Components of concrete structure, depending on desired seismic resistance, amount of damage and type of connections can be repaired or strengthened by using of resin injection, replacing separated parts, attaching sheets on surface and embedding reinforced concrete jacket or steel cages. However, successful operation for repairing and strengthening attain by reaching to high degree of bond between old and new concrete. Connecting jacket reinforcements to foundation bar, with enough braced length and passing longitudinal reinforcements from beam-column joints can help the increasing of resistance and bend stiffness in frames.
Each reinforcing strategy is useful for a structure with special characteristics. Increased resistance is particularly useful for strengthening hard structures. Retrofitting ductility is also suitable for brittle components. Increasing the stiffness is only appropriate for strengthening flexible structures, because it causes the decreasing of relative displacement of story. Of course in practice, the increasing of stiffness is usually accompanied with increasing strength. It has shown in Fig. 1.
The choice of the repair and/or strengthening method depends on the structural
behavior objectives. Strengthening strategies may be divided into increasing
the resistance to lateral loads, improving the ductility and an association
of both (Sugano,1981). Basically, the strengthening
techniques for reinforced concrete structures can be divided into addition of
new structural elements and strengthening of the existing structural elements.
Two general techniques are considered in the US for retrofitting designs. First
strategy is overall reform of structural system with increased structural walls,
steel bracing and base isolator. The second is partial reforming of structural
and unstructured components. Increasing components sections (sections of components)
for reducing displacement of story and reducing adjustability are common methods
for seismic retrofitting. It usually reduces the displacement (Mohebbimoghaddam,
Although, several methods are available for Structural rehabilitation, little
information is available and insufficient code guidelines are accessible. In
fact, most repair and strengthening designs are based on the assessment of engineers
only and, often, empirical knowledge and current practice have an important
role in the decisions to be made (Julio et al., 2003).
Thus, the objective of this study is to review one of the most commonly used retrofitting techniques: jacketing of reinforced concrete columns and beams. This method is evaluated according to different characteristics and, in order to help structural engineers to choose the most appropriate solutions, recommendations are given based on published experimental research and real case studies.
REINFORCED CONCRETE JACKET OF COLUMNS
Addition of a concrete jacket is used to enhance flexural strength, ductility
and shear strength of columns. In the usual when jacket is performed around
the column section, sometimes the combined function of old and new concrete
exclusively occurs due to natural bond between the materials. As shown in Fig.
2 this bond can be strengthen by roughing old levels and welding a series
of bent reinforcement between new and old reinforcement bars (Rahaee
and Nemati, 2004).
Jacketing procedure of RC columns step by step: This strengthening technique,
unlike other methods where steel elements are used, does not have a specialized
work demand. Its simplicity of execution makes any construction company, capable
of building with quality new RC structures, also competent to execute structural
rehabilitation using RC jacketing. In the following paragraphs, this method
is assessed according to different aspects: anchoring and slab crossing of the
added longitudinal reinforcement, interface surface preparation, spacing of
added stirrups and addition of new concrete (Julio et
Added longitudinal reinforcement
Anchoring to the footing: One advantage of RC jacketing strengthening
is the fact that the increased stiffness of the structure is uniformly distributed,
in contrast to the addition of shear walls or steel bracing. In fact, for these
procedures, it is usually necessary to execute new foundations or, at least,
to strengthen the existing ones. Generally, in the case of RC jacketing, the
steel longitudinal reinforcing bars of the added jacket can be anchored to the
original footings. Although there are several commercial products, very effective
to bond the added steel bars to the RC footing, attention must be taken when
executing this operation. Actually, the quality of the bonding can only be ensured
if some details are considered.
Crossing the slab: When continuity between floors of the RC jacketing
is required, holes must be provided in the slab to allow the longitudinal bars
of the jacket to pass through. In the case of slab-column structures, this procedure
is easy to perform and has no drawbacks. In the case of beam-column structures,
to avoid interrupting the middle bars, the longitudinal reinforcement must be
located at the corners, which can lead to excessive bundling of bars (Jara
et al., 1989).
Interface surface treatment: It is well known that the success of jacketing
to strengthen Reinforced Concrete (RC) columns is dependent on good bond between
the original column and the added jacket. To achieve this purpose, the treatment
of the interface must be carefully choosen. The common practice consists of
increasing the roughness of the interface surface and applying a bonding agent,
normally an epoxy resin. Steel connectors are also occasionally applied (Julio
et al., 2005).
As previously mentioned, two-component epoxy resin is most commonly used. However,
when an effective method to increase the surface roughness, such as sand-blasting,
is used, in this latter situation, the subsequent application of an epoxy resin
can even produce the opposite result and therefore should not be used (Julio,
Spacing of added stirrups: The monolithic performance of jacketed RC columns can be achieved if a higher percentage of transverse reinforcement is considered in the repaired solution. Thus, it has recommend that half the spacing of the original column transverse reinforcement be adopted for the jacket transverse reinforcement.
Added concrete: Normally the added concrete has a maximum aggregate
dimension of about 2 mm because of the lack of space in the jacket. This is
due to its diminished thickness associated with the volume occupied by the added
steel reinforcement (Julio et al., 2003). Concerning
the added concrete mixture and due to the reduced thickness of the jacket, the
option is usually a grout with characteristics of Self-Compacting Concrete (SCC)
and Highstrength Concrete (HSC) (Julio et al., 2005).
REINFORCED CONCRETE JACKET OF BEAM
The Reinforcement Concrete (RC) jacket can be executed by adding concrete to 3 or 4 faces of beams. We can also reinforce tensile and compressive zone of beams by concrete coat. Reinforced coating underneath face cause to increase the flexural capacity. Executing of concrete jacket on each four faces of beam is more effective style. The concrete thickness that is added to upper face of beam is in a way that can be lost in the thickness of ceiling (50-70 mm). In this way the stirrup will be controlled in upper side properly. Executing the jacket on the four faces of a beam has shown in Fig. 3.
In Diyab study (Rahaee and Nemati, 2004) by analyzing
loading-displacement curve in reinforced beams with concrete jacket, the final
loading of all reinforced beams was about double as primary specimen. Figure
4 shows the details of steel reinforcements of experimented beams, Xanthakos
(1996) drew attention to the necessity of dowels of sufficient length to
develop the required connection strength. He indicated when new concrete is
mechanically bonded to the old concrete, the new concrete will be prevented
from shrinking and therefore may crack. This cracking in turn may cause weakness
in integrated behavior of section (Coskun, 2002).
Surveys show concrete coatings or jackets are effective strategies for repairing or strengthening structures. If the cases are followed, the additional layers during the whole loading span act integrally until failure. Reduction resistance in repaired beam change 8 to 15% compare to resistance of entire beam.
|| Executing the jacket on the four faces of a beam 1-connect
of welded reinforcement
|| The details of steel reinforcements of experimented beams
Literature reviews on repair and strengthening RC structures:
Aguilar et al. (1989) performed a statistical study on the repair
and strengthening methods of 114 RC buildings damaged after the 1985 earthquake
in Mexico City. According to this work, the most commonly used techniques were
the addition of shear walls and the RC jacketing of columns.
Julio et al. (2001) performed tests on RC columns
strengthened by jacketing. The steel bars of the added Longitudinal reinforcement
were anchored to the footing of the original column by a commercially available
two-component epoxy resin. The models were submitted to monotonic tests, consisting
of a constant axial force combined with an increasing bending moment and shear
force. Initially, failure of all steel bars of the longitudinal reinforcement
of the original column and slipping of all the corresponding steel bars of the
added jacketing were observed. Pull-out tests were performed to analyse the
problem and it was concluded, without any doubt, that the bars slipped because
the holes drilled on the footings had not been adequately cleaned. So, the use
of a vacuum cleaner was enough to guarantee the change from slipping failure
to tension rupture of the added steel bars.
Julio (2001) performed slant shear tests and pull off
tests on specimens, considering different interface surface situations and also
considering the application of a commercial two-component epoxy resin. The values
of the shear and tension strength of the interface reduced when the epoxy resin
was applied on the sand-blasted surfaces, contrarily to what happened when other
roughening methods were used.
Bett et al. (1988) and Arturo
(2001) studied three models of columns (models of 1-1, 1-2, 1-3). After
primitive test, specimen 1-1 was repaired again by jacketing and then retested
as specimen 1-1 R. The remaining two specimens were strengthened prior to testing
by the same jacketing system. This Jacketing system consisted of a reinforcement
cage, not attached to the column end abutments, covered with a layer of shotcrete.
Additionally specimens 1-1 R and 1-3 were reinforced with cross-ties introduced
in drilled holes made in the original column, then cemented with epoxy adhesive
gel and tied to extra longitudinal bars placed on the middle points of the reinforcement
cage. Model of reinforced column, examined at the University of Texas has been
shown in Fig. 5. Under a constant axial and lateral load,
specimens showed displacements. This displacement was gradually increasing until
failure. Figure 6 shows test set up in Texas University.
From the test results, the following conclusions were made:
||The model 1-1 performed poorly under moderate earthquake loading,
exhibiting a shear-dominated failure
||Shear resistance of specimens increased by retrofitting columns
with shotcrete jacketing prior to lateral loading and ductility was greatly
improved compared to that of specimen 1-1
|| Model of reinforced column which was examined at the University
of Texas (Arturo, 2001)
|| The test set up of axial and lateral loading in Texas University
||The use of cross-ties as a part of the strengthening or repair
technique, was not significantly affect the columns stiffness or strength
under monotonic loading, but it improved the maintenance of such stiffness
or strength under lateral displacements exceeding from allowable limits
||The repaired specimen 1-1 R using the same jacketing technique
was proven to withstand almost the same lateral loading and drift ratio
imposed to the strengthened ones. Its stiffness and strength were almost
equivalent to those of specimens 1-2 and 1-3
||Improved behavior of the specimens would have been achieved,
in terms of ductility, if the longitudinal bars of the reinforcement cage
were fixed to the column end blocks since these bars never yielded (Bett
et al., 1988). Envelope of lateral load and displacement drift
of specimens has been shown in Fig. 7.
Studies on shear transfer between old and new concrete were done by bass and
his colleagues (Bass et al., 1989; Mohebbimoghaddam,
2008). Bass and his colleagues found such a shear strength which was increased
by increasing of connector steel, the depth embedment and strength of concrete.
Moreover the preparation of common surface didnt give any distinctive
affect as a connecting factor with results. The performance of jackets becomes
more important when the columns are the boundary component of frameB (Mohebbimoghaddam,
2008). The improvement techniques of present splice is including removal
of concrete coat, using welded overlap bar and surrounding coating splice with
reinforced concrete jacket and new reinforcement in jacket.
|| Envelope of lateral load-displacement drift of specimens
According to test results (Bett et al., 1988;
Tonkabonipour, 2005; Beer et al.,
2006). The lateral capacity of reinforced column can be predicted carefully
that its supposed the concrete jacket and main column are completely agreeable.
Of course the results for shotcrete jacket, is slightly lower than gained result
for cast-in-place jacket.
As workshop conditions are not ideal it is better the compressive strength of new concrete be 5 MPa greater than members concrete strength. Otherwise, a correction coefficient of modal ø = 0.8 have to be introduced for the calculation of stiffness and strength of repaired component.
And is monol has been used for regarding all components consisting the new and old jacket.
Valluan et al. (1993) presented the result of
retrofitting of column splice by meld and cross-steels. The results indicated
the bond in the zone of splice by melding longitudinal reinforcement decreased
the risk of buckling for this bars in order additional layer can show integrated
behavior until failure.
Ersoy et al. (1993) ran two series of tests
to study the behavior of strengthened and repaired concrete jacketed columns.
The first series compares the behavior of jacketed columns with a monolithic
reference specimen under monotonic axial loading. All the concrete for the monolithic
specimen was cast with the base column and retrofit reinforcement in place,
to provide a specimen with perfect interaction and bond between the base column
and retrofit material. Hoop reinforcement is used in the base column and retrofit
reinforcement. The jackets are applied under two conditions: after the compression
loading was applied and removed, as well as while the axial load is still applied.
It is determined that columns jacketed after unloading performed well, reaching
80 to 90% of then strength of the monolithic reference specimen. Repair jackets
applied while the column is still under load did not perform as well and only
reached 50% of the axial load carried by the monolithic specimen. The second
series of tests study the effectiveness of concrete jackets with columns tested
under combined axial load and bending. Both repair and strengthened jackets
behave adequately under monotonic and reversed cyclic loading.
Rodriguez and Park (1994) conducted further testing
on rectangular columns repaired and strengthened by concrete jackets under compressive
axial loading as well as lateral loading. Rebar hoops are provided as the retrofit
reinforcement for the concrete jackets. Concrete jackets increase the strength
and stiffness of the as-built (unretrofitted or base) columns by up to three
times. It is also shown that damage before the retrofit has no significant influence
on the performance of the jacketed columns. Overall, concrete jackets with rebar
reinforcement significantly improve stiffness, strength and ductility of typical
reinforced concrete columns, but construction is very labor-intensive.
Lehman et al. (2001) used concrete jackets to
repair severely damaged columns. Three repair methods are considered and implemented
for the damaged columns, which were built to modern seismic specifications.
Initial damage to the columns includes crushing of concrete, buckling and fracture
of longitudinal reinforcement and fracture of the spiral reinforcement, which
was the result of axial and lateral loading. Concrete jacketed columns are reinforced
with spiral transverse reinforcement. Loose concrete is removed from the cover
as well as the base column. The concrete jacket retrofitted column displays
increased stiffness and strength, comparable to the original column before damage.
In this study, 12 bars with numbers 8, 10, 12 were planted in concrete section by the use of epoxy resin. For any number of reinforcement, four samples were tested. Length of reinforcement coverage distinguishes them from each other. In order to investigate test results in meaningful way, nominal strength of concrete (fc) and yield strength of steel bars (fy) was kept constant.
Concrete: Since, the situation of concrete placing has significant influence on concrete resistance, all samples had depositing concrete at the same time and with same mix design. More details about mix design and mechanical characteristics of concrete have shown in Table 1.
Reinforcements: Round steels used in this study, were kind A II steel bars with 8, 10, 12 mm diameter. Mechanical characteristics of round steel have shown in Table 2.
Glue: The glue used in this study was two-component epoxy resin with mix ratio of A:B = 3:1. Its Characteristics have shown in Table 3.
|| Amount of existing materials in one cubic meter (m3)
of normal concrete (kg)
||Mechanical characteristics of round steel
||physical details of two -component epoxy resin
|| Braced length of reinforcements in concrete section
|| Minimum braced length required for bond stress
Test method and results: Since, determining of adequate braced length and adhesive resistance of epoxy resin are very important, steel reinforcements in three sizes and different lengths were placed in concrete section by the use of epoxy resin. characteristics has shown in Table 4.
After placing reinforcing steel in concrete sections, tension test was conducted. Table 5 offers the minimum length which is necessary to provide bond stress on the basis of yield strength of steel.
Bond stress which is created between bars and concrete by the use of epoxy resin, is recommended according to the following equation:
where, L is minimum braced length for providing bond stress. Thus, Epoxy resin bond stress between u = 3-3.5 Mpa is estimated According to above equation.
MATERIALS AND METHODS
Designing of connectors: The execution of this project was prolonged
about one year, 2009, in Mazandaran and Mohaddes university laboratory. When
jacketing is used for increasing the flexural strength and when combined function
is needed, In order to provide shear transfer between new and existing materials,
applying roughening of concrete surface before the concrete, using of shear
connector and root reinforcement are recommended.
Shearing stress that occurs on the faying surface of new and old concrete,
is figured out by following relation according to strength of mechanics principles:
When contact surface is in tensile zone:
Figure 8a and b show shear stress between
the old and new concrete.
||Shear stress between the old and new concrete: (a) strengthening
in tension zone and (b) strengthening in compressive zone
Resin layer including higher strength could be used to achieve adequate adhesion between new and old concrete. Amount of resistance obtained from following relation on contact surface should be compared with the shear strength of concrete base.
Total shear flow (T = τ0b) should be transferred through welding new and old reinforcement.
τRd is the base shearing strength design, Eq. 4.
In Eq. 5, the amount k is equal 1 when 1.6-≥1 (d is according meter):
τ0 is th axial stress because of axial loading.
Contact surface should be in compress zone: The amount of shear stress created on contact surface between new and old concrete is presented by following equation with a proper approximate according to classic theory of materials strength.
||Strain Changes of composite and noncomposite beam (Shapoor,
1999). (a) noncomposite action and (b) Composite action
If established tx1, is larger than tRd1, the total shear flow (T = tx1b) has to be transfer by shear uncuses. The final shear is tolerated by these uncuses is equal to following amount, see Eq. 6:
wher, d is the diameter of shear Uncus, fcd is the strength of concrete design and fyd is the strength of shear design.
Figure 9 shows shear uncuses between the old and new concrete
in compressive zone (Rahaee and Nemati, 2004).
Designing of shear connectors for anchoring of concrete coating of composite section: In general method, for designing of jacket and its connectors with present concrete, when the connectors between old and new materials is proper, the nominal shear strength could be figured out like combined zone method.
Designing in this method is similar to concrete slab on steel beam. In mentioned
method the affect of friction between layers has been ignored. Figure
10 shows strain Changes of composite and no composite beam (Shapoor,
In Eq. 7, the amount of total horizontal shear force that has to be transfer between the maximum point of positive moment and zero moment is equal:
||Final horizontal shear power between the maximum point of
positive moment and zero moment
||The number of required shear connectors from the point of maximum flexural
moment to the point of zero moment
||Shear strength of shear connector.
The method of establishing longitudinal shear in a section of a beam: Figure 11 shows shear in beam (horizontal section). as shown, we separate the component CDD'C' with the length of Δx from the length of member and this component goes through two sides to desirable curve surface. The force striking to this component consisting of shear force Vc and VD, slight horizontal force CdA and DdA. Longitudinal shear force ΔH that is the resultant of slight longitudinal shear forces striking on the curve surface, reaches to balance point according to following Eq. 8:
By integration on the shaded surface, the longitudinal shear striking to component of beam is identified by the Eq. 9:
The horizontal shear to length unit or shear flow and the distance of connectors
figured out according to the following Eq. 10:
|| Tridimensional model and the plan of retrofitted building
|| Vertical shear in section of beam
||The first moment of shaded surface
||The force of each connector
|| Distance of connector
The model of design: The modeling building, with concrete frame, containing seven story has built in 2007, it had much less seismic strength expected in present construction code but by the idea of using concrete jacket has been treated in seismic. It has shown in Fig. 12.
For estimating the structure and the method of treating it, by regarding the
limitation of liner method (TODCMPO, 2002), linear spectral
analysis and the software of Etabs 9.2 were used.
The present building with residential application is including regular plan, joists and filler block ceiling. Approximate size for each story is 368 m2. To improve the seismic behavior of the existing building, Idea of using lightweight materials has been used. The studied option for seismic treating is the usage of concrete jacket. The selection of strengthened members, after many different computational analyses have been considered very impressive. Beam Plan has been shown in Fig. 13.
The result of analysis, after strengthening of Strengthened members, according to Fig. 13 showed the increasing of sections accompanying with increasing in the stiffness of frame can resist against maximum moment. When the structure is treated for the affects of little fluctuation, with decreasing of usual period time the stiffness of structure increase and its displacement decrease.
As shown in Fig. 14, three different strengthening techniques
of columns with jacket were presented by Vandoros and Dritsos
(2008) and the specimens were studied in following states:
||Welding the jacket stirrup ends together (Model N)
|| Beam plan (a strengthened section is defined with hatch)
||Using of connection bars, jacket stirrup end welding and poured
concrete (model E)
||Using bent down steel connector bars welded to the original
column longitudinal reinforcement bars and using of concrete (model W)
After axial and lateral loading of specimens, results indicated that usage of concrete jacket has an impressive affect in increasing strength and damping capacity of energy of strengthened RC columns as regards primary specimen. Although, in comparison with composite specimen (model M), decreasing strength and stiffness occurrence, were not much, but they must be included in calculations. Load against displacement curve for each specimen has been shown in Fig. 15.
By studying characteristics of designing model and according to experimental results, we can improve the efficiency of this method by doing a series action outlined below:
||Surrounding present concrete section by concrete jacket and
passing additional longitudinal bars around current section, in a way that
related code is regarded, in order to facilitate working on this method
||Load against displacement curve for specimen (a) sample N,
(b) sample E, (c) sample W and (d) sample M (Vandoros
and Dritsos, 2008)
||Welding the jacket stirrup ends together, for compensating
the loss of inevitable seismic function of construction and fulfilling the
executive needs of constructions according to present building code (Moosavi
and Abbasnia, 2007; Frangou et al., 1995).
Figure 17 shows geometry of stirrup end welding
||The least thickness of concrete jacket for poured concrete
is about 100 mm and for shotcrete concrete, 50 mm is offered
||Minimum strength of concrete should be considered at least
20 Mpa. The least thickness of additional bars coating is considered as
followings: beam 35 mm, column 40 mm, foundation 75 mm and foundation 50
mm (Mohebbimoghaddam, 2008)
|| Geometry of stirrup end welding
|| Creating integrated close circle from two pieces
||In studying of special rules for flexural members in earthquake
zone and in many cases that a ring is needed, each longitudinal bar should
be located on the corner of stirrup and its interior angle should not be
more than 135°. The free distance of longitudinal bar should not exceed
more than 15 cm (Shapoor, 1988). Figure
18 shows creating integrated close circle from two pieces
The results of this study showed that using concrete jacket is affective method in increasing strength and stiffness in component of a frame and structural frame as well. Although sometimes supplying the anchorage bond of extra reinforcement concrete jacket, passing these bars from connection joint of beam-column, drilling of zones in executing stirrup and also using bent down connection bars with melded bars, cause difficulties in performing of this method, in this study, by summing up the results and assessment the requirements of this method, proper techniques for executing designing model are offered:
||For establishing complete bond of beam-column in a way that
moment could be transferred to frame components corresponding to increased
resistance, the new longitudinal reinforcements in columns should be passed
through floor and they should be surrounded in a concrete jacket too. Similar
methods like adding stirrups or continuous reinforcements across the connection
on beams can also establish bond on the floor
||For improving Inappropriate function arising from inadequate
development length and slip of reinforcement in the connection of extra
reinforcement to the column base, this length can be designed by using of
||The study of epoxy resin bond tests on concrete samples with
a fixed nominal resistance showed that by providing minimum length required,
epoxy can be used for connecting old and new concrete Instead of passing
cross-stirrup through section of old concrete
||Since, the lack of space and method of cross-reinforcement
limited volume of extra concrete and buckling of longitudinal bars, consistent
with Vandoros and Dritsos (2008) study, designed
connectors bars can be used for transfer interlayer shear flow instead of
welded bent down connection bars
||Studies by Xanthakos (1996) and Bett
et al. (1988) showed, In designing of computational models of
structures that dont have adequate seismic strength, the reduction
of stiffness and resistance for reinforced section should be considered
I would like to profoundly thank Ommehani Alizadeh for his support throughout this project. Dr. Ghasempouri also contributed valuable additional guidance before, during and after the completion of this project. It is greatly appreciated.