cmmt 2 final
TRANSCRIPT
CONSTRUCTION MATERIAL , METHODS & TECHNOLOGY
MBA IN “CONSTRUCTION PROJECT MANAGEMENT”
SEM: I SECTION: C GROUP: 2
Presentation Topic:- A failure analysis study of a Prestressed steel cable of a suspension bridge.
Team Members: ANURAG SONAWANE ISHA SHARMA SAINATH GOUD SHAIKH BILAL SYED ASAD HASSAN
ABOUT CASE STUDY• This paper presents results of a failure analysis study• The damage phenomenon that occurred in prestressed
steel cables of a suspension bridge.• This study includes: material characterization using
chemical, microstructural and hardness analysis; fractographic analysis by scanning electron microscopy (SEM); mechanical tests of the material in static tension; fatigue tests (S–N curves) and fractographic analysis of fatigue fracture surfaces
• The main cause of cable failure, which led to a collapse of the bridge, was stress corrosion cracking (SCC).
• This paper presents the study of the damage causes occurring in one of prestressed
steel cables of a suspension bridge.
• The bridge is used to cross one of the most important rivers in the centre of Portugal.
• The main structural elements of the bridge are steel cables. Each cable is composed of
84 wires, distributed by 12 (6 + 1) twisted wires
• The damage in the wires caused the collapse of the bridge. As a result there was a
significant disturbance in traffic over the bridge because it stopped for a significant
period of time and were made repair operations.
• Some wires occurred failures due to excessive oxidation, corrosion
Highlights of case study
• A failure analysis study is presented.• Damage phenomenon characterization
occurred in prestressed steel cables of a suspension bridge is discussed.
• Results from fatigue tests and fractographic study allow to the conclusion the main cause of cable failure, which led to a collapse of the bridge.
Mechanical properties
• The mechanical strength of a failed wire rope cannot always be verified.
• The mechanical properties of a wire rope result from the individual wires and their arrangement or construction.
• The inherent friction coefficient between bare steel strands mandates presence of adequate lubrication to allow the requisite wire movement.
Improvements in mechanical properties of Pre-stressed cable
• It is generally accepted that many smaller
provide better fatigue resistance, while fewer, larger wires provide better abrasion resistance.
• Independent wire rope cores (IWRC) provide better crushing resistance than fiber cores.
• Grouping the wires in two layers HIGH DENSITY POLY-ETHYLENE(HDPE) material to protect them from atmospheric effect.
Chemical analysis
• Chemical analysis of any surface residue or corrosion product can also be helpful, especially in cases of corrosion fatigue or severe pitting.
• Other useful analytical techniques include energy dispersive X-ray spectrometry, X-ray diffraction, or a comparable surface science methodology.
• The fiber core could also be analyzed for evidence of overheating, grease degradation, and organic contamination.
Chemical Composition
Chemical composition of the cable material.
Element [%]Carbon (C) 0.91Manganese (Mn) 0.59Phosphorous (P) 0.02Sulphur (S) 0.06Silicon (Si) 0.39Copper (Cu) 0.12Nickel (Ni) 0.07Chromium (Cr) 0.06Molybdenum (Mo) 0.06
Microstructure
• Rope wires are very heavily drawn, with severely cold-worked microstructures.
• The structure consists of pearlite and ferrite grains that have been drawn down so far that the grain boundaries are not easily resolved.
• Relatively low levels of nonmetallic inclusions are required in high quality rope wires, to provide better fatigue resistance and more uniform mechanical properties.
Fatigue tests
• The geometry of the fatigue specimens was the same as those in the tensile tests, i.e., cylindrical specimens with 4.35 mm diameter and 20 mm gauge length.
• The specimens were tested in a servo-hydraulic fatigue test machine with ±100 KN capacity
• The objective of the test was to simulate the effect of the static load of the bridge mainly induced by self-weight.
• The principal objective of fatigue tests was to obtain the S–N curves of the material.
One particularly damaged rope that failed via fatigue is shown in Figure
Failure analysis processFailure analysis process• The preliminary portion of an investigation is of great
importance in wire rope analyses, similar to all materials failure investigations.
• All possible information should be gathered, including the purchased rope specification, service history, service environment, estimated loads, and maintenance history.
• After the available information has been collected, the failure analysis procedure should be planned, including identification of analytical tests and the location of test samples.
Visual examination
• The rope, strand, and wire diameters should all be measured. It is especially important to measure all different wire sizes that may be present.
• This is necessary to identify the rope construction. Examination should also address the presence or absence of lubricant, corrosion evidence, and gross mechanical damage.
• The identification of cracks, kinks, doglegs, and abrasion on the rope is also important.
Nondestructive examination• Most nondestructive evaluation (NDE) techniques are not
applicable to wire rope failures. • The size of the individual wires confounds most inspection
methods, requiring the rope to be addressed as a whole construction.
• The size of the individual wires confounds most inspection methods, requiring the rope to be addressed as a whole construction.
• Applicable specifications, such as ASME and ANSI, typically consider the number of visibly broken wires in a prescribed length, or lay, to be an acceptance criterion.
Fractographic analysis of damaged wires
Fractographic analysis was performed by scanning electron microscopy.
Specimens with ductile failures (cup and cone type) identified for small magnifications starting from central zone
where as in figure b and d magnifications of that central zones identified typical morphologies of this fracture mode, designated by “coalescence of cavities”
Specimens with failures of the arrow point type.
Analysis of the fractures surfaces of fatigue specimens
• the fracture surface was inspected with SEM
o It can be observed a radial cracking of the surface
• In Fig-a the fracture surface is rougher and the steps of propagation are more visible because the crack growth rate is higher, giving a shorter life.
• The fracture surface showed in Fig-b is more uniform as a result of a small stress level and a longer life (lower crack propagation speed viewed as can be confirmed by the lower degrees or steps).
• The fracture surface showed in Fig-b is more uniform as a result of a small stress level and a longer life (lower crack propagation speed viewed as can be confirmed by the lower degrees or steps).
• Intergranular failure mode is observed in Fig-b which is one of the mechanism failures.
Project Considered for Technological Advancements
. Overview of the project• The China–Myanmar Gas Pipeline, one of the key projects in China's energy
strategy, is the main passage of energy imports in the southwestern China, which starts from Kyaukphyu off the west coast of Myanmar and enters China at Ruili City in Yunnan Province and ends at Guigang City in Guangxi Province.
• Suspension cable crossing mode is used at both the Nujiang River section and the Yangbijiang River section of the China–Myanmar Gas Pipeline, the span length of which is 380 m and 230 m respectively. Specifically, the Nujiang River crossing is a suspension cable double-pipeline crossing with the longest span in China (Fig. 1). Steel structure tower and hinged steel structure tower are respectively used in these two crossing sections, and the tower height is up to 57 m. The cable system includes main cable, sling cable, wind cable, wind-resistance cable, and stabilizing cable. The main cable is made of braided cables and parallel steel wires prefabricated by PPWS. This project was completed and delivered into operation in May 2013.
Location and characteristics
Crossing sectionPre-stressed structure of main anchor
Pre-stressed structure of wind anchor
Nujiang River Replaceable pre-stressed system (12 bunches)
Low retraction anchor (4 bunches)
Yangbijiang River Tunnel anchor cable (24 bunches), low retraction anchor (4 bunches)
Low retraction anchor (4 bunches)
Main anchor system
• 1) In the Yangbijiang River crossing section of the China–Myanmar Gas Pipeline, where the terrain structures on the east and west bank are different, two types of anchoring structures are employed respectively. Specifically, 32 bunches of M15DH low retraction anchors in pass-through layout are used at the east bank, namely, the front and back low retraction anchors are symmetrical, with the front anchor pressing tightly on the main anchor seat, and the back anchor pressing on the positioning steel plate. In contrast, because of the terrain restriction and fragmented surface rocks on the west bank, tunnel anchor structure is used.
• 2) Pre-stressed anchor cables mainly include cable material, anchoring apparatus, grouting material and anchor pier material [4]. Twenty four epoxy coated steel strands and eight-level bearing plates are used in the structure of tunnel anchor cable. The tunnel is 23 m long, and borehole of anchor cable section is 25 m deep. The anchoring end on the west bank has eight sections of anchoring cables and one section of test anchoring cable. The bearing layer is sandstone with medium weathering, and the tunnel was filled with C30 concrete after completion.
• 3) In the Nujiang River crossing section of the China–Myanmar Gas Pipeline, the main cable is PPWS fabricated in air. Therefore, the anchoring system is distinct from the anchor seat structure. To prolong the service life of the crossing structure, replaceable epoxy coated steel strand and anti-corrosion grease are used, and steel strand can be replaced years later. On the east and west banks, fourteen bunches of M15-12 anchorages in front-back symmetrical arrangement are used respectively.
• Wind anchor system-The wind anchor systems in the Nujiang River crossing and Yangbijiang River crossing are both M15DH pass-through front-back symmetrical anchoring system, the same as the main anchor used in the Yangbijiang River crossing on the east bank.
• Pre-stressed system on the tower top-Since concrete towers are used in the Nujiang River crossing, the cross beam on the tower is where vertical load of the whole bridge concentrates. Therefore, 12 bunches of M15-12 anchorages are used to arrange pre-stressed force symmetrically at the this part, to improve the stress state and optimize the quantity of concrete and steel bar used.
Optimized application of pre-stressed technology in suspension cable crossing and its
technical characteristics
As an important method of crossing, suspension cable crossing
features graceful bridge structure, reasonable mechanic property and long history of application. Moreover, its structure is special and different from the traditional bridge crossing. It mainly consists of upper and lower parts. The lower structure includes stake foundation, anchorages and bearing platform, while the upper structure includes hinged steel structure tower, main cable, truss bridge, wind cable and pipeline .
Optimization of anti-corrosion agent injection system for steel strands
• Pre-stressed tendons are very susceptible to corrosion in high stress state. Therefore, the quality of filling is directly related to the anti-corrosion capacity of pre-stressed, safety and durability of pre-stressed structure. For all traditional pre-stressed structures, finished anchor plates are used in grouting injection and discharge. But the pre-stressed structure of the crossing takes anchor seat, on which the construction of grouting injection system is unsuitable. Therefore, the grouting injection system of the anchor pier's pre-stressed system must be modified to adapt to the anti-corrosion demand of pre-stressed anchoring system of the crossing. Considering the different anti-corrosion agents for the pre-stressed system, the appearance of anchorage and the protection requirements of steel strand in later stage, lines of grouting injection and discharge system outside the anchorage are used in the China–Myanmar Gas Pipeline crossings, which optimized anti-corrosion agent injection route and enhance the construction efficiency of anti-corrosion agent significantly. It is fit for pre-stressed structure of the crossings, and ensures the anti-corrosion effect in later stage.
Inferences• The successful application of pre-stressed structure in the pipeline crossings
has improved the mechanical behavior of concrete, enhanced the anti-cracking capacity of cable anchor piers, and prolonged the service life of the crossings. So, pre-stressed structure represents the optimization direction of anchoring structure of large span suspension cable crossing. In the construction of Nujiang River and Yangbijiang River crossings in the China–Myanmar Gas Pipeline, multiple pre-stressed structures met the anchoring demand of several suspension cable crossing structures, proving that the measures taken in the pre-stressed construction can meet the structure features of the crossings and provide technical support for the smooth construction of the project. The innovative pre-stressed anchoring system was not only successfully applied in this project, but also provides alternative technical options for the anchoring construction of similar crossing structures.
Optimization of anti-corrosion agent injection system for steel strands
• Pre-stressed tendons are very susceptible to corrosion in high stress state. Therefore, the quality of filling is directly related to the anti-corrosion capacity of pre-stressed, safety and durability of pre-stressed structure. For all traditional pre-stressed structures, finished anchor plates are used in grouting injection and discharge. But the pre-stressed structure of the crossing takes anchor seat, on which the construction of grouting injection system is unsuitable. Therefore, the grouting injection system of the anchor pier's pre-stressed system must be modified to adapt to the anti-corrosion demand of pre-stressed anchoring system of the crossing.
Results
Study of Property ObservationChemical, mechanical & microstructural characterization
No difference in intrinsic microstructure
Fractrographic Analysis Failure of cable in 2 phases:1. 50% of rods affected by SCC
resulting in fracture2. Ductile Failure(Cup &
Cone,Coalescence of Cavities)
Fatigue Behaviour Fatigue Strength significantly reduced leading to reduction in fatigue life
Stress Corrosion Cracking
• Effect of pitting, dissolution & oxidation caused by corrosion.• Fracture surface is rougher
CORROSION MANAGEMENT ALTERNATIVESPreventive Measures1. Adding Inhibitors
• High-Performance ConcreteImpeding ingress of chlorides to the rebar (by reducing concrete permeability). Using lower water-to-cement ratio concrete/Low permeability concrete.
• Corrosion-Inhibiting AdmixturesAdding mineral admixtures to the concrete mix (Silica fume and fly ash)or
calcium nitrite
2. Modifying the electrode potential of the metal
Control the electrochemical reactions at the steel surface to mitigate the corrosion reactions by imposing the proper voltage field on the rebar (cathodic protection) in a manner that forces the steel to become cathodic (reduction reactions are favored and anodic reactions, which result
in metal loss, are decreased), thereby mitigating corrosion.
3. Provision of a barrier on the surface of the concrete Application of an overlay of low-slump concrete, latex-modified concrete (LMC), high-density concrete, polymer concrete, or bituminous concrete to prevent
ingress of chloride.
4. Isolation of the metal through coatings
Use Of • Metal-Coated/Clad Rebars and Solid Corrosion-Resistant Alloy Rebars
Coated Carbon Steel Rebars Galvanized Rebars
• Corrosion-resistant Alloy/Clad Rebars Stainless Steel Rebars (Solid Type 304 or 316 stainless steel rebars and stainless steel-clad rebars)
• Epoxy-Coated Rebars
The epoxy coatings are intended to prevent moisture and chlorides from reaching the surface of the reinforcing steel. The coatings achieve their toughness and adhesion to the substrate as a result of a chemical reaction initiated by heat. Since these epoxy powders are thermosetting materials, their physical properties,performance, and appearance do not change readily with changes in temperature.
• Overcoating
Benefits
1. Extension of the service life of the cable in the future and delay the time to the first required maintenance thus increasing performance
2. Low Operation & Maintenance Costs : Thought costs in the life-cycle analysis are high initially but eliminate corrosion-related deterioration repair and maintenance.
3. Long term economic benefits.
4. Providing a 'minimum intervention' method of slowing or resisting corrosion.
A suspension bridge across Durgam Cheruvu lake connecting Jubilee Hills Road No 45 and Hyderabad Knowledge City (Raidurg IT Park) has been proposed by the Andhra Pradesh Industrial Infrastructure Corporation Limited (APIIC), to meet the ever increasing traffic requirements in the IT Park at Gachibowli and Nanakramguda,.
Proposed Project:
Convincing Stakeholders
Why to consider Pre-stressed Cable• 1. Cost Effective
There is a very minimal amount of materials needed in order to construct a suspension bridge. The only other costs incurred are with the labor involved in building it.
• 2. Can Be Built High UpSuspension bridges can be built very high up over waterways. This is essential for any area that needs to be able to allow passing ships to come through.
• 3. Span Great LengthsThe way that suspension bridges are constructed, and the materials that are used, allow them to be able to span a great distance with minimal materials.
• 4. Has FlexibilityOne common reason that the choice to build a suspension bridge is reached is if it is being built in a high earthquake zone, like California. This is because suspension bridges are flexible due to the cable system they are held up by. The bridge can “move” with the wind and during natural disasters such as an earthquake.
• 5. Simple ConstructionNo access is needed from below the bridge while it is being constructed, making it a great choice for areas that ships and waterways need to stay clear.
• OVERVIEW-Conjugated cables and stabilizing cables are also added to prevent dynamic wind effect. Therefore, the suspension cable crossing bridge reflects more obvious nonlinearity than traditional suspension bridges.
• The pre-stressed concrete structure can improve the stress state of the concrete's tensile region, and expand the tensile scope of the anchorages, so that the life of concrete can be prolonged and the total concrete quantity used can be reduced.