Assessment Of Defects In Concrete Structures And Evaluation Of Safety Of Concrete

Infrastructure Training4th Day

Rehabilitation methods and Strengthening Techniques

By:MAHMOUD ABUFOUDA

MSc Civil Engineer

Content

Rehabilitation Methods

•Repair using mortars.•Dry pack•Pre-placed aggregate concrete (PAC)•Self Compacted Concrete (SCC)•Epoxy bonded concrete•Polymer concrete system•Protective seal coats •Plate bonding•Section enlargement•Propping and supporting •Fiber Reinforced Polymers (FRPs)

Strengthening

Techniques

• The need for strengthening• Flexural Strengthen of Beams• Shear Strengthening of Beams• Strengthening of Columns

Rehabilitation/Repair Methods Various methods are available Single or combination of methods can be used according to the

assessment of the distress. The basic methods are;

1. Repair using mortars.2. Dry pack3. Pre-placed aggregate concrete (PAC)4. Concrete replacement5. Shotcrete6. Self Compacted Concrete (SCC)7. Epoxy bonded concrete8. Silica fume concrete 9. Polymer concrete system

Rehabilitation/Repair Methods 10. Thin polymer overlays11. Thin epoxy overlay12. Protective seal coats 13. Ferro-cement14. Plate bonding15. Section enlargement16. Propping and supporting 17. Fiber Reinforced Polymers (FRPs)18. Chemical and electro-chemical methods

Repair Methods- Repair with Mortars It is the most common form of repair. Types of mortar repairs;

1. Portland cement mortars2. Polymer modified cement mortars3. Epoxy mortars

Repair Methods-Portland Cement Mortars

Applied for cosmetic and very small superficial repair areas not associated with critical performance of the structure.

Shall not be used for repairs to old or existing concrete Shall not be used for repairs that extend to or below the first layer of

reinforcing concrete. It is advisable to use the same proportion of cement: sand: water as

used in the preparation of parent concrete. Portland cement mortars need to be cured

Repair Methods-Modified Cement Mortar

Used only for repairing defects on exposed concrete surface. Has higher performance properties than normal cement (Fast setting,

strength, etc.). For larger repair areas with thickness in excess of 50 mm. concrete is

for thicker thickness. For larger areas, it is advisable to use steel reinforcing mesh fixed

with nails.

Repair Methods-Epoxy Mortars Depending on the type, epoxy mortars consist of resin, hardener and

silica sand. Applied over the epoxy bonding coat over the hardened concrete

surface. It attain strength within few hours. High strength, Water resistant and Abrasion resistant. Can be used in few millimeter thickness.

Repair Methods- Dry Pack

https://www.youtube.com/watch?v=XsKWCFH-FJc

Repair Methods- (PAC) Preplaced aggregate concrete is made by forcing the grout into the

voids of mass of clean graded coarse aggregate pre-packed in the form work.

The grout consists of sand, cement, pozzolana, plasticiser/superplasticiser and air entraining agents.

https://www.youtube.com/watch?v=T9r6Cwj5Ehshttps://www.youtube.com/watch?v=BmvmIUK1wAY

Repair Methods- Concrete replacement Concrete replacement shall be used. If the area of damaged or

unacceptable concrete is greater than 0.1 sqm and having a depth of more than 150 mm or extending 25 mm from the back side of the reinforcement.

Epoxy agents, latex bonding agents, dry neat cement, cement paste or cement and sand mortars shall not be used to bond.

Normally the concrete used should have a modified properties agents such as superplasticier to enhance workability.

Not efficient (has segregations, honeycomb problems, etc.).

Repair Methods-Self-Compacted Conc.

The use of SCC in renovation works is a natural result of its fabulous characteristics.

SCC can solve many constructability and placement challenges that most concrete mix designs cannot like Consolidates under its own weight. Pumping and placement into very congested formwork and reinforcing

bars. Doesn’t need mechanical compaction.

The main feature of this unique type of concrete relates to having very high slump and flowability properties without segregation.

https://www.youtube.com/watch?v=YH42V9yyGZg https://www.youtube.com/watch?v=jHUQCbakzXE

Repair Methods- Shotcrete Shotcrete is defined as pneumatically applied concrete or mortar

placed directly onto a surface. It is one of the oldest materials and the most common techniques of

repairing and strengthening of reinforced concrete structures. Sprayed concrete has been used in that field for almost 90 years. Shotcrete shall be placed by either the dry mix or wet mix process.

Dry mix sprayed concrete in which most of the mixing water is added at the nozzle

Wet mix sprayed concrete in which the ingredients , including water, are mixed before introduction into the delivery hose.

Both are used in concrete repair/strengthening work, but the use of dry mix sprayed concrete is more common.

Repair Methods- Shotcrete (Dry Mix)

https://www.youtube.com/watch?v=kMecv_n84H0

Repair Methods- Shotcrete (Wet Mix)

https://www.youtube.com/watch?v=GxN2X_PQoSo

Repair Methods- Polymer Concrete Like Methyl Methacrylate (MMA) Polymer Concrete. Used for patches, overlays, grout pads and embedment of slits, gates

and similar structures in concrete members. The physical properties (e.g. compressive, tensile, flexural strength and

modulus of elasticity) of cured repaired materials are very high.

https://www.youtube.com/watch?v=lDvKpzP2Rug&feature=youtu.be

Repair Methods- Silica Fume Concrete Silica fume concrete is a mix of Portland cement and silica fume

which is used as effective Pozzolana material in quantities that is not exceeding 10% by the weight of cement in the concrete mix.

This mix shall be used with superplasticizer. It may be used where a high strength repair concrete of low

permeability is required

Repair Methods- Epoxy bonded concrete It is defined as freshly mixed Portland cement concrete that is placed

over epoxy resin bond coat on existing hardened concrete. Mostly, it is concrete-concrete epoxy types. Usually, it is mixed with silica sands materials. Used to bond new layer of concrete with the parent concrete. In case of a deep concrete casting, anchors and fasteners should be

used with this type of repair to ensure the bonding between new concrete and parent one.

Repair Methods- Ferrocement Ferrocement can be described as a type of thin

composite material made of cement mortar reinforced with wire meshes.

The steps; removing the concrete from the cracked affected zone. layer of galvanized welded wire mesh and a layer of

skeletal steel are fixed with the original reinforcement of the slab.

concrete surface must roughened before the additional reinforcement is placed

additional reinforcement result from the design and technological restrictions are added.

Polymer modified Concrete is applied.

Repair Methods- Protective Sealcoats Seal coats are defined as liquid epoxy that are applied to the surface

of hardened concrete to prevent or decrease the penetration of liquid or gaseous media.

https://www.youtube.com/watch?v=wBeUWXvK0nU

Repair Method-Cracks Epoxy Injections

This technique used to treat the cracks in concrete by injecting epoxy into the cracks.

The process is illustrated in this video https://www.youtube.com/watch?v=7_diXmKZp_8

Repair Methods- Plate Bonding Inexpensive, multi-functional and advanced technique for

rehabilitation. It consists in bonding steel plates or steel flat bars to the structural

elements by steel bolts and epoxy. Substantially increase the stiffness, ductility and stability.

Repair Methods- Plate Bonding

Repair Methods- Section Enlargement On of the traditional techniques in the rehabilitation of reinforced

concrete structures. This technique aims to increase the structural cross section area of

the structure elements. Can be apply to all structural elements ( slabs, beams, columns,

foundation, etc. The type of bonding is a critical factor in this type of technique.

Repairing Methods- Fiber Reinforced Polymers (FRPs)

Fiber Reinforced Polymer system is a patented cost effective family of well proven & engineered, strengthening products designed to meet a wide variety of strengthening applications on concrete.

FRP materials have superior properties with respect to strength, weight, durability, creep, and fatigue.

The current commercially available FRP reinforcements are1. Continuous fibers of aramid (AFRP).2. Carbon (CFRP).3. Glass (GFRP) impregnated in a resin matrix.

Repairing Methods- Fiber Reinforced Polymers (FRPs)

Applications

Seismic Retrofitting

Structural Upgrades

Structural Repairs

Design or Construction

Errors

Blast Mitigation

Modifications to

Structural Elements

Corrosion Repairs

Load Increases

Underwater Repairs

Repairing Methods- Fiber Reinforced Polymers (FRPs) FRP composites can be produced by different manufacturing methods

in many shapes and forms. Rods (bars): used for internal concrete reinforcement. Pre-cured laminates Plates : used to replace bonded steel plates. Pre-cured laminate shells: used as jackets for columns. Fiber sheets: commonly used for external concrete reinforcement

Strengthening of RC Elements The aim of strengthening is to increase the capacity of an existing

structural element. Strengthening can be achieved by:

Replacing poor quality or defective material by better quality material Attaching additional load-bearing material Redistribution of the loading actions through imposed deformation of the

structural system  

Strengthening of RC Elements The new load-bearing material:

Load-bearing materi

al

High quality concrete

• SCC• UHPC (Ultra High Performance Concrete)

• Polymer Modified Concrete

Reinforcing steel bars

Thin steel plates and straps

Post-tensioning tendons

Using Composite

elements such as Steel W sections

FRP composite materials

• CFRP• AFRP• GFRP

Strengthening of RC Elements The need of structural strengthening

Load increases due to:

• Higher live loads,

• Increased wheel loads,

• Installations of heavy machinery

• Vibrations

Damage to structural parts

due to:• Aging of

construction materials

• Fire damage,• Corrosion of

steel reinforcement

• Impact of vehicles

Improvements in suitability for use due to:

• Limitation of deflections,

• Reduction of stress in reinforcement

• Reduction of crack widths

Modification of structural

system due to:• Elimination of

walls/columns• Openings cut

through slabs

Errors in planning or

construction due to:

• Insufficient design dimensions

• Insufficient reinforcing steel

Strengthening of RC Elements

The following slides will discuss1. Flexural Strengthening of RC Beams.2. Shear Strengthening of RC Beams.3. Strengthening of RC Columns.

Flexural Strengthening of RC Beams In most cases, the compression zone of RC beam is safer from failure

as the tension zone if it is under pure bending. The critical area for beam under bending stresses is a tension zone of

the RC beam. Mostly, the failure initiated by the development of crack from tension

zone, and extended up to compression zone before reaching to failure.

These cracks usually start from the bottom of applied load, which indicates flexural failure.

Flexural Strengthening of RC Beams There are several Flexural strengthening techniques for RC beams.

Section enlargement

Ferrocement cover

External plate bonding

External post-tensioning

Fiber Reinforced Polymer (FRPs)•Prefabricated laminates •Near Surface Mounted bars NSM

Section Enlargement Section enlargement is one of the famous methods used in

retrofitting concrete members. Section enlargement maybe easier and cheaper compared to other

approaches. Enlargement consists of the placement of reinforced concrete jacket

around the existing structural member to achieve the desired section properties and performance.

Section Enlargement

• Increasing of the load-carrying capacity• Increasing of the stiffness

• Increase in the concrete member size obtained after the jacket• Increasing loads to be attained by other elements• The need to construct a new formwork

Adva

ntag

es

Disadvantages

Section Enlargement There are two different scenarios for Strengthening RC beams by

section Enlargement;1. Enhancing negative moment capacity by adding new reinforcement and

new concrete layer to the bottom face of the structural beam. 2. Enhancing positive moment capacity by adding new reinforcement and

new concrete layer to the top face of the structural beam.

In this technique the most important problem is to ensure an appropriate bonding between “old” concrete in the existing structure and “new” concrete applied for strengthening the structure.

Formwork and applying of the repair material 

Reinforcement protection (in some cases)

Replacement or addition of the supplementary reinforcement

Surfaces cleaning and

preparation to ensure bonding with the repair

material

Corrosion removal from the

exposed reinforcement

Removal of the deteriorated

concreteTemporary supports

Section Enlargement Steps to perform section enlargement techniques for RC beams;

https://www.youtube.com/watch?v=zkaMGT2b1ck (Steps)

https://www.youtube.com/watch?v=xDQnJ1iFTNA

Ferrocement Cover Thin composite material made of cement mortar reinforced with wire

meshes. The wire meshes are uniformly distributed in continuous layers with

relatively small diameters. The Ferro cement is used to replace the damaged concrete. Strengthening with Ferro cement improves:

Cracking resistance Flexural stiffness The ultimate loads compared to the original un-strengthened element.

These improvements depend on the full composite action between the Ferro cement layers.

External Plate Bonding Steel plates or steel flat bars are bonded to the structural elements. Widely appear in strengthening of bridge structures. The bonding is ensured by:

The use of epoxy adhesives, Additional fastening by means of dowels or bolts glued to the holes drilled

in the concrete members. Disadvantages: it can be applied only to the relatively sound

structures

External Plate Bonding

Attaching the steel plates to the concrete using bolts

Putting a layer of epoxy mortar on top of the plates with a 5mm thickness.

Making holes in the

concrete surfaces and

plates.

Coating the concrete

surfaces with a bonding

epoxy material.

Roughing and cleaning the

concrete surfaces

where the plates will be

attached.

External Post-Tensioning Effective in increasing the flexural and shear capacity. Applied to reinforced and prestressed concrete members. The post-tensioning forces are delivered by:

Standard pre-stressing tendons High-strength steel rods

Usually located outside the original section. The repair system supplements minimal additional load to the

structure thus being an effective economical strengthening technique.

External Post-Tensioning

https://www.youtube.com/watch?v=4nApm5nLUh8

The tendons are connected to the

structure at anchor points, typically located at the

ends of the member

The existing cracks must be repaired

by means of epoxy injecting or other known methods

If there are existing spalls

patching must be done, because this

repairs must ensure that the pre-stressing

forces are distributed

uniformly across the section of the

member

Fiber Reinforced Polymer (FRP)

FRP

High Strength

• ×5 Steel

Multifunctional

•Suit any project

Corrosion Resistance •Durable Structure

Light Weight•Easy to install

Fiber Reinforced Polymer (FRP) FRP is a new class of composite material for the development and

repair of new and deteriorating structures in Civil Engineering. FRPs are organized in a laminate structure. each lamina (flat layer) contains an arrangement of unidirectional

fibers fabrics embedded within a thin layer of light polymer matrix material.

FRP consists of two main components: Fibers. Resin or Matrix.

Fiber Reinforced Polymer (FRP)

Fiber Reinforced Polymer (FRP)

Advantages

•Corrosion Resistance•Lightweight•Ease of installation•Less Finishing•Less maintenance•Improve Ductility•They are ideal for external application

•Anti-seismic behavior

Disadvantages

•High cost•susceptibility to deformation under long-term loads

•Temperature and moisture effects

•lack of design codes•lack of awareness

FRP Mechanical Properties-Tensile Stress

FRP Properties

FRP Strengthening Methods There are 3 types of FRP repairing Methods

Externally bonded FRP (EBR)• Sheets

https://www.youtube.com/watch?v=vsXJzwST9yI • Plates• Shells• Plates

Sprayed FRP• Vinyl Ester

Near Surface Mounted Bars NSM• Bars

https://www.youtube.com/watch?v=ev59-1nrDdI• Plates

FRP installation Step 1 Surface Grinding / Shot Blasting to open pores

FRP installation Step 2 VE primer

FRP installation Step 3 VE putty

FRP installation Step 4 VE tack coat

FRP installation Step 5 CFRP saturator

FRP installation Step 6 CFRP sheet lay up

FRP installation Step 6 Embedding of CFRP in tack coat

FRP installation Step 7 Rib rolling

FRP Installation Completed Finished CFRP- Top, Sides, Bottom

FRP Installation Completed Acrylic / Intumescent Topcoat installation https://www.youtube.com/watch?v=NSbpl9f0lO8

Shear Strengthening of RC beams

Shear failure is a sudden failure which make it more devastating than the Flexural failure.

Shear failure start occurring from the critical section at high shear zone near support.

The failure is usually occurring without giving any alarming alerts. Shear force maximum at support and the diagonal cracks start from

support to applied load. These diagonal cracks formed on either side or both sides together in

RC beam and failure occurred by widen of shear cracks in RC beam

Shear Strengthening of RC beams Shear strengthening techniques

External anchored stirrups external post-stressing plate bonding using steel plates FRP https://www.youtube.com/watch?v=b4sVzPktjn0

Strengthening of RC Columns Strengthening of reinforced concrete columns is needed when:

The load carried by the column is increased due to either increasing the number of floors or due to mistakes in the design.

The compressive strength of the concrete or the percent and type of reinforcement are not according to the codes’ requirements.

The inclination of the column is more than the allowable. The settlement in the foundation is more than the allowable.

There are many techniques for strengthening RC columns, the most famous techniques are:

1. RC jacketing 2. Steal Jacketing 3. Using FRP wraps

Strengthening of RC Columns- RC jacket The size of the jacket and the number and diameter of the steel bars

used in the jacketing process depend on the structural analysis that was made to the column.

The loads applied to the column shall be reduced or eliminated by: Putting mechanical jacks between floors. Putting additional props between floors.

In case of corrosion in Reinforcement;1. Remove the concrete cover.2. Clean the steel bars using a wire brush or sand compressor.3. Coat the steel bars with an epoxy material that would prevent corrosion.

https://www.youtube.com/watch?v=aXP4oNzi6YM

Strengthening of RC Columns- RC jacket

Strengthening of Columns- Steel jacket

This technique is chosen when the loads applied to the column will be increased, and at the same time, increasing the cross sectional area of the column is not permitted.

Strengthening of Columns- Steel jacket

Strengthen RC Columns by FRP Wraps FRP improve the confinement of the RC columns leading to:

Improving Axial Capacity Buckling Resistance Bomb Blast Resistance

Thank YouAny Questions ???