Presentation on theme: "Strengthening Anchorage Details in Vintage Reinforced Concrete Bridges A Possible Titanium Application Christopher Higgins, PhD, PE Deanna Amneus, MS Student."— Presentation transcript:
1Strengthening Anchorage Details in Vintage Reinforced Concrete Bridges A Possible Titanium ApplicationChristopher Higgins, PhD, PEDeanna Amneus, MS StudentLaura Barker, MS StudentDecember 6, 2012Thank you Dr. Higgins. And thank you everyone for attending my presentation this afternoon entitled, “Flexural Steel Anchorage Performance at Diagonal Crack Locations.” This research project was made possible with funding from the Oregon Department of Transportation and with guidance from my advisor Dr. Chris Higgins.
2Vintage Reinforced Concrete Bridges Many constructed in 1950s (Eisenhower Defense Highway System)Design standards have changed since builtLoads more frequent and heavierMany are now cracked and deficientA significant number of reinforced concrete deck girder bridges constructed during the 1950s have developed diagonal cracking in the stems. Since the 1950s the design provisions regarding shear design have changed and the service loading has increased.In the 1950s standardized deformed reinforcing bars came into use. Compared to proprietary reinforcing bars, standard deformed bars were believed to provide adequate anchorage without the need for hooks and bends so designers terminated flexural steel without special detailing where it was no longer needed.
3Research ProblemProblem with poor details (reinforcing steel terminated in locations that show cracking) that can lead to member failureMany existing bridges with these reinforcing details cannot carry required truck loads by calculationNeed new methods to strengthen these bridgesTitanium looks promising: high strength, good stiffness match with adhesives, bendable and field adjustable, and durable. Material cost may not drive issue. Labor!
4Typical Specimen Cross-Section The No. 11 flexural steel was arranged in two layers. Each beam had two cutoff bars in the top layer, two hook bars in the bottom layer, and three specimens also had a straight bar in the bottom layer. No. 4 bars were placed in the deck to simulate deck mat reinforcement.:40
5Typical Poor Cutoff Detail View along length of beam showing where a diagonal crack intersects a bad anchorage detail
6(Modified from Higgins et al., 2004) Load SetupAll of the specimens to be tested in the Structural Engineering Research Laboratory at Oregon State University. A reaction frame constructed on the strong floor allowed for four-point loading using a 500 kip servo-hydraulic actuator. The steel spreader beam applied the load at two points spaced 2 ft apart, centered about the middle of the specimen.Specimen height 4 ft, Length 28 ft, weigth = 20,000 lbs:22(Modified from Higgins et al., 2004)
7Typical Cracking at Anchorage Starting around the 100 kip load cycle, anchorage cracks like these started to form. They were characterized by periodic vertical cracks extending from the location of the cutoff bar to the bottom surface of the beam. The vertical cracks were connected by mainly horizontal cracks at the level of the cutoff bar. As the applied load increased, the extent of the anchorage cracks increased.:24
8Typical Anchorage Failure This is the failure video from specimen 2. This specimen has the most predominant evidence of crack sweeping as the beam failed.:45
9Retrofit Approach for Strengthening Poor Cutoff Detail: Near-Surface Mounted Reinforcing Add External Reinforcing to strengthen beam
10Near-Surface Mounted Reinforcing Saw-Cut slots in concreteClean & DryEpoxy placed in slotsInsert external reinforcing in slotsGrooves were saw-cut into the web of the specimen. We pressure washed them and allowed them to dry and then epoxied in strips of the CFRP.The CFRP we used is Hughe’s Brothers Aslan 500 tape. One of the few made specifically for NSM repair and it’s the only one made in the USA, used in previous research, and has a unique rough surface which should help it.The adhesive we used was Concressive It is on the Hughe’s Brother’s list of recommended adhesives, it is cheaper than others, readily available, and has been used in other research.
11Failure Modes for Carbon Fiber in NSM Application Outer shell peelingSlip of CFRPTitanium may eliminate these modes (with hooks)What often happened, at least with the tighter CFRP spacing, is the crack formed around the NSM-CFRP.At the top and bottom of the shear crack an outer shell made of the NSM reinforcing peeled away from the inner core of reinforcing steel and concrete. The inner core cracked diagonally, but the outer shell cracked steeply between these sections of peeling.This is a unique failure mode for NSM retrofitting and none of the current design approaches really account for it.
12Working with Titanium for Civil Engineering Shear to length as for rebarCold bend with same radius as for steel rebar but overshoot to account for spring-backIncrease friction along surface to achieve better bond with epoxy