National Academies Press: OpenBook

Guide for Conducting Forensic Investigations of Highway Pavements (2013)

Chapter: Chapter 5 - Non-Destructive Testing

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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
×
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Suggested Citation:"Chapter 5 - Non-Destructive Testing." National Academies of Sciences, Engineering, and Medicine. 2013. Guide for Conducting Forensic Investigations of Highway Pavements. Washington, DC: The National Academies Press. doi: 10.17226/22507.
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29 This chapter discusses the implementation of the investi- gation plan described in Chapter 4 as well as analysis of the non-destructive testing data, the preparation of an interim report, and the decision to continue or terminate the study based on the findings at this stage of the investigation. 5.1 Implementing the Initial Investigation Plan Conducting the NDT identified in the initial investiga- tion plan should begin as soon as possible after the pre- investigation site visit. The testing procedures (i.e., setup and operating the equipment) recommended by the equipment manufacturer should be followed at all times. 5.2 Non-Destructive Testing Analysis Interpretation of NDT results will depend on the issues being investigated. Although analysis and interpretation of the results of some non-destructive tests are relatively straightforward (e.g., profile), others (e.g., GPR and FWD) require considerable experience with analysis methods and data interpretation, which may necessitate additions to the investigation team at this stage. The following sections dis- cuss the key issues to consider when analyzing results from the various types of equipment and when considering the need for additional destructive testing to address the issues being investigated. During analysis, there is a need not only to focus on the issues being investigated, but also to recognize the possible influence of other factors. Consider the following during analysis and interpretation: • If the results provide a well supported explanation of the issues being investigated, no additional testing will be required. • If the results are inconclusive, determine what addi- tional non-destructive or destructive testing is required to explain the issues, and use available results to identify/ delineate areas where this additional investigation needs to be carried out (e.g., locations for coring, Dynamic Cone Penetrometer [DCP], or test pits). 5.2.1 Ground Penetrating Radar The interpretation of GPR data is complex and requires considerable expertise, training, and experience. The forensic investigation coordinator/team leader will need to identify an individual within the agency or engage the services of a specialist to assist with this interpretation. The following key issues should, however, be considered: • Use the radargram (example in Figure 5.1) or other GPR output to assess layer thickness and changes in construc- tion as a means for identifying specific locations where cores need to be taken to validate observations. Thickness/ depth values may not be accurate without calibration (from cores), but the visual presentation is useful for assessing variation in thickness. Highlight any problem/anomalous areas. • Use the amplitude analysis to identify and delineate prob- lem areas such as delamination, debonding, stripping, or voids. Figure 5.2 shows an example of a void under con- crete slabs in a jointed plain concrete pavement. Conclu- sive evidence is unlikely, but sufficient resolution should be available to select points where cores can be taken to validate the observation. • Use the frequency analysis to identify and delineate changes in moisture content and moisture-related problems such as stripping. Figure 5.3 shows an example of an area within the pavement that has higher moisture content than that of the surrounding materials. This area could be investigated C h a p t e r 5 Non-Destructive Testing

30 Distance Depth Time Figure 5.1. Example GPR radargram showing layer depth. Distance Depth Time Figure 5.2. Example amplitude analysis showing a void under JPCP.

in more detail for stripping. While conclusive evidence is unlikely, sufficient resolution should be available to select locations where cores can be taken or a DCP can be driven to validate the observation. • Link the GPR observations to the results from other NDT (e.g., FWD), and to the observations from the initial visual assessment to determine if the issues being investigated can be explained. 5.2.2 Falling Weight Deflectometer (FWD) A number of commercial software programs have been developed for FWD interpretation. No recommendations are provided in this guide on the suitability of any software pack- age for specific issue analysis. Use the following procedure for analyzing pavement structure issues recognizing that accurate surfacing and base layer thicknesses are required to obtain reasonable backcalculated values: • Study the as-built data and layer thicknesses determined from GPR or core measurements to provide a baseline for interpreting the FWD data (deflections and backcal- culated stiffnesses depend on the stiffness and thickness of the pavement layers and subgrade). Deflection moduli and backcalculated stiffnesses (and in some cases the raw deflections or indices based on raw deflections) can be used to identify weak or damaged layers by comparing expected values with measured values, or by comparing values in areas with good performance to those in areas with poor performance. 31 • Select the sensor or sensors that will be used in the analysis. – Deflections from the geophone directly under the load provide an indication of overall pavement structure including the subgrade. The furthest sensors from the load provide an indication of subgrade response with little influence from the pavement structure. The middle sensors provide deflection data on the layers between the surface and the subgrade. – The sensor used for assessing a specific layer’s response will depend on the total thickness of the pavement struc- ture, thickness of individual layers, and the layer type (e.g., cement stabilized or aggregate base). In general, a sensor is affected by pavement layers at depths greater than the distance of that sensor from the load. For exam- ple, a sensor located 2 ft (600 mm) from the center of the load will be affected by pavement layers that are 2 ft (600 mm) or more below the pavement surface. • Plot the measured deflections or calculated parameter (e.g., stiffness, modulus, deflection modulus, etc.) against dis- tance for the length of the project (example in Figure 5.4). Contour plots and cumulative-sum plots (1986 AASHTO Guide for Design of Pavement Structures [9]) are also useful for analyzing deflection data. • Use the plots to: – Assess the spatial variability of the deflection data in both the longitudinal and transverse directions and determine the uniformity of the pavement structure and subgrade stiffness. – Identify unique sections, weak/problem areas, or anom- alous areas. Distance Depth Frequency (MHz) Figure 5.3. Example frequency analysis showing areas of high moisture content. (Dark areas indicate higher moisture content.)

32 – Link weak/problem areas to the issues being investigated (or confirm exceptional performance). For example, areas with debonded asphalt concrete layers will typical- ly have higher deflections than areas with no debonding because the layers act individually and not as a mono- lithic single layer. – Determine whether sufficient information has been collected to address the issue being investigated and, if not, identify areas requiring additional investigation. For example, the deflection modulus calculated (from FWD Sensor 6) in an investigation of suspected sub- grade failures shown in Figure 5.4 reveals variability in subgrade stiffness in the area under investigation. Sec- tions A, C, and F have a stiffer subgrade, Sections B and D are less stiff and Section E (within Section D) is soft. Sections B, D, and E coincided with areas of pavement failure (alligator cracking). • Determine the stiffness (layer moduli) of the pavement and subgrade layers using an appropriate backcalculation method (e.g., layered elastic solutions; non-linear, finite element analysis; or dynamic solutions). Remember to take temperature and moisture conditions into consideration. • Determine the overall structural capacity of the pavement using an appropriate backcalculation method (e.g., Bur- mister two-layer solutions [equivalent pavement thickness having standard modulus and subgrade modulus]). • On asphalt pavements, compare the backcalculated stiff- ness results against expected values for different material types. Expected values will vary depending on the pavement design, pavement structure, age of the asphalt, performance grade of the asphalt, compaction, etc. Modulus ranges for different layer types, based on the authors’ experience, are listed in Table 5.1. – Identify problem areas or problem layers (i.e., lower than typical values) that could be contributing to the issues being investigated. For example, early rutting and fatigue cracking could be attributed to stripped asphalt layers, debonding of asphalt layers, weak sta- bilized layers, or saturated subgrade layers that can be identified from the deflection and backcalculated data. – Alternatively, use the data to explain observed good performance if the stiffnesses are higher than typically experienced and justify new approaches to design or construction (e.g., stricter compaction requirements, better drainage, different stabilization methods, etc.). • On jointed concrete pavements, calculate the load transfer efficiency (LTE) at mid-slab or wheelpath joints, working cracks and mid-slab edges for tied shoulders. – The load transfer efficiency can use either the simple definition of LTE (dunloaded/dloaded where d is deflection on the loaded slab and the unloaded slab on the other side of the joint) or Westergaard’s equation. It is important to note the method used because these methods give different values. – Lower than typical load transfer would explain faulting and corner cracks, especially if the pavement has an erod- ible (unstabilized) base and is in an area of high rainfall. – For a set of joints (or transverse cracks if measured), LTE will likely increase as the temperature of the slabs increases. Check the results by plotting LTE versus 0 10 20 30 40 50 60 70 80 90 100 0 20 0 40 0 60 0 80 0 1,0 00 1,2 00 1,4 00 1,6 00 1,8 00 2,0 00 2,2 00 2,4 00 2,6 00 2,8 00 3,0 00 3,2 00 3,4 00 3,6 00 3,8 00 4,0 00 4,2 00 4,4 00 4,6 00 4,8 00 5,0 00 Distance (m) (1m = 3.048 ft) D ef le ct io n M od ul us (M Pa ) Section A Sect. B Section C Section D Section F Section E Sections A, C & F: >45MPa, satisfactory subgrade strength Sections B & D: 25MPa - 45MPa, marginal subgrade strength Section E: <25MPa, weak subgrade Figure 5.4. Example plot of subgrade deflection modulus against distance.

33 surface temperature to determine if temperature is controlling the results. If it is, comparing the surface temperatures at the time of testing against surface tem- peratures across the year will help determine if the LTE results are representative of high or average tempera- tures. General ranges for LTE are: 77 Excellent — 90 to 100 percent (by the simple defini- tion) 77 Good — 80 to 90 percent 77 LTE contributing to faulting/pumping — 50 to 80 percent 77 LTE likely resulting in faulting/pumping — less than 50 percent – If LTE is low, identify potential causes and, if necessary, identify core or DCP locations to confirm these reasons and suggest corrective measures. Low LTE values could result from absence of dowels, or corroded, missing, or misplaced dowels. 77 LTE less than 50 percent usually occurs only when there are no dowels, dowels have become corroded or have become loosened due to high bearing stresses between the dowel and surrounding concrete under loading. 77 Loss of aggregate interlock (because of shrinkage or traffic damage) or voids under the corners can also reduce LTE. 77 Coring will confirm the presence and condition of dowels and voids, and the condition of the base (e.g., degradation of a cement-treated base). 77 The presence of chlorides may contribute to dowel corrosion, which can be determined by measuring the chloride content in cores. • If investigating corner, mid-slab or wheelpath cracks, or mid-slab edge deflections on JPCP, the software should report vertical deflections at these locations. – These deflections will typically be larger when the tem- perature difference between the pavement surface and the bottom of the slab is greatest, usually in the early morning. They will be smallest in the late afternoon and early evening when the surface is much hotter than the bottom of the slab. Under these conditions, the highest deflections may be an indication of voids beneath the corners or mid-slab edge. Interpret the results relative to temperatures over the rest of the year. – The effects of temperature gradient will be less pro- nounced for joints tested away from the corners and mid-slab edges than for corners. – A high joint deflection difference (the absolute value of the difference in deflections across a loaded joint) can provide an indication of potential for faulting. – Reasons for high deflection differences associated with faulting and corner breaks can sometimes be deter- mined from cores (e.g., visual observation to check for evidence of disintegrated or eroded base, thinner than design thickness, etc.). • If investigating cracking on concrete pavements, use back- calculation software that provides accurate k-values for a mechanistic-empirical evaluation (note that good layer thickness information is necessary for estimating stiffness values from backcalculation). – Premature cracking, if not related to overloading or overtrafficking, can be due to poor support of the slab as manifested by a low k-value, or low concrete strength. Layer Type Modulus Range1 Lower Bound Upper Bound psi MPa psi MPa Portland cement concrete Asphalt concrete FDR2 + cement FDR + foamed asphalt FDR + asphalt emulsion FDR/no stabilizer PDR3 + emulsion Asphalt-treated base Asphalt emulsion base Cement treated base4 Lean concrete base Aggregate base Granular subgrade Fine-grained subgrade 2,200,000 100,000 80,000 50,000 50,000 40,000 80,000 100,000 50,000 - 1,500,000 15,000 10,000 5,000 15,000 700 550 350 350 275 550 700 350 - 10,000 105 70 35 7,000,000 1,000,000 800,000 600,000 600,000 150,000 800,000 900,000 500,000 - 5,500,000 50,000 50,000 50,000 50,000 7,000 5,500 4,100 4,100 1,035 5,500 6,750 3,500 - 40,000 350 350 350 1 Ranges are highly dependent on test temperatures. 2 Full-depth reclaimed. 3 Partial-depth reclaimed/cold in-place. 4 Modulus range depends on the level of cracking. Table 5.1. Example modulus ranges for different layer types.

34 – The software should estimate the elastic modulus of the slab and the k-value of the combined underlying layers, as a minimum. Backcalculate as a two-layer system consisting of the concrete and the underlying layers. Consider an alternative two-layer system where there is a cement- or asphalt-treated base because thin cemented layers are difficult to separate from the con- crete slabs in backcalculation. Combine the concrete slab and base and keep the remaining underlying lay- ers as the second layer. – Check modulus values for reasonableness. Lower than expected values for the concrete layer (e.g., <2,200 ksi [15 GPa]) may be an indication of voids beneath the concrete or internal problems in the concrete. – If the backcalculated concrete modulus is low, plan to take cores to test for compressive strength. Modulus and compressive strength are usually correlated. Typi- cal modulus values generally range between 2,200 and 7,000 ksi (15 and 50 GPa). – The k-values of the underlying layers are an important indicator of the support being provided to the concrete by the base/subbase layers and subgrade. Use the same set of layers in the forward M-E analysis software to determine if the support to the slab would have a sig- nificant effect on expected performance. – Bonding in the vertical direction between the concrete and base layers can play an important role in cracking performance. If stiffness and k-value are as expected, then coring to determine bonding between slab and base can be performed to identify whether this is con- tributing to the issue being investigated. Good bonding contributes to long-term fatigue performance, although high friction in the horizontal direction can contribute to cracking shortly after construction. • If FWD testing was carried out in conjunction with GPR testing, compare data sets to refine the analysis discussed above. Bonding issues, voids, problem layers, etc., can be better identified through the combined use of the two techniques. • If the issues being investigated cannot be satisfactorily explained from the FWD data, use the plots to identify areas/locations for additional observation and testing (e.g., a more intensive visual assessment, coring, and/or a test pit or trench). 5.2.3 Profilometer Use the following procedure for analyzing roughness issues: • Compare the IRI values for the pavement sections investi- gated against the limits in use by the agency. IRI provides a single value that reflects the overall roughness of the segment and is useful in comparing relative roughness as well as in tracking changes in roughness over time. • Apply a high pass filter with a base length between 25 and 100 ft (8.0 and 30 m) to remove noise and obtain more detailed information regarding the nature of the rough- ness for each segment (example in Figure 5.5a and b). The FHWA ProVAL software (10) can be used for this and other data analyses. • Determine if high IRI values occur consistently over the entire project (e.g., due to poor asphalt paving, poor joints on concrete pavements, raveling), or at localized areas (e.g., pothole, example in Figure 5.5c). • Compare results with pavement management system data to determine if performance is typical of other roads with similar characteristics. • If the data shows consistently rough and worse than expected pavement when compared to the network, review the as-built records to better understand construction-related problems (e.g., subgrade issues) and the visual assessment notes to determine if raveling is a contributing factor. In the latter case, review the mix design and as-built records for deviations from the norm (e.g., binder content, choice of binder, temperature issues on asphalt, tining and grinding issues on concrete, evidence of subgrade heave, etc.). – On asphalt pavements, review the visual assessment notes to determine if the roughness appears to be sur- facing related (e.g., end-of-load segregation) or sub- grade (e.g., clay or frost heaving) related. If subgrade issues are the likely cause, compare results with FWD test data to identify weak or wet subgrade areas. Review design documentation for subgrade plasticity, frost design, local knowledge of sulfate-related problems, etc. – If high roughness occurs in isolated areas, compare results to the as-built records and visual assessment notes to determine the cause (e.g., potholes, transverse cracks, construction joints, slab joint faulting, asphalt pick-up of spills, raveling associated with an equipment breakdown, supply trucks standing for long periods, etc.). – On jointed concrete pavements, measure fault heights at transverse joints and cracks using ProVAL or analysis software provided by the profilometer equipment sup- plier following the AASHTO R36 specification (Standard Practice for Evaluating Faulting of Concrete Pavements). The data should not be filtered, and the data collection interval must be between 0.75 and 1.5 in. (19 and 38 mm). – If no satisfactory explanation is found, consider a more detailed visual assessment to check problem areas and laboratory tests to check material properties in the affected areas. Subgrade problems may require Shelby tube samples or a test pit investigation if no satisfactory explanation can be found.

35 5.2.4 Skid Resistance/Friction Use the following procedure for analyzing skid resistance/ friction issues: • Compare the friction values for the investigated pavement sections to the friction index in use by the agency. Data plots of friction against distance are useful for identifying problem areas or areas of better-than-expected perfor- mance (Figure 5.6, for example, shows difference in fric- tion values for two adjacent lanes). • Delineate sections on the project that fall within accept- able, investigatory, or intervention range. • Compare results with pavement management system data to determine if performance is typical of other roads with similar characteristics. Visit 04 Left Elevation (in) 0 100 200 300 400 500 Distance (ft) -.5 0 .5 -.5 0 .5 -.5 0 .5 Visit 07 Visit 11 a) Unfiltered Profile Data 170 172 174 176 178 180 Distance (ft) -.6 -.4 -.2 0 .2 Left Elevation (in) Visit 04 Visit 07 Visit 11 0 100 200 300 400 500 Distance (ft) 0 50 100 150 200 250 Left Roughness Profile (in/mi) b) Filtered Profile Data c) Section Roughness Profile Figure 5.5. Example profile data. 0 20 40 60 80 100 0 50 100 150 200 250 300 Test Point Sk id N um be r ( SN ) Lane-1 Lane-2 Figure 5.6. Example plots of skid resistance against distance.

36 • If the comparison indicates underperformance or better- than-expected performance, review the mix design and as-built records to determine whether aggregate selection (asphalt, surface treatment, and concrete), surface textur- ing (concrete), or other problems (e.g., slow break on a fog spray or other surface treatment) were noted. • If no satisfactory explanation is found, consider a more detailed visual assessment to check surface texture and laboratory tests to check polished stone values. 5.2.5 Tire-Pavement Noise at the Source Use the following procedure for analyzing noise-related issues: • Plot the OBSI measurements against distance. Several sec- tions can be placed within the area of interest for the inves- tigation to serve as controls, comparisons, or replicates. • Check that all measurements were taken at the same speed. OBSI is dependent on the vehicle speed with most test- ing done at 60 mph (97 km/h) on highways or 35 mph (55 km/h) on lower speed routes. • Apply corrections identified in the test method. Apply a tire correction if different tires have been used, with the correction based on OBSI testing at the same time on the same sections with the different tires. Note that each indi- vidual tire will have different sound intensity response on a given pavement section, even if they are the same type (e.g., the Standard Reference Test Tire [SRTT]). • Analyze tire/pavement noise in terms of overall OBSI, or by frequency in terms of 1/3 octave band frequencies (example in Figure 5.7 shows OBSI for several mixes of different ages plotted by frequency). • Evaluate tire/pavement noise. Humans can typically only identify changes in noise of 2 to 3 dBA or greater. Most pave- ments surfaces have overall OBSI between 95 and 115 dBA with an SRTT tire at 60 mph (97 km/h). The tire/pavement noise level is highly dependent on the tire, with more aggres- sive tread patterns, such as snow tires, causing more noise. • If tire/pavement noise is higher or lower than anticipated, then identify potential sources, in conjunction with visual assessment notes, contributing to the noise. – On dry asphalt and chip-sealed pavements, the major contributor to tire/pavement noise at low frequencies is raveling, accentuated by increasing maximum aggre- gate size, which can be evaluated using a macro-texture measurement (see Section 5.2.6) or visual condition sur- vey (see Section 4.2.1). Distresses such as cracking and high roughness can also increase tire/pavement noise. At high frequencies the major contributor to tire/pavement noise is low air permeability. Open-graded asphalt mixes that are noisy may have been over-compacted or become 70 75 80 85 90 95 100 105 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 1/3 Octave Band Analysis So un d In te ns ity L ev el s, dB (A ) less than a year old RAC-O (QP-41) 1-4 years old RAC-O (06- N467) older than 4 years OGAC (QP-23) less than a year RAC-G (QP-26) (ES-13) 1-4 years old RAC-G older than 4 years DGAC (QP-11) RAC-O – Open-graded rubberized asphalt concrete OGAC – Open-graded asphalt concrete RAC-G – Gap-graded asphalt concrete DGAC – Dense-graded asphalt concrete Figure 5.7. Example plot of OBSI against one-third octave frequencies.

37 clogged and should be checked for permeability in the wheelpath (see Section 5.2.7). Bleeding and water on the road may also contribute to noise through the sound of the tire sticking to the asphalt on the surface or the water being squeezed out from under the tire. – Good performance on asphalt pavements is typically attributed to the aggregate grading and the use of rub- ber or other modified binders. – On dry concrete surfaced pavement, the major con- tributors to tire/pavement noise are the original texture applied to the concrete surface and subsequent surface abrasion that may leave stones protruding from the sur- face. In general, transverse tined concrete is the noisiest of the different types of concrete pavement surface tex- ture. Measurements should be made on other concrete pavement sections with the same nominal texture to determine if the pavement section under investigation is noisier than normal. Concrete pavement textures can be measured using a scanning texture meter (see Sec- tion 5.2.6) to determine if the section under investiga- tion has the same texture as other sections in the inves- tigation or other pavements. – Determine if chains and studded tires are contributors, as these can significantly increase the tire/pavement noise in a very short period of time on both asphalt and concrete surfaces. 5.2.6 Texture Meter Use the following procedure for analyzing texture-related issues. Example reference standards include ASTM E1845, Standard Practice for Calculating Pavement Macrotexture Mean Profile Depth. • Mean profile depth (MPD) can be calculated from vehicle- mounted laser profilometer data measurements, from laser scanning instrument data obtained from a scan of the pavement surface in the field, or on a core brought to the laboratory. • If data is not analyzed and downloaded by the testing equip- ment, use software provided by the equipment supplier to produce an MPD statistic. For laser profilometer data, cal- culate MPD for the pavement length of interest. For laser scanning instrument data, the calculation is for the scan area (usually on the order of 4.0 by 2.5 in. [100 by 60 mm]). • Plot the data over distance or area. • MPD for pavement surfaces typically ranges from 400 to 2,500 microns for asphalt surfaces or non-directionally textured concrete surfaces. MPD does not have mean- ing for directionally textured pavement surfaces, such as tined or grooved pavement. High MPD generally indicates greater raveling or more stones protruding from the pave- ment surface. 5.2.7 Permeameter Use the following procedure for analyzing permeability issues: • Compare the permeability values for the pavement sec- tions being investigated against better performing areas on the pavement, or against agency standards. A permeability of 0.08 to 0.4 in./s (0.2 to 1.0 cm/s) is typical of new open- graded asphalt friction coarse surfacings, while a perme- ability of less than 0.04 in./s (0.1 cm/s) is typical of older, clogged surfacings (based on use of the NCAT falling head permeameter, commonly used for testing permeability on asphalt concrete surfaces). Note that very different perme- ability results can be obtained if a different type of perme- ameter is used, such as a constant head device (e.g., ASTM C1701), and that differences within each type of device (falling head, constant head) depend on the characteristics of the device and on the test method. • If the permeability is unacceptable, examine the pavement closely to determine the causes for the lower permeability. Problems could include clogging (e.g., fines washed from the side of the road, windblown fines, material spillages, organic matter from roadside activities, mud from agricul- tural vehicles, incorrect maintenance activities, etc.), bleed- ing (incorrect binder content), or poorly connected voids. • If the cause is not clear, consider removing cores from affected and unaffected areas to determine whether the problem can be attributed to the mix design (i.e., incor- rect binder content and/or aggregate grading). Dry cores (air cooled) are preferable to prevent contamination, but if wet cores are taken, ensure that coring slurry and debris are flushed from the cores to prevent clogging. 5.2.8 Magnetic Tomography Technology Use the following procedure for analyzing dowel bar place- ment and alignment issues: • Check conformity of the number, size, and location of all dowel bars with the design requirements. • Check that the dowel bars are at the correct depth, correct spacing (distance between dowels), and have equal length on both sides of the joint. • Check that the dowels are correctly aligned (parallel to direction of traffic and parallel to the surface). • Note that scans are usually sufficient to identify any prob- lems, but some core examination may be required to verify the observations.

38 5.3 Interim Report • Analysis and interpretation – Summarizes non-destructive testing (and limited cor- ing) data interpretation in terms of answering the inves- tigation questions. • Findings/conclusion – Determines whether or not the issues have been ade- quately addressed. • Decision – Documents decision to (1) terminate the study or (2) continue with additional (e.g., destructive) testing. If continuation of the study is proposed, provide a justifi- cation for the additional testing. An example cover sheet for the interim report is provided in Appendix C (example Form #12). 5.4 Decision to Continue or Terminate the Study A decision to continue with or end the study is made at this point. If the team concludes that the investigation issues have been satisfactorily addressed, the interim report becomes the final report (discussed in Section 8.2) and recommended actions based on the findings are prepared (discussed in Sec- tion 9). A record of decision (Section 8.3) is prepared and the project closed (Section 9.3). If the information collected does not satisfactorily address the issues being investigated, refine the investigation plan to include the work required to collect additional information and proceed as described in Chapter 6. Interim reports are often not completed by state highway agencies, but are encouraged in this guide to ensure that studies are adequately documented, that appropriate actions are taken, and to prevent recurrences of the problem. In the event that a study is terminated, the interim report becomes the final report. An interim report is prepared at this point to document the findings and support the decision to either (1) end the study (i.e., sufficient information has been collected from this phase of the investigation to address the issues being investi- gated), or (2) continue the study with more detailed investi- gations. Include the following in the report: • Introduction – Lists the reasons for doing the investigation. • Objectives and hypothesis – Lists the issues being investigated and the potential rea- sons (hypothesis) for the issues. • Investigation plan • Observations and measurements – Provides tables of key observations and measurements from the initial site visit and non-destructive testing that support the findings.

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TRB’s National Cooperative Highway Research Program Report 747: Guide for Conducting Forensic Investigations of Highway Pavements explores a process for conducting forensic investigations of pavements that is designed to help understand the reasons behind premature failures or exceptionally good performance. The process also allows for the collection of data for use in developing or calibrating performance-prediction models.

The report includes example forms and checklists for use during the conduct of an investigation. These forms can be modified to suit the particular requirements and procedures for the agency. The example forms are included with the print version of the report in CD-ROM format.

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