Overview The Los Angeles (L.A.) abrasion test (Figure 1) is a common test method used to indicate aggregate toughness and abrasion characteristics. Aggregate abrasion characteristics are important because the constituent aggregate in HMA must resist crushing, degradation and disintegration in order to produce a high quality HMA.

The standard L.A. abrasion test subjects a coarse aggregate sample (retained on the No. 12 (1.70 mm) sieve) to abrasion, impact, and grinding in a rotating steel drum containing a specified number of steel spheres. After being subjected to the rotating drum, the weight of aggregate that is retained on a No. 12 (1.70 mm) sieve is subtracted from the original weight to obtain a percentage of the total aggregate weight that has broken down and passed through the No. 12 (1.70 mm) sieve. Therefore, an L.A. abrasion loss value of 40 indicates that 40% of the original sample passed through the No. 12 (1.70 mm) sieve. The standard Los Angeles abrasion test is: AASHTO T 96 or ASTM C 131: Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine

Background

Aggregates undergo substantial wear and tear throughout their life. In general, they should be hard and tough enough to resist crushing, degradation and disintegration from any associated activities including manufacturing (Figure 2), stockpiling (Figure 3), production (Figure 4), placing (Figure 5) and compaction (Figure 6) (Roberts et al., 1996[1]). Furthermore, they must be able to adequately transmit loads from the pavement surface to the underlying layers and eventually the subgrade. These properties are especially critical for open or gap graded HMA, which do not benefit from the cushioning effect of the fine aggregate and where coarse particles are subjected to high contact stresses (Wu et al., 1998[2]). Aggregates not adequately resistant to abrasion and polishing may cause premature structural failure and/or a loss of skid resistance. Furthermore, poor resistance to abrasion can produce excessive dust during HMA production resulting in possible environmental problems as well as mixture control problems.

Test Concept

Summary A sample of aggregate retained on the No. 12 (1.70 mm) sieve is placed inside a rotating steel drum containing a specified number of steel spheres or “charge”. As the drum rotates, a shelf inside the drum picks up the aggregate and steel spheres. The shelf carries them around until they drop on the opposite side of the drum, subjecting the aggregate to impact and crushing. Then, the aggregate is subjected to abrasion and grinding as the drum continues to rotate until the shelf picks up the contents, and the process is repeated. The drum is rotated for a specified number of revolutions. Afterward, the aggregate is removed from the drum and sieved on a No. 12 (1.70 mm) sieve. The aggregate retained on the sieve is weighed and the difference between this weight and the original weight is expressed as a percentage and reported as the L.A. abrasion loss value. Figure 11 shows major equipment used in the L.A. abrasion test.

Approximate Test Time

3 days from aggregate sampling to final weight determination. A breakdown of testing time

follows:

Reducing a sample to testing size 5 – 10 minutes

Washing the sample 5 – 10 minutes

Drying to a constant mass 8 – 12 hours (overnight)

Time in rotating drum 15 minutes

Sieving and rewashing 30 minutes

Drying to a constant mass 8 – 12 hours (overnight)

Final weighing 5 – 10 minutes

Basic Procedure Obtain the aggregate sample to be tested, and reduce the sample to adequate size

Wash the sample and oven dry to a constant mass at 230ºF (110ºC) After drying, sieve the material into individual size fractions, and recombine to one of

four specified gradings that most nearly represents the aggregate gradation as received. Record the total sample mass. The total sample mass should be about 5000 g.

Place the sample and the specified number of steel spheres into the drum and rotate for 500 revolutions at 30 to 33 rev/min. The charge required is dependent upon the grading used.

Discharge the material and sieve the aggregate over a sieve coarser than a 1.70-mm

(No. 12) sieve.

Sieve the finer material on a No. 12 (1.70 mm) sieve. Wash the aggregate coarser than the No. 12 (1.70 mm) sieve and oven-dry to a

constant mass at 230ºF (110ºC). After cooling, determine the mass. .

Specifications

Material Value Specification HMA Distress of Concern

Coarse Aggregate % Loss Varies1 Deformation, skid resistance

Table 1: Source Property L.A. Abrasion Specifications

Note 1 There is no standard L.A. abrasion specification for Superpave mix design; specifications are typically established by state or local agencies. Typically U.S. state specifications limit the abrasion of coarse aggregate for HMA use to a maximum ranging from 25 to 55 percent, with most states using a specification of 40 or 45 percent (Figure 14). Requirements for Portland Cement Concrete (PCC) tend to be similar, while requirements for specialized mixes such as Stone Matrix Asphalt (SMA) tend to be lower; AASHTO specifies a maximum L.A. abrasion loss of 30 percent for SMA. Calculations (Interactive Equation).

Determine the percent loss as a percentage of the original sample mass.