Rockfall Concrete Barrier Evaluation and Design Criteria · Rockfall Concrete Barrier Evaluation and Design Criteria . Interim Report on Task 4 . Impact Tests of Instrumented Concrete

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

mailto:[email protected]

Patnaik and Liang 2 SJN 134640

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


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.


Figure 2.3 Three groups of test sections

Table 2.1 Details of Impacting Ball (Rock)

Figure 2.4 Impacting Balls

Ball # 1 2 3 4 5 6 7 8 9Dimension (inch) 16 16 18 15 24 30 30 24 28

Weight (lb) 150 190 260 312 720 1270 2200 2230 4030Unit weight (lb/ft3) 128 163 155 173 175 150 145 490 490 160

Steel Ball Natural Rock

1500

10Concrete Ball

60


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)


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


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).

𝑉 = �2𝑔ℎ (1) Where,

V is the velocity of the impacting ball g is the acceleration due to gravity h is the drop height


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.

Fig. 2.10 Velocities that can be Generated for Different Drop Heights The energies for different drop heights were calculated using the following equation:

𝐸 = 𝑚𝑔ℎ (2) Where,

E is the impact energy m is the mass of the rock

The velocities in ft/sec for different drop heights and energies in kJ for the ball masses used in this test program are shown in Table 2.2 for select drop heights. The energies developed for different drop heights and ball masses are also shown in Figure 2.11. The values shown in Table 2.2 and the plots of Figure 2.11 demonstrate that the test parameters used in the impact tests are representative of the velocities and energies expected for typical rockfalls at Ohio slopes. 2.5 Number of Impacts In the initial stages of testing, several impacts were made at the same location of the test barriers. Some of the impacts were duplicated using the same ball size and drop height, while some were repeated with same energy levels using various ball sizes and drop heights. The


impacts were designed so that the energy is progressively increased. This was done to understand the effect of repeated impacts and to progressively increase the energy of subsequent impact. However, this approach proved to be very conservative. Later in the test program, the impacts were made with mass and velocity combinations that produced large energy levels. Consequently, fewer impacts were delivered at any particular location.

Table 2.2 Velocities and Energies for Different Drop Heights Drop Ht.

(ft) Velocity (ft/sec)

Energy for Different Balls (kJ)

Ball # 1 Ball # 2 Ball # 3 Ball # 4 Ball # 5 Ball # 6 Ball # 7 Ball # 8 Ball # 9 Ball # 10

1 8.0 0.20 0.26 0.35 0.42 0.98 1.72 2.98 3.02 5.47 2.03

5 17.9 1.02 1.29 1.76 2.12 4.88 8.61 14.92 15.12 27.33 10.17

10 25.4 2.03 2.58 3.53 4.23 9.77 17.22 29.84 30.25 54.66 20.34

11 26.6 2.24 2.83 3.88 4.65 10.74 18.95 32.82 33.27 60.12 22.38

12 27.8 2.44 3.09 4.23 5.08 11.72 20.67 35.81 36.29 65.59 24.41

13 28.9 2.64 3.35 4.58 5.50 12.69 22.39 38.79 39.32 71.06 26.45

14 30.0 2.85 3.61 4.94 5.92 13.67 24.11 41.77 42.34 76.52 28.48

15 31.1 3.05 3.87 5.29 6.35 14.65 25.84 44.76 45.37 81.99 30.52

18 34.0 3.66 4.64 6.35 7.62 17.58 31.00 53.71 54.44 98.38 36.62

20 35.9 4.07 5.15 7.05 8.46 19.53 34.45 59.68 60.49 109.32 40.69

21 36.8 4.27 5.41 7.41 8.89 20.51 36.17 62.66 63.51 114.78 42.72

25 40.1 5.09 6.44 8.82 10.58 24.41 43.06 74.60 75.61 136.65 50.86

29 43.2 5.90 7.47 10.23 12.27 28.32 49.95 86.53 87.71 158.51 59.00

Figure 2.11 Energy Versus Drop Height for Different Ball (Rock) Sizes


2.6. Instrumentation The instrumentation used in the test program comprised of (i) a three-directional accelerometer attached to the ball, (ii) unidirectional accelerometers attached to the barrier, (iii) strain gages on concrete external surface, (iv) strain gages on steel reinforcing bars that were embedded in concrete, (v) high speed cameras, and (vi) simple surveying instruments. The details of the data collected and the data analysis will be presented in later reports.

3.0 FAILURE MODES 3.1. PCB Failure Modes PCB test specimens failed by extensive cracking followed by crumbling of concrete (Figure 3.1a and b), rupture of steel welded wire fabric (WWF) as seen in Figure 3.1b, and excessive sliding of the base (Figure 3.1c). Concrete crumbling was particularly severe in the unreinforced areas of the PCB test specimen (Figure 3.1b). The impacts at the hinges did not severely impede the energy absorption any less than impact elsewhere in the test sections. However, spalling of concrete was observed at the end of #4 bars (Figure 3.1d). Therefore, suitable adjustments to the reinforcing details are needed to improve the performance of PCBs under impact loading. It was observed in the tests that multiple impacts at the same location of the test section reduced the energy absorption of the test barriers. Therefore, two tests were conducted on PCBs with WWF that were designed to fail each PCB with one impact. The specimens for these tests were placed on the prepared base next to the asphalt pavement, and the 1270 lb concrete ball was used to impact these barriers one at a time. From these two tests, the average energy absorbed by PCBs was determined to be 24 kJ (17.8 kip-ft). 3.2. CIP Barrier Failure Modes CIP barrier specimens also failed by extensive cracking followed by spalling of concrete, as shown in Figure 3.2. The shape of the failed segment of the wall indicated that the failure was by punching shear. The angle of failure lines with respect to the horizontal was about 30 to 40 degrees as seen in this figure. The maximum impact energy that was resisted by CIP barriers was recorded to be 57.5 kJ (42.6 kip-ft). Severe lack of bond between the embedded epoxy coated bars and the surrounding concrete was noted, as seen in Figure 3.2. Crumbling of concrete was also observed at the locations where the reinforcing bars were provided at large spacing (18 to 24 inches). Multiple impacts at a spacing of 8 to 10 feet along the length of the barrier (Figure 3.3) were resisted well by the CIP barriers, demonstrating that closely spaced impacts can be adequately resisted by CIP


barriers. The length of influence of each impact as defined by the failure plane, was about 7 to 8 ft. in length measured at the top of the barrier.

Figure 3.1 PCB Failure Modes

Figure 3.2 Failure Modes of CIP Barriers

(a) (c)

(b) (d)


Figure 3.3 Failure due to Multiple Impacts

4.0 FINITE ELEMENT SIMULATIONS Finite element models are currently being developed using a commercial computer program called LS-DYNA to simulate the impact tests of this project. The results from the preliminary simulations are encouraging and will be refined to match with the test results. Detailed reporting on the finite element simulations will be prepared in the coming months.

5.0 DISCUSSION Repeated low energy impacts at the same location cause significant cracking and weakening of concrete barriers, leading to a reduction in energy absorption relative to the energy absorption due to a single high-energy impact event. The test barriers demonstrated that they can withstand much larger impact energy if no prior impact had occurred at the same location. The practical implication of this finding is that it is preferable to replace PCBs immediately after a rockfall has impacted the barrier, even if it was a low-energy impact with minimum visible cracking. Similarly, it is also preferred to repair or replace the portion of a CIP barrier that was subjected to a low-energy rockfall with no visible signs of failure.


5.1 ODOT Standard PCB PCBs were able to withstand a maximum impact energy of about 24.1 kJ (17.9 kip-ft), if this limit is based on one occurrence. PCB test specimens were able to contain the concrete ball and did not allow the ball to jump over the barrier. The PCB test specimen tipped about the toe, losing contact at the heel immediately following the impact. While the specimen rotated significantly, causing a lateral displacement of over 10 inches at the top, it did not overturn. Significant sliding was observed (as much as 12 to 15 inches as seen in Fig. 3.1c) suggesting that the placement of PCB at the edge of the roadway may result in sliding of the PCB closer to the traffic when a large impact rockfall occurs. WWF provided in accordance with the details required in ODOT standard PCB drawings ruptured under impact loading. Severe spalling and crumbling of concrete occurred at the unreinforced regions within the PCBs. Spalling and crumbling caused the PCBs to become ineffective in absorbing impact energies. The current reinforcement details may need to be modified, reducing the unreinforced areas within PCBs so as to improve the energy absorption capacity. The bolted hinge details of ODOT standard PCB worked well. The response of the test specimens under a direct impact at the hinge was found to be similar to the response of the specimen when the impact was at mid-length of the middle PCB section. However, spalling was observed at the ends of #4 bars near the hinge as seen in Fig. 3.1d. From the response of the test PCBs, there is an indication that shorter length barriers may function better than the longer length PCBs. The energy absorption seems to be better if sliding of the PCB units is permitted. Sliding of PCBs may be possible if they are placed at some distance away from the pavement edge rather than at the edge. 5.2 CIP Barriers The CIP test barriers of 42” height were able to resist impact energy of up to 57.5 kJ (42.6 kip-ft), if the impact is based on one occurrence at the location. The CIP test barrier was able to contain the concrete ball after such an impact and did not allow the ball to jump over the barrier. The CIP test barriers appeared to have slightly tipped about the toe, losing contact briefly at the heel and rotating by a small amount when the impact was close to the barrier end (within 5 to 10 ft. from the free end of the barrier). However, there was no overturning or any major lateral deflection.


The vertical face of the adjacent pavement (concrete or asphalt) was able to provide the required bearing reaction to the footing at the base of the barrier. The unreinforced footing of the CIP barrier for the most part functioned well without any cracking or distress. There was no indication of failure at the cold joint between the footing and the stem of the barrier. Based on the test results obtained from the impact tests, we believe that the current detailing practice of ODOT for concrete barriers may need to be revisited. Spalling and crumbling of concrete may be minimized by reducing the diameter of the reinforcing bars and the corresponding spacing of the vertical and horizontal reinforcing bars. It is anticipated that reduced diameter with reduced spacing would lead to improved impact energy absorption by delaying the crumbling of concrete. However, this needs to be verified through suitable testing. From preliminary calculations, #4 bars at a spacing of 15 inches on center would be a suitable alternative to #5 bars at a spacing of 24 inches. Also, #3 bars at a spacing of 8.5 inches would also work, but this may not be practical because the mesh would become too congested to place concrete. Some ODOT districts use hairpin reinforcing bars (inverted U-shaped reinforcement) at the top of CIP barriers. It is expected that a CIP barrier would perform better if hairpins are provided. The test barriers used in this study were not provided with hairpins at the top, but future testing may include such a detail. Epoxy coated bars demonstrated severe lack of bond with the surrounding concrete under impact loading (see Figure 3.2). This is a cause for concern and needs to be addressed. The problem may be somewhat countered by using smaller diameter epoxy coated bars at a closer spacing. 5.3 Effect of Pavement Type Both concrete pavement and asphalt pavement were able to provide adequate lateral support to the footing of the CIP barriers. The nine-inch-thick concrete pavement was made from plain concrete and showed many transverse cracks due to the impacts, but it remained intact during the entire testing. This suggests that the barriers were able to develop the required lateral reaction at the footing level. The asphalt pavement appeared to be more impact resistant, but it was difficult to quantify the better performance, if any. No cracks were observed in the asphalt pavement. 5.4 Rock Size, Shape and Type The impacting ball size and density did not make any difference to the impact performance of the barriers. Concrete balls with straight edges made contact with the barriers over a larger length/area. When a flat face of the ball impacted the barrier, the effect of the impact was muted. When the straight edge impacted the barrier, the rock (ball) was able to rotate and


transfer a smaller impact energy to the barrier than its spherical ball counterpart. The impact seemed to be delivered in multiple stages due to the rotation of the ball after first impact. This suggests that the impact tests conducted with spherical balls (or a ball with spherical impacting surface) is more severe than a ball with multiple edges and flat surfaces. Spherical balls underestimate the impact performance of the test barrier – meaning that the energy absorption limit is a lower bound limit when determined with a spherical ball. This is because the spherical balls deliver higher impact energy than their counterparts with straight edges and flat surfaces. If the rocks in a rockfall from a natural slope are not spherical in shape, the energies delivered to the barrier are likely to be somewhat smaller. The natural rock broke into two pieces upon impact. The sharp edge of the natural rock also provided a larger length of contact at the time of impact, making the impact less severe. The impact energy delivered by a natural rock is smaller than that of the corresponding concrete ball of same mass. By using concrete balls with spherical surface, the performance of the barrier is underestimated because of the dissipation of some of the impact energy through the breakage of the natural rock. 5.5 Location of Impact The vertical location of impact influenced the amount of energy absorbed by the wall. An impact closer to the top developed a smaller failure plane, and the resulting failure occurred at smaller impact energy. The energy absorbed was much larger when the impact occurred at about the mid-height of the barrier. The horizontal location of impact along the barrier length did not influence the energy absorption to any notable extent. Adjacent impacts at a spacing of about 6 to 8 ft did not reduce the impact resistance of the barrier in any way because the failure planes did not overlap. The energy absorption was diminished when the impact was delivered closer to the ends. In this case, the failure mode changed from punching to splitting within the final 6- foot length of the barrier. This end behavior suggests that the barrier needs to extend at least 6 feet beyond the location of the potential rockfall area where a barrier is no longer needed. 5.6 Drop Height and Impact Energy It was the higher energy impact that caused failure of the barriers. The velocity (which depends on the drop height) did not affect the failure mode or the failure energy. In other words, a high-velocity impact and a low-velocity impact had similar effects as long as the energy delivered to the barrier was similar.


5.7 Number of Impacts As previously mentioned, repeated impacts at one location caused cracking and softening of the barrier, leading to failure at impact energies much smaller than those corresponding to a single-impact location.

6.0 SUMMARY OF FINDINGS The most important findings from Task 4 work are as follows: ODOT Standard PCB • PCBs made according to the current ODOT details and specifications are suitable for

maximum impact energy of up to 24 kJ (17.9 kip-ft). • It is feasible to make improvements to the energy absorption capacity of PCBs by making

suitable changes to the detailing of PCBs. CIP Barrier • CIP barriers of 42” height made according to the current ODOT details and specifications are

able to resist impact energy up to 57.5 kJ (42.6 kip-ft). • Detailing of CIP barriers may be modified to improve the performance under impact

loading.

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Rockfall Concrete Barrier Evaluation and Design Criteria · Rockfall Concrete Barrier Evaluation and Design Criteria . Interim Report on Task 4 . Impact Tests of Instrumented Concrete