LTPP Data Analysis: Influence of Design and Construction Features on the Response and Performance of New Flexible and Rigid Pavements (2005)

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1 EXECUTIVE SUMMARY This report for the project “LTPP Data Analysis: Influence of Design and Construction Features on the Response and Performance of New Flexible and Rigid Pavements” [NCHRP 20- 50 (10/16)] contains the background information, experiment status, data availability, results from analyses, and the conclusions for Specific Pavement Study-1 (SPS-1), Specific Pavement Study-2 (SPS-2) and Specific Pavement Study-8 (SPS-8) experiments of the long term pavement performance (LTPP) program. This research was conducted to evaluate the relative influence of structural and site factors on the performance of new flexible and rigid pavements, based on LTPP NIMS data (Release 17 of DataPave) for SPS-1 and SPS-2 experiments. The effects of environmental factors, in absence of heavy traffic, were also studied based on the LTPP NIMS data for the SPS- 8 experiment. The SPS-1 experiment was designed to investigate the effects of hot-mix asphalt (HMA) layer thickness, base type, base thickness, and drainage on flexible pavement performance, while the SPS-2 experiment is aimed at studying the effects of Portland cement concrete (PCC) slab thickness, base type, PCC flexural strength, drainage, and lane width on jointed plain concrete pavement (JPCP) performance. In this report, a detailed description of the experiment designs and their current status are presented. A summary of data availability, extent, and occurrence of distresses in the test sections within each of the experiments are also included. A brief description of each analyses method and a synopsis of the salient findings from all the analysis are also presented. A summary of findings from a comprehensive evaluation of all the experiments, based on “mid- term” performance trends, is followed by a discussion on the limitations of the findings from this research and recommendations for future data collection and research. Only two sites each are located in Dry Freeze (DF) and Dry No Freeze (DNF) zones for the SPS-1 experiment. A total of three sites are located in DF zone while two sites are located in DNF zone for the SPS-2 experiment. In light of this, the research team chose not to draw conclusions on the effects of these climates based on analyses of test sections located in these zones. However, the data from test sections in these climates were used for analyzing the effects of design factors.

2 Although most of the findings support the existing understanding of pavement performance, the results from this study provide a systematic outline of the interactions between design and site factors, as well as new insights on various design options. In addition, the analysis methodology outlined in this research will be useful for future data analysis. A detailed discussion of the effects of the experimental factors on pavement performance and response can be found in the concluding portion of the report (Chapter 8). A brief summary of the main findings regarding pavement performance from this study follows. Effects of Design and Site Factors on Pavement Performance— SPS-1 Experiment One of the main purposes behind the SPS-1 experiment is to investigate the interaction effects of key design and site factors on pavement performance. For this the test sections were constructed in various site conditions (soil-climate combinations). The “mid-term” assessment of the performance of the test sections in the SPS-1 experiment has thus highlighted some of these effects. Fatigue performance All the experimental factors were found to be affecting fatigue cracking, though not at the same level. Of the design factors in the experiment, base type has the greatest influence on the fatigue performance of flexible pavements, especially when built with in-pavement drainage. Pavements with ATB have shown the best performance. The interaction effects among design factors and between design and site factors are presented below. • Among un-drained pavements, on average, an increase in HMA surface thickness from 4- inch (102 mm) to 7-inch (178 mm) has a slightly higher effect on fatigue cracking for pavements with DGAB than for pavements with ATB. However, this effect is not statistically significant. • On the whole, pavements with “thin” 4-inch (102 mm) HMA surface layer have shown more fatigue cracking than those with “thick” 7-inch (178 mm) HMA surface layer. This main effect of HMA surface thickness is more significant for sections built on coarse-grained soils. • Among pavements built on fine-grained soils, the effect of drainage is seen only in those sections with DGAB; i.e., those with drainage have less fatigue cracking than those without

3 drainage. For drained pavements built on fine-grained soils, those with 8-inch (203 mm) base have more cracking than those with 12-inch (305 mm) and 16-inch (406 mm) base. Hence, for pavements built on fine-grained soils, drainage improves the fatigue performance, especially if built with thicker bases. • The main effect of HMA thickness, discussed above, is mainly seen among sections located in WNF zone. This may be an indication that an increase of HMA thickness from 4-inch (102 mm) to 7-inch (178 mm) is not sufficient in resisting fatigue cracking for pavements in WF zone as compared to WNF zone. • Among sections located in the WF zone, those with DGAB have shown the highest amount of cracking while those with ATB have the least cracking. In addition, those with 16-inch (406 mm) drained base have the least amount of fatigue cracking. This suggests that among pavements located in WF zone, “thick” 16-inch (406 mm) treated bases with drainage are less prone to cracking. The effects of HMA thickness and base thickness discussed above imply that, among sections located in WF zone, an increase in base thickness to 16-inch (with drainage) has a greater impact than an increase in HMA thickness from 4-inch (102 mm) to 7-inch (178 mm), suggesting that using a thicker base with drainage helps in reducing frost effects. Structural rutting performance The extent of structural rutting among the test sections in the SPS-1 experiment is 6.5 mm, on average, with a standard deviation of 2.4 mm. Their average age is about 7 years with a range between 4.5 and 10 years. The amount of rutting for the majority of these sections is within the normal range at this point in time. Therefore, the results at this point may only show initial trends. The interaction effects between design and site factors are presented below. • Among the pavements built on coarse-grained soils, those with 7-inch (178 mm) HMA surface have shown slightly less rutting than those with 4-inch (102 mm) HMA surface. However, this effect is not operationally significant at this point. This effect suggests that for sections built on fine-grained soils an increase in HMA thickness from 4-inch (102 mm) to 7- inch (178 mm) may not be sufficient in reducing the amount of rutting.

4 • Among pavements built on fine-grained soils, a marginal positive effect of drainage is seen in sections with ATB. • Among drained pavements located in WF zone, those with DGAB have shown more rutting than those with ATB. Also, among sections located in WF zone and built with ATB, those with drainage have shown significantly less rutting than those without drainage. This implies that, among pavements located in WF zone, those with ATB and drainage perform better than those with other combinations of base type and drainage. • Among un-drained sections located in WNF zone, those with 12-inch (305 mm) base have less rutting than those with 8-inch (203 mm) base. For sections built on DGAB and located in WNF zone, those with drainage have shown slightly less rutting than those without drainage. This effect was found to be marginally significant. These early trends imply that the importance of drainage among pavements with DGAB is considerable in improving rut performance among sections located in WNF zone. On the other hand an increase in base thickness from 8-inch (203 mm) to 12-inch (305 mm) improves rut performance for un- drained sections, irrespective of base type. Roughness (IRI) All the experimental factors were found to be affecting roughness, though not at the same level. Of the design factors in the experiment, base type has the greatest influence on the change in roughness of flexible pavements, especially when built on fine-grained soils. Pavements with ATB have shown the best performance, while DGAB has contributed to the worst performance. The interaction effects among design factors and between design and site factors are presented below. • Among pavements built on fine-grained soils, an increase in HMA thickness from 4-inch (102 mm) to 7-inch (178 mm) has a significant positive effect on change in roughness (∆IRI). Also for un-drained pavements, those with ATB have significantly lower ∆IRI than those with DGAB. Finally the effect of drainage is significant only for sections with DGAB. These effects suggest that, for pavements built on fine-grained soils, higher HMA thickness and/or treated base will help inhibit the increase in roughness. Also, drainage appears to be more

5 effective in preventing an increase in roughness for sections with DGAB, especially among those located in WF zone. • For un-drained pavements built on coarse-grained soils, an increase in base thickness from 8- inch (203 mm) to 12-inch (305 mm) causes slightly lower ∆IRI. Transverse cracking Transverse cracking seems to be associated with wet-freeze climate. Pavements located in WF zone have shown significantly more transverse cracking than those located in WNF zone. This confirms that transverse cracking occurs mainly in freezing environment. The interaction effects between design and site factors are presented below. • Among drained pavements built on coarse-grained soils, those with ATB performed better than those with DGAB. • Among pavements with DGAB and built on fine-grained soils, those with drainage have shown significantly less transverse cracking than those without drainage. Longitudinal cracking-WP On the whole, longitudinal cracking-WP seems to be more prevalent in WF climate, especially when built on fine-grained soils. Base type including drainage are the most critical design factors; pavements with ATB have shown the best performance. The interaction effects between design and site factors are presented below. • On average pavements in WF zone have shown higher levels of longitudinal cracking-WP than those in WNF, especially among pavements built on fine-grained subgrade. This effect was found to be only marginally significant. • Among pavements built on fine-grained soils, those built with DGAB have shown more longitudinal cracking-WP, and those built with ATB have shown the least amount of cracking. Also, drainage has a significant effect on longitudinal cracking, and this effect is more pronounced in pavements built with DGAB. This trend implies that if a pavement on fine-grained subgrade is constructed with a DGAB base, better performance (in terms of

6 longitudinal cracking-WP) can be achieved by providing drainage. These effects are seen in both WF and WNF zones. Longitudinal cracking-NWP The initial trends indicate that longitudinal cracking-NWP is caused by “freeze” climate (frost effects), and that pavements without drainage may be more prone to it. In general, more longitudinal cracking-NWP was observed among sections located in “freeze” climate compared to those in “no-freeze” climate. It was also found that, the effect of drainage is more pronounced (with marginal statistical significance) among pavements located in “freeze” climate. However, this effect is not of practical significance. In summary, based on the analysis of the SPS-1 data, base type seems to be the most critical design factor for fatigue cracking, roughness (IRI) and longitudinal cracking (wheel- path). This is not to say that the effect of HMA surface thickness is not significant. In fact, the effect of base type should be interpreted in light of the fact that an asphalt-treated base (ATB) effectively means thicker HMA layer. Drainage when combined with base type also plays an important role in improving flexible pavement performance, especially in terms of fatigue and longitudinal cracking. Base thickness has secondary effects on performance, especially in the case of roughness and rutting. Subgrade soil type seems to be playing an important role in flexible pavement performance. In general, pavements built on fine-grained soils have shown the worst performance, especially in the case of roughness. Also, climate is a critical factor in determining flexible pavement performance. Longitudinal cracking and transverse cracking appear to be affected by climate. Longitudinal cracking (wheel-path) and transverse cracking seem to be associated with Wet Freeze environment, while longitudinal cracking (non wheel-path) seems to be dominant in “freeze” climate. Effects of Design and Site Factors on Pavement Performance— SPS-2 Experiment A majority of SPS-2 sections are showing “low” occurrence and extent of distresses at this point in time. From an engineering viewpoint it can be said that the sections are exhibiting

7 “good” performance. Thus the results presented in this report at this point are an indication of initial trends at best. It should be noted that the effects presented herein are statistically significant unless mentioned otherwise. Transverse cracking The PCC slab thickness and base type have significant influence on the occurrence of transverse cracking. Pavement sections built on PATB (with drainage) have the least occurrence of transverse cracking while sections built on LCB have exhibited highest occurrence of cracking. Considerable amount of cracking in LCB layer, which probably caused reflection cracking in the PCC slab, can be attributed to shrinkage cracking as per the construction reports. The effects of the experimental factors are summarized below: • The occurrence of transverse cracking among pavements with 8-inch (203 mm) PCC slab is higher than pavements built with 11-inch (279 mm) PCC slab. • The occurrence of transverse cracking among pavement sections constructed on LCB is higher than pavement sections built on PATB-over-DGAB or with DGAB. Pavements sections constructed on PATB-over-DGAB have shown the “best” performance (least occurrence of cracking). • Sections without drainage (sections with DGAB) have a slightly higher likelihood of cracking than sections with drainage (sections with PATB-over-DGAB). • On average, among sections built with LCB, those with 8-inch (203 mm) PCC slab have higher occurrence of cracking than those with 11-inch (279 mm) PCC slab. It is important to interpret these results in light of the construction issues, i.e. shrinkage cracking in LCB. • Pavements built on fine-grained soils have slightly higher chances for the occurrence of transverse cracking than those built on coarse-grained soils. The effect is marginally significant Longitudinal Cracking The PCC slab thickness and base type have the greatest influence on the occurrence of longitudinal cracking on rigid pavements. Pavements constructed on PATB have least

8 occurrence of longitudinal cracking while those with LCB have the highest occurrence of cracking. The effects of the experimental factors are summarized below: • The occurrence of longitudinal cracking among pavements with 8-inch (203 mm) PCC slab thickness is higher than among those with 11-inch (279 mm) PCC slab thickness. • The occurrence of longitudinal cracking among pavements constructed with LCB is higher than among those with PATB-over-DGAB or with DGAB. Pavements with PATB-over-DGAB have shown the “best” performance (least occurrence of cracking). • On average, among sections built with LCB, those with 8-inch (203 mm) PCC slab have higher occurrence of cracking than those with 11-inch (279 mm) PCC slab. It is important to interpret these results in light of the construction issues i.e. shrinkage cracking in LCB. Faulting A majority of SPS-2 sections are exhibiting “good” performance with respect to joint faulting, at this point in time. About 33% of the sections have less than 20% of the joints with faulting more than 1.0 mm, and 5% of the sections have more than 20% of the joints that are faulted more than 1.0 mm. Therefore, the results at this point may only indicate the initial trends/observations. It would thus be premature to draw any conclusions on the influence of design and site features on joint faulting. Roughness The results suggest that the change in roughness can be inhibited by constructing pavements with PATB-over-DGAB, as compared to sections with DGAB or LCB, especially in the case of pavements built on fine-grained soils. In addition, PCC slab thickness also plays an important role in increase of roughness. The effects of the experimental factors are explained below: • Pavements constructed with PATB have shown lower change in IRI (∆IRI) compared to those with DGAB or LCB, while pavements with DGAB have the highest change in roughness.

9 • Among pavements constructed with standard lane width [12’ (3.7 m) wide lane], sections with DGAB have shown slightly higher ∆IRI than those with LCB or PATB. The effect is marginally significant. • Among pavements built on fine-grained soils, those with 8-inch (203 mm) PCC slab have higher ∆IRI than those with 11-inch (279 mm) PCC slab. This effect is more prominent among sections located in WF zone. • A positive effect of drainage is more noticeable among sections located in WF zone and built on fine-grained soils. In summary, based on the findings from analysis of the SPS-2 data, base type and PCC slab thickness appear to be the most critical design factors in explaining cracking (transverse and longitudinal) and roughness (IRI). DGAB and drainage, when combined also play an important role in improving rigid pavement performance, especially in terms of cracking (transverse and longitudinal) and roughness. The effects of PCC flexural strength and lane width on pavement performance are inconclusive, at this point in time. However, sections with widened lane have shown lesser faulting occurrences than those with standard lane. The site conditions (climate and subgrade soil type) seem to be having marginal effects on cracking (transverse and longitudinal) and roughness. Effects of Environment on Pavement Performance—SPS-8 Experiment The SPS-8 pavements have “low” occurrence and extent of distresses, at this point. Most of the pavements in the experiment are performing at comparable levels. No formal statistical methods could be employed due to this. Therefore the observations presented here are just based on average performance of the distressed pavements. The observations need to be considered as initial trends, in light of these limitations. Flexible Pavements: On average, pavements in WF zone have more fatigue cracking, longitudinal cracking (non wheel-path), and roughness than pavements in other climates. Also, in general, pavements constructed on “active” (frost susceptible or expansive) subgrade soils have more longitudinal cracking (non wheel-path), transverse cracking, and fatigue cracking than pavements on “non-active” soils.

10 Pavements located in “wet” climate, on average, have higher change in IRI than those in “dry” climate. Furthermore, pavements located in WF zone and those built on active soils have the higher changes in IRI. Rigid Pavements: Longitudinal spalling, on average, was more prevalent in sections located in “wet” climate. Spalling was not observed in any of the pavements located in the dry-freeze (DF) zone and in any of the pavements constructed on coarse-grained subgrade soil. Transverse cracking was not observed in any of the pavements constructed with thicker PCC slabs and in any of the pavements constructed on coarse-grained subgrade soils. The results of this research should be useful for highway agencies and pavement engineers in assessing the relative importance of design and site factors for pavement design. As the results are based on data from controlled field experiments these are expected to be helpful in evaluating the existing design methods, and developing improved design options.