Experimental determination of energy absorption capacity for prestressed concrete sleepers under impact loads

Extreme loading conditions on railway tracks may include dynamic impact loads with very high magnitude but short duration. These loading conditions are caused by wheel or rail abnormalities such as flat wheels, dipped rails, etc. A high-capacity drop weight impact testing machine was constructed at the University of Wollongong, in order to evaluate the ultimate capacity of prestressed concrete sleepers under impact loads. This paper presents the experimental investigations to evaluate failure modes, flexural toughness, and energy absorption mechanisms for railway prestressed concrete sleepers under static and impact loadings. Energy absorption capacity of the prestressed concrete sleepers was also evaluated to determine the amount of energy required to fail the sleeper under impact load. Static and impact tests were carried out on the Australian-manufactured prestressed concrete sleepers. The residual capacity of the prestressed concrete sleepers after impact has also been highlighted. Figure 1. Typical components of railway tracks. ties, contact zone stiffness, frequency of loading, precision of impact, and locally energy-absorbed area (Hughes and Al-Dafiry, 1995). Regarding to railway sleepers, Ye, et al. (1994) and Wang (1996) investigated the resistance of concrete railroad ties to impact loading. Their study focused on the effect of material uses on the ultimate capacity of prestressed concrete sleepers. However, it was unclear whether strain rate has an effect on the behaviors of concrete sleepers or not, and whether there could be a simplified prediction for the ultimate capacity of concrete sleepers. The key hindrance was about how rail pad really affect the system impact responses and how much of that effect. Wakui and Okuda (1999) have later proposed a simplified technique to predict the ultimate capacity of concrete sleepers but they failed to prove it. In the proposal, strain rate and loading rate have been taken into account in moment capacity calculation on the basis of sectional analysis and only steel tendons’ failure mechanisms. So far, the ultimate behaviors of prestressed concrete sleepers under impacts are currently unclear and there is no method to predict the ultimate moment capacity under impact loading. This paper examines the ultimate behaviour of railway prestressed concrete sleepers subjected to static and impact loading. The prestressed concrete sleepers were designed complied with Australian Standard: AS1085.14 (2003). The test specimens were kindly supplied by an Australian manufacturer, ROCLA. Static energy absorption capacity can be obtained from the static tests. Drop-weight impact hammer was used to apply extreme impact loading to the specimens at certain drop heights on the basis of the test arrangement. The impact pulses were recorded using the high capacity load cell connected to the National Instrument data acquisition system. After applying the ultimate impact load, the sleeper was re-tested for residual capacity and energy absorption. The comparative study of both static and impact energy absorption of prestressed concrete sleepers was carried out. The damage and failure modes were identified in this paper. 2 EXPERIMENTAL OVERVIEW 2.1 Testing specimens The typical full-scale prestressed concrete sleeper, which is often used in broad gauge tracks, was selected for these tests. The dimensions and shape of the prestressed concrete sleeper are shown in Table 1. The high strength concrete material was used to construct the prestressed concrete sleepers, with design compressive strength at 28 days of 55 MPa, and the prestressing steels used were the high strength with rupture strength of 1860 MPa. The cored samples, drilled from the sleepers, were taken and tested, as per the Australian Standard AS1012.14, as shown in Figure 2. It was found that the average compressive strength at the test age of about two years was 80 MPa. It is believed that the high strength prestressing wires are of high quality and the strength will not change during time. Cross section of the prestressed concrete sleepers at railseat can be seen in Figure 3. 2.2 Experimental program In the experiments, a steel plate was used to distribute impact load to concrete sleepers. The width of the plate is equivalent to railseat and effective zone described in AS1085.14 (2001). The supports were considered as a simple support with influential span due to elastic support. These supports provide restraints to the translational and rotational deformation. The weight of the projectile was set as 5.81 kN, and therefore, the drop height becomes the only variable. The experimental setup thus required for specific energy absorption capacity for particular sleeper, in order to back calculating for the optimum drop height. A sleeper was performed the static tests in the conventional manner as shown in Figure 4. An electronic load cell was used to measure the applied load in order to keep load accurate and consistent, while LDVT was mounted at the mid-span to obtain the corresponding deflection. The device was connected to computer for recording. Figure 2. Cored concrete samples Figure 3. Cross section of sleepers at railseat Figure 4. Static test setup. Figure 5. Impact test setup. Table 1. Dimensions and masses of the test sleepers At railseat (m) At centre (m) Mass