2INTRODUCTIONNon – Destructive testing [NDT] is an activity to assess the variation from the design and monitor performance.With the increasing number of structures showing signs of distress a greater need is felt to apply more advance assessment techniques and equipment in the bridge inspection.This presentation focuses on the available techniques , and two nos case studies of NF Railway , where the NDT has been used successfully.
3NDT Techniques Compressive Strength Rebound Hammer Test, Windsor Probe Test , Core Test , Lok Test, Capo TestFlexural StrengthBreak- Off TestDirect Tensile StrengthPull- Off TestHomogeneityUltrasonic Pulse Velocity Test , Acoustic Emission Method , Pulse – echo method. Gamma- Ray Radiography , Endoscopy, Integrity Testing of PilesPermeabilityInitial Surface Absorption Test , Figg’s Air and Water Permeability Test
4NDT Techniques [… Contd] Chemical CompositionWet Chemical Analysis for Total Chloride , Potential Measurement Technique for Chlorides, Determination Carbonation TestCracksUltrasound Pulse Velocity Test, Acoustic Crack Detector , Dye Penetration Test , X-Ray Radiography , Gamma Radiography
5NDT in NF Railway`Of the above NDT techniques so detailed , NF Railway has gained some experience in the following techniquesA. Rebound Hammer TestB. Core TestC. Ultra Sound Pulse Velocity MethodD. Integrity Testing of PilesE. Chemical Analysis of the Concrete Material
6NDT in NF RailwayThe experienced so gained in the last two years on NF Railway shall be explained in the remaining part of the presentation using two nos case studies of NF Railway
7NDT in NF Railway --- Br No 40 Br No 40 is an Important Bridge on New Jalpaiguri – New Bongaigaon section of NF Railway having an span of 20 x 150 feet over Teesta River. Its design discharge is cusecs. It has Cement Concrete Well Foundation designed for BGML Standard with 30 Feet Well Dia and 100 feet depth of well below HFL. The bridge was initially constructed as 13x150 feet spans . During 1988 floods eastern approach was washed away and the bridge was extended by 7 spans of 150ft on the east in 1975.For the last few years cracks were developing on Pier No. P1, P2 , P3 and P4.
8NDT in NF Railway --- Br No 40 PANORAMIC VIEW OF BRIDGE NO 40
9NDT in NF Railway --- Br No 40 ZIG ZAG CRACK PATTERN
10NDT in NF Railway --- Br No 40 CRACKS IN WELL CAP
11NDT in NF Railway --- Br No 40 SHOWING CASTING LIFT VARIATIONS
12NDT in NF Railway --- Br No 40 HORIZONTAL CRACKS FORMED DUE TO SEPERATION OF COLDJOINT IN COURSE OF TIME
14NDT in NF Railway --- Br No 40 SEVERE VERTICAL CRACKS
15NDT in NF Railway --- Br No 40 CRACKS CONCENTRATION FROM DIFFERENT DIRECTIONS ,RESULTING IN SEVERE DETERIORATION AT MEETING POINT
16NDT in NF Railway --- Br No 40 These cracks were prominent in Piers P1, P2, P3 and P4.After this visual examination , certain points remained unansweredIt was not clear , as to what was the depth of these cracks?What was the resultant compressive strength of the concrete mass used in Pier?Whether the concrete has undergone any long term deterioration due to excessive sulphur or chloride content?
17NDT in NF Railway --- Br No 40 In order to assess the above points it was decided to undertake the following examinationsUltrasonic Pulse Velocity TestConcrete Core extraction for evaluation of compressive strengthChemical Analysis to ascertain approximate mix proportions of concrete
18NDT in NF Railway --- Br No 40 M/s S. K. Mitra and Associates of Kolkatta were entrusted the work, who carried out the same in March – April 2003Test Methods and the deductions on the basis of the test results are explained in the following slides.
19NDT in NF Railway --- Br No 40 Ultrasonic Pulse Velocity Test The aim of the Ultrasound pulse velocity test was to identify weak locations possessing loss of integrity , presence of void , quality of concrete and depth of cracks which could not be ascertained by visual inspection.The ultrasonic pulse is generated by an electro-acoustical transducer. When the pulse is induced in the concrete from a transducer, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves is generated which includes longitudinal , shear and surface waves. The receiving transducer detects the longitudinal waves which is the fastest.
20NDT in NF Railway --- Br No 40 Ultrasonic Pulse Velocity Test The velocity of an ultrasonic pulse through any material depends upon its density , modulus of elasticity , presence of reinforcement steel and Poisson’s ratio.The underlying principle of assessing the quality of concrete is that comparatively higher velocities are obtained when the quality of concrete in terms of density , homogeneity and uniformity is good. In case of poorer quality , lower velocities are obtained.If there is a crack , void or flaw inside the concrete which comes in the way of transmission of the pulses, the pulse strength is attenuated and lower velocities are obtained,
21NDT in NF Railway --- Br No 40 Ultrasonic Pulse Velocity Test Pulse Velocity by Cross Probing [Km/sec]Concrete Quality Grading [IS 13311Part –I-1992]1Above 4.5Excellent23.5 to 4.5Good – Very Good33.0 to 3.5Medium4Below 3.0Doubtful
22NDT in NF Railway --- Br No 40 Ultrasonic Pulse Velocity Test-Interpretations There are two dimensions to the result interpretationFirst – General Quality Assessment of Concrete- The velocity ranged fro 3.1 to 4.0 Km/sec. This indicates that quality of concrete was good to very goodSecond- Depth of Crack – It was assessed based on the velocity of propagated waves on both sides of the crack. Crack Depth were found to be varying from 31mm to 684 mm .
23NDT in NF Railway --- Br No 40 Concrete Core Extraction The basic purpose to extract cores was to evaluate directly the compressive strength of concrete on the basis of cylinder strength and co-relate it to conventional cube strength.This gives fairly a good idea about the compressive strength reached
24NDT in NF Railway --- Br No 40 Concrete Core Extraction CORE DRILLING IN OPERATION
25NDT in NF Railway --- Br No 40 Concrete Core Extraction CORE LOCATION AFTER DRILLING
26NDT in NF Railway --- Br No 40 Concrete Core Extraction 5 Cores of 55mm Diameter were extracted.To assess crushing strength , cores were trimmed at the end and tested for voids and density also. Prior to test , cores were capped with sulphur and tested after soaking for 48 hoursEquivalent concrete cube strength was assessed as per IS The results obtained were found nearer to M-20 grade concrete.As per the completion plan , a mix proportion of 1:3:6 is mentioned in the drawing for construction of pier , and the bed block and well cap was constructed using mix proportion of 1:2:4. The test results confirmed that the compressive strength of the concrete is above the assumed value.
27NDT in NF Railway --- Br No 40 Concrete Core Extraction- Test Results Sample NoLength [cm]% of Water AbsorptionCrushing Strength in N/sq mmEq Cube Strength in N/sq mmC115.82.4723.7829.2C216.43.0419.8324.8C313.83.9817.9122.39C4123.5918.4023.00C514.64.9618.5423.17
28NDT in NF Railway --- Br No 40 Chemical Analysis The aggregates of Cement Concrete sample was separated from the mortar mechanically and chemical analysis of mortar portion of the cement concrete was carried out generally on the lines of ASTM CPH value , Chloride content and the Sulphate Content were analysed.
29NDT in NF Railway --- Br No 40 Chemical Analysis- Test Results PH at 31Deg C>10.5Chloride in PPM247742597Sulphate in PPM30874013349851455728
30NDT in NF Railway --- Br No 40 Chemical Analysis- Test Results As per IS , the PH should not be less than .Test results reveals that concrete is basic in nature and so no deterioration due to acidic nature was envisaged.As per IS 456 – 2000 , the chloride content should be below 3000 ppm . In all the samples the Chloride content was found to be much below thisThe limiting value of the sulphate content is 4000 ppm . In this case the sulphate content were found to be near or slightly more than the limiting value. As per IS , higher values of sulphates can cause expansion and disruption of concrete.
31NDT in NF Railway --- Br No 40 Chemical Analysis- Test Results IS Clause states that, some aggregates containing particular varieties of silica may be susceptible to attack by alkalis originating from cement and other sources producing an expansive reaction which can cause cracking and disruption of concrete.Aggregates retrieved from the extracted core was subjected to aggregate reactivity test as per IS:2386 and the results ruled out the possibility of Alkali- Aggregate reactivity
32NDT in NF Railway --- Br No 40 CONCLUSIONS Compressive Strength of Concrete was found to be more than the design value.Crack depth was found to be varying between 31mm to 684 mmHorizontal Cracks , noted in between the lifts of casting in some cases were due to cold joints formed during construction.Higher sulphate content might have resulted in the cracks.
33NDT in NF Railway --- Br No 184 Br No 184 is a major bridge in Guwahati – Lumding section of NF Railway . This has span of 1x 12.20m and 3x 18.30m . The bridge was built between It is having well foundationsInterestingly the bridge pier is of concrete with one brick thick cladding all around the pier.AEN/Lumding reported a hollow echoing sound emanating from the piers peropheri , when the piers were struck by hammers at different points along the cross section and height of the piers.Sectional Sr DEN imposed a restriction of 20 kmph over the bridge. The Division proposed rebuilding of the bridge at an approximate cost of Rs 5.16 Crores
34NDT in NF Railway --- Br No 184 CBE’s inspection revealed that apart from the usual weathering of the bricks , there was no distress noticeable on the outer face of the piers where the hollow sounds were coming fromCBE decided to carry out the NDT and have an insight into the bridge structural behaviour before going for the complete rebuilding.
35NDT in NF Railway --- Br No 184 M/s S K Mitra & Associates were entrusted to carry out the NDT using the following techniquesUltra sound pulse velocity testCore extraction and TestingChemical test to evaluate sulphate . Chloride content and PH value
36NDT in NF Railway --- Br No 184 UPV Test MEASURING ULTRA SONIC PULSE VELOCITY
37NDT in NF Railway --- Br No 184 UPV Test MEASURING PULSE VELOCITY
38NDT in NF Railway --- Br No 184 UPV Test Pulse Velocity by Cross Probing [Km/sec]Concrete Quality Grading [IS 13311Part –I-1992]1Above 4.5Excellent23.5 to 4.5Good – Very Good33.0 to 3.5Medium4Below 3.0Doubtful
39NDT in NF Railway --- Br No 184 UPV Test Interestingly , the velocity of pulse ranged from to 1.45km/sec for Pier No 2 and from to 1.32 Km/sec for Pier No P3This placed the concrete quality as doubtful according to the UPVvalues as per IS:13311(Part I)Next Logical step was to carry out the Concrete Core Strength Tests and do the Chemical Analysis.
40NDT in NF Railway --- Br No 184 CONCRETE Core Cutter Test CORE EXTRACTION IN PROGRESS
41NDT in NF Railway --- Br No 184 CONCRETE Core Cutter Test AFTER CORE EXTRACTION
42NDT in NF Railway --- Br No 184 CONCRETE Core Cutter Test Total 7 Nos cores of 50 mm dia were extracted. Out of these , 2 Cores were extracted from the brick layer to assess the quality of the bricksIn all cases , water absorption test was carried out and strength determined as per the procedure described in IS:516. To assess crushing strength , cores were trimmed at the end and tested for void and density.
43NDT in NF Railway --- Br No 184 CONCRETE Core Cutter Test Sample NoLength [cm]% of Water AbsorptionCrushing Strength in N/sq mmEq Cube Strength in N/sq mmC1206.47.859.81C21916.46.113.016.25C3186.711.013.75C4193.534.743.40C55.018.322.90
44NDT in NF Railway --- Br No 184 CONCRETE Core Cutter Test The results obtained were nearer to M15 , excepting in two cases where these were found nearer to M10 and M40 respectively.Hence the quality of concrete was found to be good in generalConcrete cores also displayed a lot of voids and honeycombing at the interface of the concrete core and brick cladding.
45NDT in NF Railway --- Br No 184 Chemical Analysis From the cores extracted , the aggregate was separated from the mortarFrom the chemical test result it was concluded that , the concrete was basic in nature. Chloride and Sulphate content of the concrete was well below the limit prescribed by IS456.
46NDT in NF Railway --- Br No 184 Condition Monitoring and Useful Life Assessment The results as obtained were conveyed to Prof P. Banerjee of IIT-Powai. Pixel Networks of Mumbai arranged for the site visit and investigation of the bridge .On 14th January 2004, Prof Banerjee visited the site and . Two more cores were extracted from the bridge in his presence as requested by him.He submitted his detailed report afterwards
47NDT in NF Railway --- Br No 184 CONCLUSIONS based on the NDT and Prof Banerjee’s Report The hollow sound emanating form the pier was due to the voids and honeycombing formed at the face of pier and brick cladding.Lower pulse velocity was also due to the voids and honeycombing at the interfaceConcrete Core was of fairly good quality and was found to be having sufficient strength to carry the present day loadingThere was no deterioration due to acidity , higher chloride content or higher sulphate content.
48NDT in NF Railway --- Br No 184 CONCLUSIONS based on the NDT and Prof Banerjee’s Report While calculating the strength of the bridge it was assumed that the brick cladding was not participating in any load sharing arrangement.Hollow sound was not a cause of concern.The Railway got confidence in the structural integrity of the bridge and the rebuilding of the bridge was not considered necessary
49NDT in NF Railway --- Br No 184 The Cost of the NDT was Rs 1.6 Lakh and the cost of Conditional monitoring by Prof Banerjee was Rs 2.5 LakhThe cost of rebuilding of the bridge was Rs 5.2 Crores.Not only the NDT resulted in the huge saving , it also initiated an approach which is scientific , logical and extends beyond the realms of judgments based solely on the visual examinations