Following is a description of each of the General Pavement Study (GPS) experiments:

GPS-1: Asphalt Concrete (AC) on Granular Base

Sections in this experiment include a dense-graded hot mix asphalt concrete (HMAC) surface layer, with or without other HMAC layers, placed over an untreated granular base. One or more subbase layers may also be present but are not required. "Full-depth" AC pavements are also included in this study. The pavements include an HMAC surface layer and usually one or more HMAC layers beneath the surface with a minimum total HMAC thickness of 150 mm (6 inches) placed directly upon treated or untreated subgrade. Seal coats or porous friction courses are allowed on the surface but not in combination. Sampling design factors for this study are moisture, temperature, subgrade type, traffic rate, AC stiffness, base thickness, and AC thickness. All factors have two levels except AC thickness, which has three.

GPS-2: AC on Bound Base

Pavements studied for GPS-2 include a dense-graded HMAC surface layer with or without other HMAC layers, placed over a bound base layer. To account for a variety of bound base types in the sampling design, two classifications of binder types—bituminous and nonbituminous—are defined as the factor levels. Bituminous binders include asphalt cements, cutbacks, emulsions, and road tars. Nonbituminous binders include all hydraulic cements, lime, fly ashes, and natural pozzolans, or combinations thereof. Stabilized bases with lower quality materials, such as sand asphalt or soil cement, are also possible. Stabilization practices of primary concern for this study are those in which the structural characteristics of the material are improved due to the cementing action of the stabilizing agent. Seal coats or porous friction courses are permitted on the surface but not in combination. Project selection included those constructed on both fine and coarse subgrades. Sampling design factors for this study are moisture, temperature, subgrade type, traffic rate, surface thickness, base thickness, and binder type. All factors have two levels.

GPS-3: Jointed Plain Concrete Pavement (JPCP)

Pavements studied in GPS-3 include a jointed, unreinforced portland cement concrete (PCC) slab placed over most types of base layers (excluding a cracked and seated PCC layer or a soil-aggregate mixture with a predominantly fine-grained soil). One or more subbase layers may also be present but are not required. The joints may have either no load transfer devices or smooth dowel bars. Jointed slabs with load transfer devices other than dowel bars are not acceptable. A seal coat is also permissible above a granular base layer. Sampling design factors for this study are moisture, temperature, subgrade type, traffic rate, dowels, PCC thickness, and base type. All factors have two levels.

GPS-4: Jointed Reinforced Concrete Pavement (JRCP)

Pavements in the GPS-4 study include a jointed, reinforced PCC pavement with doweled joints spaced less than 6.1 m (20 ft) apart. The slab may rest on most types of base layers (excluding a cracked and seated PCC layer, a soil-aggregate mixture with a predominantly fine-grained soil, and unstabilized coarse-grained subgrade). A base layer and one or more subbase layers may exist but are not required. A seal coat is also permissible above a granular base layer. Jointed reinforced concrete pavements without load transfer devices or using devices other than smooth dowel bars at the joints are not included. Sampling design factors for this study are moisture, temperature, subgrade type, traffic rate, joint spacing, and PCC thickness. All factors have two levels.

GPS-5: Continuously Reinforced Concrete Pavement (CRCP)

Pavements studied in GPS-5 include continuously reinforced PCC slab placed over most types of base layers (excluding a cracked and seated PCC layer, a soil-aggregate mixture with a predominantly fine-grained soil, and unstabilized coarse-grained subgrade). One or more subbase layers may exist but are not required. A seal coat is also permissible above a granular base layer. Sampling design factors for this study are moisture, temperature, subgrade type, traffic rate, percent reinforcing, and PCC thickness. All factors have two levels.

GPS-6: AC Overlay of AC Pavement

The GPS-6 data include sections that were part of the original Long-Term Pavement Performance (LTPP) experimental design for rehabilitated pavements, as well as those that have been added in response to changes in practice. The GPS-6A and -6B experiments are part of the original design. Sections that are classified as GPS-6C, -6D, or -6S have been retained in the LTPP program but do not have an experimental design associated with them.

Pavements included for GPS-6A, Existing AC Overlay of AC, and GPS-6B, Planned AC Overlay of AC, have a dense-graded HMAC surface layer with or without other HMAC layers placed over a previously existing AC pavement meeting the requirements of GPS-1 or GPS-2. Seal coats or porous friction courses are allowed but not in combination. Fabric interlayers and Stress Absorbing Membrane Interlayers are permitted between the original surface and the overlay. The total thickness of HMAC used in the overlay is at least 25 mm (1.0 inch). Pavements that have been overlaid more than once since they were originally constructed are not studied.

Sampling design factors for GPS-6A are moisture, temperature, subgrade type, traffic rate, original pavement structural number, overlay stiffness, and overlay thickness.

Sampling design factors for GPS-6B are moisture, temperature, subgrade type, traffic rate, original pavement condition number, original pavement structural number, and overlay thickness. All factors have two levels. Data requests for sections in GPS-6B will contain the information from the previous GPS-1 or -2 study period.

Rehabilitated AC surfaced pavements may also be classified as GPS-6C, -6D, or -6S. The table below shows how a section may evolve to obtain such a designation.

GPS-7: AC Overlay of PCC

The GPS-7 data include sections that were a part of the original LTPP experimental design for rehabilitated pavements, as well as those that have been added in response to changes in practice. The GPS-7A and -7B experiments are part of the original design. Sections that are classified as GPS-7C, -7D, or -7S have been retained in the LTPP program, but do not have an experimental design associated with them.

Pavements studied in GPS-7A, Existing AC Overlay of PCC, and GPS-7B, Planned AC Overlay of PCC, include a dense-graded HMAC surface layer with or without other HMAC layers placed on a JPCP (GPS-3), JRCP (GPS-4), or CRCP (GPS-5). The slab may rest on any combination of the base or subbase layers excluding a cracked and seated PCC layer or a soil-aggregate mixture with a predominantly fine-grained soil. The previously existing concrete slab may also have been placed directly on lime or cement-treated fine or coarse-grained subgrade or on untreated coarse-grained subgrade. Slabs placed directly on untreated fine-grained subgrade are not included. Seal coats or porous friction courses are allowed but not in combination. Fabric interlayers and stress absorbing membrane interlayers are allowed when placed between the original concrete surface and the overlay. Overlaid pavement with aggregate interlayers and open-graded AC are not included in this study. The total thickness of HMAC used in the overlay must be at least 38 mm (1.5 inches). Pavements that have been overlaid more than once since they were originally constructed are not included.

Sampling design factors for GPS-7A are moisture, temperature, subgrade type, traffic rate, original pavement type, overlay stiffness, and overlay thickness.

Sampling design factors for GPS-7B are moisture, temperature, subgrade type, traffic rate, original pavement condition level, original pavement type, and overlay thickness. All factors have two levels except the original pavement type, which has three levels. Data requests for sections in GPS-7B will contain the information from the previous GPS-3, GPS-4, or GPS-5 study period.

Rehabilitated PCC surfaced pavements may also be classified as GPS-7C, -7D, -7R, -7F, or -7S. The table below shows how a section may evolve to obtain such a designation. Data requests for sections in these categories will contain the information from the previous study period(s).

GPS-9: Unbonded PCC Overlays on PCC Pavements

Study pavements for GPS-9 include unbonded JPCP, JRCP, or CRCP overlays with a thickness of 125 mm (5 inches) or more placed over an existing JPCP, JRCP, or CRCP. An interlayer used to prevent bonding of the two slabs is required. The overlaid concrete pavement may rest on most base and subbase types or directly on the subgrade. Sampling design factors include moisture, temperature, overlay thickness, original pavement type, and overlay type. All factors have two levels except original pavement type and overlay type, which have three.