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Chapter 5. LTPP Data Analysesof Flexible Pavements

Publication Number: FHWA- HRT-17-095 Date: September 2017

Publication Number: FHWA- HRT-17-095
Date: September 2017

Pavement Performance Measures and Forecasting and The Effects of Maintenance and Rehabilitation Strategy on Treatment Effectiveness (Revised)

CHAPTER 5. LTPP DATA ANALYSES OF FLEXIBLE PAVEMENTS

For all LTPP test sections in SPS-1 through -7 and GPS-6, -7, and -9, the time-series pavement conditions, distresses, and some of the FWD data were downloaded and organized in spreadsheet format for analyses. The data from these LTPP test sections and a few pavement sections from the CDOT, LADOTD, and WSDOT databases were modeled using the proper mathematical function and were subjected to analyses. The procedures and the results of the analyses of the LTPP flexible pavement condition and distress data are presented in this chapter, while the procedure and results for rigid pavement condition and distress data are presented in chapter 6. Results of the analyses of the FWD data are presented and discussed in chapter 7. Finally, the results of the analyses of the CDOT, LADOTD, and WSDOT data are presented and discussed in chapter 9. The information in this chapter is arranged in the following sections:

Modeling of the Time-Series Pavement Condition and Distress Data.
Impacts of Climatic Regions, Drainage, and AC Thickness on Pavement Condition and Distress Using the LTPP SPS-1 Test Sections.
Summary, Conclusions, and Recommendations, LTPP SPS-1.
Results of the Analyses of the LTPP SPS-3 Test Sections.
Summary, Conclusions, and Recommendations, LTPP SPS-3.
Results of the Analyses of the LTPP SPS-5 Test Sections.
Summary, Conclusions, and Recommendations, LTPP SPS-5.
Results of the Analyses of the LTPP GPS-6 Test Sections.
Summary, Conclusions, and Recommendations, LTPP GPS-6.
ORCSE Method.
Summary, Conclusions, and Recommendations, ORCSE Model.

MODELING THE TIME-SERIES PAVEMENT CONDITION AND DISTRESS DATA

The time-series pavement condition and distress data of all test sections in the SPS-1 through -7 and GPS–6, -7, and -9 experiments were downloaded, organized, and modeled using the proper mathematical functions based on the type of pavement condition (IRI) or distress (rut depth, and cracking). The mathematical functions listed in table 27 and shown in figure 50 were selected based on reported trends and mechanisms of pavement deterioration.^(37,81,78) For example, rutting typically occurs early in the asphalt pavement’s life, and its accumulation rate decreases over time as the pavement materials densify under traffic loads.

Table 27. Description of the mathematical functions used in the analyses of the pavement distress and condition data.

Pavement Condition or Distress	Function Type
IRI (inches/mi (m/km))	Exponential function
Rut depth (inches (mm))	Power function
Cracking (length, area, or percent)	Logistic (S-shaped)

Figure 50. Graph. Exponential, power, and logistic (S-shaped) curves.

Therefore, a power function is typically used to model the time-series rut depth data. On the other hand, pavement roughness usually increases exponentially as the pavement ages, deteriorates, and becomes uneven causing increases in the dynamic effects of traffic loads. Hence, an exponential function is typically used to model the pavement roughness (IRI). Finally, the propagation of pavement cracks usually follows three stages. In the first stage, a few cracks appear in the early pavement life; their number and length increase exponentially. In the second stage, the number and length of cracks increase almost linearly over time. In this stage, a few new cracks are initiated and most existing cracks approach their maximum possible lengths (lane width or the pavement section length). In the third stage, the number of cracks and their length reach equilibrium as shown by the logistic curve in figure 50. Given this scenario, the modeling of crack propagation over time could be achieved using two different functions, depending on the availability of the data. If the cracking data are available over a short period of time after construction (stage one data only), an exponential function could be used to model the data. On the other hand, if the cracking data are available when the pavement is old (stage three only), a power function could be used. The modeling of the crack propagation using the logistic function cannot be confidently achieved unless at least four data points are available spanning the three crack propagation stages. To reduce the effect of the problem and to increase the number of test sections included in the analyses, one crack saturation point was assumed for each type of cracking. The assumed crack saturation points used throughout this study are listed in table 28. The assumption of the crack saturation points was based on engineering logic. For example, the saturation point for alligator cracking is the entire surface area of the pavement section, whereas the saturation point for longitudinal cracking is three cracks along the entire pavement section length. Note that the square, circle, and triangle symbols in figure 50 represent measured data. The solid portions of the curves are fit to the measured data while the dashed portions are forecasted based on the mathematical function fitting the data. It should be noted that the three mathematical models fit the LTPP pavement condition and distress data for all test sections and State data.

Table 28. Crack saturation values used in the analyses of the pavement cracking data.

Cracking Type	Saturation Value			Reason
Cracking Type	Per 500 ft (152.4 m) LTPP Test Section	Per 0.1km	Per 0.1mi	Reason
Alligator cracking	5,906 ft² (549 m²)	360 m²	6,336 ft²	100-percent of section cracked (12-ft (3.66-m) lane width)
Longitudinal cracking^¹1,500 ft (457.2 m)	1,500 ft (457.2 m)	300 m	1,584 ft	Three cracks along the entire section length
Transverse cracking (length), flexible pavements	500ft (152.4 m)	100 m	528 ft	One crack every 12ft (3.65 m)
Number of transverse cracks, flexible pavements¹	42	28	44	One crack each 3.65 m (12ft)
Transverse cracking (length), rigid pavements	375ft (114 m)	75 m	396 ft	One crack per slab (16-ft (4.87-m) joint spacing)
Number of transverse cracks, rigid pavements¹	31	21	33	One crack per slab (16-ft (4.87-m) joint spacing)
¹Data included for convenience. The analyses were conducted using the measured crack lengths and alligator cracked areas.

After selecting the proper mathematical function, the least squares regression technique was used to determine the statistical parameters of the selected mathematical functions. The least squares regression technique is based on minimizing the sum of the squared differences (error) between the calculated and the measured data.⁽³⁾ To expedite the analyses, a MATLAB®-based computer program was written to complete the following functions for each pavement condition and distress dataset of each LTPP test section and for each pavement treatment type (see the program flowchart in figure 51):

Read all available time-series data from a Microsoft® Excel spreadsheet.
Separate the data into two parts, before and after treatment.
Check the available number of time-series data points before and after treatment.
If three or more data points are available that were collected before and/or after the treatment, use the proper mathematical function to fit the data and obtain the statistical parameters of the function.
Organize the results (section identification, treatment date and type, climatic region, traffic, the last collected before treatment data point, the first collected after treatment data point, the number of before treatment and after treatment data points, and the statistical parameters) in a Microsoft® Excel spreadsheet format.

Figure 51. Illustration. Flowchart of the MATLAB® program.

The MATLAB® output data were then subjected to further analyses to estimate the following parameters for each test section and for each treatment type:

The before treatment RFP, RSP, and CS.
The after treatment RFP, RSP, and CS.
Any changes in RFP or RSP resulting from the pavement treatment. The change in RFP or RSP is defined as the differences between the before treatment RFPs or RSPs and the after treatment RFPs or RSPs.
The time period for the reoccurrence of the previous pavement conditions (the last collected data point before the application of the preservation treatment).
Instantaneous change in the pavement conditions and distresses due to treatment (also called PJ).

IMPACTS OF CLIMATIC REGIONS, DRAINAGE, AND AC THICKNESS ON PAVEMENT PERFORMANCE USING THE LTPP SPS-1 TEST SECTIONS

Recall that the main objective of the SPS-1 experiment is to study the effects of the conditions in climatic regions and the following structural factors on pavement performance:⁽⁶⁵⁾

Presence or absence of a drainage layer.
AC thickness (4 or 7 inches (102 or 178 mm)).
Base type (dense-graded aggregate base, asphalt-treated base, permeable asphalt-treated base, or a combination thereof).
Base thickness (8, 12, or 16 inches (203, 305, or 406 mm).

The analyses of the impacts of the various variables were accomplished in the following steps:

Step 1: For each pavement test section in the SPS-1 experiment, each of the time-dependent pavement condition (IRI) and distress (rut depth, and alligator, transverse, and longitudinal cracking) data were used to calculate RFP and RSP of that section from the time of construction to the time when the pavement condition or distress reached the appropriate threshold values. The reason for calculating RFP and RSP from the construction data (surface age is 0 years) was that the dates of construction and last data collection for different test sections were not the same. This implies that the reference time for all SPS-1 test sections was taken as the date of construction.
Step 2: For each pavement condition and distress type, the resulting RFPs and RSPs and other inventory data (such as SHRP ID, State, AC thickness, drainage, base type and thickness, and so forth) were then organized into a Microsoft® Excel spreadsheet format.
Step 3: For each SHRP ID and for each pavement condition and distress type, the minimum and maximum RFPs and RSPs and their averages were calculated and listed in the Microsoft® Excel spreadsheet.
Step 4: The data were then divided into the following climatic regions, groups, and subgroups. The main objective of the division was to separate the design variables affecting pavement performance.
- 1—Climatic Regions: The results were divided into four climatic regions: WF, WNF, DF, and DNF.
- 2—AC Thickness Groups: The results in each climatic region were then divided into two groups based on the thickness of the AC (4 and 7 inches (102 or 178 mm)).
- 3—Drainage Subgroup: The results in each AC thickness group were then divided into two drainage subgroups (presence and absence of drainage). It should be noted that various attempts were made to divide the results in each drainage subgroup into base thickness and base type subgroups and into three traffic levels. Unfortunately, none of the attempts was successful because any further division yielded an insignificant number of test sections in each subgroup such that no decisions could be made with any level of certainty. Therefore, the impacts of the base thickness and type were not studied any further.

The impacts of the conditions in the four climatic regions (WF, WNF, DF, and DNF), the AC thickness (4 and 7 inches (102 or 178 mm)), and drainable and undrainable bases on the pavement performance in terms of RFP and RSP were analyzed. The detailed results of the analyses (the calculated and rounded minimum, maximum, and average RFPs or RSPs for the SPS-1 test sections) were submitted to FHWA and are available from the LTPP Customer Support Services.⁽⁷⁹⁾ For convenience, the detailed results are summarized in table 29 through table 34.

The data in table 29 through table 34 address the impact of the climatic regions, the AC thickness, and drainable and undrainable bases on the pavement performance (in terms of RFPs and RSPs) of the SPS-1 test sections. The figures in the tables (which are rounded to whole numbers) indicate the differences in years in RFPs or RSPs of the SPS-1 test sections having the column heading parameters compared with RFPs and RSPs of the SPS-1 test sections having the row heading parameters. Thus, in table 29 through table 33, the diagonal indicated by asterisks represents the line of symmetry. If the table were folded along the diagonal, the aligned numbers from above and from below the diagonal would be the same but with different sign. The proper reading of the data in the tables is illustrated in the two examples following the tables.

Table 29. Summary of the results of analyses of the impacts of design factors on RFP of LTPP SPS-1 test sections based on IRI (years).

Climatic Region, AC Thickness, and Drainage Subgroup			Climatic Region, AC Thickness, and Drainage Subgroup
			WF				WNF				DF				DNF
			4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC
			D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND
WF	4-inch AC	D	*	–5	0	–2	2	2	2	2	0	2	2	2	2	1	2	2
	4-inch AC	ND	5	*	4	3	7	7	7	7	5	7	7	7	7	6	7	7
	7-inch AC	D	0	–4	*	–1	2	2	2	2	1	2	2	2	2	1	2	2
	7-inch AC	ND	2	–3	1	*	4	3	4	4	2	4	4	4	4	3	4	4
WNF	4-inch AC	D	–2	–7	–2	–4	*	0	0	0	–2	0	0	0	0	–1	0	0
	4-inch AC	ND	–2	–7	–2	–3	0	*	0	0	–1	0	0	0	0	–1	0	0
	7-inch AC	D	–2	–7	–2	–4	0	0	*	0	–2	0	0	0	0	–1	0	0
	7-inch AC	ND	–2	–7	–2	–4	0	0	0	*	–2	0	0	0	0	–1	0	0
DF	4-inch AC	D	0	–5	–1	–2	2	1	2	2	*	2	2	2	2	1	2	2
	4-inch AC	ND	–2	–7	–2	–4	0	0	0	0	–2	*	0	0	0	–1	0	0
	7-inch AC	D	–2	–7	–2	–4	0	0	0	0	–2	0	*	0	0	–1	0	0
	7-inch AC	ND	–2	–7	–2	–4	0	0	0	0	–2	0	0	*	0	–1	0	0
DNF	4-inch AC	D	–2	–7	–2	–4	0	0	0	0	–2	0	0	0	*	–1	0	0
	4-inch AC	ND	–1	–6	–1	–3	1	1	1	1	–1	1	1	1	1	*	1	1
	7-inch AC	D	–2	–7	–2	–4	0	0	0	0	–2	0	0	0	0	–1	*	0
	7-inch AC	ND	–2	–7	–2	–4	0	0	0	0	–2	0	0	0	0	–1	0	*
* Indicates the line of symmetry along the diagonal of the table. 1 inch = 25.4 mm. D = Drainable base. ND = Undrainable base.

Table 30. Summary of the results of analyses of the impacts of design factors on RFP/RSP of LTPP SPS-1 test sections based on rut depth (years).

Climatic Region, AC Thickness, and Drainage Subgroup			Climatic Region, AC Thickness, and Drainage Subgroup
			WF				WNF				DF				DNF
			4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC
			D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND
WF	4-inch AC	D	*	–5	–1	–3	5	5	5	5	4	6	6	6	6	6	6	6
	4-inch AC	ND	5	*	4	2	10	10	10	10	9	11	11	11	11	11	11	11
	7-inch AC	D	1	–4	*	–3	6	6	5	6	5	7	7	7	7	7	7	7
	7-inch AC	ND	3	–2	3	*	8	9	8	9	7	9	9	9	9	9	9	9
WNF	4-inch AC	D	–5	–10	–6	–8	*	0	0	1	–1	1	1	1	1	1	1	1
	4-inch AC	ND	–5	–10	–6	–9	0	*	0	0	–1	1	1	1	1	1	1	1
	7-inch AC	D	–5	–10	–5	–8	0	0	*	1	–1	1	1	1	1	1	1	1
	7-inch AC	ND	–5	–10	–6	–9	–1	0	–1	*	–2	1	1	1	1	1	1	1
DF	4-inch AC	D	–4	–9	–5	–7	1	1	1	2	*	2	2	2	2	2	2	2
	4-inch AC	ND	–6	–11	–7	–9	–1	–1	–1	–1	–2	*	0	0	0	0	0	0
	7-inch AC	D	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	*	0	0	0	0	0
	7-inch AC	ND	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	0	*	0	0	0	0
DNF	4-inch AC	D	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	0	0	*	0	0	0
	4-inch AC	ND	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	0	0	0	*	0	0
	7-inch AC	D	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	0	0	0	0	*	0
	7-inch AC	ND	–6	–11	–7	–9	–1	–1	–1	–1	–2	0	0	0	0	0	0	*
* Indicates the line of symmetry along the diagonal of the table. 1 inch = 25.4 mm. D = Drainable base. ND = Undrainable base.

Table 31. Summary of the results of analyses of the impacts of design factors on RSP of LTPP SPS-1 test sections based on alligator cracking (years).

Climatic Region, AC Thickness, and Drainage Subgroup			Climatic Region, AC Thickness, and Drainage Subgroup
			WF				WNF				DF				DNF
			4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC
			D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND
WF	4-inch AC	D	*	–4	–3	–3	0	0	2	2	–3	–5	–2	–3	–5	–6	–1	–2
	4-inch AC	ND	4	*	1	1	4	3	5	5	0	–2	2	1	–1	–2	3	2
	7-inch AC	D	3	–1	*	0	3	2	4	4	–1	–3	1	0	–2	–3	2	1
	7-inch AC	ND	3	–1	0	*	3	3	5	5	0	–2	1	0	–1	–3	2	2
WNF	4-inch AC	D	0	–4	–3	–3	*	–1	1	1	–4	–6	–2	–3	–5	–6	–1	–2
	4-inch AC	ND	0	–3	–2	–3	1	*	2	2	–3	–5	–2	–3	–4	–6	–1	–1
	7-inch AC	D	–2	–5	–4	–5	–1	–2	*	0	–5	–7	–4	–5	–6	–8	–3	–3
	7-inch AC	ND	–2	–5	–4	–5	–1	–2	0	*	–5	–7	–4	–4	–6	–8	–3	–3
DF	4-inch AC	D	3	0	1	0	4	3	5	5	*	–2	1	1	–1	–3	2	2
	4-inch AC	ND	5	2	3	2	6	5	7	7	2	*	3	2	1	–1	4	4
	7-inch AC	D	2	–2	–1	–1	2	2	4	4	–1	–3	*	–1	–2	–4	1	0
	7-inch AC	ND	3	–1	0	0	3	3	5	4	–1	–2	1	*	–2	–3	2	1
DNF	4-inch AC	D	5	1	2	1	5	4	6	6	1	–1	2	2	*	–1	3	3
	4-inch AC	ND	6	2	3	3	6	6	8	8	3	1	4	3	1	*	5	4
	7-inch AC	D	1	–3	–2	–2	1	1	3	3	–2	–4	–1	–2	–3	–5	*	0
	7-inch AC	ND	2	–2	–1	–2	2	1	3	3	–2	–4	0	–1	–3	–4	0	*
* Indicates the line of symmetry along the diagonal of the table. 1 inch = 25.4 mm. D = Drainable base. ND = Undrainable base.

Table 32. Summary of the results of analyses of the impacts of design factors on RSP of LTPP SPS-1 test sections based on longitudinal cracking (years).

Climatic Region, AC Thickness, and Drainage Subgroup			Climatic Region, AC Thickness, and Drainage Subgroup
			WF				WNF				DF				DNF
			4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC
			D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND
WF	4-inch AC	D	*	–1	–1	0	5	6	6	5	1	0	–1	0	6	5	6	7
	4-inch AC	ND	1	*	0	1	6	7	7	6	1	1	0	1	7	6	6	8
	7-inch AC	D	1	0	*	1	6	7	7	6	2	1	0	1	7	6	7	8
	7-inch AC	ND	0	–1	–1	*	5	6	6	5	0	0	–1	0	6	5	5	7
WNF	4-inch AC	D	–5	–6	–6	–5	*	1	1	0	–5	–5	–6	–5	1	0	0	2
	4-inch AC	ND	–6	–7	–7	–6	–1	*	0	–2	–6	–6	–7	–6	–1	–1	–1	1
	7-inch AC	D	–6	–7	–7	–6	–1	0	*	–1	–6	–6	–7	–6	0	–1	–1	1
	7-inch AC	ND	–5	–6	–6	–5	0	2	1	*	–4	–5	–5	–5	1	0	1	2
DF	4-inch AC	D	–1	–1	–2	0	5	6	6	4	*	0	–1	–1	5	4	5	7
	4-inch AC	ND	0	–1	–1	0	5	6	6	5	0	*	–1	0	6	5	5	7
	7-inch AC	D	1	0	0	1	6	7	7	5	1	1	*	1	7	6	6	8
	7-inch AC	ND	0	–1	–1	0	5	6	6	5	1	0	–1	*	6	5	6	7
DNF	4-inch AC	D	–6	–7	–7	–6	–1	1	0	–1	–5	–6	–7	–6	*	–1	0	1
	4-inch AC	ND	–5	–6	–6	–5	0	1	1	0	–4	–5	–6	–5	1	*	1	2
	7-inch AC	D	–6	–6	–7	–5	0	1	1	–1	–5	–5	–6	–6	0	–1	*	2
	7-inch AC	ND	–7	–8	–8	–7	–2	–1	–1	–2	–7	–7	–8	–7	–1	–2	–2	*
* Indicates the line of symmetry along the diagonal of the table. 1 inch = 25.4 mm. D = Drainable base. ND = Undrainable base.

Table 33. Summary of the results of analyses of the impacts of design factors on RSP of LTPP SPS-1 test sections based on transverse cracking (years).

Climatic Region, AC Thickness, and Drainage Subgroup			Climatic Region, AC Thickness, and Drainage Subgroup
			WF				WNF				DF				DNF
			4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC		4-inch AC		7-inch AC
			D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND	D	ND
WF	4-inch AC	D	*	1	2	1	3	3	4	3	–3	2	–3	0	–1	2	2	0
	4-inch AC	ND	–1	*	2	1	2	2	3	2	–4	1	–4	0	–1	1	1	0
	7-inch AC	D	–2	–2	*	–1	1	0	1	1	–6	0	–5	–2	–3	–1	0	–2
	7-inch AC	ND	–1	–1	1	*	2	1	2	2	–5	1	–4	–1	–2	0	1	–1
WNF	4-inch AC	D	–3	–2	–1	–2	*	–1	0	0	–7	–1	–6	–3	–4	–1	–1	–3
	4-inch AC	ND	–3	–2	0	–1	1	*	1	1	–6	0	–5	–2	–3	–1	0	–2
	7-inch AC	D	–4	–3	–1	–2	0	–1	*	0	–7	–1	–6	–3	–4	–2	–1	–3
	7-inch AC	ND	–3	–2	–1	–2	0	–1	0	*	–7	–1	–6	–3	–4	–1	–1	–3
DF	4-inch AC	D	3	4	6	5	7	6	7	7	*	6	1	4	3	5	6	4
	4-inch AC	ND	–2	–1	0	–1	1	0	1	1	–6	*	–5	–2	–3	0	0	–2
	7-inch AC	D	3	4	5	4	6	5	6	6	–1	5	*	3	2	5	5	3
	7-inch AC	ND	0	0	2	1	3	2	3	3	–4	2	–3	*	–1	1	2	0
DNF	4-inch AC	D	1	1	3	2	4	3	4	4	–3	3	–2	1	*	2	3	1
	4-inch AC	ND	–2	–1	1	0	1	1	2	1	–5	0	–5	–1	–2	*	0	–1
	7-inch AC	D	–2	–1	0	–1	1	0	1	1	–6	0	–5	–2	–3	0	*	–2
	7-inch AC	ND	0	0	2	1	3	2	3	3	–4	2	–3	0	–1	1	2	*
* Indicates the line of symmetry along the diagonal of the table. 1 inch = 25.4 mm. D = Drainable base. ND = Undrainable base.

Table 34. Summary of the results of analyses of the effects of climatic regions on the performance of the LTPP SPS-1 test sections.

Condition or Distress Type	Climatic Region	Climatic Regions and the Percent of Test Sections Where RFP/RSP Was Better, Equal, or Worse Than Other Climatic Regions
		WF			WNF			DF			DNF
		Better	Same	Worse	Better	Same	Worse	Better	Same	Worse	Better	Same	Worse
IRI	WF	—	—	—	58	4	38	56	28	17	58	42	0
	WNF	38	4	58	—	—	—	77	6	17	8	84	8
	DF	17	28	56	17	6	77	—	—	—	17	72	11
	DNF	0	42	58	8	84	8	11	72	17	—	—	—
Rut depth	WF	—	—	—	83	17	0	82	13	5	73	27	0
	WNF	0	17	83	—	—	—	23	68	9	9	91	0
	DF	5	13	82	9	68	23	—	—	—	0	100	0
	DNF	0	27	73	0	91	9	0	100	0	—	—	—
Alligator cracking	WF	—	—	—	67	8	25	42	4	54	42	8	50
	WNF	25	8	67	—	—	—	38	8	54	13	0	87
	DF	54	4	42	54	8	38	—	—	—	46	17	38
	DNF	50	8	42	87	0	13	38	17	46	—	—	—
Longitudinal cracking	WF	—	—	—	88	8	4	46	4	50	92	8	0
	WNF	4	8	88	—	—	—	8	29	63	42	42	16
	DF	50	4	46	63	29	8	—	—	—	67	29	4
	DNF	0	8	92	16	42	42	4	29	67	—	—	—
Transverse cracking	WF	—	—	—	54	33	13	42	12	46	46	21	33
	WNF	13	33	54	—	—	—	38	54	8	7	50	42
	DF	46	12	42	8	54	38	—	—	—	42	46	12
	DNF	33	21	46	42	50	7	12	46	42	—	—	—
— Indicates no data.

Example 1: This example illustrates how to read the data in table 29 through table 33 using example data from table 29. The first four numbers in the first row below the top headings imply the following:
- The average RFP of test sections located in the WF region having 4-inch (102-mm)-thick AC layers and undrainable bases was 5 years shorter (–5) than compatible test sections with drainable bases.
- The average RFP of test sections located in the WF region having 7-inch (178-mm)-thick AC layers and drainable bases was the same (0 years) as those having 4-inch (102‑mm) thick AC layers with drainable bases.
- The average RFP of test sections located in the WF region having 7-inch (178-mm)-thick AC layers and undrainable bases was 2 years shorter (2 years) than those having 4‑inch (102-mm)-thick AC layers with drainable bases.
- The average RFP of test sections located in the WNF region having 4-inch (102‑mm)-thick AC layers and drainable bases was 2 years longer (2 years) than those located in the WF region having 4-inch (102-mm)-thick AC layers and drainable bases.
Example 2: This example illustrates how to read the data in table 34. The data in the table address the impact of the climatic regions on the pavement condition (IRI) and distresses (rut depth and alligator, longitudinal, and transverse cracking). The numbers in the table indicate the percent of the test sections with column heading parameters that performed better, the same, or worse compared with the test sections having the row heading parameters. For example, for IRI, the six numbers (three numbers in each of the top two populated rows in the three columns under the heading WF) and for rut depth, the six numbers (three numbers in each of the next two populated rows) under the same three columns heading (WF) imply the following:
- In terms of IRI, 38 percent of the test sections located in the WF region performed better than those located in the WNF region, 4 percent performed the same, and 58 percent performed worse.
- In terms of IRI, 17 percent of the test sections located in the WF region performed better than compatible sections located in the DF region, 28 percent performed the same, and 56 percent performed worse.
- In terms of rut depth, none of the test sections located in the WF region performed better than those located in the WNF region, 17 percent performed the same, and 83 percent performed worse.
- In terms of rut depth, 5 percent of the test sections located in the WF region performed better than compatible sections located in the DF region, 13 percent performed the same, and 82 percent performed worse.

Consistent with this explanation of table 29 through table 33 and table 34, in the following subsections, the discussion of the results shown in those tables is organized according to the pavement condition and distress.

IRI

The average calculated RFPs listed in table 29 indicate that the differences between the average RFP of test sections located in different climatic regions and that have 4- or 7-inch (102- or 178‑mm)-thick AC layers with drainable and undrainable bases varied from –1 to 7 years. Because the 1-year difference was not significant and was within the data variability, it was considered a value of zero in the following discussion. Nevertheless, the data in table 29 indicate the following:

In the WF region, the average RFP of test sections having 4-inch (102-mm) AC thickness and undrainable bases was about 5 years shorter than the average RFP of compatible test sections with drainable bases. The 5-year difference decreased to 2 years when the AC thickness of the undrainable test sections increased from 4 to 7 inches (102 to 178 mm). That is, the average RFP of test sections having 7-inch (178-mm)-thick AC layers and undrainable bases was about 3 years longer than test sections having 4-inch (102-mm)-thick AC layers and undrainable bases. Finally, for test sections having 7-inch (178-inch)-thick AC layers and drainable bases, the average RFP was the same as test sections having 4-inch (102-mm)-thick AC layers and drainable bases and 4years longer compared with test sections having 4-inch (102-mm)-thick AC layers and undrainable bases.
Also in the WF region, the average RFP of the test sections having 4-inch (102 mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases was 2 to 7 years shorter than compatible test sections located in the other three climatic regions.
In the WNF, DF, and DNF climatic regions, RFP of the test sections having 4-inch (102 mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases was 2 to 7 years longer than compatible test sections located in the WF region.
In the WNF region, RFP/RSP of the test sections having 4-inch (102 mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases was almost the same.
In the DF region, the average RFP of the test sections having 4-inch (102-mm)-thick AC layers and drainable bases was about 2 years shorter than test sections having 4-inch (102 mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases located in the WNF and DNF climatic regions.
In the DNF region, test sections having 4-inch (102 mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases performed almost the same. That is, the existence of drainable bases and thicker AC layers did not affect the pavement performance in terms of IRI.

The impact of the climatic regions on pavement performance in terms of IRI is summarized in table 34. The data in the table indicate the following:

Fifty-eight percent of the test sections located in the WF regions performed worse, 4 percent performed the same, and 38 percent performed better than compatible test sections located in the WNF regions.
Fifty-six percent of the test sections located in the WF regions performed worse, 28 percent performed the same, and 17 percent performed better than compatible test sections located in the DF regions.
Seventy-seven percent of the test sections located in the DF region performed better than compatible sections located in the WNF region while test sections located in the DNF and WNF performed almost the same.

There are several significant initial conclusions regarding pavement performance that could be drawn from the findings listed in table 29 and table 34. However, these conclusions would be based on pavement roughness only. To make these conclusions a part of the global pavement performance, the research team decided to include them in the summary, conclusions, and recommendations section that follows the discussion of all the other distress types.

Rut Depth

The average calculated RFPs or RSPs listed in table 30 indicate the following:

In the WF regions, the average RFP/RSP of the SPS-1 test sections having 4-inch (102‑mm)-thick AC layers and undrainable bases was 5 years shorter than the average RSP of compatible test sections with drainable bases. Further, the average RFP/RSP of SPS-1 test sections having 7-inch (178-mm)-thick AC layers and undrainable bases was 3years shorter than compatible sections with drainable bases.
All SPS-1 test sections having 4- or 7-inch (102- to 178-mm)-thick AC layers and drainable or undrainable bases located in the WNF, DF, and DNF climatic regions performed significantly better than compatible test sections located in the WF regions. The differences in RFP/RSP varied from 5 to 11 years.

The impact of the climatic regions on pavement performance in terms of rut depth is summarized in table 34. The data in the table indicate the following:

Eighty-three percent, 82 percent, and 73 percent of the SPS-1 test sections located in the WF regions performed worse than compatible test sections located, respectively, in the WNF, DF, and DNF regions.
Almost all SPS-1 test sections located in the DNF region performed the same as compatible test sections located in the WNF and DF climatic regions.

The conclusions are included in the summary, conclusions, and recommendations section following the discussion of the other distress types.

Alligator Cracking

The average calculated RSPs listed in table 31 indicate the following, on average:

In the WF regions, the average RSP of test sections having 4-inch (102-mm)-thick AC layers and undrainable bases was 4 years shorter than the average RSP of test sections having 4-inch (102-mm)-thick AC layers and drainable bases. Further, RSP of SPS-1 test sections having 7-inch (178-mm)-thick AC layers and drainable and undrainable bases was almost the same.
SPS-1 test sections having 4- and 7-inch (102- to 178-mm)-thick AC layers and drainable or undrainable bases located in the WNF region had longer RSPs compared with compatible test sections located in the WF region.
All SPS-1 test sections located in the DNF regions had shorter RSPs than those located in the WNF region. The differences in RSP varied from 1 to 8 years.

The impact of the climatic regions on pavement performance in terms of alligator cracking is summarized in table 34. The data in the table indicate the following:

Sixty-seven percent of the SPS-1 test sections located in the WF regions showed worse, 8percent showed the same, and 25 percent showed better performance than compatible test sections located in the WNF region.
Fifty-four percent and 50 percent of the SPS-1 test sections located in the WF regions performed better than compatible test section located, respectively in the DF and DNF regions. Forty-two percent performed worse than compatible test sections located in the DF and DNF regions. Hence, statistically speaking, SPS-1 test sections located in the WF region had slightly better performance than those in the DF and DNF regions.
Forty-six percent of the SPS-1 test sections located in the DNF regions performed better than compatible test sections located in the DF region, whereas 38 percent performed worse.
Finally, 87 percent of the SPS-1 test sections located in the DNF region performed worse than compatible test sections located in the WNF region, and only 13percent performed better.

The conclusions are included in the summary, conclusions, and recommendations section following the discussion of the other distress types.

Longitudinal Cracking

The average calculated RSPs listed in table 32 indicate the following, on average:

All SPS-1 test sections having 4-inch (102-mm)-thick and 7-inch (178-mm)-thick AC layers and drainable and undrainable bases and located in WF climatic region showed almost the same RSP in terms of longitudinal cracking.
RSPs of most SPS-1 test sections having 4-inch (102-mm)-thick and 7-inch (178‑mm)-thick AC layers and drainable and undrainable bases located in the WNF, DF, and DNF regions were longer than compatible test sections located in the WF region. The differences in RSP varied from 1 to 8 years.
RSPs of almost all SPS-1 test sections having 4-inch (102-mm)-thick and 7-inch (178‑mm)-thick AC layers and drainable and undrainable bases located in the DF and DNF regions were shorter than the RSP of compatible test sections located in the WNF region. The differences in RSPs varied from 1 to 7 years.
Finally, the RSP of all SPS-1 test sections located in the DNF region was longer than the RSP of compatible sections located in the DF region. The differences in RSPs varied from 1 to 7 years.

The impact of the climatic regions on pavement performance in terms of longitudinal cracking is summarized in table 34. The data in the table indicate the following:

Eighty-eight percent and 92 percent of the SPS-1 test sections located in the WF regions performed worse than compatible test sections located, respectively, in the WNF and DNF regions.
Statistically speaking, the performance of SPS-1 test sections located in the WF region was almost the same as those located in the DF region. The data indicate that 50 percent performed better, and 46 percent performed worse.
Forty-two percent of the SPS-1 test sections located in the DNF regions performed better than compatible test sections located in the WNF region, 42 percent performed the same, and only 16 percent performed worse.
Sixty-seven percent of the SPS-1 test sections located in the DNF regions performed better than compatible test sections located in the DF regions while 29 percent performed the same.

Once again, the conclusions are included in the summary and conclusions section following the discussion of the other distress types.

Transverse Cracking

The average calculated RSPs listed in table 33 indicate the following:

In the WF region, the average RSPs of test sections having 4- and 7-inch (102- and 178‑mm)-thick AC layers and undrainable bases were almost the same as compatible sections with drainable bases. Further, the average RSP for SPS-1 test sections having 7‑inch (178-mm)-thick AC layers and drainable bases was 2 years longer than test sections having 4-inch (102-mm)-thick AC layers and drainable bases.
All SPS-1 test sections located in the WNF regions had 1- to 4-year longer RSPs than compatible sections located in the WF region.
The majority of the SPS-1 test sections located in the DF and DNF regions had shorter RSPs than compatible sections located in the WF and WNF regions.
The majority of the SPS-1 test sections having 4- and 7-inch (102- and 178-mm)-thick AC layers and drainable and undrainable bases and located in the DNF region had longer RSPs than compatible test sections located in the DF region.

The impact of the climatic regions on pavement performance in terms of transverse cracking is summarized in table 34. The data in the table indicate the following:

Fifty-four percent of the SPS-1 test sections located in the WF regions performed worse, 13 percent performed better, and 33 percent performed the same as compatible test sections located in the WNF region.
Statistically speaking, the performance of SPS-1 test sections located in the WF and DF and DNF regions was the same.
Forty-two percent of the SPS-1 test sections located in the WNF regions performed better, 50 percent performed the same, and 7 percent performed worse than compatible test sections located in the DNF region.
Forty-two percent of the SPS test sections located in the DNF regions performed better, 46 percent performed the same, and 12 percent performed worse than compatible test sections located in the DF region.

Summary, Conclusions, and Recommendations for LTPP SPS-1

The performance of each SPS-1 test section was analyzed using the available time-series IRI; rut depth; alligator, longitudinal, and transverse cracking data; and the proper mathematical functions. The results of the analyses were then expressed in terms of RFP for IRI, RFP/RSP for rut depth, and RSP for each cracking type. The test sections and their performance (RFP and RSP) were then tabulated using the SHRP IDs, climatic regions, AC thicknesses, and drainable or undrainable bases. Based on the results, the following conclusions were drawn:

The climate in WF regions had a significant impact on pavement performance in terms of IRI, rut depth, and cracking. This conclusion was expected because of the repeated volume changes caused by freezing and thawing. These effects were reported previously by many researchers. Results of the analyses suggest that base drainage and AC thickness should be carefully examined in the pavement design in the WF region.
Use of drainable bases decreased the impact on pavement performance in WF regions. This conclusion was expected, and it was reported in the 1993 AASHTO Guide for Design of Pavement Structures.⁽³⁶⁾
Increasing the thickness of the AC layer from 4 to 7 inches (102 to 178 mm) increased the frost protection of the lower layers, and hence it decreased the impact of the WF region. However, this option was not a cost-effective one.
In the WF region, the inclusion of drainable base had slightly more favorable impact on pavement performance than increasing the AC thickness from 4 to 7 inches (102 to 178mm).
The other three climatic regions (WNF, DF, and DNF) did not affect the pavement performance in terms of rutting potential.
The DF region had more adverse effects on cracking potential than those in the DNF region. This could be attributed to higher oxidation (aging) potential of the AC layer in the DF region.
In the DNF regions, there were significantly higher adverse effects on cracking potential than in the WNF region. This could be attributed to higher solar radiation in the DNF region, which oxidizes the asphalt binder and makes AC more susceptible to cracking.
The inclusion of drainable bases in the DF and DNF regions did not affect pavement performance in terms of RFP or RSP. This was expected because the volume and frequency of available water were low, and most rainfall occurred over short periods of time with most water running off the surface and not penetrating the pavement layers.
In the WNF and DNF regions, the pavement performance in terms of IRI and rut depths of most SPS-1 test sections having 4- and 7-inch (102- and 178-mm)-thick AC layers and drainable and undrainable bases was very much similar. This scenario was substantially different for cracking potential as stated in previous conclusions. Similarly, in the DNF and DF regions, the pavement performance in terms of IRI and rut depths of most SPS-1 test sections having 4- and 7-inch (102- and 178-mm)-thick AC layers and drainable and undrainable bases was similar. Once again, this scenario was substantially different for cracking potential.

IMPACTS OF MAINTENANCE TREATMENTS ON PAVEMENT PERFORMANCE USING THE LTPP SPS-3 TEST SECTIONS

The main objective of SPS-3 experiment was to compare the performance of different maintenance treatments on flexible pavements compared with the control (untreated) test sections. The 81SPS-3 test sites were initiated between 1990 and 1991 and were distributed across the United States and Canada. Each of the SPS-3 test sites consisted of 4 test sections for a total of 324 test sections. Fifty-one of the 81 test sites had control sections labeled 340. Each of the other 30sites were linked to a GPS test section (listed in table 35 along with their SHRP ID), which could be used as control sections.⁽⁶⁰⁾

Table 35. Linked GPS sections that serve as control sections.⁽⁶⁰⁾

Site ID	Linked GPS Section ID
04_A300	4_1036
04_B300	4_1021
04_D300	4_1016
05_A300	5_3071
08_B300	8_2008
12_A300	12_9054
12_B300	12_3997
12_C300	12_4154
16_A300	16_1020
16_B300	16_1021
16_C300	16_1010
28_A300	28_1802
30_A300	30_1001
32_A300	32_1021
32_C300	32_2027
40_B300	40_1015
40_C300	40_4088
47_A300	47_3101
47_B300	47_3075
47_C300	47_1023
48_D300	48_2172
48_G300	48_1169
49_A300	49_1004
49_B300	49_1017
49_C300	49_1006
53_A300	53_1008
53_B300	53_1501
53_C300	53_1801
56_A300	56_1007
56_B300	56_7775

Each of the four SPS-3 test sections in each test site was subjected to one of the following treatments (note that the numbers in parentheses are the LTPP designation of the treatment; for example, the designation of the joint and crack sealing is 410):

Thin overlay (310).
Slurry seal (320).
Crack seal (330).
Aggregate seal coat; chip seal (350).

Several variables affect the performance of the treated pavement sections. These include climatic region, traffic, subgrade type, and the before treatment pavement condition and distress. Unfortunately, these variables could be separated to analyze the effects of each on pavement performance. The reason is that separating the variables yields statistically insignificant numbers of test sections to be used in the analyses.

To illustrate, table 36 lists the number of test sections that were available for analyses based on separation of the following variables:

Four treatment types.
One pavement condition (IRI).
Four pavement distress types.
Four climatic regions (WF, WNF, DF, and DNF).
Three traffic levels.

Table 36. Number of test sections that have before treatment and after treatment pavement condition, distress, and traffic data.

Condition or Distress Type	Treatment Type	Number of Test Sections by Climatic Region and Traffic Level
		WF			WNF			DF			DNF
		L	M	H	L	M	H	L	M	H	L	M	H
IRI	Thin overlay	8	4	4	8	2	6	3	4	3	1	0	1
	Slurry seal	6	4	4	6	3	6	3	3	2	1	0	1
	Crack seal	7	4	4	2	1	6	3	3	2	1	0	1
	Aggregate seal coat	5	4	4	7	2	5	3	3	3	0	1	1
Rut depth	Thin overlay	4	2	2	4	2	7	2	1	2	0	0	1
	Slurry seal	4	1	2	4	2	8	2	2	2	1	0	1
	Crack seal	4	1	3	2	0	6	3	2	2	0	0	1
	Aggregate seal coat	1	2	1	5	1	9	2	2	1	0	0	1
Alligator cracking	Thin overlay	4	2	4	4	2	5	1	0	0	0	0	0
	Slurry seal	1	0	3	5	0	5	0	0	0	0	0	0
	Crack seal	1	0	1	3	2	2	0	0	0	0	0	0
	Aggregate seal coat	2	0	3	4	0	3	0	0	0	0	0	0
Longitudinal cracking	Thin overlay	4	2	4	4	2	5	1	0	0	0	0	0
	Slurry seal	2	0	3	5	0	5	0	0	0	0	0	0
	Crack seal	1	0	3	2	3	3	0	0	0	0	0	0
	Aggregate seal coat	3	0	3	4	0	4	0	0	0	0	0	0
Transverse cracking	Thin overlay	4	2	4	4	2	5	1	0	0	0	0	0
	Slurry seal	1	0	3	5	0	4	0	0	0	0	0	0
	Crack seal	2	0	3	3	2	3	0	0	0	0	0	0
	Aggregate seal coat	2	0	3	4	0	3	0	0	0	0	0	0
Note: For each pavement condition and distress type, the test section was analyzed if the database contained at least one data point before treatment and/or three or more data points after treatment that could be modeled. L = low traffic (0 to 60,000 yearly ESAL). M = medium traffic (61,000 to 120,000 yearly ESAL). H = high traffic (> 120,000 yearly ESAL).

It can be seen from the table that in some cells, especially in the DF and DNF regions and for some pavement distress types, the number of available test sections for analyses was not significant (ranges from 0 to 2). Therefore, the analyses were conducted to assess the impact of each treatment type in each climatic region and for each pavement condition and distress type. That is, the data were not separated based on traffic level, type of base and subbase, or type of roadbed.

Nevertheless, the analyses of the impacts of each of the four treatment types on pavement performance were accomplished using the following steps:

Step 1: For each treated pavement test section in the SPS-3 experiment, the available pavement condition (IRI) and distress data in the LTPP database were used to calculate RFP and RSP of that section after treatment.
Step 2: For each SPS-3 test section, each pavement condition and distress type, and for each pavement treatment type, the minimum and maximum RFPs and RSPs were calculated and tabulated. Further, the averages of RFP and RSP of all test sections located in the same climatic region were also calculated and tabulated.
Step 3: The time-dependent pavement condition and distress data of each control section and/or linked GPS section in the SPS-3 experiment were used to calculate RFP and RSP of that section after the assignment date.

Results of the analyses are discussed per pavement condition and distress type in the next five subsections.

IRI

Listed in table 37 are the calculated minimum, maximum, and average RFPs based on IRI data for the SPS-3 test sections that were subjected to the same treatment type and located in the same climatic region. The table also includes the same data for the associated control sections. To assist the reader in interpreting the data, the numbers listed in the first row of the table, for example, indicate the following:

There were 19 SPS-3 test sections in the WF region that were subjected to thin overlay and accepted for analyses. The minimum, maximum, and average RFP of the 19 SPS-3 test sections were 4, 20, and 16 years, respectively.
There were 21 control sections in the WF region with a minimum RFP of 0 years, a maximum RFP of 19 years, and an average RFP of 11 years.
The difference in the average RFP of the test sections and the average RFP of the control sections was 5 years. That is, on average, RFP of the treated sections was 5 years longer than the control sections.

Table 37. Impacts of various maintenance treatments and control section on pavement performance in terms of RFP based on IRI.

Climatic Region	Treatment Type	Remaining Functional Period (Years)								Difference in RFP (Years)
		Test Sections				Control Sections
		Number of Sections	Min	Max	Average	Number of Sections	Min	Max	Average
		Number of Sections	Min	Max	Average	Number of Sections	Min	Max	Average
WF	Thin overlay	19	4	20	16	21	0	19	11	5
WNF		23	8	20	18	29	3	19	14	4
DF		13	5	20	17	13	2	19	12	5
DNF		3	3	13	9	4	3	18	11	–2
WF	Slurry seal	15	0	20	12	21	0	19	11	1
WNF		22	4	20	19	29	3	19	15	4
DF		13	4	20	14	13	2	19	13	1
DNF		2	9	20	15	4	3	18	11	4
WF	Crack seal	18	0	20	11	21	0	19	11	0
WNF		12	1	20	16	29	3	19	14	2
DF		13	3	20	15	13	2	18	12	3
DNF		4	6	20	14	4	3	18	11	3
WF	Aggregate seal coat	16	0	20	13	21	0	19	11	2
WNF		21	14	20	19	29	3	19	15	4
DF		13	1	20	14	13	2	19	13	1
DNF		3	4	10	7	4	3	18	11	4
Max = Maximum. Min = Minimum.

Examination of the results of the analyses listed in table 37 indicates the following:

Test sections that were subjected to thin overlay treatment performed better than the control sections by 5, 4, and 5 years in the WF, WNF, and DF regions, respectively, while they performed worse in the DNF region by 2 years. The reason for the latter was that the construction of the overlay caused increases in the IRI of all test sections in the DNF region. For example, the IRI of test section 04B310 increased from 88.564inches/mi (1.3978 m/km) before the overlay to 95.901 inches/mi (1.5136 m/km) after the overlay.
Test sections that were subjected to slurry seal performed better than the control sections by 1, 4, 1, and 4 years in the WF, WNF, DF, and DNF regions, respectively.
Test sections that were subjected to crack seal performed better than the control sections by 2, 3, and 3 years in the WNF, DF, and DNF regions, respectively. Further, crack sealing had no impact on pavement performance in the WF region.
Test sections that were subjected to aggregate seal coat performed better than the control sections by 2, 4, and 1 year in the WF, WNF, and DF regions, respectively, while they performed worse in the DNF regions by 4 years. Once again, the reason for the decreasing performance in the DNF regions was that construction of the aggregate seal coat increased the IRI of two of the three test sections.

For some of the SPS-3 test sections, the LTPP database contained one or more IRI data points before the sections were subjected to maintenance treatments. To assess the impact of the before treatment pavement conditions on the after treatment pavement performance, for each maintenance treatment type, RFPs after treatment were plotted against the last collected IRI data point before treatment. The results are shown in figure 52 through figure 55 for thin overlay, slurry seal, crack seal, and aggregate seal coat, respectively. Although the data in the figures are widely scattered, the general trend is that the higher the IRI is before treatment, the lower the RFP is after treatment. This finding was expected and supports the notion that maintaining pavement sections in good conditions pays higher dividends than treating deteriorated sections. Nevertheless, the scattering of the data in figure 52 through figure 55 was likely caused by differences in the original pavement cross sections, pavement materials, roadbed soil, climatic region, and traffic level. Unfortunately, the number of test sections subjected to the same traffic level bracket was so small that no decision regarding the impacts of traffic could be made with any level of certainty. Note the solid best fit curves in figure 52 through figure 55 are not intended to model the data. They show only the global trend, and therefore the inclusion of statistics such as standard error would be meaningless. As stated previously, the data in the figures are a function of many other variables that were not included in the analyses. Because separation of variables yielded data for few test sections (two or fewer), no decision could be made with any degree of certainty.

1 inch/mi = 0.0158 m/km.

Figure 52. Graph. After-treatment RFP versus before-treatment IRI of LTPP SPS-3 test sections subjected to thin overlay.

1 inch/mi = 0.0158 m/km.

Figure 53. Graph. After-treatment RFP versus before-treatment IRI of LTPP SPS-3 test sections subjected to slurry seal.

1 inch/mi = 0.0158 m/km.

Figure 54. Graph. After-treatment RFP versus before-treatment IRI of LTPP SPS-3 test sections subjected to crack seal.

1 inch/mi = 0.0158 m/km.

Figure 55. Graph. After-treatment RFP versus before-treatment IRI of LTPP SPS-3 test sections subjected to aggregate seal coat.