Raphael Crowley

Computational Modeling of Bed Material Shear Stresses in Piston-Type Erosion Rate Testing Devices

AbstractThe sediment erosion rate flume (SERF) device was computationally modeled using CD-adapco’s Star-CCM+ at varying flow rates and sample roughnesses so that wall shear stresses could be evaluated during a piston-style erosion test. Shear stress data were matched between the model and data from previous physical tests. Pressure differential upstream and downstream from an eroding specimen displayed similar behavior during both physical and modeled tests in that as eroding sample roughness increased, pressure differential did not appear to change. A series of complicated bed configurations were added to the computational model to quantify the effects of blocking, chunking, or sample overadvancement during an erosion test. Results appeared to indicate that small deviations in sample geometry may have large effects on localized shear stresses. Another series of models was run to provide an explanation for the beginning of blocking or chunking as a result of shear stress development over a rough sample. ...

The Sediment Erosion Rate Flume (SERF): A New Testing Device for Measuring Soil Erosion Rate and Shear Stress

A new apparatus that measures soil erosion in a flume is described. It is designed to measure a bed material’s erosion rate and approximate its corresponding applied shear stress (erosion function). The apparatus, or Sediment Erosion Rate Flume (SERF), is unlike similar devices, in that during an erosion test, sample advancement is computer-controlled through a feedback loop via a series of lasers, ultrasonic depth sensors, and a Servo-stepper motor. Because of this computerized control mechanism, the SERF can be used to measure near-instantaneous erosion rate as opposed to average erosion rate over an elapsed time domain. Also, included with the SERF is a device that can measure shear stress directly for a sample with a certain uniform roughness. Using the SERF, erosion versus time curves were used to estimate erosion rate for both synthetic and natural samples. For synthetic samples, a roughness approximation was conducted to estimate shear stress. Tests with the shear stress sensor appear to indicate that smooth-wall approximations used in previous flume-style erosion rate testing apparatuses under-predict shear stress. However, further analysis of erosion results indicates that the smooth-wall approximation is conservative; and until a direct method for measuring shear stress is discovered, it may be appropriate for devices like this. Using this rationale, erosion functions were estimated for natural samples.

The Rotating Erosion Testing Apparatus (RETA): A Laboratory Device for Measuring Erosion Rates versus Shear Stresses of Rock and Cohesive Materials

This paper describes an apparatus for testing the erosion rate of erodible rock and stiff clay as a function of water-flow-induced shear stress. The device, referred to as the Rotating Erosion Testing Apparatus, or RETA, is similar in design to other devices but has a number of advantages. The RETA uses a real-time control unit that monitors the shear stress via the torque imparted to the sample and adjusts its rotational speed to maintain a nearly constant shear stress throughout the test. RETA test results have been used to estimate local sediment erosion rates at structures founded in these types of bed materials.

The Creep Testing Apparatus (CRETA): A New Testing Device for Measuring the Viscoelasticity of Joint Sealant

A new device, the Creep Testing Apparatus (CRETA), has been developed for use in determining the viscoelastic properties of joint sealant. Creep tests were conducted on two types of silicone sealant: self-leveling (SL) and non-self-leveling (NS). The results appear to indicate that the silicone sealant tested is a linear viscoelastic material and that its creep response does not appear to be sensitive to temperature variations from 0 °C to 60 °C. Further creep tests were conducted under artificial aging conditions. Hot water aging appears to cause both SL and NS sealant to become softer and more ductile. Freeze-thaw aging did not appear to have an effect on the viscoelastic properties. Oven aging did not appear to affect SL sealant, but it did cause NS sealant to become stiffer. These results might be useful in a long-term joint performance model.

Cellulose Fiber Reinforced Concrete Fracture Mechanisms and Damage Detection Using Acoustic Emission

ASTM C78 standard tests for flexural strength of concrete were conducted on cellulose fiber reinforced concrete (CFRC) specimens using varying percentages of ultimate load. During testing, Acoustic Emission (AE) signals were recorded while after testing, a scanning electron microscope (SEM) was used to visually observe surface features. SEM results appear to illustrate three important stages of the fracture process: cement cracking, fiber-cement debonding, and fiber breakage/pullout. AE results were used to bound ranges for a relationship between AE amplitude and CFRC fracture mechanisms.

Estimation and Measurement of Bed Material Shear Stresses in Erosion Rate Testing Devices

AbstractThe sediment erosion rate flume (SERF) is used to evaluate various methods to estimate shear stresses in a flume-style erosion rate testing device. Results show that direct shear stress measurements most closely correspond to shear stress estimates obtained from the Colebrook equation using a roughness height of approximately 77% of the sediment particle diameter. Using pressure drops to estimate shear stress appears to underestimate rough sample (grain sizes >0.25  mm) stresses. Likewise, smooth-wall assumptions and analytical methods also appear to produce lower than expected results for similar heights. Colebrook equation results should be interpreted as “instrument specific” because of the different devices’ hydraulic diameters.

Performance-Based Maintenance Contracting in Florida: Evaluation by Surveys, Statistics, and Content Analysis

AbstractIn recent years, the Florida Department of Transportation (FDOT) adopted a form of performance-based maintenance contracting (PBMC) that it calls the asset maintenance contracting (AMC) pro...

The large-scale soil box: A new device for testing the performance of buried pipe

A new apparatus that measures pipe deflection as a function of simulated overburden stress is described - the large-scale soil box (LSSB). While the LSSB is similar to previous quasi-full scale instruments, it is capable of load testing two pipes simultaneously while monitoring deflection along their crowns. A series of tests were conducted on 24 and 36-in., 10-ft-long pipe sections. Pipe material varied throughout testing; materials included polyvinylchloride (PVC), high-density polyethylene (HDPE), steel, and aluminum. During load application, lasers allowed for deflection curves to be developed for each of the pipes as a function of simulated overburden produced via constrained lift bags. Analysis of the results appeared to indicate that loading in the LSSB is not necessarily uniform. This is likely due to non-uniform soil densities that were present prior to testing even though strict procedures and quality control checks were used prior to each test. Therefore, it would be difficult to use data from a device like this as a comparison against finite element models. However, results from these tests still may be used to assess the relative levels of effect of different pipe types and installation procedures.

Moisture Effects on Cellulose Fiber Reinforced Concrete Properties

ASTM C78, the Flexural Strength tests were conducted on Cellulose Fiber Reinforced Concrete (CFRC) samples subjected to difference moisture-levels to quantify the effect of moisture on them. Results indicated that modulus elasticity did not change along the increase in moisture. However, flexural strength and yield strength appeared to be affected under certain conditions.