DISTRESS OF CONCRETE STRUCTURES & THEIR REPAIR

TECHNIQUESCONSTRUCTION & MATERIALS –III

PRESENTED BY: MOHAMMAD ZAID & MOHAMMAD FARZAN

PRESENTED TO : AR. KHALID HASAN

INTRODUCTIONIf a building has given about 25v to 30 years of service without much maintenance or repair then it is reasonable to expect that it would need some repair sooner or later.

CATEGORIES OF REASONS DISTRESS OF CONCRETE STRUCTURES

1. WEATHERING

2. AGEING

3. ENVIRONMENTAL EFFECTS

4. INADEQUATE MAINTENANCE

5. POOR DESIGNING AND CONSTRUCTION QUALITY

6. CHANGE OF LOADING PATTERN OR NON CONVENTIONAL LOADING ON STRUCTURE

7. WATER LEAKAGE LEADING TO CORROSION OF CONCRETE STRUCTURE

PERFORMANCE OF CONCRETE STRUCTURE DEPENDS UPON:

1. QUALITY OF CONCRETE

2. AMBIENT CONDITIONS

EXPECTED SERVICE LIFE OF STRUCTURESLife span of RCC is generally taken as 100 years

EXPECTED CONVENTION ABOUT DESIGN LIFE SPAN Monumental structure like temple ,mosque or church is 500 to 1000 yrs. Steel bridges ,steel buildings or similar structures is 100 to 150 yrs. Concrete bridges or hign rise buildings or stone bridges is 100 yrs. A house or general building is 60 to 80 yrs. Highways :concrete pavement for about 30 to 35 yrs and bituminous

pavements for about 8 to 10 yrs.

COMMON FAILURE MODES OF R.C.C.Cracks, potholes and erosion of the top layer of concrete, rusting of reinforcement, splitting etc.

Cracking is the most common phenomenon in concrete structures.Cracking of concrete section cannot be prevented but size and location of the cracks can be limited & controlled by reinforcement, placement of control joint, and better concreting and curing

Poor workmanship : untrained man power and bad quality of constructionInadequate cover to reinforcement.

Loosely fitted joints.

Excessive water cement ratioThe IS: 456:2000 has laid down upper limits for water cement ratio, normally it need about 15 to 18 lts. Of water per 50kg bag of cement.

With this quantity of water concrete will be stiff and uncompact able so we mix additional water for workability. This extra water after evaporation leaves out minute capillary pores which lead to corrosion 0f reinforcement.

CAUSES OF EARLY DETERIORATION OF CONCRETE STRUCTURES

Inadequate concrete cover Bars are not bent accurately or may not be effectively fixed to ensure that specified gap is left between bars and form work while often not only bars touch formwork but also the binding wires loose ends and the steel bars are seen at the surface of concrete.

In that case, they are prone to moisture.

Honey comb or Un vibrated concretePorous concrete is a major source of weakness in concrete.

Un-compacted concrete results in honey combing which gives low compressive strength and permits moisture to enter, resulting in corrosion of steel bars.

Cold joints or bad construction joints A cold joint is a joint where fresh concrete is placed against a previous un-compacted concrete which has already hardened due to lapses of time that is more than concrete setting time.

Inadequate reinforcementMany a contractor does not put enough reinforcement in concrete structural member s and hence it fails under tension.

Wrong placing of reinforcementMany times contractors do not know the correct location of reinforcement in structural member so it is not placed at proper position.

Inadequate cement quantityIS: 456:2000 has laid down the minimum quantity in concrete.

Minimum quantity of cement is needed not only to coat the fine and coarse aggregate particles but also to fill the voids between the aggregate particles and to provide a thicker fill of cement grout for easy workability.

Congested reinforcement bars Too many steel bars in narrow and slender RCC columns, walls or beams results in NO room for having cover in concrete nor for inserting a needle vibrator to ensure full compaction leading to honey combed concrete.

Initially rust steel bars Alkali – aggregate reactivityUnder most conditions the reaction causes excessive expansion and cracking of concrete after few months or year.

Effect of weather and water leakageWeather causes structure to delay.

Prolong exposure to polluted environ,ent and acid rain can give weathered concrete anfd corrode metal ties and fastenings.

Porous cover blocks Having incorrect mix proportions and water cement ratio.

These are usually fixed to the steel bars at about one meter centers and if they are porous they become the starting source of decay as they permit the ingress of moisture.

EFFECTS OF CRACKING ON LIFE OR DURABILIY OF STRUCTURE

Understanding of cracks in concrete will help us to avpoid failures of concrete on one hand, avoidable worries and expenditure on other hand . Reduce loading capacity of structure Progressive failure Loss of appearance Leakages • Apprehension of failure in mind

Controlling cracks Better concrete mix design Friendly environment condition at the time of concreting at

site. Have dense concrete • Use low heat of pozzolanic cement in mass concreting.

IDENTIFICATION OF DISTRESSED LOCATIONS ON STRUCTURESPRELIMINARY INSPECTIONCarry out a preliminary inspection possibly with some testing in order to get characterize the nature of problem and to plan the detailed investigations.

Detailed inspection Checking the strength of concrete Chemical analysis on concrete Corrosion test

Conclusion from investigation In majority of cases ,the cause of damage can be tracked to the following singly or in combination: Low cover to steel Permeable concrete

• High chloride levels

REPAIR Repair refers to modification of structure, damaged in its

appearance or serviceability, to restore, partly or wholly, the pre-existing characteristics of serviceability, load-bearing capacity and if necessary, to improve its durability.

Repair techniques are used to restore the structural integrity and shape of a concrete element and generally include removal of damaged concrete and replacement of new concrete.

GENERAL REPAIR PROCEDURESurface preparation and protection of reinforcement Chipping of defective or deteriorated concrete should be done till all

defective concrete is removed. During repairs structural load carrying members must be supported. Remove all rust from steel bars if it is reinforced member. In concrete

rust is like cancer. The old concrete of masonry surface should be made rough to give

proper mechanical bond. Wash and clean the entire surface before taking up repairs Bonding material / epoxy should be applied between old and new

concrete. Spraying of water at regular intervals on inclined or vertical members

or we may also apply curing compound.

MATERIALS AND METHODS FOR CRACK REPAIR

Repair materials can be of following types: Cement bound Polymer cement bound Resin hardener bound(epoxy)

Following materials are generally used in crack repair:1. Low viscosity epoxy materials2. Epoxy modified mortar3. Polyurethane based coating over epoxy mortar4. Elastomeric concrete5. Fiber reinforced concrete6. Ferro cement

Low viscosity epoxy materialDesigned for grouting gaps of low width of about 0.5 to 10mm. Epoxy compound like RESICRETE of SWC or CONVEXTRA EP10 of FORSROC which is normally supplied as two component product consisting of base and hardener that is filler and polymer can be used.

Epoxy modified mortarEpoxy mortar is a common material used for repair of concrete surfaces in general. It cannot be used in open surfaces as it is non UV resistant

Polyurethane based coating over epoxy mortarAs epoxy mortars are non UV resistors ,but still have good strength so they can be used with a coating of this material.Elastomeric concreteAn epoxy material used in repairs not only in slabs and roads but also in structures and concrete surface.

Fiber reinforced concreteIt is a concrete with small sized aggregates, cement sand mortar and some type of hair .Various types of hair can be: Steel fibers Carbon fibers Glass fibers’ Polypropylene fibers

Generally polypropylene fiber is used.

Ferro cementTerm Ferro cement is applied to a mixture of Portland cement and sand reinforced with layers of woven or expanded steel mesh and closely spaced small diameter steel rods. Construction made with this is more resistant to earthquakes.

SOME SPECIFIC REPAIR TECHNIQUE FOR CONCRETE SURFACE1. Cracking of concrete surface up to certain depth:a) Cracks up to depth of 0 to 20 mm –category Ab) Cracks between depth of 20 to 50-category Bc) Cracks to depth of more than 50mm-category C1. Weather effects on concrete or rusting of reinforcement –category D2. Blowholes and streak marks on concrete-category E3. Honeycombing in concrete –category F4. Holes in concrete or potholes or holes in road –category G

Repair methods-anyone of the above materials can be used with following described methods Application by hand Casting into formwork fixed into existing concrete surface Spraying through purposely designed equipment.

ASSESMENT OF QUALITY OF STRUCTURE SOON AFTER ITS CONSTRUCTIONAs the repair of structural element of building is over, it is desirable hat soundness of structure is assed with the help of non-destructive testing technique. Some NDTs which are available are: Pulse velocity measurement method (PVM) Cover meter Rebound hammer or similar methods Core cutting and testing of critical spots

Through these methods the strength of concrete, depth of cover, cracks, voids, if any can be found.

REQUIREMENT FOR TRAINING FOR CONCRETE REPAIR AND CONCRETE WORKERS

To achieve good quality of work it is necessary to have skilled workers at site.

Like masons know the importance of water, right placement of reinforcement, mixing ingredients etc. Training should look that construction workers are trained for correct process and quality procedure with practical sessions and hands on work.

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