Los Angeles Abrasion

Publish date: JUNE 28, 2015 | Author: Pavement Interactive

Cite

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.

Figure 1: L.A. abrasion testing equipment.

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.

Figure 2: Aggregate manufacturing.

Figure 3: Aggregate stockpiles.

Figure 4: HMA production.

Figure 5: Placing HMA.

Figure 6: HMA Compaction.

Because of the preceding issues, a test to predict aggregate toughness and abrasion resistance is valuable. The L.A. abrasion test is the predominant test in the U.S.; it is used by 47 States (Wu et al., 1998[2]).

Test Concept

The L.A. abrasion test measures the degradation of a coarse aggregate sample that is placed in a rotating drum with steel spheres (Figure 7). As the drum rotates the aggregate degrades by abrasion and impact with other aggregate particles and the steel spheres (called the “charge”). Once the test is complete, the calculated mass of aggregate that has broken apart to smaller sizes is expressed as a percentage of the total mass of aggregate. Therefore, lower L.A. abrasion loss values indicate aggregate that is tougher and more resistant to abrasion.

Figure 7: Steel spheres.

Test Adequacy

The L.A. Abrasion test is an empirical test; it is not directly related to field performance of aggregates. Field observations generally do not show a good relationship between L.A. abrasion values and field performance. Wu et al. (1998[2]) found that L.A. abrasion loss was unable to predict field performance. Specifically, the test may not be satisfactory for some types of aggregates. Some aggregates, such as slag and some limestones, tend to have high L.A. abrasion loss but perform adequately in the field. L.A. abrasion loss seems to be reasonable well correlated with dust formation during handling and HMA production in that aggregates with higher L.A. abrasion loss values typically generate more of dust.

Micro-Deval Test: an Alternative to L.A. Abrasion

Wu et al. (1998[2]) found the Micro-Deval apparatus to be the only commonly used test that had adequate predictive abilities concerning toughness and abrasion resistance. The Micro-Deval also uses a rotating drum (Figure 8) with steel spheres but the drum is much smaller as are the spheres (Figure 9). The result is that the Micro-Deval test tends to polish (smoothen) aggregate particles (Figure 10) while the L.A. abrasion test tends to break them. Video 1 shows a Micro-Deval test in progress.

Figure 8: Drum used in the Micro-Deval apparatus.

Figure 9: Steel spheres used in the Micro-Deval test.

Figure 10: Aggregate particles before and after Micro-Deval test.

The standard Micro-Deval test is:

• AASHTO TP 58: Resistance of Coarse Aggregate to Degradation by Abrasion in the Micro-Deval Apparatus

Test Description

The following description is a brief summary of the L.A. abrasion test. It is not a complete procedure and should not used to perform the test. The complete testing procedure can be found in:

• AASHTO T 96 or ASTM C 131: Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine

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.

Figure 11: 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

1. Obtain the aggregate sample to be tested, and reduce the sample to adequate size (Figure 12).
   

   Figure 12: Splitting an aggregate sample for the L.A. abrasion test.
2. Wash the sample and oven dry to a constant mass at 230ºF (110ºC).
3. 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.
4. Place the sample and the specified number of steel spheres into the drum and rotate for 500 revolutions at 30 to 33 rev/min (Video 2). The charge required is dependent upon the grading used.
5. 
   
6. 
   
7. 
   
8. 
   
9. 
   
10. 
    
11. 
    
12. Video 2: LA abrasion test.

1. Discharge the material and sieve the aggregate over a sieve coarser than a 1.70-mm (No. 12) sieve (Figure 13).
   

   Figure 13: Sieving the sample after the L.A. abrasion test.
2. Sieve the finer material on a No. 12 (1.70 mm) sieve.
3. 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.

Results

Parameters Measured

L.A. abrasion loss as a percentage by weight.

Specifications

Table 1: Source Property L.A. Abrasion Specifications

Material	Value	Specification	HMA Distress of Concern
Coarse Aggregate	% Loss	Varies1	Deformation, skid resistance

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.

Figure 14: Agency L.A. abrasion specifications. Includes 49 states (Maine uses the Micro-Deval), FHWA, FAA and a California District 2 specification for a total of 52 "agencies". (from Uhlmeyer, 2003)

Typical Values

Typical L.A. abrasion values are shown in Table 2.

Table 2: Typical L.A. Abrasion Loss Values

Rock Type	L.A. Abrasion Loss (by percent weight)
General Values
Hard, igneous rocks	10
Soft limestones and sandstones	60
Ranges for specific rocks
Basalt	10 – 17
Dolomite	18 – 30
Gneiss	33 – 57
Granite	27 – 49
Limestone	19 – 30
Quartzite	20 – 35

Figure 15 shows typical before and after L.A. abrasion aggregate samples.

Figure 15: Samples before and after the L.A. abrasion test.

Calculations (Interactive Equation)

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

Where:

Moriginal = original sample mass (g)

Mfinal = final sample mass (g)

Report this value as the percent loss.

Kingdom of Saudi Arabia

Ministry of Communications

Materials and Research Department

Standard Method of Test for

Resistance to Abrasion of Small Size Coarse Aggregate

by Use of the Los Angeles Machine

MRD Test Method 309

(Adaptation of AASHTO T 96‑77)

1. Scope

1.1 This method covers a procedure for testing sizes of coarse aggregate smaller than 37.5 mm (1½ in.) for resistance to abrasion using the Los Angeles testing machine.

Note 1: A procedure for testing coarse aggre­gate larger than 19.0 mm (¾ in.) is covered in the Method of Test for Resistance to Abrasion of Large Size Coarse Aggregate by the Los Angeles Machine (ASTM C 535).

2. Apparatus

2.1 Los Angeles Machine‑The Los Angeles abrasion testing machine equipped with a counter and conforming in all its essen­tial characteristics to the design shown in Fig. 1, shall be used. The machine shall consist of a hollow steel cylinder, closed at both ends, having an inside diameter of 711 ± 5 mm (28 ± 2 in.) and an inside length of 508 ± 5 mm (20 ± 0.2 in.). The cylinder shall be mounted on stub shafts attached to the ends of the cylin­der but not entering it, and shall be mounted in such a manner that it may be rotated with the axis in a horizontal position within a tolerance in slope of 1 in 100. An opening in the cylinder shall be provided for the introduction of the test sample. A suitable, dust tight cover shall be provided for the opening with means for bolting the cover in place. The cover shall be so designed as to maintain the cylindrical contour of the interior surface unless the shelf is so located that the charge will not fall on the cover, or come in contact with it during the test. A removable steel shelf extending the full length of the cylinder and projecting inward 89 ± 2 mm (3.5 ± 0.1 in.) shall be mounted on the interior cylindrical surface of the cylinder or on the inside surface of the cover, in such a way that a plane center between the large faces coincides with an axial plane. The shelf shall be of such thickness and so mounted by bolts or other suitable means as to be firm and rigid. The position of the shelf shall be such that the distance from the shelf to the opening, meas­ured along the outside circumference of the cyclinder and direction of rotation shall not be less than 1270 mm (50 in.).

Note 2: The use of a shelf of wear resistant steel, rectangular in cross section and mounted independently on the cover, is preferred. However, a shelf consisting of a section of rolled angle, properly mounted on the inside of the cover plate, may be used provided the di­rection of rotation is such that the charge will be caught on the outside face of the angle. If the shelf becomes distorted from its original shape to such an extent that the requirements given in A.2 of the Appendix to this method are not met, the shelf shall either be repaired or replaced before additional abrasion tests are made.

3.2 Sieves‑Conforming to Sieves for Testing Purposes (MRDTM 10 1).

3.3 Balance‑The balance shall have a capacity of 5 kg or more and shall conform to a Class E balance as described in MRDTM 102, Weights and Balances Used in Testing of High­way Materials.

3.4 Oven‑The oven shall be capable of maintaining a uniform temperature of 110 ± 5 C.

4. Abrasive Charge

4.1 The abrasive charge shall consist of steel spheres averaging approximately 46.8 mm (1 27/32 in.) in diameter and each weighing be­tween 390 and 445 g (Note 3).

Note 3: Steel ball bearings 46.0 mm (1 13/16 in.) and 47.6 mm (1 7/8 in.) diameter, weighing ap­proximately 400 to 440 g each, respectively, are readily available. Steel spheres 46.8 Mm (1 27/32 in.) in diameter weighing approximately 420 g are also used. The abrasive charge may consist of a mixture of these sizes conforming to the weight tolerances of Sec 4.1 and 4.2.

4.2 The number of spheres used in the abrasive charge will depend on the grading of test sample as described in Table 1 and shall be as follows:

Grading	Number of Spheres	Mass of Charge, g
A	12	5000 ± 25
B	11	4584 ± 25
C	8	3330 ± 20
D	6	2500 ± 15

5. Test Sample

5.1 The test sample shall consist of clean aggregate representative of the material under test. If the aggregate is dirty or coated, wash until clean (Note 4). The aggregate shall be dried to constant mass, separated into individ­ual size fractions, and recombined to the grad­ing of Table 1 most nearly corresponding to the range of sizes in the aggregate as furnished for the work. The mass of the sample prior to test shall be recorded to the nearest 5 g.

Note 4: If the aggregate is essentially free from adherent coatings and dust, the requirement for washing before and after test may be waived. Elimination of washing after test will seldom reduce the measured loss by more than about 0.2 percent of the original sample mass.

6. Procedure

6.1 The test sample and the abrasive charge shall be placed in the Los Angeles abra­sion testing machine and the machine rotated at a speed of 30 to 33 rpm for 500 ± 1 revolutions. The machine shall be so driven and so counterbalanced as to maintain a sub­stantially uniform peripheral speed (Note 5). After the prescribed number of revolutions, the material shall be discharged from the machine and a preliminary separation of the sample made on a 4.75 mm (No. 4) sieve. The finer portion shall then be sieved on a 1.70 mm (No. 12) sieve in a manner conforming to Sec 5.1 of MRDTM 204, Sieve Analysis of Fine and Coarse Aggregates. The material coarser than the 1.70 mm (No. 12) sieve shall be weighed to the nearest 5 g.

Note 5: Back‑lash or slip in the driving mecha­nism is very likely to furnish test results which are not duplicated by other Los Angeles abra­sion machines producing constant peripheral speed.

7. Calculation

7.1 The difference between the original mass and the final mass of the test sample shall be expressed as a percentage of the original mass of the test sample. This value shall be reported as the percentage of wear.

8. Report

8.1 Test results shall be recorded on Form No. MRDWS 309.

Table 1

Gradings of Test Samples

Sieve Size				Mass of Indicated Sizes, g
				Grading
Passing		Retained		A	B	C	D
mm	(in.)	mm	(in.)
37.5	(1½)	25.0	(1)	1250 ± 25	-	-	-
25.0	(1)	19.0	(¾)	1250 ± 25	-	-	-
19.0	(¾)	12.5	(½)	1250 ± 10	2500 ± 10	-	-
12.5	(½)	9.5	(3/8)	1250 ± 10	2500 ± 10	-	-
9.5	(3/8)	6.3	(¼)	-	-	2500 ± 10	-
6.3	(¼)	4.75	(No. 4)	-	-	2500 ± 10	-
4.75	(No. 4)	2.36	(No. 8)	-	-	-	5000 ± 10
Total ¼¼¼¼¼¼¼¼¼¼¼¼¼¼¼¼¼				5000 ± 10	5000 ± 10	5000 ± 10	5000 ± 10

Appendix

Maintenance of Shelf

A.1 The shelf of the Los Angeles Ma­chine is subject to severe surface wear and im­pact. With use, the working surface of the shelf is peened by the balls and tends to develop a ridge of metal parallel to and about 32 mm (1¼ in.) from the junction of the shelf and the inner surface of the cylinder. If the shelf is made from a section of rolled angle, not only may this ridge develop but the shelf itself may be bent longitudinally or transversely from its proper position.

A.2 The shelf should be inspected period­ically to determine that it is not bent either lengthwise or from its normal radial position with respect to the cylinder. If either condition is found, the shelf should be repaired or re­placed before further abrasion tests are made. The influence on the test results of the ridge developed by peening of the working face of the shelf is not known. However, for uniform test conditions, it is recommended that the ridge be ground off if its height exceeds 2 mm (0.1 in.).

Footnotes    (↵ returns to text)

1. Roberts, F.L.; Kandhal, P.S.; Brown, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.↵
2. Wu, Y.; Parker, F. and Kandhal, K. (1998). Aggregate Toughness/Abrasion Resistance and Durability/Soundness Tests Related to Asphalt Concrete Performance in Pavements. NCAT Report 98-4. National Center for Asphalt Technology. Auburn, AL. http://www.eng.auburn.edu/center/ncat/reports/rep98-4.pdf. Accessed 23 June 2004.↵