TACA Conveyor - Winter 2007 - (Page 19) Quality Control of Aggregates Using the Micro-Deval Abrasion Test By Caroline H. Herrera, P. E. Texas Department of Transportation T he Texas Department of Transportation (TxDOT) implemented test method Tex-461-A, “Degradation of Coarse Aggregate by Micro-Deval Abrasion” in Items 302, 340, 341, 342, 344, and 346 in the 2004 Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges to control the quality and consistency of aggregates at the project and at the mixing plant. The test is an indicator of a coarse aggregate’s durability and toughness (resistance to degradation caused by traffic loading). The procedure requires a 1500-gram sample of aggregate, graded in accordance with TxDOT’s normalized gradations specified in test method Tex-411-A, “Soundness of Aggregate Using Sodium Sulfate or Magnesium Sulfate” (magnesium sulfate soundness). The sample is placed in a stainless steel jar with 2.0 liters of water and an abrasive charge consisting of 5,000 grams of 3/8 inch diameter steel balls. The jar is rotated at a rate of 100 revolutions per minute for two hours. Afterwards, the sample is washed, oven-dried and sieved to determine the amount of loss. The micro-deval abrasion test (micro-deval) is an excellent tool for aggregate producers to use as a quality control test to monitor their daily production. It has good repeatability, has quick turnaround, and the equipment is fairly inexpensive. In comparing the magnesium sulfate soundness test to the microdeval, it is evident that the degradation mechanism of the two tests is different, and as such, cannot replace each other on a one-for-one basis. However, the aggregate quality trends indicated by both tests correlate fairly well. This relationship allows the producer to use the micro-deval as a screening tool to indicate when the magnesium sulfate soundness of the aggregate is approaching unacceptable levels. A pit-specific maximum micro-deval value can be determined. The quality control micro-deval test results can then be plotted on a control chart and correlated to mining locations within the pit. Production managers can use this as a tool to help make decisions about production changes. Examples can be seen in Figure 1 and Figure 2. TxDOT’s Geotechnical, Soils and Aggregates Branch of the Construction Division performs the micro-deval on all coarse aggregate samples tested for the Department’s Aggregate Quality Monitoring Program (AQMP). TxDOT uses the test results in conjunction with magnesium sulfate soundness and Los Angeles Abrasion results to monitor aggregate quality. Currently, TxDOT specifications require sampling and testing hot mix asphalt concrete and surface treatment aggregate stockpiles by Tex-461-A at the mixing plant or aggregate quarry at a minimum rate of once per ten days production per project. TxDOT uses the result obtained from Tex-461-A to decide whether to sample and test the stockpile to determine magnesium sulfate soundness loss. If the decision is ‘yes,’ TxDOT rejects the stockpile if the sample fails to meet the specification requirements for magnesium sulfate soundness. A procedure similar to this can be used to monitor concrete aggregate during construction. For more information about this topic, please contact Caroline Herrera, P. E., at 512-506-5907 or cherrer@dot.state.tx.us. Caroline is the Manager of TxDOT’s Geotechnical, Soils and Aggregates Branch. She is also a member of TACA’s Specification Committee and works closely with industry and academic experts on a variety of soils and aggregate issues. Aggregate A Concrete Magnesium Soundness versus Micro-Deval 40 30 y = 1.7195x - 33.416 R2 = 0.9435 % Mg(conc) Loss 35 25 20 15 10 5 0 30 Perfect Fit 25 % Loss 20 Mg Con Micro-Deval 15 10 5 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 24 28 32 36 0 Sample No. (Earliest to Latest) % Micro-deval Loss TACA Conveyor • Winter 2007 19
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