Featured · Valley Metro, Phoenix

Two million load cycles on a fiber-reinforced track slab

This 12 inch slab carries a rail but no rebar, only hooked steel fibers. After one load cracked it, the slab was pushed down two million times at 4 Hz, the crack opening and closing with every cycle, and it lost 8.4% of its stiffness. The same fiber mix now forms the track slab of the Valley Metro Northwest Extension Phase II in Phoenix.

Load cycles

0/ 2,000,000

Load0kN

Deflection0mm

Crack width0mm

Stiffness—kN/mm

The 3D test needs WebGL. The charts beside it still replay the record.

DIC strain Deformation ×15

Load, deflection and stiffness are the laboratory records of slab SFRC2_#2, tested at ASU in August 2019. The strain band and the crack width are the DIC of the same slab, from the recorded burst nearest the cycle count. Deformation is magnified, and the laboratory around the slab is drawn for illustration.

2,000,000
load cycles at 4 Hz, 0 to 18 kips (80 kN) on the rail
8.4%
stiffness lost over those cycles, 178 to 163 kip/in (31 to 28.5 kN/mm)
$12.1 M
saved per mile of track against the rebar design, 69% of its cost
110 days
less construction per mile, 121 days against 231

The real test

What the cameras recorded

A camera above the slab filmed the rig running, and a second camera filmed the speckle pattern on the side face at 800 frames a second, in nine bursts spread over the two million cycles. Correlated, those images show one crack that opens and closes with every cycle. From 250,000 cycles on it opened 0.84 to 0.87 mm at the soffit under 16 kips, and it neither grew upward nor ratcheted open before the test ended.

Slab SFRC2_#2 under the actuator, with the timber cage and pipe clamps that held the loading box on the rail for six days.

DIC of the same slab over one load cycle, at 1,000 and at 2,020,000 cycles, with the load against the crack width at the soffit.

The fibers

Hooked steel fibers instead of rebar

Two steel fibers were tested in full-size slabs. The 25 mm twisted fiber at 15 kg/m³ (0.2% by volume) could not hold its crack, and its fatigue test stopped after 70,000 cycles. The 60 mm hooked fiber at 39 kg/m³ (0.5%) carried the two million, so the hooked mix went into the track.

A hooked-end steel fiber about 60 mm long beside a ruler — **Hooked, 60 mm**39 kg/m³, 0.5% by volume, 2,000,000 cycles

A twisted steel fiber about 25 mm long beside a ruler — **Twisted, 25 mm**15 kg/m³, 0.2% by volume, stopped at 70,000 cycles

Where it runs

Northwest Extension Phase II, Phoenix

The extension carries the line 1.6 miles from 19th Avenue and Dunlap across I-17 to Metro Parkway, with three new stations. It opened on January 27, 2024, a year ahead of its planned 2025 opening, and finished 2.3% under budget with more than $6.5 million of contingency left.

The 3D map needs WebGL.

Drag to turn the map. Cars move at the average daily density of the ADOT counts. On the current schedule 182 trains a weekday pass 25th Ave/Dunlap.Aerials USDA NAIP. Terrain USGS 3DEP. Traffic ADOT 2024. Schedule Valley Metro GTFS. Map data © OpenStreetMap contributors.

On site

Built without rebar

Without rebar the crews no longer cut, tied and welded cages, or tested the welds for stray-current continuity. The field cost data put the in-place slab at $1,013 per track foot against $3,309 for the rebar design, and one mile of track took 121 days instead of 231, with 187,740 lb of hooked fiber replacing 482,000 lb of bars in each mile.

Formwork and rails in their rubber boots ready for the fiber concrete, North Mountain behind

Workers placing the fiber concrete between the rails from a pump hose

A concrete pump boom over the track slab formwork

The hooked-fiber track slab going in on the Northwest Extension, May 5, 2023.

Sources

D. Patel, C. Pleesudjai, A. Haines, T. Santana and B. Mobasher, Implementation of Fiber-Reinforced Track Slabs in Light-Rail Systems, manuscript in review. Test results, fiber mixes and field cost data.
ASU Structural Engineering Laboratory, Structural Testing and Design Verification of Fiber Reinforced Concrete Structural Slabs for Phoenix Metro Light Rail, final report to Kiewit McCarthy JV, February 2020. Test records and photographs.
DIC reduction of the side-face images of slab SFRC2_#2, nine bursts from 1,000 to 2,020,000 cycles, 2026. Strain maps, crack path and crack width at the soffit.
Valley Metro, Northwest Extension Phase II fact sheet, March 2024. Hill International, Northwest Extension Phase II project page. Schedule and budget.
USDA National Agriculture Imagery Program, USGS 3D Elevation Program, ADOT 2024 Average Annual Daily Traffic, Valley Metro GTFS (City of Phoenix open data) and OpenStreetMap contributors. Map layers.