The largest use of aggregates is for highway construction. Furthermore, the majority of sales for other than highway uses is dependent upon compliance with state highway specifications because it is widely accepted that, if an aggregate is satisfactory for state highway uses, it will be safe for almost any other construction purpose. It, therefore, is essential for an aggregate producer to have a source of material that meets the specifications of the state highway department and to take all necessary steps in the processing to prevent the stigma of rejection.
Abrasion Test
Probably the first abrasion test was the Deval test, developed in France in 1878 and is still used by a few state highway departments. This test consists in subjecting a sample weighing 5,000 grams to 10,000 revolutions in a tumbling barrel and determining the amount of material passing the No. 12 sieve as the percentage of wear. There are two ASTM methods of making the Deval test: one, ASTM D 289 is for graded Coarse aggregate which includes gravel, crushed gravel, crushed stone, and crushed blast furnace slag; and the other, ASTM D 2 is for rock. The test sample for gravel is a graded sample to which is added a charge of six 1 7/8 in. steel balls, and for stone the sample consists of 50 cubical pieces hand broken from ledge rock with no charge of steel balls. Test results are not comparable between the two types of aggregate. The Deval test has been almost completely replaced by the Los Angeles test; however, the few states still using the Deval test probably will not change for a long time because the correlation between the two tests is not good with aggregates of certain mineral compositions .
It is likely that the first question which comes to mind is that if this Los Angeles test is so advantageous, why does not everyone use it. This brings us to the heart of our subject “Specifications.” The aggregates industry has developed under a specification which requires a Deval loss of not more than, say, 6 for crushed stone and 10 for gravel. Also, the performance or service record of materials as supplied was good. The problem is how to change to the Los Angeles abrasion test without opening up the specifications to inferior materials or making them more restrictive so as to reject presently acceptable material.
Soundness
All aggregate specifications have a requirement for soundness where weathering is a problem. The acceptance test most often referred to is the sodium or magnesium sulfate soundness test which is empirical, lacking in precision, has not been correlated with field performance, and has not been accepted as an ASTM standard. Nevertheless, thirty-four states use it as an acceptance test. A typical specifications is ASTM C 33-57 for Concrete Aggregates which states in part-“Coarse aggregate subjected to five cycles of the soundness test shall show a loss, weighted in accordance with the grading of a sample complying with the designated limitations set forth in Section 7, not greater than 12 per cent when sodium sulfate is used or 18 per cent when magnesium sulfate is used.” Section 7 referred to gives nine sizes or gradings used in concrete, ranging from 3 ½ to 1 ½ in. down to ¾ in. to No. 4, which must comply with the one value in the specifications.
“While reliance on service records is frequently advocated in ASTM methods of test, rarely is it used in practice. Aggregates must comply with the specifications on the basis of acceptance test results. Bloem says: “The lack of more dependable methods has led specifying agencies to put inflexible limits on soundness loss and to disregard the need for engineering judgment in the interpretation of the test results. These arbitrary limits have caused rejection of highly acceptable aggregates and permitted acceptance of materials that contributed to extensive concrete disintegration.”
As good deposits of gravel become depleted, there has been a need to recover the good gravel from marginal deposits which had been by-passed as uneconomical. Large gravel deposits in the Ohio River were badly contaminated with coal of lower specific gravity than the gravel, which led the Dravo Corporation to install, in 1951, the first heavy media separation plant for the beneficiation of gravel in this country. Gravel has been upgraded not only by removal of coal but also by the removal of unsound material such as shale, chert, and soft pieces of low specific gravity.
Shape of Particle
The foregoing discussion of abrasion and soundness encompasses the vital considerations that affect the very life of an aggregate operation. There are other requirements of specifications that must be met but may be thought of as secondary since they hardly ever necessitate the abandonment of an operation, but do have an effect on the efficiency and cost. These are shape of particle, deleterious substances, and gradation.
Specification requirements on shape of particle for crushed stone have been a source of great concern for the past few years. For years, the general statement that stone shall be free of flat and elongated pieces was in all specifications. Then, complaints became numerous on the lack of proper workability of concrete due to excessive amounts of poorly shaped stone.
Unpublished data obtained in our laboratory indicated that as the sizes became smaller, the percentage of flat and elongated pieces increased. Subcommittee C-12 of Committee D-4 of ASTM on Surface Treatments made a survey of the state highway departments last year to obtain data or experience statements as to what should be the limitation on shape of particle for this type of construction. The data from this survey formed the basis of a recommendation that was accepted and should appear in the revised specification.
Gradation
Lack of acceptance of standard gradation specifications for aggregate is the prime cause of waste and efficiency in aggregate production. The processing and stockpiling of aggregate of relative small differences in grading to comply with specifications of different agencies for the same use is far too prevalent a practice. Often a producer will set up his plant to make a grading to meet one specification and when sufficient material is in stockpile, will change the plant to make another size. If the aggregate in stockpile soon moves to the job, well and good but if open stockpiles are left to the weather for any period of time, they may become contaminated and be rejected; then the material has to be reprocessed.
Probably, the greatest departure from Simplified Practice Recommendations is in the matter of tolerances. Tolerances are a prime requirement for determining the cost of any manufactured article or processed material. A number of years ago, Severinghaus of Consolidated Quarries made a study of the effect of different grading requirements for railroad ballast. The rate of production of the standard AREA grading was 200 tons per hour and for the most restrictive grading the rate was reduced to 65 tons per hour. It is quite probable that many producers are not fully aware of the effect of tolerances on the rate of production, and consequently the cost, because few break down their cost for each size made. This matter of reduction in tolerance may best be explained by an example with surface treatment aggregate, ½ in. to No. 4:
Deleterious Substances
Nearly all specifications for aggregate contain a paragraph on deleterious substances which, from a production point of view, have seemed innocuous because soundness requirements usually suffice. Yet, this paragraph may be described as a “catch-all” and all requirements under its heading should be considered important and should be studied carefully as to their possible effects on production. The purpose of having limitations on deleterious material probably was to prevent disfigurement of concrete surfaces caused by pop-outs due to such material as clay balls, unsound chert, shale, soft pieces, coal, and lignite. These materials would all fail in a soundness test; however, the allowable loss in the soundness test may be 12 per cent, while for deleterious materials it is usually, 1 per cent, except for soft pieces which may be 5 per cent.
