Micro mechanics of railway ballast degradation

The strength of ballast is usually determined by revolving particles in a drum and measuring the amount of damage produced (MA, LAA, Deval Tests). However, the micro mechanics of deterioration in these tests is different from that beneath the track under traffic loading: a quasi-static test should be used. The Aggregate Crushing Value (ACV) test is problematical because the test is performed on particles of size less than 14mm. For particles of a given size, there is a distribution of strengths and consequently a size effect on strength such that smaller particles are statistically stronger because they contain fewer and smaller flaws. This size effect will be different for different ballasts. Consequently, the ACV gives no information about the strength of 50mm particles. Furthermore the sample aspect ratio means that wall friction may cause the axial stress at the top of the sample to be significantly different from that at the bottom. Here, the strengths of ballast particles have been measured directly by diametral compression between flat platens. The strengths follow the Weibull distribution, and it is shown that there is a size effect on average strength. The ACV test has been replaced by a large oedometer test and the results of oedometer tests on 37.5-50mm ballast and Railtrack specification ballast show that the standard ACV for 10-14mm ballast does not give a fair indication of the performance of the ballast used beneath the track. It is proposed that large oedometer tests on aggregates of specification ballast be used in addition to single particle crushing tests, in order to measure strength. For the covering abstract see ITRD E122683.