Thin and Ultra-Thin Whitetopping (2004)

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66 Currently, a number of states have thin whitetopping (TWT) and ultra-thin whitetopping (UTW) projects that are signifi- cant data sources for the industry. These include projects in Minnesota, Colorado, Iowa, Tennessee, Georgia, Missouri, and Virginia. The following sections outline project details of sites from each of these states. MINNESOTA The Mn/ROAD Project was constructed in 1994 to provide a full-scale testing facility for the study of climate, materials, and traffic interactions (50–52). A total of 40 portland cement concrete (PCC), hot-mix asphalt (HMA), and aggregate sur- faced roads are subjected to low- and high-traffic volumes. In October 1997, the Minnesota Department of Transportation (MnDOT) constructed six trial whitetopping sections that are subject to Interstate traffic conditions. These TWT and UTW sections are reinforced with fibers based on the assumption that the fibers can hold the crack widths tight. Typically, white- topping is used on low-volume roads and at deteriorating intersections, so this test simulates the effects of accelerated loading and investigates the viability of UTW as a rehabilita- tion technique on highly trafficked roads. In addition, the Minnesota climate is one of the more severe environments that a whitetopping overlay will have to endure. Temperatures can vary greatly there, and the Minnesota spring frost can result in extensive damage to the pavement by traffic. This site is of interest because it has been heavily instru- mented with dynamic, static, moisture, and temperature sen- sors. In addition, data from these sensors can be used to assess the bond between the UTW and the HMA layers. Bond is believed to be one of the key performance quantifiers of the UTW. If the bond is strong, the two layers behave in a mono- lithic manner to reduce load-related stresses. If the bond is broken, then the layers act independently, and the benefits of UTW are reduced significantly. Mn/ROAD Design As Table B1 illustrates, six sections of fiber-reinforced white- topping overlay were constructed on an existing 340 mm (13.5 in.) HMA pavement on Minnesota I-94. Traffic on the Interstate is approximately 1 million equivalent single-axle loads (ESALs) per year. To reduce the effects of curling, the whitetopping panels are cut into 1.2 × 1.2, 1.5 × 1.8, and 3.0 × 3.6 m (4 × 4, 5 × 6, and 10 × 12 ft) sections. The joints were sealed as a precaution against Minnesota’s temperature extremes and freeze–thaw cycles. Under traffic loading, these shorter panels deflect downward instead of bending, as typi- cal 4.5-m (15-ft) jointed PCC pavements do. The existing HMA pavement contains a Pen 120/150 binder type. Whitetopping Instrumentation The test section is subdivided into six cells with various thicknesses, joint patterns, and types of fibers, as indicated in Table B1. Each cell is instrumented with dynamic and static strain, temperature, and moisture sensors. This allows for the measurement of both static and dynamic pavement response under various applied and environmental loading conditions. The hourly strains produced by environmental and applied loads will be recorded for these whitetopping designs. Distress Survey The first transverse cracks and corner breaks were found in June 1997. Most of the corner breaks occurred between March and June 1997, and are believed to be the result of high neg- ative temperature gradients (measured in May 1997). When the corners are subjected to a negative temperature gradient, the corners of the whitetopping curl upward, and a gap forms between the pavement support and the overlay slab. Traffic loading subsequently causes the cracks to form as a result of this cantilever effect. It is believed that the cracks remain tight owing to the bridging of the synthetic fibers. Transverse cracks were found in the whitetopping in Jan- uary 1999 and are believed to be attributable to the reflection cracking mechanism. Seventy percent of the total number of cracks in the whitetopping is believed to be the result of reflection cracking. Most of the cracks formed in the 75-mm (3-in.) whitetopping overlay, and there are no cracks in the thicker 150-mm (6-in.) sections. However, until the overlay has been subjected to additional loading, it is too early to draw conclusions as to the optimal mix and design. Results The bond between the whitetopping and the HMA is believed to be one of the critical properties of the whitetopping system. If the bond is high, then the neutral axis of the PCC–HMA pavement system is lowered and the pavement acts as a mono- lithic system. If the bond is broken, then the overlay and under- lying pavement deform separately, and the life of the overlay will be shortened. To quantify the bond of the overlay to the existing pavement, strain gauges were installed at the top and bottom of the overlay. The strain readings indicate the degree of bonding of the overlay to the existing HMA pavement. APPENDIX B Detailed Case Studies

67 Measurements from the first year show that strains at the bot- tom of the 75 mm (3 in.) overlay are less than 10 microstrains in tension. Therefore, the overlay is predominately in com- pression. In the 100 mm (4 in.) overlay, the strains at the bot- tom are less than 20 microstrains in tension. Flexural rigid- ity of the system increases as PCC thickness increases, as the neutral axis shifts up, and as more load is carried by the over- lay than by the HMA pavement. Pavement strains are highly dependent on temperature. As temperature decreases, the HMA’s resilient modulus increases, and the tensile stresses at the bottom of the overlay increase. Conversely, as temperature increases, the resilient modulus decreases, and the bottom of the PCC overlay can go into compression. Conclusions The hardened properties of the fiber-reinforced concrete (FRC) using polyolefin and polypropylene fibers were found to be similar to those of conventional PCC. The bond between the overlay and the underlying HMA is good at the Mn/ROAD site, but subsequent loading will determine if the bond deteri- orates. The factors that significantly affect the strains at the bottom of the overlay are HMA stiffness and overlay thick- ness. The neutral axis of the pavement system can move through the section dependent on temperature changes and on overlay thickness. Increasing the overlay thickness does not always lower the tensile strains at the bottom of the overlay. Instead, they can increase, so it is necessary to select the opti- mal design from continued testing of the Mn/ROAD sections. Evaluation of the Mn/ROAD sections has revealed that some of the cracking in the underlying HMA reflected through the UTW layer. This phenomenon is believed to be a func- tion of a high bond strength and stiff HMA layer. It was hypothesized that when the flexural stiffness of the HMA approaches that of the overlay, this distress can occur (50). COLORADO Whitetopping sections in Colorado, 125 to 175 mm (5 to 7 in.) thick, with joint spacing of up to 3.6 m (12 ft) were instru- mented to measure critical stresses and strains owing to traf- fic loads and temperature differentials (17,20). These results were used to develop a thickness design procedure for the state of Colorado’s whitetopping overlays (see chapter four of this synthesis for more information). Eleven slabs were instrumented at three different sites in Colorado to obtain the field data. Strain gauges were placed primarily in the center of the slab and along the longitudinal joint. Traffic loads were simulated using Colorado DOT (CDOT) trucks. Objectives The objectives for the field testing in Colorado were to 1. Determine the critical stresses and strains in the white- topping overlays as a result of traffic loads, 2. Examine and measure the interfacial bond between the PCC and the HMA layers, and 3. Experimentally calibrate the theoretical finite-element method (FEM) stress prediction equations. Experimental Whitetopping Test Sections Three different Colorado sites (CDOT1, CDOT2, and CDOT3) were instrumented in this study. The PCC mix design for the Colorado whitetopping is given in Table B2 (19). Information on whitetopping thickness, HMA thickness, joint spacing, dowels, HMA surface preparation and the modulus of sub- grade reaction are given in Table B3. All sections included tie bars along the longitudinal joints. Field Instrumentation Field instrumentation varied from site to site. For the first site (CDOT1), all of the test slabs were instrumented with 12 strain gauges. To instrument the interface of the PCC and the HMA, two strain gauges were placed along the transverse centerline on top of the HMA and 12.5 mm (0.5 in.) above the HMA surface in the PCC. These gauges were placed at both longitudinal edges and at the slab center. Gauges were also installed on top of the PCC surface—three above the sets of embedment gauges and three along the corner diagonal line. Section ID Thickness [mm (in.)] Joint Spacing [m (ft)] Fiber Reinforcement Cell 93 75 (3) 1.2 × 1.2 (4 × 4) Polypropylene 1.8 kg/m3 (3 lb/yd3) Cell 94 100 (4) 1.2 × 1.2 (4 × 4) Polypropylene 1.8 kg/m3 (3 lb/yd3) Cell 95 75 (3) 1.5 × 1.8 (5 × 6) Polyolefin 14.8 kg/m3 (25 lb/yd3) Cell 96 150 (6) 1.5 × 1.8 (5 × 6) Polypropylene 1.8 kg/m3 (3 lb/yd3) Cell 97 150 (6) 3.0 × 3.6 (10 × 12) Polypropylene 1.8 kg/m3 (3 lb/yd3) Cell 97b 150 (6) 3.0 × 3.6 (10 × 12) w/dowels Polypropylene 1.8 kg/m3 (3 lb/yd3) TABLE B1 MN/ROAD WHITETOPPING DESIGN

68 For the slabs at CDOT2, eight strain gauges were used per test slab. Similar to the instrumentation pattern used for CDOT1, the interface between the PCC and the HMA was instrumented at the slab center and at one of the tied longitu- dinal edges. Likewise, strain gauges were installed above these embedded gauges on the pavement surface. For the slabs at CDOT3, only four surface strain gauges were used. Two were placed at the slab center and along the longitudinal tied slab edge, and the other two were placed at the slab corner—one in the transverse and the other in the longitudinal direction. Thermocouples were installed in all the pavements to measure the temperature profile. Five thermocouples were installed through the overlay depth—one at the PCC surface, one at PCC mid-depth, one at the PCC–HMA interface, one 63 mm (2.5 in.) into the HMA, and the last near the bottom of the HMA layer. To measure slab curling, invar reference rods were driven into the subgrade before construction. Rel- ative elevations of the test slabs were recorded using these rods. A Dipstick profiler recorded the test slab profiles. Laboratory Testing Cores were taken from the test slabs for determination of layer thicknesses. In addition, the strength of the interfacial bond was measured using the direct shear test. Properties (modulus of elasticity, compressive strength, and flexural strength) of the PCC were measured using the cylinders and beams that were cast during overlay construction. PCC–HMA Interfacial Bond The effect of HMA surface preparation on interfacial bond was also examined on sections CDOT1 and CDOT2. This com- parison could not be made for the CDOT3 sections because all sections were milled. The results, shown in Table B4, indicate that the interfacial shear strength between the PCC and the milled HMA layer is low at 28 days. The bond between the PCC and the new HMA pavement surface is higher. How- ever, after 1 year, the interfacial shear strengths were found to be comparable. The effect of milling on new and existing HMA layers was examined using the load-induced strain measurements. For an existing HMA pavement, milling its surface reduces the strain in the whitetopping by 25%. For new HMA pave- ment, milling increases the strain in the whitetopping by 50%. More research will have to be conducted to examine the use of milling and its effect on the whitetopping interfacial bond and load-induced strains. Strains at the top of the HMA layer were also measured and compared with the strains at the bottom of the PCC slab. Typically, strains in the HMA layer were less than the strains in the PCC. This finding may indicate that the two layers are not fully bonded. Stress Results The location of maximum stress was found to be at the free longitudinal edge (such as a curb and gutter). Because it is unlikely that this joint will experience heavy traffic loading, the tied longitudinal joint was taken as the one with maxi- mum stress. Profile Results The profiles of all the instrumented CDOT test slabs were obtained at 28 days and 1 year. Profiles of the 100-mm (4-in.) thick CDOT1 whitetopping sections were obtained along the slab diagonal. It was found that the slab is typically curled upward. The difference in deflection from the slab edge to the slab center is approximately 2.5 mm (0.1 in.) for both the measurements taken at 28 days and 1 year. For the 125-mm (5-in.) CDOT1 whitetopping sections, curling is not as evi- dent and these slabs are relatively flat. Profiles of the CDOT2 slabs are relatively flat as well. For the CDOT3 sections, the difference in displacement between the slab edge and center, as measured on the centerline, is less than 0.5 mm (0.02 in.). The shorter joint spacing is believed to reduce slab curling. Pavement Loading The pavement was loaded several times during the day to measure the load-induced stresses as a function of the temper- ature differential. This correlation was made so that the load- induced stress at a zero-temperature gradient could be obtained. Quantity Material kg/m3 lb/yd3 Type I Cement 335 565 Fly Ash 67 113 Coarse Aggregate 421 710 Fine Aggregate (Sand) 771 1,300 Intermediate Aggregate 546 920 Water 166 280 Air Entrainer (AEA) 913 ml/100 kg cement 14 oz/cwt Water Reducer 2,217 ml/100 kg cement 34 oz/cwt AEA = air entraining agent; cwt = hundred weight. TABLE B2 PCC MIX DESIGN FOR A COLORADO WHITETOPPING SECTION

69 Findings A number of interesting and important observations were made at the Colorado experiments. For TWT overlays, it was found that tie bars were helpful in maintaining horizontal and vertical alignment between adjacent lanes along the construction joint. In addition, it was found that, at least initially, the bond strength between the PCC and the HMA is very low if a new HMA level up course is used instead of placing it directly atop the existing surface. The additional observation of a “tender” HMA mix should also be considered. This finding was reversed as the newer HMA aged and the bond was later tested. The bond strength appeared to increase over time, and it was found to be equal to or better than the bond of the PCC to existing HMA. IOWA The state of Iowa, through the Iowa DOT, has provided valu- able information for the understanding of variables affecting the performance of whitetopping overlays. Whitetopping experimental projects conducted by Iowa include one con- ducted on Road R16 to study the effect of different prepara- tion methods to enhance the bond strength between the exist- ing HMA pavement and the PCC overlay. A second effort, on Route 21, investigated the effect on performance by a number of variables. Name Slab No. Whitetopping Thickness [mm (in.)] HMA Thickness [mm (in.)] Joint Spacing [m (in.)] HMA Surface Dowels Preparation Modulus of Subgrade Reaction (psi/in.) 1 119 (4.7) 114 (4.5) 1.47 × 1.47 (58.0 × 58.0) N New 150 2 147 (5.8) 150 (5.9) 1.46 × 1.46 (57.6 × 57.6) N New 150 CDOT1 3 152 (6.0) 137 (5.4) 1.49 × 1.49 (58.5 × 58.5) N New milled 150 1 130 (5.1) 84 (3.3) 1.85 × 1.85 (73.0 × 73.0) N Existing 340 2 137 (5.4) 117 (4.6) 3.80 × 3.07 (149.5 × 121.0) N New 340 3 160 (6.3) 86 (3.4) 1.89 × 1.82 (74.5 × 71.5) N New 340 4 185 (7.3) 86 (3.4) 3.80 × 1.83 (149.8 × 72.0) N Existing milled 340 CDOT2 5 173 (6.8) 71 (2.8) 3.80 × 3.92 (149.8 × 154.5) Y Existing milled 340 B 188 (7.4) 178 (7.0) 3.05 × 3.58 (120.0 × 141.0) Y Existing milled 225 E 173 (6.8) 168 (6.6) 1.80 × 1.83 (71.0 × 72.0) Y Existing milled 225 CDOT3 F 142 (5.6) 168 (6.6) 1.82 × 1.75 (71.5 × 69.0) Y Existing milled 225 Notes: N = no; Y = yes. TABLE B3 THREE COLORADO FIELD TEST SITES FOR WHITETOPPING CONSTRUCTION Name Slab No. HMA Surface Preparation 28-Day Interfacial Shear Strength [kPa (psi)] 1-Year Interfacial Shear Strength [kPa (psi)] 1 New 310 (45) 552 (80) CDOT1 3 New milled 69 (10) 552 (80) 1 Existing 689 (100) — 4 Existing milled 448 (65) 689 (100) CDOT2 5 Existing milled — 1,069 (155) TABLE B4 EFFECT OF HMA SURFACE PREPARATION ON PCC–HMA INTERFACIAL BOND

70 Road R16 Project A whitetopping test section was constructed on Road R16, in Dallas County, to investigate the effectiveness of various sur- face preparation techniques in improving the bond strength between the existing HMA pavement and the PCC overlay. The existing pavement, built in 1959, consisted of a 63-mm (2.5-in.) HMA surface on top of a 150-mm (6-in.) rolled stone base, over 100 mm (4 in.) of soil base. In 1971, a 75-mm (3-in.) HMA overlay was constructed. The initial condition of the pavement surface included ruts in excess of 25 mm (1 in.), random and transverse cracking, and some areas with alliga- tor cracking. Twelve test sections were constructed, as shown in Table B5 (79). Data Collected Data collected on this research effort include rut-depth mea- surements, Road Rater structural measurements, beam and cylinder strengths, core shear strengths, slump, and entrained air. Compressive strengths in the range of 23.2 MPa (3,364 psi) to 28.4 MPa (4,118 psi) and flexural strengths in the range of 4.34 MPa (629 psi) to 4.75 MPa (689 psi) for Sections 2 to 5 were reported. The average shear strengths obtained ranged from 600 kPa (87 psi) to 1,500 kPa (218 psi). Some of the cores extracted presented no bond at the PCC–HMA inter- face, or the HMA was broken into pieces. In the years of 1994 and 1996, additional cores were extracted to test for shear strength at the PCC–HMA inter- face. Mack et al. (77) contains a detailed table with shear strength information, location, and pavement thicknesses at the core locations for a total of 142 cores. Pavement condi- tion surveys were performed in 1992, 1994, and 1996. Findings From results shown in Table B6, it can be concluded that sec- tions with milling as surface preparation developed higher bond shear strengths. In addition, it was observed that tack coat might reduce strength when a cationic emulsion is used. Sections with cement and water grout as a bonding agent did not show any contribution to bond strength. Different PCC types, thicknesses, planing, and air blasting did not affect bond strength significantly. It was also found in the study that the increase in shear strength does not correlate to an increase in structural contri- bution of the old HMA pavement. According to the study, only sufficient bond is necessary to anchor the PCC to the old HMA, to use some of the underlying HMA pavement struc- ture. It was concluded that the structural evaluation did not provide enough information to determine the bond strength or the level of support provided by the HMA layer. In subsequent monitoring of the test sections, it was found that the PCC–HMA bond was degrading over time in the outside wheelpath for all bonding methods, except tack coat. The total pavement thicknesses correlated very well with the pavement structural capacity tested with the Road Rater. The primary observations from the condition surveys are that Section* Surface Preparation Bonding Agent Planing Design Thickness (mm) As-Built Thickness (mm) Mix (Iowa DOT classification) 2 Broomed None No 130 124 B 3 Milled None No 100 106 B 4 Milled None No 100 117 C 5 Milled None Yes 100 125 C 6 Broomed None No 100 114 C 7 Broomed None No 130 142 C 8 Broomed w/air blast None No 130 143 C 9 Milled None No 130 149 C 10 Milled None Yes 130 153 C 11 Milled Cement and water grout No 130 146 C 12 Broomed Cement and water grout No 130 135 C 13 Broomed Tack emulsion No 130 133 B *Section 1 is not mentioned in the reference (79). TABLE B5 EXPERIMENTAL DESIGN FOR THE IOWA ROAD R16 PROJECT

71 • Most cracks observed were longitudinal, attributed to weakness of the base; and • The cracks are concentrated in Sections 11, 6, 3, and 4 (in decreasing order). No connection was found between the cracking and the surface preparation methods used. However, some correla- tion was found between cracking and PCC thickness and between cracking and planing. It was found that cracking was predominant on the thinnest PCC sections that were not planed. Route 21 Project In 1994, an experimental whitetopping overlay was con- structed in Iowa County. The project, 11.6 km (7.2 mi) in length, was constructed on Iowa Route 21, which carried aver- age daily traffic of approximately 1,000 at the time. The objec- tive of the project was to evaluate the performance of various whitetopping sections with a variety of design factors, includ- ing PCC thickness, use of fibers, and joint spacing. The project consists of 41 test sections with PCC thick- nesses ranging from 50 to 200 mm (2 to 8 in.) overlaying an 88-mm (3.5-in.) HMA surface, 175-mm (7-in.) cement-treated base, and 150 mm (6 in.) of granular subbase (rolled stone base) built in 1961. In addition, 24 sections were used to tran- sition between test sections. Different slab lengths were used, from 0.6 to 4.5 m (2 to 15 ft). Sections with PCC thicknesses greater than 100 mm (4 in.) had their joints sealed with hot- pour sealant. Sections with thicknesses of 100 mm (4 in.) or less were not sealed, except for five sections at a rate of 1.8 kg/m3 (3 lb/yd3). Monofilament and fibrillated polypropy- lene fibers were added to the PCC mix for designated sec- tions. Three test sections were overlaid with 113 mm (4.5 in.) of HMA for comparison purposes. The typical section was 213 m (700 ft) in length. Soil type for the length of the proj- ect, classified according to both the Unified Classification System and AASHTO, is detailed elsewhere (114). Conditioning of the pavement for the whitetopping con- sisted of patching and scarifying, patching only, and cold in- place recycling (CIPR). For the CIPR, 95 mm (3.75 in.) was removed and combined with 2.3% CSS-1 emulsion (by weight of material). This rejuvenated material was placed back onto the milled surface 1 month in advance of the construction of the whitetopping sections. The type of fibers used in each section is presented in Table B7, and the experimental design for this project is summarized in Table B8. Although it was not initially considered, the contractor sprayed the HMA surface with water immediately ahead of the paver. This procedure was stopped after the first 15 sec- tions had been constructed. The difference in the first 15 sec- tions is therefore considered as a variable inadvertently intro- duced. As of 1994, the average annual daily traffic was 1,090 vehicles per day (vpd) and average annual truck traffic was 142 vpd. Data Collected The information obtained during placement of the white- topping project is presented in Wilde et al. (116) and includes the following: • Daily inspection reports of PCC; • Daily plant reports for HMA; • Sawcut time; • Slump and air; • Beam and cylinder strengths at 7, 14, and 28 days; • Profilograph results; • Slab thicknesses; • Paver vibrator revolutions per minute; • Concrete and air temperatures at the time of placement; • Documentation of distresses made with photographs and condition surveys; Section Core Shear Strength in 1991 [MPa (psi)] Surface Preparation Contribution of Old HMA (SN) 5 10.3 (1,500) Milled 1.08 3 9.0 (1,300) Milled 1.15 8 8.3 (1,200) Broomed w/air blast 1.35 4 7.9 (1,150) Milled 1.14 11 6.9 (1,000) Milled 1.26 2 6.6 (950) Broomed 1.38 9 6.6 (950) Milled 1.51 12 6.2 (900) Broomed 1.46 10 6.2 (900) Milled 1.79 6 5.5 (800) Broomed 1.54 7 5.5 (800) Broomed 1.25 13 4.1 (600) Broomed 0.97 SN = structural number. Source: Grove et al. (79). TABLE B6 AVERAGE SHEAR STRENGTH AND SURFACE PREPARATION Section From To Type 1–2 2335+6 2341+0 Convention 2–10 2341+0 2386+7 Fibrillated 10–14 2386+7 2412+7 Monofilam 14–15 2412+7 2415+0 Fibrillated 17–33 2425+0 2505+0 Convention 35–37 2515+0 2539+0 Convention 37–54 2539+0 2632+2 Fibrillated 54–64 2632+2 2703+9 Convention Source: Heyer and Marks (119). TABLE B7 TYPE OF FIBERS USED FOR EACH SECTION

72 • Pullout (pull-off) testing; and • Structural ratings with a road tester. Traffic loading was monitored with a weigh-in-motion device installed in each lane. Condition Surveys Late sawed joints resulted in transverse cracking at Sec- tions 36, 39, 41, 43, and 45. The rest of the overlay did not show transverse cracks caused by late sawcutting. No change was observed from initial construction to approximately 1.5 years after construction. During the second year, new cracks were observed that have been attributed to severe temperature changes. Figures B1 through B5 present the distress progression during the first 2 years. Only sections that had cracking during that period are plotted. Potential debonding of the test sections was monitored with manual soundings. Only Sections 23 and 62, with a PCC thickness of 50 mm (2 in.), showed possible debonding dete- rioration. In summary, the following observations were made for the condition surveys during the first 2 years: • Only sections with 50 and 100 mm (2 and 4 in.) of PCC indicated some distress. • Only two of the eight sections with 50 mm (2 in.) PCC thickness exhibited some debonding and cracking Notes: N/A = not applicable; P&S = patch and scarify (milling); P Only = patch only; CIPR = cold-in-place recycle; D = dowels; ND = no dowels; F = fibers present; NF = no fibers present. Source: Heyer and Marks (119). Section Number Begin Station End Station Length (ft) Design Thickness (in.) As-Built Thickness (in.) Fiber Joint Spacing (ft) Surface Prep. 1 2335+60 2340+00 440 8 200 N/A 20 N/A 3 2342+00 2349+00 700 6 150 F 12 P&S 4 2349+00 2356+00 700 6 150 F 6 P&S 6 2357+00 2364+00 700 4 100 F 6 P&S 7 2364+00 2371+00 700 4 100 F 2 P&S 8 2371+00 2378+00 700 4 100 F 4 P&S 10 2380+00 2387+00 700 2 50 F 2 P&S 11 2387+00 2394+00 700 2 50 F 4 P&S 13 2396+00 2403+00 700 6 150 F 6 P&S 14 2403+00 2414+00 1100 6 150 F 12 P&S 16 2415+00 2425+00 1000 4.5 110 HMA N/A P&S 18 2426+00 2433+00 700 6 150 NF 12 P&S 19 2433+00 2440+00 700 6 150 NF 6 P&S 21 2441+00 2448+00 700 4 100 NF 2 P&S 23 2449+00 2456+00 700 2 50 NF 2 P&S 25 2458+00 2460+00 200 6 150 NF 6 P&S 26 2460+00 2468+00 800 6 150 NF 6 P Only 27 2468+00 2479+00 1100 6 150 NF 12 P Only 29 2480+00 2487+00 700 4 100 NF 4 P Only 31 2489+00 2496+00 700 8 200 NF 15 ND P Only 32 2496+00 2503+00 700 8 200 NF 15 D P Only 34 2505+00 2515+00 1000 4.5 110 HMA N/A P Only 36 2516+00 2538+00 2200 6 150 NF 6 P Only 38 2540+00 2547+00 700 2 50 F 2 P Only 39 2547+00 2554+00 700 2 50 F 4 P Only 41 2555+00 2562+00 700 4 100 F 4 P Only 42 2562+00 2569+00 700 4 100 F 2 P Only 43 2569+00 2576+00 700 4 100 F 6 P Only 45 2577+00 2585+00 800 6 150 F 12 P Only 46 2585+00 2593+00 800 6 150 F 6 CIPR 48 2594+00 2601+00 700 4 100 F 6 CIPR 49 2601+00 2608+00 700 4 100 F 2 CIPR 50 2608+00 2615+00 700 4 100 F 4 CIPR 52 2616+00 2624+00 800 2 50 F 2 CIPR 53 2624+00 2631+00 700 2 50 F 4 CIPR 55 2633+00 2640+00 700 6 150 NF 6 CIPR 56 2640+00 2653+00 1300 6 150 NF 12 CIPR 58 2654+00 2661+00 700 4 100 NF 6 CIPR 60 2662+00 2689+00 2700 6 150 NF 12 CIPR 62 2691+00 2698+00 700 2 50 NF 4 CIPR 65 2704+00 2714+08 1008 4.5 110 HMA N/A CIPR TABLE B8 SUMMARY OF EXPERIMENTAL SECTIONS FOR IOWA ROUTE 21 PROJECT

05 10 15 20 25 30 35 Ju l-9 4 Au g- 94 Se p- 94 O ct -9 4 N ov -9 4 D ec -9 4 Ja n- 95 Fe b- 95 M ar -9 5 Ap r-9 5 M ay -9 5 Ju n- 95 Ju l-9 5 Au g- 95 Se p- 95 O ct -9 5 N ov -9 5 D ec -9 5 Ja n- 96 Fe b- 96 M ar -9 6 Ap r-9 6 M ay -9 6 Ju n- 96 Ju l-9 6 Au g- 96 Se p- 96 O ct -9 6 Date of Survey N um be r o f C ra ck s Sec. 10 Sec. 23 Sec. 53 Sec. 39 Sec. 11 Sec. 62 Sec. 46 Sec. 45 Sec. 14 Sec. 60 Sec. 1 FIGURE B1 Iowa Route 21—transverse cracking. FIGURE B2 Iowa Route 21—longitudinal cracking. 0 5 10 15 20 25 30 35 Ju l-9 4 Au g- 94 Se p- 94 O ct -9 4 N ov -9 4 D ec -9 4 Ja n- 95 Fe b- 95 M ar -9 5 Ap r-9 5 M ay -9 5 Ju n- 95 Ju l-9 5 Au g- 95 Se p- 95 O ct -9 5 N ov -9 5 D ec -9 5 Ja n- 96 Fe b- 96 M ar -9 6 Ap r-9 6 M ay -9 6 Ju n- 96 Ju l-9 6 Au g- 96 Se p- 96 O ct -9 6 Date of Survey N um be r o f C ra ck s Sec. 23 Sec. 53 Sec. 39 Sec. 62 Sec. 41 Sec. 29 Sec. 43 Sec. 36 Sec. 45 Sec. 32 HMA Sec. 16 HMA Sec. 34 HMA Sec. 65

05 10 15 20 25 30 35 Ju l-9 4 Au g- 94 Se p- 94 O ct -9 4 N ov -9 4 D ec -9 4 Ja n- 95 Fe b- 95 M ar -9 5 Ap r-9 5 M ay -9 5 Ju n- 95 Ju l-9 5 Au g- 95 Se p- 95 O ct -9 5 N ov -9 5 D ec -9 5 Ja n- 96 Fe b- 96 M ar -9 6 Ap r-9 6 M ay -9 6 Ju n- 96 Ju l-9 6 Au g- 96 Se p- 96 O ct -9 6 Date of Survey N um be r o f C ra ck s Sec. 23 Sec. 53 Sec. 39 Sec. 62 Sec. 50 Sec. 29 Sec. 45 Sec. 3 Sec. 60 Sec. 32 Sec. 1 0 5 10 15 20 25 30 35 Ju l-9 4 Au g- 94 Se p- 94 O ct -9 4 N ov -9 4 D ec -9 4 Ja n- 95 Fe b- 95 M ar -9 5 Ap r-9 5 M ay -9 5 Ju n- 95 Ju l-9 5 Au g- 95 Se p- 95 O ct -9 5 N ov -9 5 D ec -9 5 Ja n- 96 Fe b- 96 M ar -9 6 Ap r-9 6 M ay -9 6 Ju n- 96 Ju l-9 6 Au g- 96 Se p- 96 O ct -9 6 Date of Survey N um be r o f C ra ck s Sec. 52 Sec. 10 Sec. 29 Sec. 1 Sec. 65 FIGURE B3 Iowa Route 21—corner cracking. FIGURE B4 Iowa Route 21—diagonal cracking.

75 distress. The area that presented distress cracking is only 2% for each section. • The two 50 mm (2 in.) sections with debonding problems and cracking distress contained no fibers. Two 50-mm (2-in.) thick sections with cracking, but no debonding, contained fibers. The remaining four 50-mm (2-in.) thick sections with no cracking or debonding contained fibers. • One of the sections with debonding and cracking was reported to have a construction problem. • Transverse cracking has been observed in the three HMA sections. Cracking at those sections started approxi- mately 1.5 years after construction. Laboratory Testing Sixty-four PCC–HMA composite beams were fabricated and tested under static and flexural loading. For this experiment, a factorial was used to test for different conditions, including HMA surface preparation (milled or not milled), PCC thick- ness of 50 or 100 mm (2 or 4 in.), and use of fibers. Instrumentation The instrumentation for this project included the use of deflectometers assembled to monitor PCC strains. The instru- mentation was performed before paving operations at 35 of the 41 PCC sections. Sections 1, 16, 25, 34, 45, and 65 were not instrumented. No instrumentation was performed on the HMA sections. At each site, two deflectometers and a thermo- couple were installed. Deflection Testing Deflection testing was performed with falling weight deflec- tometer (FWD) equipment before and after construction of the whitetopping section. Before construction, the existing pavement was tested on the outer wheelpath at 91 m (300 ft) intervals and at the instrumentation locations. After construc- tion, deflection testing was performed at the center of the pan- els located in the outer wheelpath of the instrumented lane. Findings After 2 years of performance, the following conclusions were made (48): • All whitetopping sections have performed well. 0 5 10 15 20 25 30 35 Ju l-9 4 Au g- 94 Se p- 94 O ct -9 4 N ov -9 4 D ec -9 4 Ja n- 95 Fe b- 95 M ar -9 5 Ap r-9 5 M ay -9 5 Ju n- 95 Ju l-9 5 Au g- 95 Se p- 95 O ct -9 5 N ov -9 5 D ec -9 5 Ja n- 96 Fe b- 96 M ar -9 6 Ap r-9 6 M ay -9 6 Ju n- 96 Ju l-9 6 Au g- 96 Se p- 96 O ct -9 6 Date of Survey N um be r o f S pa lls Sec. 10 Sec. 21 Sec. 29 Sec. 43 Sec. 19 Sec. 27 Sec. 31 Sec. 34 FIGURE B5 Iowa Route 21—spalling.

76 • Initial distresses on the 50 mm (2 in.) and 100 mm (4 in.) sections may indicate that 50 mm (2 in.) sections are more affected owing to construction procedures or weaknesses in the base. • Milling seems to enhance bonding and reduce distress for all pavement sections. • The thinnest sections with fibers have showed better performance than did similar sections with no fibers. No results on the laboratory testing of the composite beams, monitoring of the instrumented sections, or results from the deflection testing, were found in the reports reviewed for this project. TENNESSEE AND GEORGIA A number of UTW sections have been constructed in the states of Tennessee and Georgia. The performance of these sections has been observed at regular intervals. Nine UTW projects, built from 1992 to 1994, were selected for pavement condition monitoring at frequent intervals (10). Data on these sections are presented in Table B9. Differences in joint design and performance resulted in the division of Cusick Street into two different sections: Cusick Street (out- side lane) and Cusick Street (inside lane). A similar division for the I-85 weigh station was also warranted owing to dif- ferences in mix design into “approach side” and “leave side.” Location City/County State When Built Joint Spacing [m (ft)] Thickness [mm (in.)] Fibers Observations as of September 2000 Belvoir Ave. and Brainerd Rd. Chattanooga TN Nov. 93 1.5 (5) 64–75 (2.5–3) Yes Still functioning 28th Street Chattanooga TN July 92 1.5 × 1.1 (5 × 3.75)* 64–75 (2.5–3) Yes Most HMA layer was removed during milling. Overlaid with ACC 4 years later Green St. and North Jackson St. Athens TN Jan. 94 0.9 × 1.1 (3 × 3.67)* 64 (2.5) Yes Still performing very well SH 56 McMinnville TN Sep. 93 0.9 (3) 64–75 (2.5–3) Yes No observations Concorde St. and Kingston Pk. Knoxville TN Nov. 92 1.2 × 1.0 (4 × 3.3)* 89–100 (3.5–4) Yes Still in very good condition Cusick St. and Harper Ave. Maryville TN Aug. 93 1.2 × 1.5 (4 × 5)* 65–75 (2.5–3) Yes No observations I-85 Weigh Station Near Lavonia GA May 93 0.6 (2) 50 (2) F & NF No observations Wesley Chapel Rd. DeKalb County GA Sep. 93 1.2 (4) 64 (2.5) Yes No observations Marbut Rd. and Lithonia I. Blvd. DeKalb County GA Sep. 93 1.2 × 1.1 (3.83 × 3.5)* 64–75 (2.5–3) Yes No observations *Transverse—Longitudinal joint spacing. Sources: Cole (10) and J. Norris, personal communication (Information on UTW Projects in Tennessee) to T. Ferragut, Sep. 4, 2000. TABLE B9 TENNESEE AND GEORGIA UTW SECTIONS

77 Construction Procedures Typical construction procedures reported include the follow- ing (10): • All the UTW projects had similar mixture proportions. Typical Tennessee mixture proportions are reported in Table B10. • The existing HMA surface was milled and broomed. • No bonding agent was applied. • Ready-mixed concrete was used and placed with a vibrat- ing screed. • The surface was bull floated and textured with a broom. • White-pigmented curing compound was applied and joints were sawed with early sawing techniques. • Ambient temperature varied from project to project from less than 4.4°C (40°F) to more than 32.2°C (90°F) dur- ing placement. Sherwood (141) provides additional detail on placement temperatures. Traffic Loading Of all the UTW sections, only I-85 has detailed traffic infor- mation available. The traffic counts for the other sections were based on several traffic counts (Green Street, Tennessee SH 56, Concorde Street, and Cusick Street) or estimated from observations (Belvoir Avenue, Wesley Chapel Road, and Marbut Road). Table B11 presents the estimated daily traffic and ESALs for the years of 1995 and 1996 for all the UTW sections, except 28th Street. Data Collected All UTW sections were surveyed using the PAVER System protocol. Cracking condition surveys and pavement condi- tion index (PCI) using the PAVER System protocol were taken for each year from 1995 through 1998. The PAVER System protocol uses a numerical index from 0 (Very Poor) to 100 (Excellent) to rate the condition of the pavement. A summary of the performance of the UTW sections in terms of cracking and the PCI is given in Table B12. A detailed summary of the different types of cracking per section for the years 1995 and 1996 is presented by Cole (10). Findings The following findings were based on the evaluation of the UTW sections: • Corner cracking, linear cracking, and divided slabs are the most prominent distress types. • All UTW sections, except 28th Street in Chattanooga, Tennessee, presented mostly low-severity cracking. The UTW on 28th Street was surveyed after 3 years, showing a poor condition with a PCI of 32. This was attributed to the very poor condition of the existing HMA pavement or absence of this layer according to cores taken in that project. That section was not sur- veyed in later years. • Comparison of the PCI with age shows that the PCI is decreasing with time, as would be expected. • Comparison of the PCI with ESALs shows that PCI is decreasing with increasing ESALs. However, three of the four sections with the lowest PCI also have low ESAL accumulations. The two sections with the highest PCI have the most ESALs accumulated, as is the case for I-85. Good performance of the sections with the most ESALs accumulated may be attributed to the depth of the HMA and the joint spacing, which for I-85 is approx- imately 275 mm (11 in.) and 0.6 m (2 ft), respectively. A more detailed investigation of other factors affecting the performance of those test sections is required. • A significant concentration of the cracking observed occurred near the approach or leave ends of the UTW sections. The higher concentration of cracking at the ends of the UTW sections is potentially attributed to Constituent Dosage Cement 802 lb Coarse Aggregate #7 1,711 lb Fine Aggregate 1,098 lb Water 280 lb Water Cement Ratio 0.35 Fibers 3 lb Superplasticizer 15 oz Air Entrainment 6 oz Source: J. Norris, personal communication (Information on UTW Projects in Tennessee) to T. Ferragut, Sep. 4, 2000. TABLE B10 TYPICAL MIX PROPORTIONING FOR TENNESSEE UTW SECTIONS Project Vehicles (millions) Trucks (thousands) ESALs in 1998 (thousands) Belvoir Ave. and Brainerd Rd. 3.35 134 101 Green St. and North Jackson St. 6.9 414 311 State Highway 56 8.47 508 381 Concorde St. and Kingston Pike 4.40 176 132 Cusick St. and Harper Ave. (outside) 10.05 402 302 Cusick St. and Harper Ave. (inside) 6.98 268 201 I-85 Weigh Station (approach) 0.16 160 650 I-85 Weigh Station (leave) 0.16 160 650 Wesley Chapel Road 9.26 463 347 Marbut Road and Lithonia I. Blvd. 1.22 122 92 Note: From construction date to condition survey date. Source: Cole (78). TABLE B11 ESTIMATED ACCUMULATED TRAFFIC LOADING

78 impact loading, free edge condition, and debonding of the PCC overlay. • It was observed that the sections with the highest per- centage of cracking also have the largest panel size. MISSOURI Several test slabs at the Spirit of St. Louis Airport were instrumented at the time of construction (February 2–3, 1995), so that the behavior of whitetopping overlays could be assessed. This facility is a general aviation airport located in Chesterfield, Missouri. Similar to the experimental work done in Colorado (see the earlier section on Colorado), strain gauges were installed, as were thermocouples for the measurement of critical stresses and strains caused by traffic load and by chang- ing climatic temperatures. One focus of this experiment is measuring the bond strength between the PCC and the HMA. Field Instrumentation and Testing Six slabs were instrumented and load tested at the airport. Slab dimensions were 88 × 1250 × 1250 mm (3.5 × 50 × 50 in.), and polypropylene fibers were used as microreinforcement. Mix proportions are given in Table B13. The joint design was varied in this series of experiments. Three different joints were used. One joint type was a normal contraction joint over an HMA layer with no cracks. Another joint type was designed to simulate the free edge condition (the joint was sawed to the bottom of the HMA layer). The final joint type was designed to simulate isolated test slabs, with all four joints sawed to the bottom of the HMA layer. Location of Strain Gauges Plan views of the embedment strain gauge locations are shown in Figure B6. The embedment strain gauges were placed at the slab centers and joints on the surface of the HMA layer and 1 in. (25 mm) above the surface of the HMA in the PCC. Thermocouples were used to measure the temperature pro- file through the thickness of the whitetopping overlays. Thermocouples were placed at the top, middle, and bottom of the PCC overlay and 12.5 mm (0.5 in.) into the HMA layer. Percentage of Panels with Cracks Pavement Condition Index Location 1995 1996 1997 1998 1995 1996 1997 1998 Belvoir Ave. 13 21 21 ND 90 87 85 80 Green St. 3 8 8 9 98 95 92 89 State Highway 56 9 15 19 19 89 86 85 83 Concorde St. 2 3 4 5 97 96 96 95 Cruisick St. (outside) 15 16 16 16 93 91 89 89 Cruisick St. (inside) 44 51 58 58 75 71 66 66 I-85 Weigh Station (approach) 2 2 3 4 98 98 98 97 I-85 Weigh Station (leave) 7 7 9 20 95 94 95 74 Wesley Chapel Rd. 7 11 12 ND 96 92 91 ND Marbut Rd. 4 7 8 10 95 91 89 88 ND = no data. Source: Cole (78). TABLE B12 CONDITION OF UTW TENNESSEE AND GEORGIA SECTIONS Strain gauges Joints (a) (b) Quantity Material kg/m3 lb/yd3 Type I Cement 303 510 Class C Fly Ash 47 80 Coarse Aggregate (#57 limestone) 1,115 1,879 Fine Aggregate (sand) 750 1,263 Water 110–113 183–190 Air Entrainment 359 ml/100 kg cement 5.5 oz/cwt Polypropylene Fibers 1.79 3 Low-Range Water Reducer 1108 ml/100 kg cement 17 oz/cwt TABLE B13 MIX PROPORTIONS USED IN THE WHITETOPPING SECTIONS AT THE SPIRIT OF ST. LOUIS AIRPORT FIGURE B6 Plan view of the location of embedment strain gauges: (a) typical pattern; (b) pattern for slabs with four free edges.

79 Interfacial Bond Between PCC and HMA The existing HMA pavement was milled and cleaned by air blasting before PCC placement. Bond strength between the PCC layer and the HMA layer was measured by using both the interface bond strength (direct shear) test and the pull-off test for tensile bond strength. The average value of the inter- facial shear strength is 0.70 MPa (102 psi). Past studies have indicated that bond strength of 0.69 MPa (100 psi) is suffi- cient. The average value of the direct tensile pull-off test was 0.51 MPa (74 psi). Using the data from the strain gauges, the bond between the PCC and the HMA layer can be assessed. The strain at the bottom of the PCC is extrapolated from the readings obtained at the surface and 25 mm (1.0 in.) from the bottom of the PCC layer. This extrapolated strain is compared with the measured strain at the surface of the HMA layer. The dif- ference between these two strains ranges from 7.9 to 8 micro- strain regardless of the joint condition. This finding was inter- preted to be a result of the partial bonding of the PCC layer to the underlying HMA layer at the Spirit of St. Louis Airport. Slab Surface Profile A Dipstick profiler was used the morning after construction to record relative elevations. A plot of relative elevation ver- sus diagonal distance for the slab with no free edges is shown in Figure B7. To obtain the degree of curling, a linear regression of the data was first performed to ascertain the slope of the slab (constructed for drainage). Then this linear regression was subtracted from the elevation data to obtain the difference. Because this difference contains components of slab curling, construction roughness, and interaction with adjoining slabs, it is difficult to extract only the curling profile. A quadratic regression was performed and the difference in relative ele- vations between the diagonal corner and the slab center was 0.89 mm (0.035 in.). A similar analysis was performed on the whitetopping slabs having one free edge and four free edges; the extracted amount of curling was 0.79 mm (0.031 in.) and 0.64 mm (0.025 in.), respectively. This analysis showed that these slabs had a curled-downward profile. Curling of the slabs was measured again in May and Sep- tember. It is assumed that the relative movement of the slabs was measured throughout the day by using a Dipstick and the early morning measurements were used as the reference point. The maximum differences in the relative movements ranged from 0.36 to 0.64 mm (0.014 to 0.025 in.) for all the slabs. For a typical pavement with thickness of 250 mm (10 in.), the maximum difference can be 2.5 mm (0.10 in.). Therefore, the whitetopping slabs appear not to be lifting off the HMA layer but are maintaining good contact with the foundation. The short joint spacing contributes to the good contact between the PCC and the HMA layer. Load Testing Before load testing, surface gauges are attached to the PCC surface in the pattern, as shown in Figure B8. Gauges are placed on the unloaded side of the slab for determining load transfer efficiency. Loading was provided by a rental truck having a front-axle load of 16 kN (3.6 kips) and a rear-axle load of 44.5 kN (10 kips) for the May test. For the September test, the rear axle load was increased to 53 kN (11.9 kips). Load positions are also shown in Figure B8. y = 0.0428x 2 - 0.0747x + 0.0848 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Diagonal Distance (m) R el at iv e El ev at io n Relative Elevation data points Linear Regression Difference between data and regression Quadratic Regression FIGURE B7 Plot of relative elevation versus diagonal distance for whitetopping slab with no free edges.

80 The strains in all the gauges for the load testing in May and September are reported by Wu et al. (19). If none of the joints is cut, it was found that the stresses in the middle of the slab are comparable to the ones at the slab edges. It was also found that strain was higher in the pavements with cut joints than in those without free joints. Load transfer at the joint in the whitetopping slabs was also examined. Load transfer was calculated based on stress. The stress on the unloaded side of the slab was divided by the stress on the loaded side of the slab. In May, load transfer was found to be 69% for the normal joints. For the slab with four free joints load transfer was 41%. These joints are assumed not to be completely free. The effect of temperature on load transfer was also examined. In September, load transfer was found to be 39% for the normal joints and 35% for the four free joints. Laboratory Testing Cores of the pavements were taken for the determination of layer thickness. Average thickness of the PCC was 91 mm (3.6 in.), and average of the HMA was 79 mm (3.1 in.). The properties of the PCC were elastic modulus of 23,442 MPa (3.4 × 106 psi), compressive strength of 33 MPa (4,800 psi), indirect tensile strength of 4.24 MPa (615 psi), and unit weight of 2137 kg/m3 (133 lb/ft3). The modulus and unit weight of the PCC are slightly lower than those for normal PCC. For the HMA, the resilient modulus was 5,247 MPa (761 ksi) at 25°C and 12,308 MPa (1.8 x 106 psi) at 5°C. Its indirect ten- sile strength was 1.01 MPa (146 psi), and its unit weight was 2147 kg/m3 (133 lb/ ft3). Findings The original researchers identified several findings as a result of the field testing of the six whitetopping slabs at the Spirit of St. Louis Airport: 1. The whitetopping overlay is only partially bonded to the HMA pavement underneath. The strain at the bottom of the PCC layer and the strain in the HMA are not equal. 2. The whitetopping slabs do not experience significant curling movements daily after construction. 3. Stresses at the center of the slabs and at the joints are comparable in the whitetopping slabs. When the joints are cut for a free edge, the stresses increase. 4. Load transfer at the whitetopping joints was measured. For normal joints, load transfer efficiency is 69%. For free edges, the load transfer decreases to 41%. VIRGINIA Since the 1980s, the FHWA has maintained several pavement test sections made up of HMA at its Accelerated Loading Facility (ALF). In 1998, eight lanes of UTW overlays were placed over a portion of these HMA pavements. The test sec- tions, located at the Turner–Fairbank Highway Research Cen- ter in McLean, Virginia, presented an excellent opportunity for evaluation of a UTW pavement under controlled loading conditions. The experiment resembles a real-world applica- tion of UTW, because the underlying HMA pavements had been previously loaded. The new UTW overlays were placed at varying thicknesses, joint spacings, additions of fibers, and HMA base types (141). Figure B9 includes a schematic of the eight lanes, including the observed crack patterns at the end of loading. During the summer of 2000, the performance of the UTW pavements was evaluated as part of a research effort sponsored Strain gauges Joints Load positions (a) (b) FIGURE B8 Pattern for surface strain gauges and truck loading: (a) typical pattern; (b) pattern for slabs with four free edges. Lane 5 Lane 6 Lane 7 Lane 8 Lane 9 Lane 10 Lane 11 Lane 12 FIGURE B9 Schematic of UTW lanes at the FHWA ALF.

81 by the Innovative Pavement Research Foundation and the FHWA (2). As part of this evaluation, a number of observa- tions were made, and as a result, hypotheses were developed as to the probable modes of failure. Background At the FHWA ALF, the eight UTW test lanes were con- structed at dimensions of 3.7 m (12 ft) wide and 14.6 m (48 ft) long. Half of the lanes were constructed using fiber (polypropylene)-reinforced concrete and the other half with plain concrete. Two different UTW thicknesses and three dif- ferent joint spacings were also constructed. Table B14 in- cludes the test factorial used at the ALF. The UTW was placed as an inlay—on top of HMA that had been milled so that the final UTW surface would match the existing grade. The underlying structure, before milling, consisted of 200 mm (8 in.) of HMA atop 460 mm (18 in.) of unbound crushed aggregate base atop 610 mm (24 in.) of AASHTO A-4 uniform subgrade. Each of the lanes (except for Lanes 6 and 10) was constructed with a different type of HMA. Before the UTW experiment, each of the HMA lanes was loaded with the ALF, and the permanent deformation (rutting) was recorded at periodic intervals. A summary of the HMA types and performance is provided in Table B15. Before construction, resistance strain gauges were installed in the UTW along both the longitudinal and transverse direc- tions at three different depths. For a typical lane, a total of nine longitudinal and three transverse strain gauges were installed. Three extra longitudinal strain gauges were installed in Lane 5. In addition, strain rosettes were installed at the pavement cor- ners at two different depths. Linear variable displacement transducers were also installed to measure deflection at the slab center and at the joint. A schematic of the typical instal- lation locations is given in Figure B10. During the ALF loading, strain and deflection informa- tion was discretely sampled for every sensor at short incre- ments of load applications (usually one set of readings per day of testing). The test load applied was set constant to 55 kN (12.3 kips), except for the first 310,000 load applications on Lane 12 where a load of 44 kN (10 kips) was used. The speed of the ALF was set constant at 16.6 kph (10.3 mph). Each unidirectional pass of the ALF has a duration of approximately 3 s. Sensor sampling rate was approximately one-thousandth (0.001) of a second. Loading of the various lanes is illustrated in Figure B11. As can be seen, the number of load applications applied ranged from 200,000 to more than 1 million. Over the course of the loading, distresses such as cracking progressed and were mon- itored through periodic visual surveys of the conditions. Typical Distresses During the field visit, the project team observed a number of different structural distress types. The primary distress types observed included • Mid-slab transverse cracking, • Mid-slab longitudinal cracking, • Corner cracking, • Joint faulting, and • Spalling. UTW Design Thickness [mm (in.)] Joint Spacing [m (ft)] Fiber Concrete Plain Concrete 1.2 (4) Lane 5 Lane 6 64 (2.5) 0.9 (3) Lane 7 Lane 8 1.8 (6) Lane 9 Lane 10 89 (3.5) 1.2 (4) Lane 11 Lane 12 TABLE B14 LANE ASSIGNMENTS FOR UTW DESIGNS AT THE FHWA ALF Lane Binder Type Mix Type ALF Wheel Passes for 20 mm Total Rut Depth % Rut Depth in HMA at 20 mm Total Rut Depth 5 AC-10 Surface 1,160 82% 6 AC-20 Surface 1,790 97% 7 Styrelf Surface 23,200 63% 8 Novophalt Surface 39,600 32% 9 AC-5 Surface 480 85% 10 AC-20 Surface 1,790 97% 11 AC-5 Base 5,450 82% 12 AC-20 Base 22,100 85% TABLE B15 HMA CHARACTERISTICS AT THE FHWA ALF

82 Figure B12 illustrates a typical transverse crack found at the ALF UTW. Longitudinal cracks were observed to be of a similar appearance. Figure B13 shows a typical corner crack- ing. It should be noted that corner cracks were often observed to occur in tandem (roughly symmetrical about a transverse crack), as is illustrated here. Figure B14 is a photograph of the joint faulting distress. The most significant faulting was along the longitudinal joint, owing to the channelized nature of the loading, although some transverse faulting was observed as well. Finally, Figure B15 shows typical spalling. Most of the observed spalling was of low severity. In most cases, the spalled concrete has remained in place. In some of these fig- ures, the distress has been graphically enhanced for clarity. Summary of Observations At the time of the field visit in 2000, Lanes 6 and 10 were being loaded by the ALF as part of another innovative research effort. However, observations were made of the remaining lanes as follows: Lane 5 Features were as follows: 64-mm (2.5-in.) UTW design thick- ness—1.2 m (4 ft) panels—fibers 194,500 loads—HMA: AC-10 binder, surface mix. Note: Gauges A, B, and C were installed only in Lane 5. LVDT = linear variable displacement transducer. Top View Transverse Section A A’ C C’ B B’ A -A’ B -B’ C- C’ LVDT Strain Gage Rosette Direction of Travel V, W, X I, J, K L, M, N O, Q, S T, U V W X I J K L M N O Q S T U 1 2 3 D D’ A B C D - D’ A B C FIGURE B10 Typical schematic of sensor locations. 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 A p r-9 8 J u l-9 8 N o v-9 8 F e b -9 9 M a y-9 9 A u g -9 9 D e c -9 9 D a t e Cu m ul at iv e Lo ad A pp lic at io ns (in th ou sa nd s) L a n e 1 2 L a n e 1 1 L a n e 9 L a n e 8 L a n e 7 L a n e 6 L a n e 5 L a n e 1 0 FIGURE B11 Loading history of the ALF UTW pavements.

83 The panels in this lane were heavily damaged. With the exception of the southernmost panel, every panel along the loaded column of slabs had some form of cracking. The major- ity of these panels demonstrated corner cracking. Three of the panels also had transverse cracking. A significant degree of joint faulting along the longitudinal joint adjacent to the load was also observed. In the panels adjacent to the loaded column of panels, some additional cracking was observed. It appeared that most of this cracking was reflected across the longitudinal joint from the center-loaded column of panels. Lane 7 Features were as follows: 64-mm (2.5-in.) UTW design thick- ness—0.9 m (3 ft) panels—fibers 283,492 loads—HMA: Styrelf binder, surface mix. The panels in this lane appeared to have no observed dis- tress. A few cracks were observed at the impact zone, where the wheel load first exerts an impact on the pavement at load- ing. No joint faulting was detected either. Overall, this lane appeared to be in excellent condition. Lane 8 Features were as follows: 64-mm (2.5-in.) UTW design thick- ness—0.9 m (3 ft) panels—no fibers 625,838 loads—HMA: Novophalt binder, surface mix. Lane 8 also appeared to be in very good condition. In addition to the cracking found at the impact zone, a few addi- tional cracks were observed. They included tandem corner cracks on the southern end of the lane as well as a corner crack and a partial longitudinal crack on the north end of the lane. In addition to these cracks, five spalls were observed. Each of the spalls occurred at a corner and was approximately FIGURE B12 Typical UTW transverse cracking at the FHWA ALF. FIGURE B13 Typical UTW corner cracking at the FHWA ALF. FIGURE B14 Typical UTW longitudinal faulting at the FHWA ALF. FIGURE B15 Typical UTW deflection spalling at the FHWA ALF.

84 25 to 50 mm (1 to 2 in.) in size. Lane 8 also was observed to have a minor degree of faulting along the longitudinal joint. Lane 9 Features were as follows: 89-mm (3.5-in.) UTW design thick- ness—1.8 m (6 ft) panels—fibers 265,913 loads—HMA: AC-5 binder, surface mix. Lane 9 appeared to be moderately to heavily damaged. Every panel in the loaded column of slabs was cracked. Cor- ner cracks prevailed, with every cracked slab containing at least one of this type. The corner cracks appeared to occur in tandem, mirrored across a transverse joint. In one case, cor- ner cracks occurred on four adjacent panels, mirrored across both the transverse and longitudinal joints. Half of the loaded column of slabs also had longitudinal cracking, and one trans- verse crack was found. A moderate degree of joint faulting was observed along the longitudinal joint. Lane 11 Features were as follows: 89-mm (3.5-in.) UTW design thick- ness—1.2 m (4 ft) panels—fibers 1,071,302 loads—HMA: AC-5 binder, base mix. Minimal damage was observed on this lane. With the exception of the impact zone, only three cracks were noted: one corner crack, one partial longitudinal crack, and one partial transverse crack. However, some joint faulting was observed. Lane 12 Features were as follows: 89-mm (3.5-in.) UTW design thick- ness—1.2 m (4 ft) panels—no fibers 1,071,312 loads—HMA: AC-20 binder, base mix. Lane 12 was also observed to be in good condition. One full and one partial transverse crack were noted on the south- ern end of the lane. No significant faulting was observed. Of those structural distresses observed at the ALF UTW, corner cracking seemed to be the most prevalent, followed by transverse and some longitudinal cracking. Faulting of the longitudinal joint was also observed to be somewhat signifi- cant. However, it should be recognized that the channelized nature of the ALF loading on the UTW pavements does not indicate in-service pavements that are subject to vehicle wan- der. The type of longitudinal faulting observed here would probably not occur to the same degree in those cases. Trans- verse cracking was also found to some degree, as well as some longitudinal cracking. Finally, spalling was found to exist on some of the lanes. Pavement Distress Mechanisms Both during and after the field visit, a review was made of the various types of information collected at the ALF. Included were data on the UTW as well as the underlying HMA support layers, including strain, deflection, and temperature data col- lected during the accelerated loading. In addition, laboratory test data, climatic information, and design and construction records were reviewed. Hypotheses were drafted on the var- ious failure mechanisms occurring at the ALF by comparing results of the observed UTW pavement performance with the available information. A relationship between the observed pavement distress levels and the nature of the underlying HMA was realized from this analysis. Before the construction of the UTW at the ALF, the underlying HMA pavements were loaded by the ALF device as part of an experiment, to characterize perma- nent deformation (rutting) of the different mixes. Figure B16 illustrates the rutting of the HMA material used as the sup- port layer for the UTW. To make a rough quantitative comparison, the degree of cracking observed on each of the lanes was quantified with the use of a cracking index. The cracking index is a weighted average of the number of cracks observed on each lane. The index is a summation of the number of cracks per lane, with FIGURE B16 Typical HMA rutting at the FHWA ALF.

85 a full-panel crack assigned a value of 1.0, a partial-panel crack of 0.5, and a small chip or break of 0.1. That given value is then compared with the rutting that is quantified as the number of ALF load applications to reach 20 mm (0.8 in.) of total rutting. The results of this comparison are shown in Figure B17. Although not given here, a similar relationship was found between the rates of change of ride quality for each lane compared with the rutting susceptibility of the underlying HMA (49). On the basis of these observations, the following sections describe hypotheses developing in regard to the nature of the various structural distresses observed at the ALF UTW. Corner Cracking As mentioned, corner cracking was the most commonly observed structural distress at the ALF UTW. This distress is caused when the fatigue limit is exceeded in the concrete mate- rial. The fatigue limit is commonly defined as a function of the stress-to-strength ratio. In most of the lanes, the strength of the concrete over the period of loading remained approximately constant. However, the stress state in the concrete most likely changed with the number of load applications. The source of such a finding is believed to be the result of a change in the sup- port conditions owing to permanent deformation of the support layers. That effect is shown in Figures B18 and B19. In Figure B18, the action of repeated loading across the slabs leads to a permanent deformation of the HMA beneath the UTW. This void, in turn, causes a cantilever effect that increases the stresses in the concrete at the top surface. Once the fatigue limit is reached in the concrete, the UTW failure comes in the form of a corner crack. This phenomenon is shown in Figure B19. It should be emphasized that if cores were to be removed from the edge, the PCC and HMA materials may appear to be full intact (bonded). A combination of residual tensile stresses, microstructural damage (microcracking), and/or a loss in density of the HMA in proximity to the interface is possible. The result is a weakened area that may lead to a loss of support, even if the interface is not visibly debonded. Therefore, an expression such as “virtual void” may be more appropriate in describing the suspected loss of support. Another possible explanation for the corner cracking is that the cracks may have been initiated on the loaded (longi- tudinal edge) of the lane and propagated diagonally toward the closest intersecting joint with each successive wheel load. This hypothesis is derived from the belief that changes to the stress state are induced by the moving wheel load in the slabs. Mid-Slab Cracking As with corner cracking, mid-slab cracking is caused when the concrete loading exceeds the fatigue limit. As illustrated in Figure B20, one hypothesis is that the mid-slab cracking is initiated at the bottom of the slab. As the wheel load passes directly over the mid-slab, the stresses are highest directly 0 5 10 15 20 25 100 1,000 10,000 100,000 1,000,000 No. of ALF Loads for 20 mm Total Rutting Cr ac ki ng In de x FIGURE B17 Comparison of HMA rutting to UTW cracking. xxσ FIGURE B19 Corner cracking mechanism in a UTW slab. FIGURE B18 Permanent deformation of an HMA base.

86 beneath the load at the edge. These stresses can be further compounded by the presence of a void, or soft area, beneath the slab in the support layer system. A second hypothesis is that the cracks are initiated at the top of the slab, induced by tensile stresses at the top as the wheel load rolls onto the slab in question. Strain gauges instrumented in the slab verify a stress reversal in the top of the slab, indi- cating this effect, as shown in Figure B21. Joint Faulting Faulting was observed along both the longitudinal and trans- verse joints at the ALF UTW. For the longitudinal joints, the mechanism is unique owing to the channelized nature of the wheel loading. Although the ALF has the ability to test with transverse wheel wander, during the testing of the UTW pave- ments, the wheel loads were constrained along the same line. As a result, the high vertical stresses introduced into the sup- port layers resulted in permanent deformation. This effect can be seen in Figure B22. Figure B23 illustrates the hypothesized mechanism for the transverse faulting also observed at the ALF UTW. Under wheel loading, both vertical and shearing forces were intro- duced into the support layer materials. These forces resulted in a vertical and shear deformation of these layers, which, under repeated loading, caused a transverse fault along the joint. Joint Spalling There exist two common types of joint spalling. The first type, sometimes termed “delamination spalling,” is caused by the combined effect of horizontal microcracking intro- duced during the early-age concrete construction and traffic loading that eventually weakens the horizontal crack. The result is a flat-bottom spall. The second common type of spalling is sometimes termed “deflection spalling.” This type of spalling is common on airport pavements where high deflections in the slabs cause a localized crushing of the mate- rial at the joints. Owing to the thin geometry of the UTW slabs (with respect to the heavy loading), deflection spalling is believed to be the governing mechanism. Figure B24 illus- trates this phenomenon. Findings Various failure mechanisms at the ALF UTW have been identified and summarized in this appendix. From observa- tions made after the experimental loading was completed, along with a review of the project information, hypotheses were formed and documented on the various mechanisms leading to the observed distress types. It is concluded that a common element of each of the observed distress types is the permanent deformation char- acteristics of the support layers. The UTW pavements that xxσ FIGURE B20 Mid-slab cracking mechanism in a UTW slab. Note: Sensor is from Lane 5. Refer to Figure 2 for sensor location. (+) = tension. -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 40.00 50.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 Time (sec) St ra in (m e) Sensor L FIGURE B21 Measured strain response of a top mid-slab longitudinal gauge.

87 were constructed on the softer HMA sections (those more prone to rutting) performed more poorly than did those con- structed on stiffer HMA materials. It is also worth noting that the ALF loading in this exper- iment was conducted in a channelized manner along the slab edge. In practice, the loading of the slabs will vary from slab edge to center. However, the slab edge loading is believed to be the critical position, and the majority of design procedures are based on this assumption. xxσ FIGURE B22 Longitudinal joint faulting mechanism in ALF UTW slabs. FIGURE B23 Transverse joint faulting mechanism in ALF UTW slabs. FIGURE B24 Joint spalling mechanism in ALF.