Repair, Strengthening & Maintenance of structures – A brief review

Authors
Dr. K K Ganguly
Director, Devlopment Architect Pvt Ltd

Body

This article is in continuation to the article appeared in Oct-Dec 2011 Issue
CONCRETE REPAIR MANAGEMENT
It will be important to carry out regular inspection to maintain good condition of the structure. Some simple items such as clearing drainage points, mending leaks, removal of plants/detritus and cleaning of concrete surfaces etc. are to be carried out regularly. Such activities will avoid major remedial works in the future and provide longer service life of the structure.
A) It will be essential to maintain minimum records (13) of the work carried out. This will include–
a) As built records of the concrete repair work;
b) Details of the materials used;
c) Construction method details;
d) Test Certificates of the materials used;
e) Some photographs of repair work (i.e. at different stages)
f) Details of construction operations, environmental data and dates undertaken;
g) Problems faced, if any, during construction;
h) Approval of method/procedure adopted for the work;
i) Contract documents.
B) Inspection of the structure
All structures must be inspected regularly although frequency of inspection may vary depending on the type, location and usage of the structure. For some important construction including highway structures, the following approach may be adopted.
a) Superficial inspection (frequently);
b) Visual inspection (typically may be once in two years);
c) Detailed / principal inspection (typically every six to ten years);
d) Special inspection (to investigate particular defects).
The above approach should be developed and may be considered for all exposed concrete structures including multi-storied buildings, marine structures, etc.
First inspection will provide valuable inspection details base to be followed later. Detailed inspection may be done with non-destructive tests to assess the health of the structures. Routine tests often include hammer tap tests, cores samples, tests for chloride and carbonation and tests for detecting onset of corrosion.
The inspection must be carried out by experienced engineers or engineering technicians with the knowledge and training on the performance of materials used.
The inspection shall cover both the original and the repaired areas and attempts should be made to identify the extent and severity of any defects.

C) Assessment of the following tests
The assessment related to the condition of the structure in distress may be carried out in stages.
a) First Level – Preliminary assessment
Review of existing construction documents, site inspection, visual examination and to draw preliminary conclusions.
b) Second Level – Detailed examination
The items mentioned in First Level are to be examined much more in detail. Without making proper assessment of the structural condition including the possible reasons for damage, repairing method, materials etc. cannot be finalized. The tests may including cover surveys, sample coring, crack measurement, half cell and resistively testing and carbonation testing.
When the test results are available it will be possible to assess the implication of the defects. Depending on the severity of the defects, its rate of continuation, safety of the structure and its users, the action plan has to be developed accordingly. When the repair work has been completed recently, the contractor / applicator should be immediately contacted for appropriate investigation and to determine cause and rectification in the event of some problems arising. The following will be important in this connection.
i) Undertake a structural assessment – When the damage is severe and extensive and the structural safety appears to have been compromised.
ii) Instigate periodic monitoring of the defects by visual inspections and or technical monitoring i.e. by crack measurements, use of strain gauging or movement sensors.
iii) Instigate repair programme.
iv) Install safety measures and temporary works that may be required to make the structure safe.
OBJECTIVES OF STRUCTURAL INSPECTION
a) What is the present state of deterioration and structural distress?
b) Is the structure adequate for its intended purpose in respect of strength, serviceability and durability?
c) What will be the future state of deterioration and structural distress?
d) What, if any, remedial works are required now or in the future?

 

SIGNS OF DISTRESS
The following may be observed on concrete elements :
a) Cracking
b) Surface deterioration
c) Surface deposits
d) Deformation
e) Construction defects
f) Construction features
g) Signs of water leakage
& on Non-concrete components
Cracking &
Distortion
RECOMMENDATION AFTER INITIAL APPRAISAL
a) Recommend immediate actions (e.g. Propping, shoring, evacuation) if the structure is found to be in imminent danger of collapse
b) Define areas of the structure for further investigation
c) Specify the most appropriate testing techniques to be used in the principal investigation.
d) Postulate the causes of defects, whilst keeping an open mind until completion of further investigations.
e) Estimate the cost of the principal investigation.
f) Recommendation for any minor repairs or monitoring that may be considered necessary.
g) The importance of periodic inspection and maintenance should be emphasized.
REPAIRING METHODS
Depending on the problem, the following methods are often adopted for repair / restoration of concrete structures.

 

a) Injection method of repair –
This method is related to sealing of cracks in concrete elements. Injection method of repair of concrete elements may be done in stretches. The cracks can develop in the structural elements and the width of cracks may develop beyond the serviceability limit. This will call for sealing of the crack lines. Detailed study related to the reasons for the development of cracks must be done before starting the repair work. Different materials from cementitious to epoxy may be used to seal the cracks. However, cementitious materials can be used for cracks say more than 0.6 mm wide and low viscosity epoxy will be required for fine cracks up to say 0.2mm. The injection materials are pushed through the nozzles under pressure located along with the crack lines and at the interval of one to half a metre depending on the nature of problem. Injection of cementitious materials will definitely be cheaper than that of epoxy. After placing the nozzles the crack lines are to be sealed. The injection work has to be carried out very carefully from bottom to top as applicable.
The injection of selected materials into cracks is primarily to –
i) re-establish structural strength/integrity (i.e. glue it together);
ii) to fill the cracks to stop water entering or leaving the structural elements.
The cracks that are formed due to thermal movement may not provide satisfactory results if the same is filled up with low viscosity epoxy resin to re-establish structural integrity. The structure may develop the joints again from expansion, in some other places, which may occur in a more problematic location than before. The cracks developing from the corrosion of reinforcing bars inside concrete need not be treated with injection material as the repairing solution may be a short term one.
While developing the specification for repair of cracks it will be important to know –
i) the crack width;
ii) whether the crack is live one or not;
iii) whether there is any water present or likely to be present during injection work.
When the crack width is about 0.2 mm., the injection material shall be of low viscosity and the same has to be pushed under appropriate pressure to fill the crack line fully. The execution has to be done very carefully as the injection under high pressure is risky and may cause damage to concrete especially when the crack is located close to unconfined edge.
According to the standard BS EN-1504 (8) the work related to performance characteristics is covered with three generic material types –

 

i)
materials capable of transmitting forces (F) i.e. force filling of cracks
generally cement based materials, epoxies and polyesters;
ii)
materials capable of remaining ductile (D) in cracks i.e. flexible to accommodate future movement
generally polyurethanes;
iii)
materials capable of swelling to fill the cracks (S)
generally polyurethanes and acrylics.
More information on the materials will be required for each type. These are –
i)
Basic characteristics
adhesion, shrinkage, glass transition temperatures, etc.
ii)
Workability characteristics
injectability, viscosity, expansion ratio etc.
iii)
Reactivity characteristics
workability time, tensile strength, setting time etc.
iv)
Durability
Compatibility, adhesion after cyclic loading, sensitivity to wetting and drying.
For each item characteristic tests will need to be done. The tests are to be conducted by the material manufacturers and the results should match with that of the standard as applicable.
b) Patch repair
Patch repair may be required to restore durability, structural strength or for the improvement of appearance. Normally defective or unsound concrete is removed as a first step. Saw cut at the edges will be preferable for a neat finish. Corroded bars if present shall be cleaned and treated to control further corrosion. If required, additional reinforcement may be considered. Corrosion-inhibiting chemicals may be used. Bonding treatment will be required to the cut concrete areas for developing proper bond with the repairing materials.
c) Jacketing Concrete
Jacketing technique of repair work is generally considered to repair the load carrying elements like column/pier etc. The repairing items will exhibit cracking and spalling of concrete with signs of the requirement of additional strengthening.

 

This method will consist of providing additional layer of special concrete around the affected structural elements often with admixtures. The execution may include additional reinforcement and the same is connected/welded to existing bars, use of proper bonding coat on the original surface and water tight shuttering. Specially prepared concrete (i.e. often micro concrete) is poured around the periphery of the affected elements as prepared and treated like normal concrete elements for curing etc. With this treatment it will be possible to strengthen the load bearing elements for carrying additional loads.
d) Sprayed concrete repairs
Sprayed concrete repairs will provide a thin layer of high quality fine concrete that can be sprayed on to the surface of a structure to which it will bond strongly restoring the protective cover to steel reinforcement, making good concrete that has spalled or become abraded and producing an unacceptable appearance. Shot-creting or guniting are the terms for repair work used in this connection depending on the size of aggregates used. Admixtures are generally required for such applications.
Both dry and wet processes of spraying concrete can be used. In all cases in order to develop a good bond the prepared interface must be sound, rough and homogeneous.
Welded steel fabric or steel or other fibres are often used. The minimum thickness for sprayed concrete that will fully cover the fabric at over laps and allows for tolerance in fixing will be about 30 to 35 mm.
e) Large volume repairs
The defective concrete must be removed and the concrete surface with reinforcement may be exposed and cleaned properly. Anti corrosive treatment to reinforcement may be done in the usual manner. In large volume repair work, application of repairing materials by hand or spraying may not be required to fill the gap with concrete or grout. Application of bonding aid will be preferable. Form work is often required for such applications. The form work may have to be built in stages as the work proceeds or to provide temporary openings through which access can be obtained. The repair material may be concrete that may be placed by the conventional manner, it may be also formed by injecting grout into mass of aggregates or it may be entirely grout.
f) Industrial floor repair
The floors may be damaged with development of cracks and for local settlement in the ground. Cracks may have been caused due to heavy loadings especially the

 

application of heavy point loads for which the floor slab was not designed originally. Loss of support to ground floor slab may take place due to settlement of sub-grade. A proper survey after thorough cleaning will be required other wise the defects may be hidden. Existing services in the floor must be known and plotted to avoid damage during repair work. In many cases by providing a topping on the whole surface after applying bonding coating may be adequate. Local patch repair may be required with strengthening of sub-grade. For providing proper wearing surface an adequate topping layer must be selected. Specially prepared floor hardener may be selected for the top layer for better results.
g) Repair/restoration of waterproofing treatment
Different types of materials are available. Existing treatment may have to be removed during the application of the selected treatment in most cases. Careful selection of the material will be important.
h) Repair of leakage from joints of service pipes
Joints are to be plugged/sealed carefully. Considering the material, the procedure of plugging and use of proper sealant will need to be decided. In this connection checks are also required for cracks in the pipe lines and point leakage through pipe walls.
i) Leak sealing
In many cases leakage of water may take place from concrete structures. There may be different reasons for such leakage. The important issues will be availability of water, water pressure and the sectional area of leak path. In actual practice concrete construction will be done by adopting separate pours i.e. introducing construction joints. In many cases such construction joints especially in water retaining structure may allow passage of water. Often pressure grouting to construction joints will solve many problems. Movement joints, in a structure, are often subjected to leakage problems. Careful detailing and use of sealant will be important to seal leakage areas.

To be continued in the next issue

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