1The Ohio Department of TransportationOffice of Statewide Planning and ResearchResearch Section1980 West Broad StreetColumbus, OH 43207614-644-8135Research@dot.state.oh.uswww.dot.state.oh.us/Research Executive Summary Report

Rockfall Concrete Barrier Evaluation and Design Criteria

FHWA Report Number: FHWA/OH-2015/3TBDReport Publication Date: TBDFebruary 2014ODOT State Job Number: 134640Project Duration: 4137 MonthsStart Date: 11/26/2011Completion Date: 12/30/201404/30/2015Total Project Funding: $1,074,043.23Research Agency: The University of AkronResearchers: Dr. Anil Patnaik and Dr. Robert LiangODOT Project Manager: Mr. Paul Painter and Mr. Stephen TaliaferroODOT Subject Matter Experts: Mr. Paul Painter and Mr. Stephen Taliaferro

For copies of this final report go to http://www.dot.state.oh.us/research.

Project Background

Rockfall is the movement of rocks down a slope, which may be in the form of freefall, bouncing, rolling and sliding based on the characteristics of slopes and the nature of the rocks. Rockfalls have long been recognized as a problem in Ohio. Placement of 32″ high precast concrete barriers (PCB) next to the roadway or construction of 42″ or 50″ high cast-in-place (CIP) concrete barriers along the edge of the pavement are common solutions practiced in Ohio to protect roadway users from falling rocks. These standard barriers were developed by crash-testing from the road side of the barriers without consideration of the impact of rocks from the ditch side. While ODOT’s goal is to achieve a rockfall catchment effectiveness of 95%, there is no documented verification of the containment effectiveness of rockfall concrete barriers. Impact tests were conducted to develop the required basis to design PCB and CIP concrete barriers that are also effective in containing rockfalls.

Rockfall characteristics such as velocities, bounce heights and energies delivered by falling rock are generally determined based on computer simulation of the slope profiles, slope properties, and site conditions. The Colorado Rockfall Simulation Program (CRSP) is widely used to determine such design values. Even though the predictions obtained from CRSP were validated through testing elsewhere, such validations were needed for Ohio conditions. Therefore, rollout tests were conducted to validate the results obtained from CRSP analyses for slopes that are representative of Ohio geological conditions.

Study Objectives

Quantification of the limitations of concrete barriers by establishing energy absorption of 32″ PCB and 42″ or 50″ CIP concrete barriers through impact testing

Development of design alternatives and/or details to improve the energy absorption capacity and performance of concrete barriers under rockfall loading

Determination of rockfall impact velocities, bounce heights and energies delivered at the location of the barriers through rollout tests, and validation of the predictions obtained from CRSP simulations

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Development of a basis for potential revisions to ODOT practicesDescription of Work

Impact TestsImpact tests were conducted in two phases to evaluate the energy absorption capacity of PCB and CIP concrete barriers. The test setup (Figure 1) used for the impact tests resembles a pendulum mechanism. The drop heights and masses of the test rocks were varied in order to experimentally simulate a practical range of energies that can be expected from typical rockfalls for which concrete barriers are generally used. CRSP analyses were performed to estimate the range of expected velocities and energies.

Figure 1 Impact Test Setup (Left) and Pavement Pads with Test Barriers (Right)

In Phase I tests, concrete barriers conforming to current ODOT standard details were tested. The test rocks used in the impact testing were of different shapes and sizes, and they consisted of reinforced concrete rocks, steel balls, and natural rock. The PCBs tested were those currently used by ODOT with welded wire reinforcement option. The PCB test sections were placed next to the asphalt pavement over a prepared base (Figure 1). The footing of each CIP concrete test barrier was cast against a vertical saw-cut edge of the asphalt or the concrete pavement. Among the variables included in the impact testing were the size, shape and type of rock; the drop height; the impact energy; the location of impact (along the longitudinal and vertical axes); and the number of impacts. The failure modes were characterized by extensive cracking followed by spalling and splashing of concrete (Figures 2 and 3). Severe lack of bond between epoxy coated bars and the surrounding concrete was evident for CIP concrete barriers.

General Failure Mode of PCB Spalling and Crushing of Concrete

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Figure 2 Failure Modes of PCBs without Fiber

Figure 3 Failure Modes of CIP Concrete Barriers without Fiber

Phase II impact tests were performed on PCB and CIP concrete barriers with revised details after addressing the deficiencies of concrete barrier details identified in Phase I testing, and different reinforcement alternatives were also considered. The design modifications developed for PCBs in this phase of the project include the use of polypropylene or steel fiber in concrete. A new PCB design with foam board core was also developed and evaluated with and without the addition of steel fiber. For CIP concrete barriers, the use of smaller diameter bars at closer spacing, or the use of WWR (welded wire reinforcement) with or without the addition of polypropylene or steel fiber to concrete were also evaluated. A single impact was made to determine the limit of energy absorption capacity of the test barriers.

The various design alternatives studied in this project demonstrated significant improvements relative to the ODOT standard designs and details currently in use. An addition of 7.5 lb/yd3 of polypropylene fiber or 40 lb/yd3 of steel fiber in concrete was demonstrated to have increased the energy absorption capacity of the barriers with revised details, in some cases by as much as 100%. Addition of fiber also greatly controlled the extent of cracking, spalling, and splashing of concrete debris.

Rollout TestsThe rollout tests were conducted on three different slopes with inclinations of 30°, 45°, and 55° (Figure 4). Test rocks of different sizes and shapes were rolled from the top of the slope to impact the concrete barriers constructed along the ditch at the bottom of the slope. The velocities and bounce heights along with the trajectories of the rocks while rolling down the slopes were digitally captured with high speed cameras. A typical calibration layout for the test rock and a test slope used for the digital measurements is shown in Figure 5. The results obtained from CRSP were validated using the field measurements obtained from these rollout tests. Typical rockfall trajectories of the falling rocks are shown for several rocks in Figure 5 alongside the predicted profiles obtained from the CRSP simulation.

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Figure 4 Two of the Test Slopes (45° and 55°) and Dropping of Test Rock (Right)

Figure 5 Calibration of Test Rock and Slope (Left) and Rockfall Trajectories (Right)

Research Findings and Conclusions

Impact Tests

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• Severe cracking, spalling and splashing of concrete can occur when the currently used barriers are subjected to rockfall impact loading.

• Revised PCB and CIP concrete barrier designs were developed with steel or polypropylene fiber and other changes to the reinforcement details to increase the energy absorption capacity by as much as 100%. These changes led to less brittle failure modes with reduced cracking and spalling of concrete.

Rollout Tests

• The bounce heights determined using CRSP simulations were found to be greatly underestimated compared to the corresponding test values for the three test slopes. The bounce heights predicted from CRSP analysis need to be adjusted upward so as to achieve 95% rockfall containment.

• CRSP provides acceptable prediction of rock velocities at the ditch location but underestimates the velocities at the impact location.

• The energies delivered from rockfalls as determined from CRSP analysis may be underestimated, and they need to be adjusted upward for the barrier designs to ensure the safety of roadway users.

Recommendation for Implementation of Research Findings

Recommendations for suitable adjustments to the predictions from typical CRSP analyses were identified mainly for bounce heights and the average total energy delivered by the falling rocks. The suggested design methodology comprises (i) the determination of bounce heights and then deciding on a suitable height of the barriers; and (ii) a comparison of the predicted energy with the energy absorption capacity of the selected barrier with the relevant details to ensure adequacy of the selected type of barriers during potential rockfall events.