Introduction
A
novel technique used to improve the performance of existing structural elements
is the application of additional Ultra High Performance Fibre Reinforced
Concrete (UHPFRC) layers or jackets in connection to the existing elements. The
efficiency of this technique has not been adequately studied, and there are not
any published studies on the evaluation of this method with comparisons to
other traditional strengthening methods such as the use of Reinforced Concrete
(RC) layers and jackets. The technique of strengthening using additional RC
layers and jackets is one of the most commonly used techniques in seismic areas.
There are several published experimental and theoretical studies on beams and
columns strengthened with conventional concrete. A crucial parameter in this
technique, which can considerably affect the durability and the performance of
the strengthened structures, is the concrete shrinkage strain of the additional
layers/jackets. Additional stresses are induced in strengthened elements, and
cracking of the new layer and/or debonding may occur. The use of UHPFRC could
potentially improve both durability and resistance due to its superior
mechanical properties.
Reinforced concrete beams prior to
strengthening: Numerical modelling and experimental
validation
The
Initial, prior to strengthening, Beam (IB) examined in this study is based on a
previous experimental program. Initial beam’s cross sectional dimensions were
150 mm by 250 mm and the length was equal to 2200 mm. The reinforcement
consisted of two bars with a diameter of 12 mm (2H12) made of steel with a
characteristic yielding stress value of 500 MPa in the tensile side with a
cover of 25 mm. The characteristic cylinder concrete compressive strength of
the initial beam at 28 days was found equal to 39.5 MPa. The effective span was
equal to 2000 mm and the beam was tested under a four-point bending loading
with an imposed deflection rate of 0.008 mm/s. The distance between the two loading
points in the middle of the span was equal to 500 mm. For the finite element
analysis, ATENA software was used. Concrete was simulated with an eight-node
element, with nonlinear behaviour and softening branches in both tension and compression
using SBETA constitutive model. The ascending compressive branch of this model
is based on the formula.
recommended by CEB-FIP model code 90, while its
softening law is linearly descending from the peak stress until a limit
compressive strain, which was defined by the plastic displacement and the band
size, using the fictitious compression plane model. In tension, linear
ascending branch and exponential softening branch based on the fracture energy
needed to create a unit area of a stress free crack were used. In all the
analyses smeared crack approach was used. For the simulation of steel bars,
linear elements with bilinear behaviour were used. The numerical results (IBnum)
are compared to the respective experimental for (IBexp) and the results are
presented in. From the results presented in, very good agreement between the
numerical and the respective experimental results was observed. The same
assumptions were used for the modelling of RC layers and beams presented in the
following sections.
3. Experimental investigation and numerical
modelling of UHPFRC
3.1. UHPFRC material preparation
UHPFRC is a material with enhanced strength in tension
and compression and significantly high energy absorption in the post-cracking
region. One of the main characteristics of UHPFRC is the enhanced homogeneity
which is achieved by using fine aggregates only. In the mix design of the
present study, silica sand with maximum particle size of 500 lm was used together with silica fume
and Ground Granulated Blast Furnace Slag (GGBS). Silica fume, with particle size almost 100 times smaller than
cement, improve not only the density of the matrix but also the rheological properties,
while GGBS is used as a partial replacement of cement. High steel fibre content
(3%) of straight fibres with 13 mm length and 0.16 mm diameter were used. The
mix design is presented and it was based on a previous experimental
investigation.
For
the preparation of UHPFRC the dry ingredients were mixed first for 3 min in a
high shear mixer Zyklos (Pan Mixer ZZ 75 HE), then the water and the
superplasticizer were added to the mix and, at the end, the steel fibres were
added gradually. The specimens were cured in a steam curing tank at 90 _C for 3 days and the testing was
conducted 14 days after casting. These curing conditions were found to be
appropriate for the acceleration of the curing, since the strength achieved
after 3 days in the steam curing tank was the same with the strength achieved 3
months under normal curing conditions.
3.2. Compressive and direct tensile tests and numerical modelling
The
standard cube compressive tests (100 mm side) were conducted and the mean
compressive strength was found equal to 164 MPa while for the tensile strength,
direct tensile tests of 6 dog-bone specimens were carried out.
A
constant loading rate of 0.007 mm/s was used to control the tests which is in
agreement with the loading rate used by Hassan et al. leading to comparable
results. The extension of the specimens was recorded using Linear Variable
Differential Transformers (LVDTs). The setup of was used to measure the average
extension over a gauge length of 105 mm, and the stress versus strain
(extension normalized to the gauge length) results of all the 6 specimens
together with the average curve are presented in.
The experimental results indicate a variation of the
tensile strength between 11.74 MPa and 14.20 MPa. An average stress– strain
curve was calculated and the average strength was found equal to 12 MPa. The
Young’s modulus was experimentally obtained from the slope of the initial
linear part of the stress–strain graph in the linear part and a value of 57.5
GPa was calculated.
The setup used for the optical measurement of the
crack together with the strain distribution at the moment when the first crack
appeared, are presented in, alongside with stress– strain results for one of
the examined specimens.
Digital
Image Correlation (DIC) system was used during the testing in order to monitor
the crack opening and the strain distribution. According to these results, the
strain is uniformly distributed along the specimen in the elastic part of the stress–strain
distribution (strain values below 0.001). Then, in the second phase (strain
between 0.001 and 0.005) there is a combination of micro cracks and elastic
strain in the neck of the dog bone specimen and, in this phase, the multiple
micro cracks opening was taken into account as an average crack opening along
the monitoring length. For strain values higher than 0.005, all the extension
of the specimen was due to the crack opening.
Fig. 5. Stress–strain results for the direct tensile tests
Fig.
4. Direct tensile tests
(a)\geometry and (b)
experimental setup.
3.3.
Experimental and numerical investigation of UHPFRC layers under
flexural loading
In
this section, experimental and numerical results of layers with 50 mm depth
were examined. During the casting process, the specimens were cast by pouring the
material centrally along the long side and filling the first layer up to
approximately 90% of the height of the specimen before the compaction by
external vibrator. Afterwards, the moulds were filled and compacted as
described in BS EN 14651:2005, and then the specimens were rotated over 90_ around their longitudinal axis
for testing. Three identical specimens were tested under 4point loading, with
100 mm breadth, span length of 300 mm, and distance between the two loading
points 100 mm. Two LVDTs were used to record the deflection of the layers in
both sides, and the tests were conducted using a displacement control of 0.001
mm/s. An external yoke was used in order to exclude any additional displacement
at the supports. The testing setup is presented in and a typical crack pattern
is illustrated.
The examined layers
were modelled using the assumptions presented in Section 3.3.
The numerical model is presented in, and the strain contours together with the
crack pattern at midspan deflection equal to 10 mm, are presented in. A
deflection equal to10 mm was selected as this was the typical mid-span deflection
at the end of the tests.
The
examined layers were modelled using the assumptions resented in Section 3.3. The numerical model is presented in
Fig. 11a, and the strain contours together
with the crack pattern at midspan deflection equal to 10 mm, are presented in
Fig. 11b. A deflection equal to 10 mm was selected as
this was the typical mid-span deflection at the end of the tests (Fig.
10b).
The
strain localization in the middle of the span observed in the experimental investigation (Fig. 10b) was in agreement with the numerical
simulation’s results presented in Fig. 11b.
The
load deflection predictions are compared with the respective experimental
results of all the three specimens [34] and the results are presented in Fig. 12.
The numerical results are in good
agreement with two of the examined layers, while in one of the examined
specimens the strength was considerably lower compared to the other two. This could
be attributed to local deficiencies due to the fibre distribution in the mix
which resulted to a premature failure of the specimen.
The
assumptions presented in Section 2 and in Section 3.3, for
the initial beam and the UHPFRC layers respectively, were used for the simulation
of the strengthened beams with UHPFRC.
3.5.
Experimental investigation of UHPFRC shrinkage
In
case of strengthened elements with concrete jackets, crucial parameters for the
response of these ‘composite’ elements are the interface between the old and
new concrete, and the shrinkage of the ‘new’ concrete [15]. Shrinkage
strain measurements were recorded over the time for UHPFRC with 3% steel fibres
and for Ultra High Performance Concrete (UHPC) without fibres. The mix design
of Table 1 was used, and standard prisms 75 mm by 75 mmby 280
mmwere cast. The specimens were stored in a room with relative humidity 42% and
temperature 20 _C
(Fig. 13a) conditions similar to the standard climate
(temperature 23 ± 2 _C
and relative humidity or RH 50 ± 5) proposed by DIN 50014.
The
shrinkage strain distribution with the time is presented in Fig. 13b.
As a starting point for these measurements (Day 0 in Fig. 13b),
the third day after casting was used, when the initial curing was completed.
The
results presented in Fig. 13b indicate that the presence of the steel fibres considerably
reduces shrinkage strain values and an average reduction of the shrinkage
strain with the time of 30% was observed. In case of UHPFRC with 3% steel
fibres (Table 1), the shrinkage strain 90 days after casting was
measured equal to 565 microstrains. Shrinkage strain values are highly affected
by a number of parameters including the mix design, the curing conditions and
the geometry of the examined specimens. Yoo et al. used a special setup to
simulate the conditions of free UHPFRC shrinkage effects on slabs and a very
steep increase of shrinkage at the very early age was observed. Kamen and Kamen
et al. Conducted measurements on specimens with similar geometry to those of
the current study (prisms and dog-bone shaped specimens) and a distribution
similar to the one presented in Fig.
13b was observed. Also, according
to Kamen et al. In the first 2 days there is a chemical shrinkage state after
the water–cement contact, followed by a swelling, and the main part of
shrinkage strain is developed after the first two days.
5.
Strengthening with additional RC layers. Comparisons of the two techniques
In this section, the results of strengthened
beams with UHPFRC are compared to respective experimental results of
strengthened beams with traditional strengthening techniques using conventional
concrete.
The
results of a previous experimental investigation were used. The geometry and the
material properties of the initial, prior to the strengthening beam (IB), are
presented in Section 2.
Beams
strengthened in the tensile side with RC layer were examined (ST_RCL_TS). In
this case, strengthening was performed by adding a new concrete layer of 50 mm
thickness in the tensile side with two bars with a diameter of 12 mm (2H12),
made of steel with a characteristic yielding stress value of 500 MPa, and
concrete cover of 25 mm (Fig. 23a). The 28 days characteristic cylinder concrete compressive
strength of the layer was found equal to 39.5 MPa. The reinforcement of the
layer was consisted of two bars with a diameter of 12 mm (2H12) with a total
amount (volume) of steel exactly the same with the total volume of steel
fibres used in
the layer of ST_UHPFR_TS with 3% steel fibres and 12
MPa average tensile strength.
Beams
strengthened in the compressive zone were also examined (ST_RCL_CS) and, in
this case, plain concrete layer with 28 days characteristic cylinder
compressive strength of 45.4 MPa, and 50 mm thickness was placed on the top of
the beams (Fig. 23b). In this case, a loading rate of 0.008 mm/s was
used.
This loading rate is equivalent to the one used for the testing of the prisms
(0.001 mm/s) presented in Section 3.4. The distance between the load and the support in the
full scale elements (750 mm) was almost 8 times higher compared to the
respective
distance in the bending test of the prisms (100 mm)
and a loading rate 8 times higher was used in order to obtain comparable
results.
The
beams were tested under a four-point bending as described in Section 2 and the
failure pattern is illustrated in Fig.
24a and b for specimens ST_RCL_TS
and ST_RCL_CS respectively.
The load deflection results for specimens ST_RCL_TS
and ST_RCL_CS together with the experimental results of the Initial Beam (IBexp)
are presented in Fig. 25.
From
the results presented in Fig. 25, the moment at yield and failure for the strengthened
specimens were calculated together with the respective increment, in comparison
with the Initial Beam’s (IB) results of Table
3 (Table 4). In case of ST_RCL_TS, the yielding
moment was defined by the yielding of the reinforcement of the additional
layer, which occurs before the yielding of the reinforcement of the initial
beam.
These
results were compared to the respective values calculated for the beams
strengthened with UHPFRC with 12 MPa ultimate tensile strength, and the
comparisons of My and Mu for the various techniques are presented in Fig. 26a
and b.
The
results of Fig. 26 indicate that very high moment increments were
observed for beams strengthened with an additional RC layer in the tensile side
(ST_RCL_TS), since increments of 150% and 97% were observed for My and Mu. The
respective increments for specimens strengthened with UHPFRC in the tensile
side (ST_UHPFRC_TS) were found to be lower and equal to 29% and 31%. The
results of Fig. 26 indicate that the highest moment increment was
observed for a three side UHPFRC jacket and this was found equal to 167% and
178% for My and Mu respectively.
In
case of beams strengthened in the compressive zone, application of UHPFRC
(ST_UHPFRC_CS) resulted to an increment of 19% and 28% for My and Mu, while the
respective increment was slightly lower (25% for My and 22% for Mu) when
normal concrete was used (ST_RCL_CS).
The
technique of strengthening in the tensile side with combination of UHPFRC layer
and two steel bars was also examined. The same geometry and reinforcement with
the one presented in Fig. 23a was used with the only difference that instead of
normal concrete, UHPFRC with 12 MPa ultimate tensile stress was used (ST_UHPFRC_TS_12
MPa and steel bars). The load deflection results were compared to the
respective results of ST_RCL_TS and with the initial, prior to strengthening,
beam (IB) (Fig. 27).
The
results presented in Fig. 27 indicate that the initial stiffness and the ultimate
load capacity were increased when conventional concrete was replaced by UHPFRC
in reinforced concrete layers applied to the tensile side. The increment of the
yield and ultimate bending moment values are presented in Table 5.
The
results of Table 5 indicate that there is an increment of 7% in the yield
bending moment and 9% in the ultimate bending moment when the normal concrete
of the layer is replaced by UHPFRC. The results indicate that even if the
performance is overall enhanced, the contribution of the UHPFRC in this case
was not fundamental.
Reference
1.http://fulltext.study/preview/pdf/265998.pdf
6. Conclusions
In this study, extensive experimental and numerical investigation was
conducted to investigate the efficiency of UHPFRC for the strengthening of
existing beams. Mechanical testing and shrinkage strain measurements were
conducted in order to determine UHPFRC material properties, and these results
were used for the numerical modelling. The following two conclusions were drawn
regarding the behaviour of UHPFRC in tension and regarding the shrinkage
strain.
1.The proposed stress–strain model in tension,
which was consisted of an initial linear elastic part and a tri-linear post
elastic behaviour can accurately predict the response of UHPFRC.
2.Shrinkage strain measurements were also
presented for UHPFRC with 3% steel fibres and without steel fibres. The shrinkage
strain was 30% reduced in case of UHPFRC with 3% steel fibres, compared to the
respective measurements of plain UHPC.
Extensive
numerical modelling was conducted on beams strengthened with UHPFRC.
Strengthened beams with additional layer in the compressive, in the tensile
side, and with a three side jacket were examined. A parametric study with
different values of UHPFRC tensile strength was also conducted and the
following observations were made.
1.As expected, in case of specimens
strengthened with UHPFRC in the compressive side, increment of the tensile
strength of UHPFRC was not considerably affecting the response of the strengthened
specimens. Ultimate moment increment less than 4% was observed when UHPFRC
tensile strength was increased from 8 MPa to 16 MPa.
2.In case of strengthened specimens with UHPFRC
in the tensile side, the ultimate moment was increased by 31% when UHPFRC tensile
strength was increased from 8 MPa to 16 MPa.
3.The respective increment for
strengthened specimens with three side jackets (ST_UHPFRC_3SJ) was
significantly higher and equal to 53%.
The
effect of the post-peak (softening) stress–strain behaviour of UHPFRC on the
overall performance of the strengthened beams was also investigated.
1.As expected, for beams
strengthened in the compressive side, the effect of the post-peak behaviour of
the tensile stress–strain model was negligible.
2.In case of beams strengthened in
the tensile layer with UHPFRC layer, when the stresses in the post-peak branch
were increased by 50%, the ultimate strength and moment were increased by 8%,
while, when 50% lower stresses were used, the ultimate load and moment were
reduced by 11%.
3.In case of beams strengthened
with a three side jacket the respective ultimate strength increment was 2%, for
50% higher post-peak stresses, while reduction equal to 3% was observed for
stress–strain model with 50% lower post-peak stresses.
Sensitivity analysis was also conducted for the
examined strengthened with UHPFRC beams, using different shrinkage strain values
for UHPFRC. The main conclusion of this study was that in this case the
response of the strengthened elements was not affected considerably by
variations in the shrinkage strain value of UHPFRC.
Critical comparisons of this novel technique with the
traditional method of strengthening with RC layers were also conducted and the
following conclusions regarding the efficiency of the two techniques were
drawn.
1.The highest moment increment was
observed for a three side UHPFRC jacket and this was found to be equal to 167%
and 178%, for My and Mu respectively. When three side jacket was used, the slip at the
interface was considerably reduced, compared to the respective values of beams
strengthened in the compressive or tensile side.
2.The increment in case of
specimens strengthened with UHPFRC in the tensile side was found equal to 29%
and 31% for My and Mu. Considerably higher increment was observed in case of beams
strengthened with an additional RC layer in the tensile side (150% increment of
My and 97% increment of Mu).
3.In case of beams strengthened in
the compressive zone, the addition of UHPFRC resulted to an increment of My and Mu 29% and 28%
respectively, while the increment was slightly lower (25% for My and 22% for
Mu) when normal concrete was used, which indicates that there is no
need for high strength concretes when strengthening in the compressive zone.
For beams strengthened in the tensile side,
combination of steel bars and UHPFRC was also investigated and an increment of
7% in the yield bending moment and 9% in the ultimate bending moment was
observed, compared to the respective values of beam strengthened with RC
layers. The results indicate that even if the performance is overall enhanced,
the contribution of the UHPFRC in this case was not fundamental.
The main conclusion of this study is that superior
performance can be achieved by the use of three sides UHPFRC jackets. In
practise, UHPFRC could be used following the same procedure with the one used
for RC jackets. The application of UHPFRC could be done using formworks by
adapting the rheological properties of UHPFRC [26]. This novel technique has a
great potential for the structural upgrade of the existing structures.
Reference
1.http://fulltext.study/preview/pdf/265998.pdf
