ow) Repair, Sirengihening and Retrofiting of Structures Unit 2 MAINTENANCE ‘Maintenance is preventive in nature, Activities include inspection and works necessary to fulfil the intended function or to sustain original standard of service. The maintenance of structure is done to meet the following objective: Prevention of damages due to natural agencies and to keep them in good appearance and working condition, ii, Repair ofthe defects occurred inthe structure and strengthen them, if necessary Importance of Maintenance of Structures Improves the service life of structure Improved life period gives better return on investment Better serviceability of elements and components Aesthetically appealing appearance of structure is maintained Leads to quicker detection of defects and hence remedial measures may be applied to prevent damage. Prevents major deterioration that leads to collapse Ensures safety to occupants Ensures feeling of confidence by the user Classification of Maintenance Maintenance work is broadly classified as: |. Preventive Maintenance i. The maintenance work done before the defect has occurred or damage is developed in the structure is called preventive maintenance. ‘It includes thorough inspection, planning the programs of maintenance and executing the work. i. It depends upon the specifications, condition and use of structure. 2. Remedial Maintenance It is the maintenance done after the defects or damage occurs in the structure. It involves the following basic steps. Finding the deterioration Determining the causes a Evaluating the strength of the existing structure Evaluating the need of the structure vi. Selecting and implementing the repair procedure }. Routine Maintenance i. Itis the service maintenance attended to the structure periodically. ‘The nature of work done and interval of time at which itis done depends upon specifications and materials of structure, purpose, intensity and condition of use. iii, Itincludes white washing, parch repair to plaster, replacement of fittings and fixtures, binding of road surface. |. Special Maintenance i. the wr denn i ae i ec ue we owe i Lo ala new se et cree its serviceabi iii, Itmay, include particular or complete renewalDr. S. M. Laskar, NSUT it, Collection of samples and carrying out tests, both in-situ and in laboratory. iv. Studying the documents including structural aspects v. Estimation of loads acting on tl the structure, vi. Estimate of environmental effects including soil structure interaction, vii. Test data analysis and preparat ion of inferences viliTaking preventive steps to prevent further damage ix, Retrospective analysis to validate the inferences X. Assessment of stability of structure xi, xi Recommendation regarding cor Post repair evaluation through mndition of the structure tests xiii. Load test to study the behaviour The flow chart of the activities involv stages. i, Pre-repair evaluation ii, Post-repair evaluation ed for condition assessment. The assessment procedure spans two distinct koe Material Tex Seaver Documents Fatination of Coad oo Environmental eects ~ Reirospecive ‘Analyaie Asening oF ‘Structural aa a Se] Visual Inspection The first stage of condition assessment consists of visual inspection. I ‘Adeuaey | ac examination of a material to identify flaws through ‘human eyes. It is the simplest and oldest non-destructive method. It is essentially the foremost stage of structural audit. It may be classified as Aided visual inspection - carried out wit Unaided visual inspection - carried out Visual Inspection is executed to ~ th the help of optical aids such as magnifying glasses, mirrors, light, ete. with the help of optical aids, j. identify the types of structural and non-structural defects; iti, identify the alterations in the m iv, collect general information of t lentify possible cause of the defects; xembers of the structure or the usage; the structure,~ Repair, Strengihening and Retrofiting of Sruetures Destructive and Non-destructive Testing Non-destructive testing (NDT) methods are techniques used to obtain information about the properties or internal condition of an object without damaging the object, Non-destructive testing is a descriptive term used for the examination of materials and components in such way that allows materials to be examined without changing or destroying their usefulness. NDT is a quality assurance management tool which can give impressive results when used correctly. It requires an understanding of the various methods available, their capabilities and limitations, knowledge of the relevant standards and specifications for performing the tests. NDT techniques can be used to ‘monitor the integrity of the item or structure throughout its design life Though non-destructive testing procedures are very straightforward to use, analyzing and interpreting test findings is more difficult. Asa result, analyzing the hardened characteristics of concrete necessitates specialized expertise. Because the specimen is not loaded to failure in non-destructive testing, the strength inferred or calculated cannot be anticipated to give absolute values of strength. As a result, these approaches seek to test certain additional characteristics of concrete in order to determine its strength, durability, and elastic parameters. Hardness, resistance to projectile penetration, rebound number, resonance frequency, and capacity to transmit ultrasonic pulse velocity are some of the characteristics of concrete. Concrete’s electrical characteristics, as well as its ability to absorb, scatter, and transmit X-rays and Gamma-ray, ‘as well as its reaction to nuclear activation and acoustic emission, enable us to determine its moisture content, density, thickness, and cement content, ‘The greatest disadvantage of the conventional methods of testing concrete lies in the fact that in-situ strength of the concrete can not be obtained without damaging the actual structure. Also the test specimens are destroyed, ‘once the test is performed and subsequent testing of the same specimens is not possible. Thus, the effect of prolonged curing, weathering action and other time dependent characteristics can not be correctly calculated. No matter how well a concrete mix is designed, there are variations in mixing conditions, amount of compaction or ‘curing conditions at site which cause the variations in the final product. Conventional method of testing is not sufficient to predict the performance of the structures under adverse conditions e.g. exposure to liquid, gas, and ‘chemicals radiation, explosion, fire, extreme cold or hot weather, marine and chemical environment. All such ‘severe exposure conditions may induce deterioration in concrete and impair the integrity, strength and stability of the structure. Thus, conventional strength test does not give idea about the durability and performance of the actual ‘conerete in the structure. This gave the impetus to the development of non-destructive methods for t structural concrete in-situ. “Advantages of non-destructive testing of concrete are: ‘The equipment is simple to use. Defects can be found without causing damage to the components, ‘The methods are rapid and precise. Electrical, magnetic, and chemical characteristics can be used to sort ¢ {t's simple to keep track of test results and other information, Disadvantages of non-destructive testing of concrete are: ‘operations need the attention of skilled experts. ‘Manual Inspection of irregular components might be difficult. 3. The couplant’s requirement. 4, Water-resistant test items are required, ‘Thus, NDT methods are extremely valuable in assessing the condition of str ‘elevated service reservoirs and highways, ete, The principal objectives of the t in situ is to assess or more of the following properties of structural cosal fin dv alt Dr. S.M. Laskar, NSU; 7. Thickness of structural members having only one face exposed 8. Position and condition of stee! reinforcement 9. Concrete cover over the reinforcement. '0. Reliable assessment of the integrity or detection of defects of concrete members even when they are accessible only from a single surface NDT being fast, easy to use at site and relatively less expensive can be used for the following: J Totest actual structure instead of representative cube samples 2. To test any number of points and at any location 3+ Quality control and quality assurance management tool 4. To assess the structure for various distressed conditions & Pamage assessment due to fire, chemical attack, impact, age et. & To detect cracks, voids, fractures, honeycombs and week locations 7, 8 9. Tomonitor progressive changes in properties of concrete & reinforcement. To assess overall stability of the structure Monitoring repairs and rehabilitation systems 10. Scanning for reinforcement location, stress locations The various non-destructive and semi-destructive tests can be grouped as below: Group - I A: Non-destructive Tests for Concrete * Surface Hardness Test - Rebound Hammer Test * Ultrasonic Pulse Velocity Test Group -1 B: Partially Destructive Tests for Concrete + Penetration Resistance Test - Windsor Probe + Pull-out Test + Pull-off Test + Break-off Test * Core Cutting Group -Il: Tests for Properties of Concrete at Fresh and Hardened State and For Durability 2. Chemical Tests b. Cement Content & Aggregate / Cement Ratio © Sulphate Determination Test 4. Chloride Determination Test © Alkalinity Test £ Casbonation Test . Absorption & Permeability Tests Crack Monitor i, Moisture Measurement J. Abrasion Resistance Test k. Fresh Concrete Tests For W/C Rat Group - III: NDT for reinforcement details 1. Rebar Locator & bar sizer 2. Comtosion mapping 4. Half-cell Potentiometer fi, Resistivity meter Group - IV; Miscellaneous Test 1. Radiographic Test i. X-Ray ii, Cobalt Gamma ray tio and Compressive Strength and condition Selection of NDT and Sem-NDT MethodRepair. Strengthening and Retrofining of Structures Table Selection of NDT and Semi-NDT Method = Testing Methods 1. Windsor Probe 2. Ultrasonic Pulse Velocity . 3. Core Compressive Strength of Concrete 4. Capo 5. Pull-out 6. Combined Methods 7. Rebound Hammer Flexural Strength Break-off Direct Tensil |. Direct Tensile Strength ‘ensile Strength fipaPsie - Ultrasonic Pulse Velocity . Pulse Echo . Endoscopy ._ Gamma Ray Radiograph . Rebound Hammer . Ultrasonic Pulse Velocity . Ultrasonic Pulse Velocity . Acoustic Crack Detector Dye Penetration Test, . X-Ray Radiography . Gamma-Ray Radiography 3. Crack Scope Rebar Locator . Rebar Scanner . Half-Cell Potential Resistivity . Carbonation |. Chloride Content Condition Endoscope/Borescope 1. Tapping 2. Pulse-Echo i e 3. Acoustic Emission Integrity & Performane nee 5. Load Test Parameter Concrete Quality and Homogeneity Damage — Fire; Blast Cracks - Water Tanks; Pavements Steel — Location; Cover; Size Corrosion SP PepolapeeNePHPPrePps ‘A. Schmidt's Rebound Hammer Test eae The rebound hammer method could be used for the following: ae 1. Assessing the compressive strength of conerete with the help of suitable eo-r index and eompresive strength 2. Assessing the uniformity of the conerete. 3. Assessing the quality of concrete in relation to the standard req 4. Assessing the quality of one element of concrete in relation to a Principle of RH Test The teats based on the prineple thatthe rebound ofan elastic mas which it impinges. When the plunger of the rebound hammer p controlled mass rebounds and the extent of such rebound depend uporDr. SM. Laskar, NSUT surface hardness and therefore the rebound is taken to be relation to the compressive strength of concrete. The rebound is read off along a graduated scale and is designated as the rebound number or rebound index. 2.1 Basic Features of Rebound Hammer Working of Rebound Hammer A schematic cut way view of Schmidt’s rebound hammer is shown in Fig. 2.1. The hammer weight about 1.8 kg and is suitable for use both in a laboratory and in the field, When the plunger of rebound hammer is pressed against the surface of concrete, a spring-controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The rebound distance is measured on a graduated scale and is designated as rebound number. Basically, the rebound distance depends on the value of kinetic energy in the hammer, prior to impact with the shoulder of the plunger and how much of that energy is absorbed during impact, The energy absorbed by the concrete depends on the stress-strain relationship of concrete. Thus, a low strength low stiffness concrete will absorb more energy than high strength concrete and will give a lower rebound number, Bn RGR Romiansed’ =) cant Fig. 2.2 Schematic Cross-section of Rebound Hammer and Principle of Operation Method of testing using Schmidt’s RH 7 To prepare the instrument for a test, release the plunger from its locked position by pushing the plunger against the concrete and slowly moving the body away from the concrete, This causes the plunger to extend from the body and the latch engages the hammer mass to the plunger rod, 1, Hold the plunger perpendicular to the concrete surface and slowly push the body towards the test object. (The surface must be smooth, clean and dry and should preferably be formed, but if trowelled surfaced are unavoidable, they should be rubbed smooth with the carborundum stone usually provided with the equipment. Loose material can be ground off, but areas which are rough from poor compaction, grout loss, spalling or tooling must be avoided, since the results will be unreliable). uSpee q Repair. Strengrhening and Retrofiting of Structures 2. As the body is pushed, the main spring connecting the hammer mass to the body is stretched. When the. body is pushed to the limit, the latch is automatically released and the energy stored in the spring propels the hammer mass towards the plunger tip. The mass impacts the shoulder of the plunger rod and rebounds. 3. During rebound, the le indicator travels with the hammer mass and records the rebound distance. A button on the side of the body is pushed to lock the plunger in the retracted position and the rebound number is read from the scale, The test can be conducted horizontally, vertically upward or downward or at any intermediate angle. Due to different effects of gravity on the rebound as the test angle is changed, the rebound number will be different for the same concrete, This will require separate calibration or correction charts, given by the ‘manufacturer of the hammer, Correlation Procedure Each hammer is provided with correlation curves developed by the manufacturer using standard cube specimens. However, the use of these curves is not recommended because material and testing conditions may not be similar to those in effect when the calibration of the instrument was performed. A typical correlation procedure is given as below: 1. Prepare a number of 150 mm cube specimens covering the strength range to be encountered on the job site. Use the same cement and aggregates as are to be used on the job. Cure the cubes under standard moist ‘curing room conditions. 2. After capping, place the cubes in a compression testing machine under an initial load of approximately 15% of the ultimate load to restrain the specimen, Ensure that cubes are in saturated surface dry conditions. 3. Make 5 hammer rebound readings on each of four moulded faces without testing the same spot twice and minimum 20 mm gap from edges. ‘Average the readings and call this the rebound number for the cube under test. Repeat this procedure for all the cubes. | Test the cubes to failure in compression and plot the rebound numbers against the compressive strength on | a yh. 7, Fita curve ora lin by the method of least squares, It is important to not that some of the curves deviate considerably from the curves supplied with the hammer. awe Limitations ‘Although the rebound hammer provides a quick inexpensive means of checking the uniformity of conerete, it has ‘serious limitations and these must be understood clearly for interpretation of test results. Factors affecting rebound number The results of Schmidt's rebound hammer are significantly influenced by the following factors: (a) Smoothness of Test Surface i (b) Size, Shape and Rigidity of the Specimen (c) Age of Test Specimen (4) Moisture Condition (e) Type of Coarse Aggregate (f) Type of Cement (g) Type of Mould (h) Surface Carbonation Influence of these factors has different magnitudes. Hammer orientation will also. although correction factors can be used to allow for this effect. Precautions to be taken while using rebound hammer ‘The following precautionary measures are taken while using the rebound errorThe test hammer should not be used within about 20 mm from the edge of the specimen, {Rebound hammer should not be used over the same Points more than once, Pee wbound test must be conducted closely placed te tes Points, on atleast 10 to 12 locations while taking ‘he average extremely high and low values ofthe indes number should be neglected. B. Ultrasonic Pulse Velo ity Test The ultrasonic pulse velocity test is used for hon-destructive testing of plain, reinforced and prestressed concrete whether it is Precast or cast in-situ Objectives of UPV Test The main objectives ofthe ultrasonic Pulse velocity method are to establish + The Homogeneity of the Concrete {The Presence of Cracks, Voids and othe * Changes in the Structure ofthe Conerete Caused by the Exposure Condition, Corrosion, Wear ete, which may occur with time, + The Quality of the Concrete in Relati and mechanical vibration into Pind soon curacy of + 0.1 microseconds Transducers with natural frequencies between are available, ts ina change in a pulse velocity, . Thus lowering of the density weer by increase in water-cement ratio decreases bork ‘he compressive strength of concrete a8 well as the velocity of a pulse transmitted through it Pulse Velocity method is a con investigating structural conerete, The underlying ve higher velocities are obtained when the quality eensity and uniformity is good, In case poorer quality of concrete, lower cl Side the concrete which comes in the way of transmission ated and it passes around the disconti uity, thereby making path length, velocities are obtained, The actual pulse velocity obtained depends primarily upon the material and the mix proportion of the concrete, Density and modulus of elasticity of aggregate also Significantly affect the pulse velocity Transducers: Piezoelectric KHz of natural frequency, {ful magneto stitive types of transducer are available in the range of 20 kHz to 150 Generally, high frequency transdueers aye preferable for short path length and low 13