Strengthening of Building Structures

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25 September 2018 - 16:33, by , in Concrete Repair, Comments off

Structural Strengthening

 

 

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

The new load-bearing material:

  • High quality concrete
  • Reinforcing steel bars
  • Thin steel plates and straps
  • Post-tensioning tendons
  • FRP composite materials

 

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

 

Factors affecting selection of strengthening method

  • Magnitude of strength increase
  • Effect of changes in relative member stiffness
  • Size of project
    – methods involving special materials and methods may be less cost-effective on small projects
  • Environmental conditions
    – methods using adhesives might be unsuitable for applications in high temperature environments
    – external steel methods may not be suitable in corrosive environments
  • In-place concrete strength and substrate integrity
    – the effectiveness of methods relying on bond to the existing concrete can be significantly limited by low concrete strength
  • Dimensional/clearance constraints
    – section enlargement might be limited by the degree to which the enlargement can encroach on surrounding clear space
  • Accessibility
  • Operational constraints
    – methods requiring longer construction time might be less desirable for applications in which building operations must be shut down during construction
  • Availability of materials, equipment, and qualified contractors.
  • Construction cost, maintenance costs, and life-cycle costs
  • Load testing to verify existing capacity or evaluate new techniques and materials

 

Strategies for structural strengthening

1. Design faults

– Lower performance before taken into service
– Strengthening slightly above the desired performance level

2. Damaged due to an accident

– Collision, fire or overload that damaged the system to a level where performance requirements were not fulfilled
– Repaired and strengthened to a new performance level

3. Structure demands changes

– Higher load bearing capacity was required
– Strengthening to meet the performance level
– possible to keep the structure in service

 

Performance History of a Structure

 

Strategies for structural strengthening

 

Strengthening techniques

  • Section enlargement
  • External plate bonding
  • Post-Tensioning
  • Ferrocement laminates
  • Sprayed concrete
  • FRP

 

Section enlargement (Jacketing)

Jacketing

– Adding R.C. jacket section
– achieve the desired section properties and performance

Advantages

– Increasing of the load-carrying capacity
– Increasing of the stiffness.

Disadvantages

– Increase in the concrete member size obtained after the jacket
– The need to construct a new formwork.

 

Stages of section enlargement

i. Temporary supports
ii. Removal of the deteriorated concrete
iii. Corrosion removal from the exposed reinforcement
iv. Surfaces cleaning and preparation to ensure bonding with the repair material
v. Replacement or addition of the supplementary reinforcement
vi. Reinforcement protection (in some cases)
vii. Formwork and applying of the repair material

 

 

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.

 

 

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.

 

Stages of the post-tensioning strengthening

i. 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

ii. The existing cracks must be repaired by means of epoxy injecting or other known methods

iii. The tendons are connected to the structure at anchor points, typically located at the ends of the member

 

Ferrocement laminates

• 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.

 

Sprayed concrete (shotcrete)

• 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.

• There are two processes for applying sprayed concrete

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

 

Strengthening using Fibre Reinforced Polymer (FRP)

FRP Materials

Lightweight – Easy to install

Corrosion resistant – Durable structures

High Strength – 5x steel

Highly versatile – Suit any project

 

• Plastic and polymer materials that are reinforced with structural fiber such as

– fiberglass (GFRP)
– carbon fiber (CFRP)
– aramid fiber.

 

Common methods

• Externally bonded FRP (EBR).

– FRP plate bonding.
– FRP Confining or jacketing

• Sprayed fiber reinforced polymers

• Sprayed carbon or glass chopped fiber with vinyl ester resin upon concrete structures

• Benefits of using vinyl ester resin:

– it takes shorter time to harden the resin than epoxy resin
– the mechanical properties of vinyl ester resin are the same as the one of epoxy resin

 

Strengthening of concrete structures

• Foundations

• Slabs

• walls

• Beams

• Columns

• Masonry walls

 

Strengthening the foundations

• Columns foundations need strengthening in the case of applying additional loads.

• Widening and strengthening of existing foundations may be carried out by constructing a concrete jacket to the existing footings.

• The new jacket should be properly anchored to the existing footing and column neck in order to guarantee proper transfer of loads.

• This can be accomplished by drilling holes into existing concrete of footing and epoxy grouting the longitudinal reinforcement of jacket.

 

Strengthening of R.C. slabs

Strengthening of reinforced concrete slab by removing the concrete cover and applying new bottom steel rods to flexural zones of slab to enhance its positive flexural capacity.

Applying new top steel rods to flexural zones of cantilever slab to enhance its negative flexural capacity.

• Strengthening the slab with stressing method

• Increasing punching shear capacity of the slab

– Bolting through the slab
– Strengthening hollow-core slabs

Strengthening the R.C.walls

The dimensions of the wall and its reinforcement are in creased by the following steps:

1. Roughing the total area of the concrete surface.

2. Installing steel connectors for the whole surface at 25-30cm spaces in both directions.

3. Installing steel connectors into the wall footings, with the same number and diameter of the main vertical steel bars, using an epoxy material.

4. Installing the steel mesh and fasten it by steel wires to the steel connectors.

5. Coating the surface of the wall with an appropriate epoxy material.

6. Pouring the concrete jacket using low shrinkage concrete before drying of the epoxy material.

 

Strengthening of R. C. beams

• Reinforced concrete beams need strengthening when the existing steel reinforcement or cross section in the beam is unsafe or insufficient

or

• when the loads applied to the beam are increased due to the reasons mentioned before

• Adding reinforcement steel bars to the main steel without increasing the beam’s cross sectional area

• Increasing both the reinforcing steel bars and the cross – sectional area of concrete

• Tensile and/or shear capacity is to be increased by gluing steel sheets / carbon fiber reinforced laminates into a lower and/or side surface of the beam

• Reinforced concrete beam is to be strengthened by adding new steel bars

 

Strengthening of R. C. beams

ADDING STEEL PLATES TO THE BEAM

1. Roughing and cleaning the concrete surfaces where the plates will be attached.

2. Coating the concrete surfaces with a bonding epoxy material.

3. Making holes in the concrete surfaces and plates.

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

5. Attaching the steel plates to the concrete using bolts

 

Strengthening of R. C. beams

• Concrete beam is to be substituted with steel beam
• Steel profile is to be fixed into a concrete beam
• Steel profile is to be wedged below the beam

• Strengthening of reinforced concrete columns:

– Load increasing due to due to the increasing in floors number
– Load increasing due to design mistakes
– Compressive strength of concrete is not according to the code requirements
– Percent or type of reinforcement is not according to the code requirements
– Column inclination is more than allowable
– foundation settlement is more than allowable.

Strengthening of R. C. columns

• Entirely or partly jacketing

• Strengthening the column with the steel form

• Increasing cross sectional surface with steel profiles

• Substituting the concrete column with the steel column

 

Case study: Section enlargement using SCC

Section enlargement using SCC

Repairing process of the supporting walls, parapet and slab of the balcony using self-compacting concrete. (Batch 1, October 2004, laboratory condition)

Repairing process of the supporting column and slab of the balcony using self-compacting concrete (Batch 2, November 2004, outdoors condition)

Case study: conclusions

• An objective with the work was to find the minimum cover depth of the selfcompacting concrete (SCC) suitable for the repair method studied.

• Fresh and hardened concrete tests for the self-compacting concrete used showed that the concrete fulfilled the requirements for balcony repairing.

• The results of slab frost resistance tests show the scaling and flexural strength change of self-compacting concrete was very small after 300 cycles and the changes in ultrasound transmission time were small, so there is no sign of inner deterioration.

• Drying shrinkage of self-compacting concrete was higher than the theoretical shrinkage for the normal concrete because self-compacting concrete contained more fine aggregate. Using reinforcement mesh reduces the shrinkage of selfcompacting concrete

• The bond between the old concrete, which has a lower strength than the repairing self-compacting concrete, is poor specially, when casting concrete in the lower surface of concrete slab.

 

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