Bending beam rheometer, the low-temperature binder test. A small beam of PAV-aged binder, about 127 mm long and 6.35 mm thick, sits in a cold bath and takes a constant load at midspan for 240 seconds while its deflection is recorded. The results, read at 60 seconds, are creep stiffness S, which must stay at or below 300 MPa, and the m-value, the rate the binder relaxes stress, which must be at least 0.300. The test runs 10 C warmer than the grade temperature, so a -22 binder is tested at -12 C.
BBR m-value came in at 0.296, that binder's failing low temp, call the supplier before another load goes in the tank.
Binders carrying lots of recycled material tend to fail on m-value before stiffness, which is why labs watch Delta Tc, the gap between the stiffness and m-value critical temperatures.
Also heard as
- bending beam rheometer
Related terms
PG
Performance Grade: the binder rating that names the hottest and coldest pavement temperatures a binder can handle.
PAV
Pressure aging vessel: the test that ages binder under heat and pressure to simulate years of service on the road.
Thermal cracking
Transverse cracks across the lane caused by the pavement shrinking and snapping in very cold weather.
DSR
Dynamic shear rheometer: the instrument that measures a binder's stiffness and elasticity by twisting a thin disk of it.