Condition Assessment of Structures (Visual inspection & non-destructive testing of structures)

Transcription

1 1 Condition Assessment of Structures (Visual inspection & non-destructive testing of structures) Course outline 2 Initiating Deterioration Process Chemical Physical Biological Factors Causing Deterioration Intrinsic Extrinsic Leads to Cracking Scaling Spalling Popouts Delamination etc. Visible Damage Repair Principles for repair and protection Assessed Condition evaluation Condition survey Visual inspection Non-Destructive Testing Destructive Testing

2 Outlines 3 Main objective of condition assessment Deterioration / Building materials Condition assessment Condition Survey Planning Visual inspection Non-Destructive testing (NDT) detection of cracks/ voids/ delamination etc. corrosion assessment, location and diameter of reinforcement and cover thickness strength estimation of concrete --- to be continued in the next lecture --- Main objective of condition assessment 4 Main objective of condition assessment are to place the building into one of the following three categories: i. The building has not shown any signs of distress and It satisfies all the safety and serviceability requirements according to relevant Codes of practice, hence no action is needed towards repairing. ii. The building is seen to be deficient (or distressed) but it can be repaired and strengthened to satisfy the Codal safety requirements or performance criteria set by the user. iii. The building is badly damaged. It is to be demolished and a new building may be built, build back better. Main steps of condition assessment will be a) To record the damage if any, and find out the causes for distress b) To assess the extent of distress and to estimate the residual strengths of structural components and the system including the foundation c) To plan the rehabilitation and retrofitting/strengthening of the building

3 Deterioration / Building materials (1/2) 5 Deterioration / Building materials (2/2) 6

4 Condition assessment planning 7 Preliminary investigation: 1. Review of relevant documents 2. Visual inspection, with documentation of defects 3. Field and laboratory testing 4. Preliminary analysis and evaluation Is further investigation required? YES Detailed investigation: 1. Review of additional documents and data source 2. Additional field observations, and field and laboratory testing 3. Detailed analysis and evaluation NO Is repairing required? YES NO Identify and analysis repair options Identify special conditions to further considered (e.g. maintenance, planning Final report Review of plans and relevant documents 8 To review documents from design and construction process as well as inspection and maintenance reports is in general the easiest way of gathering data about the structure to be assessed. It has to be assured that the reviewed documents are correct. Loads can be usually determined from current loading codes and environmental conditions may be obtained from inspection reports. Resistance properties like material and structural properties and dimension can be obtained from: Construction specifications - Codes As-built drawings--architectural, structural, mechanical, and foundation plans Construction documents (e.g. material delivery documentation) Documentation of performance, defects, maintenance, and changes (Alterations) Reports of earlier inspection and maintenance.

5 9 VISUAL INSPECTION Scope of Visual Inspection 10 Prior to the starting of visual inspection, the structural engineer is to obtain a set of the building s structural layout plans from the building owner. The availability of the structural layout plan will help the structural engineer to: a) understand the structural system and layout of the building; b) identify critical areas for inspection; c) identify the allowable imposed loads, in order to assess the usage and possibility of overloading; and d) verify if unauthorised addition or alteration works that affect the structure of the building have been carried out. Fig. Lahdensivu, J.

6 Visual Inspection Tools and Instruments 11 Simple tools and Instruments like: Camera Magnifying glass Binocular Gauge for crack width measurement Chisel and hammer are usually needed. Pocket knife, screwdriver Occasionally, a ladder or light platform/scaffold tower can be used for access to advantage. Scope of Visual Inspection 12 A visual inspection is generally carried out of: a) the condition of the structure of the building to identify the types of structural defects to identify any signs of structural distress and deformation to identify any signs of material deterioration b) the loading on the structure of the building to identify any deviation from intended use, misuse and abuse which can result in overloading c) any addition or alteration works affecting the structure of the building - to identify any addition or alteration works which can result in overloading or adverse effects on the structure If there are no signs of any structural deterioration or defects, the visual inspection should suffice and unless the structural engineer otherwise advises, no further action needs to be taken

7 Visual inspection report (example) General Information of the Building address, usage of the building, maintenance history etc. 2. Structural System of the Building reinforced concrete, prestressed concrete, steel, etc 3. Date and Scope of the Inspection 4. Survey of addition or alteration works to building structure 5. Survey of signs of structural defects, damages, distress, etc. 6. Survey of exposure to aggressive environment 7. Conclusions on the structural condition 8. Sketches, plans and photographs Examples of typical defects found by visual inspection 14 Erosion of Brick Face Efflorescence Brick Spalling/Delaminating

8 Examples of typical defects found by visual inspection 15 Concrete Crack Crack and Spall of Concrete Around Steel Member Delaminating Concrete Over Reinforcement 16 Non-Destructive testing (NDT) on reinforced concrete structure

9 NDT Objectives 17 NDT methods are extremely valuable in assessing the condition of structures, such as bridges, buildings, elevated service reservoirs and highways etc. The principal objectives of the NDT / PDT of concrete in situ is to assess one or more of the following properties: In situ strength properties Durability Density Moisture content Elastic properties Extent of visible cracks Thickness of structural members having only one face exposed Position and condition of steel reinforcement Concrete cover over the reinforcement. Reliable assessment of the integrity or detection of defects of concrete members even when they are accessible only from a single surface. NDT Advantages and Disadvantages 18 Advantages Disadvantages Access to hidden items see through walls Better investigations with NDT Rapid accumulation of data Generally less expensive than destructive testing Minimize interruption of building services Evaluation and quality assurance More than one test method may be required Environmental conditions may effect or distort results Construction details & building components may effect results Some conditions cannot be determined with a reasonable degree of accuracy without destructive testing

10 Typical situations where non-destructive testing is needed 19 Quality control of pre-cast units or construction in situ Monitoring of strength development in relation to load application or similar purpose Location and determination of the extent of cracks, voids, honeycombing and similar defects within a concrete structure Determining the concrete uniformity, possibly preliminary to core cutting, load testing or other more expensive or disruptive tests Determining the position, quantity or condition of reinforcement Increasing the confidence level of a smaller number of destructive tests Determining the extent of concrete variability in order to help in the selection of sample locations representative of the quality to be assessed Confirming or locating suspected deterioration of concrete resulting from such factors as overloading, fatigue, external or internal chemical attack or change, fire, explosion, environmental effects Assessing the potential durability of the concrete Providing information for any proposed change of use of a structure for insurance or for change of ownership. NDT Methods for Specific Distresses 20 Distresses Air Pockets and Honeycombing Alkali-Silica Reaction Chloride-Induced Corrosion Cracking Delamination Popouts Potholing (Caving) Scaling Spalling Condition Assessment Methods Chain Dragging, Ground Penetrating Radar, Hammer Sounding, Impact-Echo, Ultrasonic, Visual Inspection Visual Inspection Half-Cell Potential, Rapid Chloride Permeability, Resistivity Impact-Echo, Ultrasonics, Visual Inspection Chain Dragging, Coring, Ground Penetrating Radar, Hammer Sounding, Impact-Echo, Infrared Thermography, Ultrasonics Visual Inspection Visual Inspection Visual Inspection Visual Inspection

11 21 NDT for detection of cracks/ voids/ delamination etc. Hammer sounding Chain dragging Ground penetrating radar Impact-echo Ultrasonic pulse velocity Radiographic testing Crack width measurement NDT - Sounding 22 A qualitative evaluation of concrete can be easily obtained by just sounding it (i.e. tapping it) with a hammer. When the hammer is struck on good concrete, a ringing sound is created. On areas where delaminations or cracks occur, the striking of the hammer produces a drum-like sound. The limitation of this method is that it cannot detect defects that exist deep in the member. Also, defects lying under overlays are also difficult to find.

12 NDT - Chain Dragging 23 The objective is to detect regions where the sound from dragging the chain changes from a clear ringing sound (sound deck) to a somewhat mute and hollow sound (delaminated deck). Chain drag is a relatively fast method for determining the location of a delamination Chain Dragging is normally used on large concrete surface areas, such as bridge decks The method typically rely on the experience of the inspector to differentiate the relative sounds of similar materials NDT - Ground Penetrating Radar (GPR) [1/4] 24 RADAR Radio Detection and Ranging. Detect target in free space Determine the range Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. GPR finding and detecting buried object. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. Civil engineering applications Probing into soil to detect pipelines and tanks Cavities Thickness determination Locating reinforcement Identifying deterioration detect objects, changes in material, and voids and cracks

13 NDT - Ground Penetrating Radar (GPR) [2/4] 25 Components of GPR: Transmitting and receiving unit Control unit Display unit Power supplies NDT - Ground Penetrating Radar (GPR) [3/4] 26 An EM pulse is sent through an antenna, penetrating into the surveyed material. A portion of the energy is reflected back to the antenna when an interface between materials of dissimilar dielectric constant is encountered. The amount of reflected energy at an interface is governed by: r = e r1 r 2 1,2 e r1 + e r 2 where 1,2 is the reflection coefficient r1 and r2 are the dielectric constants. - e

14 NDT - Ground Penetrating Radar (GPR) [4/4] 27 The thickness of a layer is given by: Typical Dielectric Constants: d i = Ct 2 e i r, i where d i is the thickness of layer i, t i the total travel time through that layer, C is the speed of light r,i the dielectric constant of the layer NDT - Impact-echo [1/3] 28 Based upon evaluation of stress waves generated by an elastic impact on a concrete surface Originally developed at Cornell University and NIST by M. Sansalone and N. Carino Impact Echo can be used to measure the thickness of slabs, plates, columns and beams, and hollow cylinders. It can also be used to determine the location and extent of flaws such as cracks, delaminations, voids, honeycombing and debonding in plain, reinforced and post-tensioned concrete structures. Impact R S P R P Compression waves S - Shear waves R - Rayleigh waves By: Guy Rapaport, Ramboll

15 NDT - Impact-echo [2/3] 29 Procedure Impact Echo testing consists of measuring both the time record and frequency spectrum associated with a mechanical impact on the surface of a structure. As stress waves propagate through the structure, they reflect off internal and external boundaries and cause periodic displacements on the surface. These motions are monitored by a transducer and digitized. The waveform is transformed into the frequency domain, so that the periodicity of stress-wave arrivals can be accurately determined. As part of Impact Echo testing, direct measurements of compression (P-) wave velocity are also made. Given the P-wave velocity and the arrival period (or frequency), the depths to internal flaws or external boundaries are calculated. NDT - Impact-echo [3/3] 30 The fundamental equation of impact-echo is d = Vp/(2f), where d is the depth from which the stress waves are reflected (the depth of a flaw or the thickness of a solid structure), Vp is the wave speed, f is the dominant frequency of the signal

16 Ultrasonic Pulse Velocity 31 Ultrasonic waves are very similar to light waves in that they can be reflected, refracted, and focused. Reflection and refraction occurs when sound waves interact with interfaces of differing acoustic properties. Ultrasonic reflections from the presence of discontinuities or geometric features enables detection and location. Ultrasonic pulse velocity in concrete (UPV) ASTM C In the UPV method, the velocity of a pulse traveling through concrete is measured and correlated to its stiffness The velocity of the pulse increases with the stiffness of the concrete, but decrease with increasing density. Wave attenuation increases when concrete becomes denser, because of absorption of energy. the UPV can be used in three modes direct, semi-direct, and indirect. The direct mode, or the throughtransmission mode, is the most reliable, but needs access to both sides of the material.

17 Ultrasonic pulse velocity in concrete (UPV) ASTM C The pulse velocity can be determined from the following equation. = where V = pulse velocity (km/s), L = path length (cm), T = transit time(µs). Based on this technique, the velocity of sound in a concrete is related to the concrete modulus of elasticity. ( ) where, E = modulus of elasticity, r =density of the concrete Ultrasonic pulse velocity in concrete (UPV) ASTM C

18 Radiographic testing 35 Radiography can be used to obtain permanent image of surface and subsurface (embedded) discontinuities The same discontinuities can be radiographed again after a period of service life and the radiographs can be compared to measure the change in the size and shape of the discontinuity. Radiographic testing of the structural reinforcement in reinforced concrete elements is carried out with the use of an apparatus containing sources of gamma or X rays. The classical inspection arrangement consists of a radiation source at one side of the object to inspect and an X- ray film at the other side. There exist also radiation methods for one-side inspections, using the backscatter effect. These methods need long inspection times and are limited to single layers of reinforcement near the surface. Radiographic testing 36 The selection of the source depends on the wall thickness, which shall be inspected

19 Radiographic testing 37 Stereo radiography is used for quantitative measurement of depth and diameters of steel reinforcements. It needs two different source positions. The reconstruction can be done by a graphical back projection or a computerised one. The simple graphical backprojection is sufficient for so called simple structures. This is usually some separated steel parts or one layer of steel bars. Principle of stereo radiography Radiographic testing 38 Results flat bar in the corner pillar is shifted vertical flat bars are led around round bar running vertical for the anchor Radiation technique arrangement of the measuring point

20 Crack width measurement 39 Measuring Magnifier Crack widths are normally limited to 0.2 mm or 0.3 mm in concrete structures. Measuring Magnifier device enables accurate determination of whether cracks exceed this limit. Magnification 10x Measuring range 20 mm x 0.1 mm Field of View 32mm Crack Width Gauge Align the Crack Width Gauge where the calibration and the crack are the same width. Record the width, length and location of the crack. Crack width measurement 40 CRACK DETECTION MICROSCOPE Measure crack widths in concrete. Consisting of a high definition Microscope connected to an adjustable light source which provides a wellilluminated image under all working conditions. The image is focused by turning the knob on the side of the microscope and the eyepiece can be rotated through 360 degrees to align with the direction of the crack being examined. The 4mm measurement has a lower scale divided into 0.2mm divisions. the maximum crack widths should not exceed 0.3mm which is 15 divisions on the scale for most types of environment. Specification: Magnification x 40 Measuring Range 4 mm Divisions 0.02mm

21 41 NDT for corrosion assessment, location and diameter of reinforcement and cover thickness Cover meter Half Cell Potential test Concrete Resistivity test Cover meter test 42 Cover meter test is used to assess the location and estimate the diameter of reinforcement bars and concrete cover. Principle: based measurement of change of an elctromagnetic field caused by steel embedded in the concrete. Equipment: profometer comprise a search head, meter and interconnecting cable. The concrete surface is scanned, with the search head kept in contact with it while the meter indicates, by analogue or digital means, the proximity of reinforcement

22 Cover meter test 43 The cover meter applies a current pulse The instrument measures the amplitude of the induced current, which depends on the orientation, depth, and size of the bar. The search head is directional and maximum signal is obtained when the bar is aligned with the long axis of the search head. The pulse-induction technique is uniquely stable, is not affected by moisture in concrete or magnetic aggregates, and is immune to temperature variations and electrical interference. Cover Master, Germann Instruments: Example of the cover meter test results 44 ete/profometer_5_/english/proceq_brochure_profometer_5 E.pdf

23 Half-Cell Potential Method (ASTM C ) 45 Principle: The electrical activity of the steel reinforcement and the concrete leads them to be considered as one half of weak battery cell with the steel acting as one electrode and the concrete as the electrolyte. The electrical potential of a point on the surface of steel reinforcing bar can be measured comparing its potential with that of copper - copper sulphate reference electrode on the surface. Practically this achieved by connecting a wire from one terminal of a voltmeter to the reinforcement and another wire to the copper sulphate reference electrode. Then generally readings taken are at grid of 1 x 1 m for slabs, walls and at 0.5 m c/c for Column, beams Half-Cell Potential Method (ASTM C ) 46 The results affected by: Degree of humidity in concrete More negative potentials result for concrete with higher degree of saturation. Stray currents The presence of stray currents will significantly affect the measurements of the half-cell potential. Oxygen content near the reinforcement The lack of oxygen near the reinforcement results in more negative potentials Microcracks Localized corrosion can be generated by microcracks, which also modify the concrete resistivity, consequently affecting the corrosion potential measurement

24 Example of the Half Cell Potential test result 47 Measured Potential Corrosion Condition E corr values mv vs. SCE <-426 <-500 Severe corrosion <-276 < to to -200 >-125 >-200 High (>-90% risk of corrosion) Intermediate corrosion risk Low (10% risk of corrosion) CSE = Copper / Copper sulphate electrode, i.e. the potential values are stated with the respect to CSE Example of the Half Cell Potential test result 48

25 Galvapulse - Surface Corrosion Rate System 49 The Galvanostatic Pulse Measurements technique (GPM) was first used in the field in It provides a solution to interpretation problems found when the half cell potential methods is used in some environments, e.g. in wet concrete. estimation of corrosion rate as well, which means how much reinforcement steel is being dissolved per year. The GalvaPulse is a rapid, nondestructive polarization technique for the evaluation of reinforcement corrosion rate as well as half-cell potentials. typically been used in connection with: Swimming pools Bridges Balconies Parking houses Galvapulse - Surface Corrosion Rate System 50 Advantages Estimation of the corrosion rate in the reinforcement can be made in less than 10 seconds. Reliable evaluation of reinforcement corrosion also in wet, carbonated or inhibitor treated concrete. Half cell potential and electrical resistance of the cover layer are given. Lightweight electrode / hand held computer and easy to operate software. Durable Guard Ring system for focusing the current field to the reinforcement. Measurements possible on uneven and curved surfaces. Measurement results in Excel-format are easily transferred to PC for further processing and presentation. Threshold values

26 Concrete Resistivity test 51 The Measurement Principle measure the electrical resistivity of concrete or rock in a non-destructive test. A current is applied to the two outer probes, with the difference measured by the two inner probes. In concrete material with high electrical resistivity the corrosion process will be slow compared to concrete with low resistivity in which the current can easily pass between anode and cathode areas Where a is probe spacing [cm] V is measured potential [V] I is the current applied [A] Concrete Resistivity test result 52 The electrical resistivity of concrete was proposed as an effective parameter to evaluate the risk of reinforcing steel corrosion, particularly when corrosion is induced by chloride attack The resistivity measurement is a useful additional measurement to aid in identifying problem areas or confirming concerns about poor quality concrete. Measurements can only be considered along side other measurements. Reinforcing bars will interfere with resistivity measurements.

27 Concrete Resistivity test 53 Limitation It is difficult to measure resistivity in very close reinforcement Carbonation may affect the resistivity It cannot be used where ambient change in temperature is there. Experience operator is required to handle this equipment. Summary 54 Lecture summary Main objective of condition assessment Deterioration / Building materials Methodology of condition assessment Condition Survey Planning Visual inspection Non-Destructive testing (NDT) detection of cracks/ voids/ delamination etc. Next lecture Tests for strength estimation of concrete Destructive and laboratory testing on reinforced concrete structure Mold inspection Inspection of masonry walls Inspection of rendered facades corrosion assessment, location and diameter of reinforcement and cover thickness