Rockfall Concrete Barrier Evaluation and Design Criteria Interim Report on Task 4 Impact Tests of Instrumented Concrete Barriers for the Ohio Department of Transportation State Job Number 134640 The University of Akron, 302 Buchtel Common, Akron, OH44325 Dr. Anil Patnaik and Dr. Robert Liang Department of Civil Engineering The University of Akron Akron, OH44325-3905 Graduate Students: Abdisa Musa and Sai Ganapuram Phone: 330-972-5226 Email: [email protected] Date Submitted: Dec. 7, 2012 Revised: March 26, 2013 1.0 INTRODUCTION 1.1 Statement of the Problem Rockfalls along a slope can reach the roadway at the bottom and create a hazard to the roadway users. Rockfalls have long been recognized as a major problem in certain areas of Ohio. The relevant background and the description of the rockfall problem in Ohio are presented in Technical Note # 1 for this project. Placement of 32” high precast concrete barriers (PCBs) next to the roadway or construction of 42” or 50” cast-in-place (CIP) barriers along the edge of the pavement (at the shoulder of the road) are two common solutions practiced in Ohio for the protection of roadway users against falling rocks. Brief descriptions and the tentative findings of the impact tests conducted as a part of the project under Task #4 are summarized in this interim report. Additional details on the project are also presented in the review presentation for the project that was previously submitted. 1.2 Objectives The broad objectives of this project are to determine the limitations of concrete barriers in terms of strength and energy absorption by conducting impact tests and field tests, primarily on PCB 32” and CIP 42” or 50” concrete barriers. Rockfall impact velocities, bounce heights, and the corresponding response of these types of barriers are to be verified for typical Ohio rock- cut slopes. The specific objective of the impact tests is to evaluate the performance of concrete barriers (PCBs and CIP barriers) under direct impact. 2.0 DETAILS OF IMPACT TESTS IN TASK #4 Brief details regarding the impact tests on PCBs and CIP barriers conducted in Task 4 are presented in this section. These impact tests were conducted on the premises of Duer Construction Company in Akron, Ohio. The test setup is shown in Figure 2.1. The following variables were included in the test program: • Concrete barrier type • Pavement type • Rock size • Rock shape • Rock type • Level of impact • Drop height • Impact energy • Location of impact (along the longitudinal axis) • Number of impacts Patnaik and Liang 2 SJN 134640 2.1 Concrete Barrier and Pavement Types Two pavement pads – one asphalt and one concrete – were made adjacent to each other as shown in Figure 2.2. Two types of barriers, precast concrete barriers (PCB) and Cast-in-Place (CIP) barriers, were used in the tests. The PCBs tested were those shown in ODOT Standard Drawing RM 4.2 with welded wire fabric option. Five PCB units with bolted hinge option at each end were linked together. The total length of each test section was 60 feet (5 units of 12 feet each, in the case of PCBs). PCBs were placed next to the asphalt pavement over a prepared base (see Figure 2.2). The footing of each CIP barrier was cast against the saw-cut edge of the asphalt or the concrete pavement, according to the ODOT requirements for CIP barriers. The CIP barriers were provided with a cold construction joint at the interface between the stem and footing. The following three sets of tests were conducted (Figure 2.3): Precast Concrete Barrier (PCB 32”) • placed next to asphalt pavement Cast-in-Place (CIP 42”) Barrier • with footing cast against concrete pavement • with footing cast against asphalt pavement Figure 2.1 Test Setup Figure 2.2 Pavement pads with barriers 2.2 Rock Size, Shape and Type Impacting balls of various sizes, shapes, and types were used to simulate rocks in a rockfall. The details of the impacting balls used in the tests are listed in Table 2.1 and shown in Figure 2.4. Three types of impacting balls were used: (i) concrete, (ii) steel, and (iii) natural rock. Patnaik and Liang 3 SJN 134640 Figure 2.3 Three groups of test sections Table 2.1 Details of Impacting Ball (Rock) Concrete Ball Steel Ball Natural Rock Ball # 1 2 3 4 5 6 7 8 9 10 Dimension (inch) 16 16 18 15 24 30 30 24 28 60 Weight (lb) 150 190 260 312 720 1270 2200 2230 4030 1500 Unit weight (lb/ft3) 128 163 155 173 175 150 145 490 490 160 Figure 2.4 Impacting Balls Patnaik and Liang 4 SJN 134640 The impacting ball made of natural rock, shown in Figure 2.5, was used to impact the CIP barrier to simulate a realistic condition representing a rockfall. The impacting balls have different densities ranging from 128 lb/ft3 (for concrete ball #1) to 490 lb/ft3 (for steel balls #8 and #9). The two steel balls, which weigh 2230 lb and 4030 lb (as shown in Figure 2.4), were used to apply maximum impact energy on the barriers. Figure 2.5 Natural rock Figure 2.6 Height of impact 2.3 Location of Impact The impact tests were conducted at several locations on the test sections both vertically and longitudinally. The vertical locations of impact for the PCBs were between approximately 6 inches and 24 inches from the top. For CIP barriers, the vertical locations of impact were between 9 inches and 28 inches from the top. Diagrams showing the vertical locations of impact for both barrier types are presented in Figure 2.6. The impact tests were conducted at several locations along the length of the test sections. For PCBs, tests were primarily conducted at the mid-length of middle PCB section, at the hinge location, and mid-length of the PCB on the left as shown in Figure 2.7a. It was intended to impact the CIP barriers (Figure 2.7b) at locations that were analogous to those for the PCB test section. However, it was possible to vary the locations of impacts for the CIP barrier more readily because of the monolithic nature of the test section and the flexibility of the test setup. Therefore, impact tests were also conducted at several other intermediate and end locations. Figure 2.7(a) Location of Impact Tests on PCBs (Longitudinal Variation) Patnaik and Liang 5 SJN 134640 Figure 2.7(b) Location of Impact Tests on CIP Barrier (Longitudinal Variation) 2.4 Drop Height and Impact Energy The impact test setup used in this task resembles a simple pendulum mechanism (see Figure 2.8). The drop heights and masses of the impacting balls were varied in order to deliver the required velocity and impact energy. The purpose of determining the drop heights and ball (rock) size combinations was to experimentally simulate a practical range of energies that can be expected from rockfalls at typical Ohio slopes to determine the limitations of the barriers. A comprehensive literature review (described in Technical Note #1) revealed that the energy absorption capacity of a typical CIP barrier is likely to be about 40 kJ. Therefore, this level of energy was considered to be the minimum energy that must be targeted for the impact tests in this project. To allow for the possibility of additional energy absorption capacity of CIP barriers made to ODOT specifications, a maximum of 60 kJ was set as the maximum target energy to be delivered for these tests. Suitable mass and drop height combinations were determined for the impact tests to deliver up to 60 kJ of energy. Figure 2.8 Drop Height Details Patnaik and Liang 6 SJN 134640 Colorado Rockfall Simulation Program (CRSP) analysis was performed to correlate the range of velocities and energies expected from a typical Ohio rockfall with the energy of 40 to 60 kJ that is targeted for the impact tests conducted in this task. CRSP analyses were performed using different slope heights and inclinations (0.5H:1V, 1H:1V and 1.5H:1V) and different coefficients of surface roughness, tangential and normal restitution based on the type of slopes that are typical of Ohio geological conditions. The energies and velocities at the location of the barrier were determined for different ball sizes and ball masses. Figure 2.9 shows a typical output of a CRSP simulation performed in this project. Figure 2.9 Example Output from a CRSP Simulation of a Typical Ohio Slope The CRSP analyses showed that the velocities of the rocks just prior to impact with the barrier are between 25 ft/sec and 35 ft/sec for energies in the range of 40kJ to 60kJ for different rock masses. These velocities were interpreted as being equivalent to the velocities of the balls to be developed in the impact tests of this project. The range of velocities and energies for the pendulum mechanism were calculated based on the drop heights using a simple physics equation given in Eq. (1). = 2 (1) Where, 푉 � 푔ℎ V is the velocity of the impacting ball g is the acceleration due to gravity h is the drop height Patnaik and Liang 7 SJN 134640 The velocities generated for different drop heights are shown in Fig. 2.10. The figure shows that drop heights between 14 ft. and 25 ft. would generate velocities of 30 ft/sec to 40 ft/sec. This range corresponds to the practical velocities predicted using CRSP analysis, thereby verifying that drop heights used in the impact tests are acceptable.