHRP for the Anchorage, Alaska area according to the SHRP weather database. The low temperature SHRP PG of the binders were found to be insufficient for the area. No rutting has been observed; however, the pavements developed transverse cracks after the first winter following construction of both the runway and taxiway pavements in 1994 and 1996, respectively. The SHRP thermal cracking model failed to predict any cracking within a 10-year period for both pavements. No obvious cause for the model failure could be ascertained. The thermal stress restrained specimen test revealed no significant difference between cracking temperatures for the 3 and 6% styrene-butadiene-styrene-modified binders.     

Investigation of a Pavement Premature Failure on a Weak and Moisture Susceptible Base

An investigation was conducted to determine the root cause of the premature pavement failure. The premature pavement failure occurred in the form of rutting and alligator cracking. Although the affected portion was repaired by removing and replacing the top 75-mm asphalt concrete (AC), the repaired AC experienced recurring rutting and alligator cracking in a few weeks. Through extensive field and lab testing, it was found that the weak base is the root cause of the premature failure and the brittleness of the AC is secondary. However, both the base and AC were built according to plan and met the current material and field density requirements. It was concluded that density alone for construction quality control is not sufficient, as it was not able to protect against premature failures from occurring. Although there are many different ways to minimize premature failures, an immediate action is to include proof rolling in construction quality control. Proof rolling has been used with success to ensure proper compaction and to locate unstable areas, as the stability is greatly influenced by the degree of densification achieved during compaction.     

Forensic Evaluation of Premature Failures of Texas Specific Pavement Study–1 Sections

The Specific Pavement Study–1 pavement test section on US281 in south Texas comprise the largest Strategic Highway Research Program experimental site in the United States. The project was opened to traffic in 1997, and performance has been poor. Three of these test sections developed deep rutting within 1 year. Their surfaces were milled to restore ride quality. Three years after construction, 14 of the sections had 10 mm or more rutting. A forensic study was initiated by the Texas Department of Transportation to identify the cause of the problem. Nondestructive testing was conducted with both the falling weight deflector and ground penetrating radar. No structural problems were detected with either device, both indicating that the base and subbase layers were strong. A field investigation was initiated; the original plan was to cut nine trenches, however, after four trenches were cut, the problematic layer was identified and the trenching operation was terminated. Dynamic cone penetrometer, stiffness gauge, seismic pavement analyzer, and nuclear density gauge tests were then conducted on top of the base and subgrade layers. The trench profiles indicated that the rutting was coming primarily from the top 50-mm (2-inch) asphalt-concrete layer. Asphalt cores were taken from both rutted and nonrutted sections and bag samples of the base were tested in laboratory. The binder was recovered, and the asphalt content and penetration, aggregate gradation, and type were determined. The cause of the problem was traced to a change in aggregate screening, and also an excess of asphalt in the top layer.     

Effect of Design and Site Factors on the Long-Term Performance of Flexible Pavements

Results are presented from a study to evaluate the relative influence of design and site factors on the performance of in-service flexible pavements. The data are from the SPS-1 experiment of the Long-Term Pavement Performance program. This experiment was designed to investigate the effects of HMA surface layer thickness, base type, base thickness, and drainage on the performance of new flexible pavements constructed in different site conditions (subgrade type and climate). Base type was found to be the most critical design factor affecting fatigue cracking, roughness (IRI), and longitudinal cracking (wheel path). The best performance was shown by pavement sections with asphalt treated bases (ATB). This effect should be interpreted in light of the fact that an ATB effectively means a thicker HMA layer. Drainage and base type, when combined, also play an important role in improving performance, especially in terms of fatigue and longitudinal cracking. Base thickness has only secondary effects on performance, mainly in the case of roughness and rutting. In addition, climatic conditions were found to have a significant effect on flexible pavement performance. Wheel path longitudinal cracking and transverse cracking seem to be associated with a wet-freeze environment, while nonwheel path longitudinal cracking seems to be dominant in a freeze climate. In general, pavements built on fine-grained soils have shown the worst performance, especially in terms of roughness. Although most of the findings from this study support the existing understanding of pavement performance, they also provide an overview of the interactions between design and site factors and new insights for achieving better long-term pavement performance.     

Performance of Polymer Modified Asphalt Bridge Expansion Joints in Low-Temperature Regions

Conventional asphalt bridge expansion joints used in low-temperature regions generally show cracking within the first 2?years. To improve the low-temperature performance of these joints, the commercial MEIJIA asphalt binder commonly used in bridge expansion joint construction was modified with two polymers: thermoplastic rubber and rubber. The goal is to find an optimum combination of polymers, binders, and aggregates to improve the performance of asphalt expansion joints in low-temperature regions. The polymer modified binders and mixtures were evaluated for their low-temperature properties using ductility, penetration, indirect tension, and bending tests. The study indicates that performance of these joints at low temperature can be enhanced significantly with the right combinations of polymers, binders, and aggregates. Four expansion joints made with the polymer modified asphalt mixtures were installed on two bridges in a cold region. A construction procedure was also developed to install these joints properly to minimize low-temperature cracking along the interface between the joint and bridge deck. After 7?years of service, the four joints show good performance without any visible cracking or rutting.     

Performance Problems and Stabilization Techniques for Granular Shoulders

Shoulder rutting and pavement edge drop-offs are common performance problems associated with granular shoulders. Being hazardous to drivers and difficult to maintain, such problems require more attention. Shoulder rutting is primarily caused by bearing capacity failure of the subgrade, whereas edge drop-offs develop by surface runoff erosion, vehicle-induced wind erosion, or vehicle off-tracking. Evaluating the performance of several granular shoulders in Iowa revealed that 2/3 of the inspected sections had an edge drop-off greater than 38 mm, while 40% had a subgrade layer with a California bearing ratio less than 10. A high-speed camera was used to study vehicle tire-aggregate interaction upon off-tracking. Off-tracking was found to elevate and displace aggregate particles away from the pavement edge. Based on these findings, six test sections were stabilized and monitored. The granular layer of four sections was stabilized using polymer emulsion, foamed asphalt, Portland cement, and soybean oil. The soft subgrade layer of two sections was stabilized using Class C fly ash and biaxial geogrid. This paper discusses granular shoulder performance problems, the repair and monitoring of six stabilized test sections, and recommendations to improve the long-term performance of granular shoulders.     

Premature Cracking from Cement-Treated Base and Treatment to Mitigate Its Effect

A forensic study was conducted to investigate the premature cracking distress on an ongoing construction project on State Highway (SH) 24. Transverse cracks occurred at approximately every 9–15?m (30–50?ft) along the 9.6?km (5.9?mi) project. The field tests involved both destructive tests, including trenching and coring, and nondestructive tests, including falling weight deflectometer (FWD) and ground-penetrating radar (GPR) tests. The laboratory tests mainly included cement-treated base material and asphalt mixture material series tests. By integrating all the test results, it was concluded that the premature cracking was originating from the cement-treated base (CTB). Although CTB is not a new concept in pavement construction, stabilization of base materials is a complex process, which, if not handled properly, may lead to premature failures. The two primary factors that contributed to the premature failure are (1)?an excessive amount of cement in the CTB, and (2)?a high moisture content when the CTB was compacted (almost 2% above optimum moisture). It was believed the possibility of premature failure could be reduced significantly, if the lab verification tests were conducted on the CTB before construction started. Pavement life analysis was conducted to evaluate the current structure adequacy to sustain future traffic. In addition, extensive material design was conducted for the final wearing course asphalt mixture, which was intended to mitigate the effect of existing cracks by resisting crack propagation from the underlying structure.     

Utilizing Advanced Characterization Tools to Prevent Reflective Cracking

To prevent premature failures of rehabilitated concrete pavements, transportation authorities need tools to characterize the prerehab pavement condition of its load carrying capacity, and to determine the resistance of the overlay material to underlying crack/joint movements. Two quantitative methods, the rolling dynamic deflectometer (RDD) and overlay tester (OT), along with field performance data were employed in rehabilitation studies involving reflective cracks. The RDD is able to continuously assess vertical differential movements at joints/cracks that represent the potential for reflective cracks on existing pavements. The OT has the ability to determine the resistance of the overlay material to underlying crack/joint movements. The RDD W1?W3 deflections were used to determine areas that have a high potential for reflective cracking due to poor load transfer across joints and cracks. This paper documents results from the RDD and OT on the following five rehabilitation projects: (1) SH225; (2) US96; (3) SH12; (4) SH342; and (5) IH35W. Based on the available test results from these five projects, it was observed that the W1?W3 threshold values of 5.5 mils (0.140 mm) for exposed concrete pavement and 6.5 mils (0.165 mm) for composite pavement with existing hot mix asphalt overlay and an OT threshold value of 700 cycles correlated well with the field performance. Ignoring either of these critical factors may lead to premature reflective cracking.     

Effectiveness of Preventative Maintenance Treatments Using Fourteen SPS-3 Sites in Texas

Fourteen Texas SPS-3 test sites were studied to determine the effectiveness of preventative maintenance treatments. These sections were built on four highway classifications (IH, US, SH, and FM) in different climates and with different levels of traffic and subgrade support. Almost all 14 SPS-3 sites were given preventative maintenance treatments (thin overlay, slurry seal, crack seal, and chip seal) in Fall 1990. The distress score concept used by the Texas Department of Transportation (TxDOT) was adopted in this study to judge the effectiveness of preventative maintenance treatments. TxDOT has used this concept since the early 1980s, though the utility factors have been revised few times. The distress score quantifies the visible surface wear due to traffic and environmental influences. Only very few sections experienced premature failures on the SPS-3 sites in Texas. In many cases, superior underlying pavement conditions have been found. The chip seal has the most sites in which it is rated the best performer. The chip seals performed well on a wide range of pavement conditions. In fact, chip seals have the highest distress score for both high and low traffic areas. When initial cost is considered, crack seal provides the best alternative for low traffic routes that have a sound underlying pavement structure. For high traffic routes, chip seal is a better choice. However, a thin overlay is the most effective for rut resistance. Since the thin overlay has the highest initial cost, it is best used on high traffic routes where rutting is a major concern. If rutting is not a concern, chip seal is the best choice for a high traffic area. The treatments applied to US84 sections were too late and did not reach seven years of life as normally was expected, which reconfirms that the timing for preventive maintenance treatment is very important.     

Construction Management of a Small-Scale Accelerated Pavement Testing Facility

Research in accelerated pavement testing (APT) facilities has traditionally focused on the pavement performance such as rutting and fatigue cracking, but documentation on construction management and information of the actual pavement construction quality is limited. There are typically four critical factors that need to be considered to achieve the best possible outcome in construction: cost, schedule, construction process, and quality control, and management. With the objective of developing guidelines for planning and executing construction of a small-scale APT facility, this paper presents a case study documenting and evaluating the construction process and construction management efforts of two sensor-instrumented hot mix asphalt pavement test sections built in a small-scale APT facility. The focus of the experiment was to study bottom-up fatigue cracking of the flexible pavement structure. The presented information and lessons learned serve as a template and guide for agencies pursuing this type of research and pavement construction.     

Behavior of Bridge Asphalt Plug Joints under Thermal and Traffic Loads

An asphalt plug joint (APJ) is a type of expansion joint providing quick, easy, and cheap installation along with good surface flatness. However, APJs are known to suffer from premature failure, and their behavior, especially under thermal movement, has not yet been fully established. In this paper, the behavior of a typical APJ subjected to thermal and traffic loads is examined through a series of finite element analyzes employing a temperature-dependent viscoplastic material model. The material parameters are calibrated by using previously published test data, and the model is validated by comparing simulated responses to APJ test data. The developed models are then used to investigate stress and strain distributions, vulnerable locations to cracking failure, and local demands at those locations when a prototype APJ is subjected to various loading and temperature conditions. Sensitivity studies are also conducted to quantify the effect of debonding the bottom of the APJ and loading rate. The model results shed light about APJ response under traffic and thermal loading and provide new, fundamental information that can be used to improve the durability of APJs. For example, the simulation results suggest that intentionally debonding the interface between the gap plate and the APJ is a practical and low cost solution to mitigate the risk of premature APJ failure.     

Field and Lab Investigations of Prematurely Cracking Pavements

A forensic study was conducted to identify the cause of the premature cracking on three recently completed projects that were built with the same design. Nondestructive [ground penetration radar, falling weight deflectometer (FWD), GeoGauge, and Portable FWD], nuclear density gauge, dynamic cone penetration, and extensive laboratory tests were performed. It was found that the initial stiffness of the treated base was found to be excessively high by FWD backcalculation. Some sections of the backcalculated base moduli were over 20.7?GPa. This indicates that the layer is excessively brittle for a base material, similar to lean concrete. Six specimens (that were made without a mellowing period) exhibited cracks. There was no cracking for six specimens that had two days of mellowing. It was concluded that the culprit of the transverse cracking in the main lanes was the shrinkage of lime treated base layers. The longitudinal cracks are related to the edge drying and the transverse cracks are related to the insufficient mellowing period. Based on the findings of this study, the District implemented a 2-day mellowing period for Quicklime treated caliche base. Three newly constructed pavements (age 8, 5, and 2?months) were surveyed. No cracking can be observed so far, and the District thinks the cracking problem has been mitigated by the 2-day mellowing period. Without the mellowing period, cracking had normally occurred 1?to?2?months after construction..     

Shanghai’s Experience on Utilizing the Rubblization for Jointed Concrete Pavement Rehabilitation

As agencies continue looking for cost-effective methods to rehabilitate deteriorated jointed concrete pavement (JCP), rubblization using a resonant breaker has been experimented by the Shanghai Municipal Roadway Authority (SMRA). It was demonstrated that rubblization using a resonant breaker offers a viable option for the SMRA because the rubblized pavement sections have been performing very well with no visible distress. Based on field observation for a typical hot mix asphalt concrete (HMAC) overlay on a nonrubblized JCP, it was found the treatment normally would have reflective cracking for the same overlay thickness in the first three years. Besides the cost advantage over the reconstruction, a resonant breaker also had yielded the minimum disturbance during the rubblization. It was observed that it was very effective to use water during compaction on a rubblized JCP surface to improve compaction efficiency and to control dust. Furthermore, there is no need to apply a prime coat before the HMAC overlay, as there was no detrimental effect that could be identified. The average rubblized JCP moduli were found to be 1,323?to?1,375?MPa, which are within the range reported in the literature. It was believed that there were high possibilities to increase rubblized JCP moduli without sacrificing the performance by increasing the particle size, because a reduction of 200?mm of HMAC was observed when rubblized JCP increased from 345?to?3,445?MPa at a subgrade modulus of 138?MPa and traffic of 30 million ESAL. However, further research is needed to optimize the rubblized JCP moduli in an attempt to reduce overlay thickness without creating reflective cracking.     

Successful Application of CLSM on a Weak Pavement Base/Subgrade for Heavy Truck Traffic

Lack of proper pavement base and subgrade compaction leads to premature failures that account for millions of dollars in damages. Controlled low-strength material (CLSM) concrete was introduced in this study as pavement base material near a manhole where proper compaction is unachievable. Rut-resistant stone matrix asphalt was placed on top of the CLSM as a wearing surface layer. Dynamic cone penetrometer (DCP) testing was used to monitor CLSM construction. One day after placing, the CLSM gained sufficient strength to support construction traffic. Further, DCP results indicated that the CLSM possessed uniform characteristics of concrete that could improve the load-bearing capacity and serviceability of the pavement near the manhole. After 18 months of heavy truck traffic, maximum rutting was 5?mm, well below the failure criteria of 12.5?mm. Based on cost and performance, CLSM concrete has the potential to improve problematic areas in pavement.     

Field Investigations of Cracking on Concrete Pavements

This paper presents the results of several investigations to identify the underlying causes of longitudinal cracking problems in Portland cement concrete (PCC) pavement. Longitudinal cracking is not intended and detrimental to the long-term performance of PCC pavement. Longitudinal cracking problems in five projects were thoroughly investigated and the findings indicate that longitudinal cracking was caused by: (1) late or shallow saw cutting of longitudinal joints; (2) inadequate base support under the concrete slab; and (3) the use of high coefficient of thermal expansion (CTE) aggregates. When the longitudinal cracks were caused by late or shallow saw cutting of longitudinal joints, cracks developed at a very early stage. However, when there was adequate base support, the longitudinal cracks remained relatively tight even after decades of truck trafficking. Another cause of longitudinal cracking was inadequate base support, and cracking due to this mechanism normally progressed to rather wide cracks. Some cracks were as wide as 57?mm. Evaluations of base support by dynamic cone penetrometer in areas where longitudinal cracks were observed indicate quite weak subbase in both full-depth repaired areas and surrounding areas. This implies that the current requirements for the subbase preparation for the full-depth repair are not adequate. Another cause of longitudinal cracking was due to the use of high CTE aggregate in concrete. Large volume changes in concrete when coarse aggregate with high CTE is used could cause excessive stresses in concrete and result in longitudinal cracking. To prevent longitudinal cracking, attention should be exercised to the selection of concrete materials (concrete with low CTE) and the quality of the construction (timely and sufficient saw cutting and proper selection and compaction of subbase material).     

铸造工艺对K423A合金力学性能的影响

 系统地研究了浇注温度和模壳温度对铸造高温合金K423A室温拉伸性能和850 ℃/325 MPa下持久性能的影响。研究结果表明：浇注温度和模壳温度对铸造高温合金K423A的力学性能有较大的影响。当浇注温度为1 380 ℃、模壳温度为850 ℃~1 050 ℃以及浇注温度为1 440 ℃、模壳温度为850 ℃时,合金的室温拉伸性能和高温持久性能均较好,合金具有最佳的综合力学性能。当浇注温度和模壳温度为其余几种组合时,合金的力学性能有不同程度的下降。当浇注温度为1 440 ℃~1 500 ℃、模壳温度为1 050 ℃以及浇注温度为1 440 ℃、模壳温度为950 ℃时,合金的力学性能较差,在生产中应避免采用。     

Hardfacing of a Modified 9Cr–1Mo Steel Component Using a Nickel-Base Alloy

A dashpot piston made of modified 9Cr–1Mo steel is hardfaced with NiCr-B alloy by the Plasma Transferred Arc (PTA) process. During initial trials, a large number of cracks were observed in the hardface deposit when hardfacing was carried out directly on the modified 9Cr–1Mo steel substrate using a preheat temperature of 723 K. Both the deposit and the martensitic structure formed in the heat affected zone of the substrate during deposition are hard and hence were unable to absorb the thermal stresses generated, leading to cracking. Subsequently, hardfacing trials carried out with an intermediate layer of 2 mm thick Inconel-625 alloy, were successful and deposits were crack-free. Use of a relatively soft Inconel-625 between the hardface deposit and the substrate reduced martensite formation in the substrate, and thus the cracking susceptibility of the deposit.     

Multiple-cracking phenomenon of the galvannealed coating layer on steels under thermal and tensile stresses

The multiple-cracking phenomenon of the Fe-Zn intermetallic coating layer on the hot-dip galvannealed (GA) steels under thermal and tensile stresses was studied experimentally by tensile tests and analytically by means of the finite-element analysis. The multiple cracking of the coating layer had occurred in the as-supplied samples, and it progressed with increasing applied strain. Based on the calculated dependence of the stress of the coating layer on the crack spacing and applied strain, the multiple cracking in the as-supplied samples was accounted for by the thermally induced residual stress, and the further multiple cracking with increasing applied strain was accounted for by the increased stress of the coating layer. The experimentally observed decrease of the average crack spacing with increasing applied strain was described well, and the tensile strength of the coating layer was estimated to be 260 MPa, by application of the calculated relation between the increased stress of the coating layer and applied strain. The influences of the thickness of the coating layer and the substrate material on the multiple cracking were discussed based the stress analysis. It was revealed that the thinner the coating layer and the higher the flow stress of the substrate, the higher the stress of the coating layer becomes and, therefore, the smaller the crack spacing becomes.     

基于多传感器融合的钢包裸浇检测

钢包浇注时,其保护套管容易发生断裂和脱落,造成钢水大范围的飞溅,严重影响到钢坯的生产质量。为了有效监控钢包浇注状态,研究了基于红外测温的钢包浇注状态检测方法。通过试验验证钢包正常浇注状态和裸浇状态有较大的温度差异,可通过设置温度阈值实现钢包浇注状态的有效分类。同时,研究了基于深度学习的钢包浇注状态分类方法,通过中值滤波和平均灰度级-梯度二维Otsu分割方法对钢包浇注图像进行预处理,再基于YOLO v3改进的目标检测算法完成了模型的训练。试验结果表明,该方法预测的钢包裸浇准确率为99.2%,具有更好的泛化性能。基于以上研究,开发了一套钢包裸浇检测设备,实现了浇注状态的准确报警,具备良好的现场应用效果。