Rehabilitation and Retrofitting of structures

Rehabili Rehabili tation and Retrofi Retrofi tting of Stru ctures – I ssue 3 

Shotcrete Shotcrete is defined as pneumatically applied concrete or mortar placed directly on to a surface. The shotcrete shall be placed by either the dry mix or wet mix process. process. The dry mix process mix process shall consist of 1. Thoroughly mixing the dry materials, 2. Feeding of these materials into mechanical feeder or gun, 3. Carrying the materials by compressed air through a hose to a special nozzle, 4. Introducing water at nozzle point and intimately mixing it with other ingredients at the nozzle; 5. Jetting the mixture from the nozzle at high velocity on to the surface to receive the shotcrete. The wet-mix process wet-mix process shall consist of 1. Thoroughly mixing all the ingredients with the exception of the accelerating admixture, if used; 2. Feeding the mixture into the delivery equipment; 3. Delivering the mixture by positive displacement or compressed air to the nozzle; 4. Jetting the mixture from the nozzle at high velocity on to the surface to receive the shotcrete. If specified, fibres of steel, poly propylene or other material, as may be specified, could also be used together with the admixtures to modify the structural properties of the concrete/mortar being placed in position.

….from Teacher’s Desk (V S Reddy)

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Rehabili tation and Retrofi tting of Stru ctures – I ssue 3 

Repair Methods

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Seismic Strengthening

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Causes of Distress in Concrete Structures High Water / cement ratio Inadequate curing Poorly graded aggregates Inadequate compaction Shuttering joints not slurry tight

a) b) c) d) e) 

a) b) c) d)

Less cover thickness g) Wrong placement of reinforcement attack by Chloride, h) Chemical Sulphate, etc. i) Wrong design and details f)

CAUSES OF DISTRESS AND DETERIORATION OF CONCRETE e) Alkali-carbonate rock reaction Accidental Loadings f) Alkali-silica reaction Chemical Reactions Acid attack g) Sulphate attack h) Construction Errors Aggressive-water attack ….from Teacher’s Desk (V S Reddy)

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Rehabili tation and Retrofi tting of Stru ctures – I ssue 3

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Corrosion of Embedded Metals Design Errors Inadequate structural design Poor design details Erosion Abrasion Cavitations

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Jacketing

i) j) k) l) m) n)

Freezing and Thawing q) Settlement and Movement r) Shrinkage - Plastic Drying s) Temperature Changes t) Fire u) Weathering.

p)

Jacketing consists of restoring or increasing the section of an existing member (principally a compression member) by encasing it in new concrete. The original member need not be concrete; steel and timber sections can be jacketed. The most frequent use of jacketing is in the repair of piling that has been damaged by impact or is disintegrating because of environmental conditions. It is especially useful where all or a portion of the section to be repaired is underwater. When properly applied, jacketing will strengthen the repaired member as well as provide some degree of protection against further deterioration. However, if a concrete pile is deteriorating because of exposure to acidic water, for example, jacketing with conventional portland-cement concrete will not ensure against future disintegration. The removal of existing damage is necessary to ensure that repair material bond well with the original material. If a significant amount of removal is necessary then temporary support is required. A steel reinforcement cage may be constructed around the damaged section. Once the form is in place, it may be filled with any suitable material. Choice of the filling material should be based upon the environment in which it will serve as well as a knowledge of what caused the original material to fail. Filling may be accomplished by pumping, by tremie placement, by preplaced aggregate techniques, or by conventional concrete placement if the site can be dewatered. 

Stitching

Stitching may be used when tensile strength must be reestablished across major cracks. Stitching a crack tends to stiffen the structure, and the stiffening may accentuate the overall structural restraint, causing the concrete to crack elsewhere. Therefore, it may be necessary to strengthen the adjacent section with external reinforcement embedded in a suitable overlay. Method involves drilling holes on both sides of the crack and grouting. The stitching procedure consists of drilling holes on both sides of the crack, cleaning the holes, and anchoring the legs of the dogs in the holes, with either a no shrink grout or an epoxy-resin based bonding system. Spacing of the stitching dogs should be reduced at the end of cracks and consideration should be given to drilling a hole at each end of the crack to blunt it and relieve the concentration of stress. Both sides of the concrete section shall be stitched so that further movement of the structure will not bend the dogs. Stitching shall be done on the tension face, where movement is occurring. The crack shall be made watertight as well as stitched to protect the dogs from corrosion. If there is a tendency for the crack to close as well as to open, the dogs must be stiffened and strengthened.


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 WHAT IS STRUCTURAL HEALTH MONITORING?

SHM refers to the broad concept of assessing the ongoing, in-service performance of structures using a variety of measurement techniques. Smart structures – those structures which incorporate numerous SHM sensors of various types – have emerged as a potential solution in diagnosing infrastructure deterioration before it becomes critical. Definition of SHM structural health monitoring can be defined as a nondestructive in-situ structural evaluation method that uses any of several types of sensors which are attached to, or embedded in, a structure. These sensors obtain various types of data (either continuously or periodically), which are then collected, analyzed and stored for future analysis and reference. The data can be used to assess the safety, integrity, strength, or performance of the structure, and to identify damage at its onset. NDT/NDE normally refers to a one-time assessment of the condition of materials in the structure using equipment external to the structure. SHM normally refers to activities focussed on assessing the condition of the structure or its key components based on response to various types of loads. It generally involves on-going or repeated assessment of this response. Some parts of the sensory system are usually embedded in or attached to the structure for the complete monitoring period. 

SHM system components As mentioned previously, structural health monitoring refers to the continuous or periodic monitoring of a structure using sensors that are either embedded in it or attached to its exterior. SHM systems are applicable to all types of civil engineering structures, including bridges, buildings, tunnels, pipes, highways and railways. While the specific details of SHM systems can vary substantially, a modern SHM system will typically consist of six common components, namely: 


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1. Acquisition of data (a sensory system); 2. Communication of information; 3. Intelligent processing and analyzing of data; 4. Storage of processed data; 5. Diagnostics (i.e. damage detection and modelling algorithms); and 6. Retrieval of information as required. Classification of SHM Systems SHM systems can be classified both in terms of their level of sophistication and by the types of information (and decision making algorithms) which they are capable of providing. The classifications of SHM systems can be summarized as follows: LEVEL I: This basic level SHM system is capable of detecting damage in a structure, but cannot provide any information on the nature, location, or severity of the damage. It cannot assess the safety of the structure. LEVEL II: Slightly more sophisticated than Level I SHM systems, Level II systems can detect the presence of damage and can also provide information on its location. • LEVEL III: A Level III SHM system can detect and pinpoint damage, and can provide some indication of its severity. • LEVEL IV: This most sophisticated level of SHM systems is capable of providing detailed information on the presence, location, and severity of damage, and it is able to use this information to evaluate the safety of the structural system. Obviously, Level IV SHM systems are the most complex and costly class. 

Advantages/Benefits of SHM Structural health monitoring presents a number of key benefits for civil engineering structures, including decreased ongoing inspection and maintenance costs, increased structural safety, and an improved understanding of the behavior and durability of the monitored structure. Some of the most commonly cited benefits of SHM include : 1. Improved understanding of in-situ structural behaviour Testing and analysis is more commonly performed on small-scale specimens, which represent only small portions of the actual structures, subjected to idealized loads. In-service monitoring of civil engineering structures provides a wealth of information on how real structures actually behave when subjected to actual structural and environmental loads. The information obtained through detailed SHM programs can thus be used to improve design equations and practices. 


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2. Early damage detection Early damage detection allows repairs to be made at the onset of damage, which can drastically decrease the resulting repair costs and prevent further deterioration. 3. Assurances of a structure’s strength and serviceability Visual inspections are, in many cases, impossible or inadequate for determining structure’s safety. In addition, SHM can be used where data is needed to provide confidence in a new building material or an innovative construction technique. 4. Reduction in down time Early damage detection and an improved understanding of structural behavior result in a reduction in down time for structures which ma y require repair or strengthening. 5. Improved maintenance and management strategies for better allocation of resources Decision makers can formulate better strategies to effectively deal with infrastructure deterioration and allocate shrinking budgets and sc arce resources more efficiently. 6. Enables and encourages use of innovative materials The use of new and innovative technology requires the implementation of monitoring and inspection to ensure that these materials are performing as planned. Thus, SHM and FRP technologies are rightfully evolving together. What is monitored, how, and why? The following is a list of some of the data types that are typically monitored by SHM systems • Load: SHM can also be used to learn how the various loads are distributed within and supported by the structure. Loads can be measured directly using load cells installed within a structure, or it can be inferred through strains or other parameters measured on selected structural components. SHM can monitor actual deformations caused by all physical and • Deformation: environmental loading. Deformations and deflections can be measured with a variety of types of displacement transducers and tiltmeters. • Strain: Strains can be used to gain a wealth of information about the behaviour and ongoing performance of a structure; these are probably the most commonly used measurements in SHM systems. Strains in structural components can be directly measured at the desired locations using standard electrical resistance strain gauges, vibrating wire strain gauges, or more recently developed fibre optic sensors. • Temperature: By incorporating temperature measurements, an SHM system can provide information on how temperature changes affect a structure, and whether the temperature-induced loads and strains are as expected. Temperatures can be measured using thermocouples, integrated temperature circuits, thermistors, or certain types of FOSs. • Acceleration: Accelerations are typically measured using a class of sensors called accelerometers. • Wind Speeds and Pressures: Wind speed can be measured using anemometers. • Acoustic Emission: An emerging suite of SHM technologies by sound waves, or acoustic emission (AE) waves, can be used to determine the location and characteristics of damage in a structure. The most common example of AE SHM is in its use for monitoring unbonded posttensioned concrete structures or cable-stayed bridge ducts. • Video Monitoring: The relatively recent introduction of low cost video surveillance and webcam systems has enabled the use of video monitoring in SHM systems. 


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Techniques Of Structural Health Monitoring

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What is Underpinning and various Underpinning Methods

Underpinning is a method used to increase foundation depth or repairing faulty foundations. This might be the case if you plan to add stories to an existing structure or when the foundation has been damaged. One visible sign that your building needs underpinning are cracks appearance. When a building needs a foundation repair, some cracks especially wider than ¼ inch appear visible, meaning than underpinning needs to be done. Foundation failures could also be considered as heaved foundations, cracked or buckled walls and cracked concrete floors. 1. Underpinning: Mass Pour The most used method of underpinning is mass pour method. Excavate sections in sequence to a pre-established depth below the footing and place concrete on each pit. Repeat the method until the entire affected area has been underpinned. 2. Underpinning: Screw Piles and Brackets Underpinning with screw piles and brackets is normally used in certain instances where traditional underpinning process is not possible. Some buildings might require excavating to great depths or maybe is unfeasible to use a piling rig and the screw piles and brackets method is then selected. The screw piles and brackets can be installed by only a two man crew by hand or using small equipment such as a mini excavator. Screw piles can be installed in foundations having the capacity to work in tension and compression, withstand vertical and lateral wind forces, and vibration and shear forces. They are ideal when used with underpinning support brackets. The structure can then be lifted back to a level position and the weight of the foundation transferred to the pier and bracket system. Screw piles have many advantages over traditional pilings, such as the speed of installation, little noise and minimal vibration that may cause damage to the surrounding area. 3. Underpinning: Pile and Beam Underpinning with pile and beams is another great and preferred method to alleviate footing. Using this system requires that a min-pile must be installed on either side of the affected wall. After the piles have been installed, then brickwork is removed below the wall and reinforced concrete needle beam is used to connect the piles and support the wall. Reducing the distance between needle beams can accommodate very high loads. The bearing capacity of the underlying ….from Teacher’s Desk (V S Reddy)

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strata will determine the number, diameter, depth and spacing of piles used. Augered piles or case driven piles can be used with this method of underpinning. The advantages of underpinning with pile and beams are: a) Suitable for restricted access b) Faster than traditional underpinning c) High load capability d) Less disruption, less spoil generated and completed quickly 4. Underpinning: Piled Raft Underpinning with piled raft, must be used when the whole structure need to be underpinned. It is recommended when foundations are too deep for other underpinning methods or in areas where the soil is so hard that small equipment could not excavated up to require depth. Piles are placed at determined locations by loading conditions; then pockets below footings are broken, and reinforced needle beams are placed to bear the wall’s load. A ring beam is then built to link all needles and the structure is poured with concrete.Advantages of this system are: a) Provides lateral and traverse ties throughout the structure. b) Economical at depths greater than 1.5m. c) No need for external access. d) Reduces disruption to drainage systems. Underpinning in foundation should be addressed and supervised by an engineer.The underpinning process must be started from the corners and the working inwards. Underpinning must be made only on load bearing walls. Do not underpin below non-load bearing walls.


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Non-Destructive Evaluation (NDE) techniques

The various NDT methods for testing concrete are listed below – A. For strength estimation of concrete (i) Rebound hammer test (ii) Ultrasonic Pulse Velocity Tester (iii) Combined use of Ultrasonic Pulse Velocity tester and rebound hammer test (iv) Pull off test (v) Pull out test (vi) Break off test B. For assessment of corrosion condition of reinforcement and to determine reinforcement diameter and cover (i) Half cell potentiometer (ii) Resistively meter test (iii) Test for carbonation of concrete (iv) Test for chloride content of concrete (v) Profometer (vi) Micro covermeter C. For detection of cracks/voids/ delamination etc. (i) Infrared thermographic technique (ii) Acoustic Emission techniques (iii) Short Pulse Radar methods ….from Teacher’s Desk (V S Reddy)

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(iv) Stress wave propagation methods – pulse echo method impact echo method response method   

Concrete technologists practice NDE methods for (a) Concrete strength determination (b) Concrete damage detection 

ultrasonic pulse velocity method

The ultrasonic pulse velocity method could be used to establish: (a) the homogeneity of the concrete (b) the presence of cracks, voids and other imperfections (c) change in the structure of the concrete which may occur with time (d) the quality of concrete in relation to standard requirement (e) the values of dynamic elastic modulus of the concrete The method is based on the principle that the velocity of an ultrasonic pulse through any material depends upon the density, modulus of elasticity and Poisson’s ratio of the material. Comparatively higher velocity is obtained when concrete quality is good in terms of density, uniformity, homogeneity etc.


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Rehabilitation and Retrofitting of structures

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