Originally developed in the 1960s, the vibrating slope apparatus (Wong et al. 2000) was recently modified by the US Army Engineering Research and Development Center (ERDC) for the US Federal Highway Administration (FHWA). The device measures the workability of low slump concretes subjected to vibration at two different shear rates in order to determine a “workability index” that is related to plastic viscosity and a “yield offset” that is related to yield stress. Researchers at the ERDC selected the vibrating slope apparatus over twenty other workability test devices as a superior choice to measure the workability of low slump concretes in the field.
The vibrating slope apparatus as modified by the US Army Engineering Research and Development Center is shown in Figure 15. Concrete to be tested is placed in the chute, which can be set at a predefined angle. Three load cells continuously measure the mass of concrete in the chute during the test. Small transverse metal strips reduce slip between the concrete and the bottom of the chute. A vibrator is mounted to the bottom of the chute. Eight vibration dampers ensure that the vibration is applied to the concrete and that the entire apparatus does not excessively vibrate and interfere with load cell measurements. Readings from the load cells are transmitted to a laptop computer, where the workability index and yield offset are calculated. The entire apparatus is designed to be rugged and easily portable.
To operate the device, concrete is placed in the chute, which is set at a predefined angle (typically 10-15 degrees). The gate is opened and the vibrator is started, allowing concrete to fall from the chute into a bucket. The data from the load cells is used to calculate the discharge rate. Since the discharge rate generally decreases as concrete flows out of the chute, the maximum discharge rate is recorded. The test procedure is repeated a second time for a different incline angle. The results of the test are plotted as a graph of maximum discharge rate versus discharge angle. The straight line connecting the two data points is defined by Equation :
R = WA + C
where R = maximum discharge rate, W = workability index, A = discharge angle, and C = calculated yield offset.
The intent of the research conducted by the ERDC for the FHWA was simply to determine if the vibrating slope apparatus would operate properly, not whether the device could accurately measure concrete rheology. The results of the preliminary ERDC laboratory testing were compared only to the slump and air content of each concrete mixture. Further, no analytical treatment of the test has been presented. Wong et al. (2000) claims that the y-intercept of the discharge rate versus discharge angle plot is the yield stress and that the slope of this plot is the dynamic viscosity; however, no effort is made to relate these parameters to fundamental units or confirm the validity of the test results. Since the yield stress of vibrated concrete is lower than the yield stress of unvibrated concrete, the yield stress recorded by the vibrating slope apparatus is not equivalent to the yield stress of the unvibrated concrete and is only applicable for the specific vibration applied by the vibrating slope apparatus. Before the vibrating slope apparatus can be used on a wider basis, the validity of the test results must be verified.
The ERDC researchers encountered multiple problems in developing the vibrating slope apparatus prototype. Many of the problems were trivial and easily corrected. Other problems will require further work to resolve. The test device is large, bulky, and weighs 350 pounds. The ERDC researchers give no cost information in their report and do not compare the cost effectiveness of the vibrating slope apparatus to other test methods.
• Unlike many rheometers, the device measures the workability of low slump concretes.
• The results of the device are given in terms of parameters related to yield stress and plastic viscosity.
• The device is designed to be rugged for field use.
• The results of the device have not been verified analytically or experimentally.
• The device is large, bulky, and heavy.
• Although the researchers have proposed using an embedded electronic device to record test data, the vibrating slope apparatus at this point still requires a notebook computer.
• The results of the test are only applicable for conditions with the same vibration as the vibration applied by the device.
• The shear rate is non-uniform throughout the test. The shear rate decreases as the mass of concrete in the chute decreases.