Author(s):
C.E. Buth, H.R. Blank, R.G. McKeen
Publication Date
September 1968
Abstract
This paper reports the results of three experimental programs conducted on synthetic aggregates; physical durability, chemical durability, and mechanical durability.|In the physical durability program, five selected commercially produced and eight TTI kiln-produced lightweight coarse aggregates were investigated. Absorption characteristics and porosity values were determined for each of the aggregates. A total of 69 batches of concrete using these coarse aggregates and natural sand were mixed and tested in accordance with ASTM C290. Various degrees of saturation of the coarse aggregates at the time of mixing were obtained by immersing them in water for periods ranging from 6 hours to 180 days. The resistance of structural lightweight concrete to freezing and thawing as measured by ASTM C290 was greatly influenced by the degree of saturation of the coarse aggregate at the time of mixing. For the concretes investigated, the critical degree of saturation of the coarse aggregate at the time of mixing was found to be about 0.25.|Experiments on chemical durability centered upon the detection and possible effects of deleterious substances likely to be present, such as "underburned" clay minerals, lumps of lime and of anhydrite, and decomposable sulfides. Underburned clay minerals can sometimes be detected by petrographic and DTA methods, more definitely by X-ray diffraction. Simpler tests, such as boiling in a pressure cooker, are also reported.|Experiments on chemical durability centered upon the detection and possible effects of deleterious substances likely to be present, such as "underburned" clay minerals, lumps of lime and of anhydrite, and decomposable sulfides. Underburned clay minerals can sometimes be detected by petrographic and DTA methods, more definitely by X-ray diffraction. Simpler tests, such as boiling in a pressure cooker, are also reported.|It was found that lumps of lime disintegrate during mixing of concrete, but lumps of anhydrite hydrate more slowly and expand, causing swelling and distortion of concrete test bars.|All raw clays containing pyrite or marcasite yielded aggregates giving positive tests for small amounts of decomposable sulfides. Aggregates prepared with artificially high concentrations of FeS oxidized and corroded steel on repeated wetting and drying, even if their initial environment was alkaline. On the other hand, these aggregates cast in a block of concrete did not cause appreciable disintegration in the duration of the experiment. It seems probable that oxidation of the sulfide may do no harm as long as the surrounding concrete remains sufficiently alkaline to neutralize the acid formed. However, the long-term effects are still undetermined, and testing of synthetic aggregates for sulfide is recommended.|The mechanical durability program was designed as a pilot study of the abrasion resistance characteristics of synthetic aggregates. The test used in the investigation was ASTM C418-67T, "Method of Test for Abrasion Resistance of Concrete (Tentative)." This test measures abrasion in terms of the volume of material eroded by a stream of standard size sand particles, blasted onto a specified surface area. Laboratory tests were performed on specimens made with four commercially produced synthetic aggregates and one commercial natural gravel aggregate. All laboratory tests were performed on sawn surfaces to expose the aggregate and therefore amplify the effects on the aggregate. Reproducibility of the method in terms of coefficient of variation was found to vary with moisture content of the specimens. The results obtained on specimens soaked 24 hr. prior to testing (as specified in ASTM C41867T) were more consistent than those obtained on air dry specimens. There was no significant difference in the performance of the synthetic aggregate concretes tested. However, they all exhibited higher losses than the natural gravel concrete. The cement factor of the concrete did not significantly affect the results obtained.|Core specimens were obtained from three locations on limestone aggregate concrete pavements which exhibited satisfactory and unsatisfactory in-service performance. At each location cores were taken in the wheel path and at the pavement edge where the surfaces at the edge were undisturbed by traffic. Results indicate significantly more wear on specimens from the pavement edge at location 1 (an area exhibiting excessive wear in the wheel path), than on all other cored specimens. These results indicate that the test is capable of distinguishing between a concrete surface that will perform well under traffic and one that will not.|Loss measured on the original surface of the unsatisfactory pavement was of the same order of magnitude as the loss measured on the synthetic aggregate concrete. It follows from these results that if synthetic aggregate is exposed in a concrete pavement surface (as it was in these laboratory tests) it may abrade readily under traffic wear. The test results indicate that the test could be useful in evaluating surface quality in terms of wear under traffic.
Report Number:
81-6
Link(s):
Document/Product
http://tti.tamu.edu/documents/81-6.pdf
Publication/Product Request
TTI reports and products are available for download at no charge. If an electronic version is not available and no instructions on how to obtain it are given, contact the TTI Library.