Strengthening Techniques for Reinforced Concrete Flat Slab Structures

1Válter Lúcio – vlucio@fct.unl.pt
STRENGTHENING OF
FLAT SLAB STRUCTURES
STRENGTHENING TECHNIQUES
FOR REINFORCED CONCRETE FLAT SLAB
STRUCTURES
- RESEARCH AND IMPLEMENTATION -
Válter Lúcio

STRENGTHENING OF
INDEX
1. STRUCTURAL DEFECTS IN FLAT SLAB BUILDINGS
2. CAUSES FOR THE STRUCTURAL DEFECTS
3. STRENGTHENING TECHNIQUES
4. STRENGTHENING BY CHANGING THE STRUCTURAL SYSTEM
5. STRENGTHENING BY ADDING REINFORCEMENT AND A CONCRETE OVERLAY
6. STRENGTHENING WITH POST-TENSIONING
7. CONCLUSIONS

STRENGTHENING OF
 Ultimate Limit State of Resistance
 bending.
 punching,
Punching
failure

STRENGTHENING OF
Progressive
collapse under
vertical loading
 bending.
 punching,
 progressive collapse,

STRENGTHENING OF
Reduced ductility
to seismic actions
 bending.
 punching,
 lack of ductility
to seismic actions.

STRENGTHENING OF
Collapse caused
by seismic action
Northridge,
Los Angeles -1994
 bending.
 punching,
 lack of resistance and/or ductility to seismic actions.

STRENGTHENING OF
 Service Limit State (functionality, aesthetical or psychological effects)
 excessive deformation and
 excessive cracking.
crack
Cracks on masonry
walls due to slab
deformation.

STRENGTHENING OF
2. CAUSES FOR THE STRUCTURAL DEFECTS
 DESIGN ERRORS
 incorrect design for seismic actions,
 reduced slab thickness for vertical loads,
 detailing;
 CONSTRUCTION ERRORS;
 ERRORS DURING THE USE OF THE BUILDING.

STRENGTHENING OF
2. CAUSES FOR THE STRUCTURAL DEFECTS - DESIGN ERRORS
Lack of vertical elements to resist seismic
actions:
• shear walls;
• beam and column portal frames,
specially in the slab edges.
Edge and corner column-slab
connections are quite fragile, not only
for seismic actions but also for vertical
actions, and shall be avoided.
Design properly for punching, including,
not only the vertical load transferred
from the column to the slab, but also the
transferred moments.
Avoid holes around the columns.
DESIGN ERRORS

STRENGTHENING OF
2. CAUSES FOR THE STRUCTURAL DEFECTS - DESIGN ERRORS
To design a flat slab as a beam supported slab,
wherein the beams have the thickness of the
slab, is a huge error.
“Horizontal beams”…….. a common mistake
DESIGN ERRORS

STRENGTHENING OF
 Concrete with lower resistance than the one specified in the design;
 Slab thickness smaller than specified;
 Slab top reinforcement with a much higher cover than specified (resulting in a
very small effective depth);
 Forget the punching
reinforcement or assembling
it in a wrong way.
2. CAUSES FOR THE STRUCTURAL DEFECTS – CONSTRUCTION ERRORS
Example of bad concrete
and top reinforcement placed
with a too small effective depth.
CONSTRUCTION ERRORS

STRENGTHENING OF
2. CAUSES FOR THE STRUCTURAL DEFECTS – ERRORS DURING USE
 Changes in use and excessive loading,
 Changes in the structure (cut openings near columns,
remove columns or shear walls);
 Accidental actions, and
terrorist attacks.
Terrorist attack Cutting opening
ERRORS DURING USE

STRENGTHENING OF
 Adding reinforcement:
• steel rebars, with partial demolition and added concrete (keeping the
geometry);
• post installed rebars in drilled holes, filled with grout or injected with resin;
• steel plates or profiles, connected to the concrete surface with anchors
and/or epoxy resin;
• fabrics or laminated carbon fibre, or glass fibre, glued to the surface with
epoxy resin;
• laminated carbon fibre inserted in grooves in the cover thickness.
 Jacketing:
• addition of a concrete layer reinforced with rebars;
• addition of a concrete layer reinforced with steel fibres;
• addition of a layer of mortar reinforced with steel fibres.
3. STRENGTHENING TECHNIQUES FOR FLAT SLAB BUILDINGS

STRENGTHENING OF
 External post tensioning:
• with strands, wires or bars of high strength steel;
• with laminated carbon fibre.
 Changing the structural system:
• introduction of new supports to reduce spans;
• controlled release of supports or introduction
differential settlements;
• partial or total reduction of stiffness in localized
areas to change the internal forces distribution;
• introduction of steel bracings, concrete shear
walls or energy dissipaters for seismic retrofitting.

STRENGTHENING OF
New columns
- Changing the structural system
Introduction of new supports to reduce spans.

STRENGTHENING OF
New columns
column
capitals
Concrete overlay with reinforcement
for negative moments
- Changing the structural system
Introduction of new supports to reduce spans
and strengthening of the slab with top reinforcement at new support.

STRENGTHENING OF
 Built in 1996;
 Two floors built with a flat slab
structure;
 The ground floor is used to park
the fire vehicles;
 The top floor is used for
administrative services and training;
4. STRENGTHENING BY CHANGING THE STRUCTURAL SYSTEM
 The building was in risk of collapse
due to design errors;
 The building had a rectangular mesh
of columns with approximate spans of 10m in both directions;
 The flat slab is 0.25m thick, with a ratio h/L = 1/40;
FIRE STATION BUILDING

STRENGTHENING OF
 The shear walls are only in the lower
floor, what causes a huge stiffness
variation, at the 1st floor, for seismic actions;
 Some columns do not have continuity due to windows in the façade;
 The edge beams are not supported by columns.
4. STRENGTHENING BY CHANGING THE STRUCTURAL SYSTEM - FIRE STATION
 The slab was analysed as a beam
supported slab, but there is no beams,
so the floor structures only supports
about half the design load;
FIRE STATION BUILDING

STRENGTHENING OF
crack
crack
 Cracks on masonry walls due to
slab deformation;
 Users experienced uncomfortable
vibrations in the floor.
OBSERVED DEFECTS

STRENGTHENING OF
 Keep the parking space for the fire vehicles
in the ground floor;
 Reduce to the minimum the intervention
in the upper floor to reduce costs in the
finishings;
 Ensure structural safety in relation to
vertical loads, namely:
• punching and bending of the slab;
• slab deformation;
CONSTRAINTS TO STRENGTHENING
 Ensure safety in relation to seismic actions, namely because the building is an important
infrastructure in the event of an earthquake.

STRENGTHENING OF
 Strengthening of the foundations creating continuous footings;
 Design of a seismic resisting system with 4 new reinforced concrete shear walls,
from foundations to top slab;
 Implementation of a steel structure
to reduce the slab spans;
 This steel structure
consists on 6 portal frames
3.30m away from each other,
that support the slab at
1/3 of the span, resulting
on new slab spans of about
3.30mx3.30m.
STRENGTHENING SOLUTION
Existente Reforço

STRENGTHENING OF
 Implementation of a steel structure to support the slab, resulting on new slab spans of
about 3.30mx3.30m;
 The floor slab was stressed against the steel beams of the parking floor;
 To reduce the intervention on the top floor, the steel beams were placed over the roof slab,
and this slab was suspended by these beams.
SLAB SUPPORT
Floor slab
Roof slab
Steel beam
Steel beam

STRENGTHENING OF
 The floor slab was stressed
against the steel beams of the
parking floor, by means of
threaded rods;
 The roof slab was suspended
by the steel beams placed
over the roof, by means of
threaded rods;
SLAB SUPPORT

STRENGTHENING OF
Steel structure to support the floor slab

STRENGTHENING OF
Threaded rods to stress the floor slab.

STRENGTHENING OF
View of the steel frames in
the parking floor
(free space for parking)

STRENGTHENING OF
 The building has a rectangular mesh of columns
with spans between 5.20 m to 8.40 m;
 The flat slab is 0.25 m thick;
TWO FLOORS PARKING GARAGE

STRENGTHENING OF
 Cracks in the corners of the floor, crossing the slab
thickness. These cracks are mainly caused by
concrete shrinkage, and may affect the durability;
 Cracks on the slab top (wmax = 0.5mm), diverging
from the columns, due to negative bending
moments, denoting high stresses on the steel
reinforcement.
OBSERVED DEFECTS

STRENGTHENING OF
 The bottom reinforcement was inspected and it
was found to be in accordance with the design;
 The equipment did not detect the top
reinforcement!!!
 A core drilled in the slab showed that the top
reinforcement had an effective depth 145 mm
instead of 205 mm.
This is a typical
construction error!
INSPECTION TO THE STRUCTURE

STRENGTHENING OF
 Minimize costs;
 Ensure structural safety in relation to vertical loads, namely for punching and bending;
 Reduce the aesthetic impact in order to keep the value of the space.
CONSTRAINTS TO STRENGTHENING
1. Demolish the concrete around the columns in a thickness of 75mm;
2. Insert new top reinforcement;
3. Insert punching reinforcement;
4. Cast a new layer of self compacting concrete.
STRENGTHENING SOLUTION

STRENGTHENING OF
New top rebars and punching reinforcement.

STRENGTHENING OF
View of the
top surface
View of the
bottom surface

STRENGTHENING OF
Final view

STRENGTHENING OF
Research:
STRENGTHENING FOR
PUNCHING WITH POST
INSTALLED REINFORCEMENT
Top face André Almeida (PhD) and several MSc students
António Pinho Ramos

STRENGTHENING OF
Research:
STRENGTHENING WITH
A CONCRETE OVERLAY
Hugo Fernandes (PhD)
Válter Lúcio and Pinho Ramos

STRENGTHENING OF
Research:
STRENGTHENING WITH
A CONCRETE OVERLAY
Section of the slab
Main parameters to analyse:
• surface preparation,
• dimensions of the concrete overlay,
• anchorage of the rebar ends,
• the use of connectors. Hugo Fernandes (PhD)
Válter Lúcio and Pinho Ramos

STRENGTHENING OF
Post-tensioning system with anchorages by bonding for structural strengthening.
(a) holes in
concrete
slab
column
post-tensioning end
(b)
deviator strand
slab
column
(c)
mechanical
actuator temporary
struttemporary
anchorage
slab
column
(d)
transmission length
injection with
adherence agent
seal the hole
ends
slab
column
(e)
cut the steel
ends
slab finishing
slab
column

STRENGTHENING OF
This strengthening system is
efficient for:
 punching capacity;
 bending capacity;
 reduces crack openings;
 reduces slab deformations;
 post-collapse behaviour.

STRENGTHENING OF
Punching failure
Post-collapse failure
Section of the slab after punching

STRENGTHENING OF
1.00
1.44
1.36
1.00
1.51 1.50 1.54
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
DF1 DF2 DF3 DF4 DF5 DF6 DF7
Punching capacity
----- h = 100mm ----- -------- h = 120mm --------

STRENGTHENING OF
CONCLUSIONS
Most common defects in flat slabs structures:
 Punching failure,
 Progressive collapse,
 Lack of ductility to seismic actions,
 Large deformations.
Most common causes for defects
 Design errors (incorrect design for seismic actions, reduced slab thickness,...),
 Construction errors,
 Errors during the use of the building.
Strengthening techniques available
 Adding reinforcement (steel rebars, post installed rebars in drilled holes)
 Post tensioning,
 Changing the structural system.

STRENGTHENING OF
ACKNOWLEDGEMENTS
The Fundação para a Ciência e Tecnologia for financing two research projects
and for giving grants to our PhD students.
The support of the companies HILTI, CONCREMAT, VSL and SECIL.

STRENGTHENING OF
Researchers:
 Ana Rita Gião
 António Ramos
 Carla Marchão
 Carlos Chastre
 Fernando Pinho
 Rui Marreiros
 Válter Lúcio
RESEARCH TEAM
RESEARCH
Concrete and Masonry
Structures

STRENGTHENING OF
RESEARCH TEAM
RESEARCH
Structures
PhD Students:
 Hugo Fernandes
 André Almeida
 Nuno Dinarte Gouveia
 Noel Franco
 Yang Yongming
 Brisid Isufi
 Helisa Muhaj
 João Marques
 Amaro Catumbaiala

STRENGTHENING OF
RESEARCH THEMES:
• Flat slab structures: punching resistance, high strength concrete, strengthening,
progressive collapse, and seismic behaviour.
Flat slab
strengthening with
post-tensioning
Flat slab test under
vertical load and cyclic
horizontal action
RESEARCH
Structures

STRENGTHENING OF
RESEARCH THEMES:
• Precast concrete structures:
connections, towers for wind turbines,
and seismic behaviour.
Development of seismic
energy dissipaters for
precast structures
Precast solutions
for towers to
support wind
turbines
RESEARCH
Structures

STRENGTHENING OF
RESEARCH THEMES:
• Ancient stone masonry structures: behaviour, structural connections, rehabilitation.
Behaviour of
ancient masonry
wall under
horizontal actions
RESEARCH
Structures

STRENGTHENING OF
RESEARCH THEMES:
• Prestressed structures: anchorage zones, fibre reinforced concrete..
Optimization of post-tensioning
anchorages
RESEARCH
Structures

STRENGTHENING OF
RESEARCH THEMES:
• Rehabilitation of concrete structures: repair techniques, strengthening techniques,
carbon fibre, concrete overlay, post-tensioning, and change of the structural system.
Strengthening of beam-column
connection for cyclic actions
Strengthening columns
for cyclic actions
RESEARCH
Structures

STRENGTHENING OF
RESEARCH THEMES:
• Strengthening and protecting concrete structures: protection systems for
buildings, analysis of the structure resistance under explosions, strengthening of
structures for explosion actions, reduction of damages developing dissipative
solutions.
RESEARCH
Structures

STRENGTHENING OF
THANK YOU
FOR YOUR ATTENTION
OBRIGADO!
GRACIAS!
ありがとう!

Strengthening Techniques for Reinforced Concrete Flat Slab Structures

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