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CHAPTER 4
The Engineering Of The Strengthening Procedures Of
Concrete Structural Walls
The possibilities of realizing the concrete necessary for the
strengthening of the damaged structures can be grouped into two
types: the pouring method and the guniting one.
The guniting method is constituted of special technological
procedures of preparation, carrying into effect and compacting the
concrete by sputtering the mixture with the help of the compressedair.
The procedures can be realized with special devices called
gunite machines.
The most used one is the dried procedure which implies the
contact between the cement with the necessary quantity of water at
the extremity of the flux through a device.
It is noticed the fact that the concrete realized by the gunitingmethod has a superior compactness, high compression and bending
strengths and a low permeability.
For these reasons are recommended to perform on the
reinforcements works of impermeability and protection against
aggressive factors.
This procedure is most recommended for the concrete
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The gunited concrete is also used at the execution of new
buildings which have special shaped elements; it is used for lining
the tunnels, for realizing the wearing course of the intensely used
railways; the method is also used for solidifying the terraces in
natural slope, etc.
The guniting method can be considered as the most
performing one for the strengthening activities of the concrete
structures, both from the point of view of the strengths obtained and
for the adherence of the support surface.The guniting is the only procedure which allows the application
of the concrete in difficult places using pouring procedures such as
the coating of the pillars at the superior part of the structural walls,
the lateral sides of the girders, the lower side of the plates and
girders; another pouring method is the guniting over head [1].
4.1 The study and analysis of the means, gunitemachines
The gunite machines can be grouped considering the
constructive characteristics and the functional parameters:
- machines with cylindrical drums and batching cylinders;
- machines with under pressure chamber and batching disks.
4.1.1 The gunite machines with cylindrical drums and
batching cylinders
From a constructive point of view the machines are endowed
with a cylindrical drum with several cylindrical holes at its extremity.
The volume of one cylinder is from 6104mm3to 1106mm4.
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One system of conduits guarantees the flow of the mixture
cement aggregate from the filling container to the heading hose (2).
The cylindrical drum activated by an engine spins around and brings
cylinders successively in the filling area and then in the area of the
conduits where the heading takes place under the action of the
compressed air, figure 4.1.
Fig. 4.1 Machines with cylindrical drums and batching cylinders
The tightening of the spinning cylindrical drum has to be
realized both at the inferior part as well as the superior one with
plane and neat steel plates and rubber stuffing [2].
Because of the frictions the stuffing gets prematurely used and
it becomes necessary to replace them at small periods of time; the
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Rehabilitation of Structural Wallstightening is usually not perfectly done so the machine looses
compressed air, dust and cement.
The main functional parameters are from a rate of gunited
concrete of 3 5 m3/h with a rate of air necessary for the functioning
of 10 20 m3/minute when the pressure of (5 7) x 105Pa.
The main disadvantage of this type of machine is the fact that
it doesnt realize a continuous and homogeneous flow of the mixture.
It realizes a functioning with pulses [2].
This is because of the large volume of the cylindrical drums, of
the mixtures quantity of 600 1000 cm4.During its passage through the heading hose the mixture has
different densities and the contact between the mixture and the water
is done with difficulty.
At the evacuation from the machine there is dust, the quality of
the concrete on the support surface is not equal; because of the
variable spacing pressure the surface of the gunited concrete
presents large subsides.
4.1.2 Machines with under pressure chamber and
batching disks
From a constructive point of view the machines have on their
inferior part a disk with cells on its profile.
At the superior part of the disk there are two chambers whichare separate between them and separate from the exterior by valves
tightened with rubber stuffing [3].
By the alternant opening of the two valves, the chambers
which are under pressure can receive the mixture aggregate
cement, figure 4.2.
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Fig. 4.2 Machines with under pressure chamber and batching disks
These machines do not have a distinct batching system; the
mixture is taken by the heading conduit from the mass of the
aggregate which is in the inferior chamber, the cells having the role
of carry of the mixture.
The main parameters are the rate of the gunited concrete of 2
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Rehabilitation of Structural Walls 3 m3/h, the proper functioning of the machine needing an air feed of
10 12 m3/minute, at a pressure of (4 6) x 105Pa.
The main disadvantage of these machines is the fact that the
feed is not constant because it depends on the quantity of mixture in
the area of the evacuation port.
4.2 Improving the performances of the concrete
guniting machines
Analyzing the disadvantages of the two types of machines
there have been conceived devices and there were performed
experiments pursuing the following objectives:
- a dosing in continuous flow of the mixture cement aggregate;
- the mixing and homogenization of the components aggregate
cement water;
- the concrete jet should be as homogeneous as possible, well
defined and having a long tronconical shape;- the pressure at the contact with the support surface should be as
uniform as possible;
- the surface of the gunited concrete should have minor subsides;
In order to realize a dosing in continuous flow of the mixture
cement aggregate there was conceived a device consisting of a
dosing chamber which creates a distinct space between each part,
limited by a cell.The mixture is thus continuously taken from there; there have
been conceived cells with small volume 30 50 cm4.
In order to obtain an homogeneous mixture with the agent
realizing the transport the air the access of this one inside the
dosing chamber is done through a conduit eccentrically placed and
creating vortices of the transport through the heading hose and
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reducing the frictions, figures 4.3 and 4.4.
Fig. 4.3 Device with horizontal disk and dosing turbulent chamber
In order to realize the best mixture of the dry components
aggregate cement with water there have been conceived a hole
choke at the end of the flow having an area where the water is
introduced with a pressure superior to the air from the heading hose.
The area is formed of a pierced ring, having 20 30 perforations with
the diameter of 0.6 0.8 mm; through this ring the water is sputtered
in the mixture aggregate cement [3].
The ending part of the hole choke was conceived having a
tronconical shape with a ratio of the extreme surfaces between 1.2
1.8, experimenting inside the quality and the form of the jet of gunited
mixture.
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Fig. 4.4 The dosing turbulent chamber
There were used heading hose with the interior diameter from
22 48 mm, figure 4.5.
Fig. 4.5 Heading hose
The experiments performed in laboratories to measure the
quantity of the components aggregate cement water, and on site
to visualize the quality of the jet of gunited mixture showed an
improvement in quality in what concerns the homogeneity, the
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constant shape of a tronconical.
The jet of gunited concrete at the contact with the support
surface can vary from 80 200 mm.
The jet of the gunited concrete at the contact with the support
surface, the print, can be obtained in the size wanted simply by
varying the parameters mentioned above [2].
During the numerous experiments on site and after producing
tens of meters of gunited concrete, there could not be totally
eliminated from the room where the guniting takes place the impure
atmosphere with a high content of dust.
Fig. 4.6 Turbine with a number of blades
In order to eliminate this shortcoming which makes it difficult to
visualize the guniting process and at the same time being noxious for
the operator, there was conceived a new area of improving the
homogeneity of the mixture aggregate cement water, as a sequel
of the water mixing area.
This area is constituted of a turbine with a number of blades
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Rehabilitation of Structural Wallswhich are moved by the stream of air under pressure, figure 4.6.
It is therefore realized a more intimate contact between the
components.
The assembly works as a small concrete mixer with forced
mixing.
The experiments on site showed no traces of dust in the
atmosphere; they also showed an increased possibility for the
operator to visualize the guniting process, figures 4.7 and 4.8.
Fig. 4.7 Atmosphere with no dust
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Fig. 4.8 The quality of the jet of gunited concrete
The improvements brought to the gunite machines allowed
energetic savings as well. It is possible to realize the guniting
process with an air consumption of 3 5 m3/minute.
The creation of new devices which improve the possibilities of
the concrete guniting procedures made possible the consolidation of
linear elements.
It also created the possibility of realizing limited concreted
areas in accordance with the damages and the selective necessities
of consolidation, figures 4.10 and 4.11.
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Fig. 4.9 The guniting process of a horizontal strap
Fig. 4.10 The guniting process of a vertical strap
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4.3 Experiments regarding the improvement of the
adherence between the support surface and thegunited concrete layer
It is well known that the calcium hydroxide migrates at the
beginning of the concretes hardening towards the surface of the
elements, in time it carbonates itself, then it transforms into calcium
carbonate forming a layer with reduced mechanical properties.
In a couple of years this layer will be 3 4 mm thick and
getting larger with the passage of time.
The guniting process at a surface where the carbonated layer
was not removed does not guarantee any adherence.
In order to verify the quality of the adherence between the
gunite and the support surface there was experimentally applied a
layer of gunite on a support surface of carbonated concrete [2].
Fig. 4.11 The breaking of the gunited concrete through the
carbonated concrete layer
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Rehabilitation of Structural WallsWhen trying to remove it, after the hardening process was
over, the breaking was through the carbonated concrete layer and
not at the contact gunite support surface, figure 4.11.
It is therefore necessary to clean off the carbonated concrete
layer from the support surface.
By removing this layer of cement with a steel brush, by bush
hammering, chiseling or sand blasting lead to no results.
In the case of the experiments there were conceived a number
of devices having a key type breaking of the carbonated concrete
layer.It was obtained a well worked, coarse surface with cleaved
aggregate granules, figure 4.12 and 4.14.
Fig. 4.12 The concrete surface after the carbonated layer was removed
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Fig. 4.13 The spalled aggregate particles after the removal of the carbonated
concrete layer
4.4 Procedures of reducing the material aggregate cement lost by bouncing
Realizing the concrete layers by the guniting method leads to
serious losses of the material used in the process.
Thus, when applying the gunited concrete on a vertical
surface the quantity of material aggregate cement which does not
remain in the layer is 15 25 %.When applying the gunited concrete on the lower side of the
elements, over top guniting, the losses are around 30 40 %.
During the experiments, analyzing the causes of these losses
which mainly take place during the first stage of applying the gunited
concrete layer, it was noticed that the aggregate particles with
maximum dimensions are bounced at the impact with the support
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Fig. 4.15 The segregation of the excessive mortar towards the surface of the
gunited concrete layer
The mortar in excess from the surface of the gunited concrete
may serve for the finishing activities.
By applying this layer of mortar the losses have been reduced
to 40 50 %.
4.5 Procedures of controlling the pre compression
force
In the case of selective consolidation of certain areas of the
damages elements with straps there were experimented the
possibilities to introduce in some reinforcements controlled tensions
to favor the co working in some activities.
The control of pre-compression forces was done using two
procedures a torsion dynamometer and a conceived and calibrated
device which measures the arrow of the loaded strap perpendicularly
acted by a force, figures 4.16 and 4.17.
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Fig. 4.16 Measuring the pre compression tension with the torsion dynamometer
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Fig. 4.17 Measuring the pre compression tension with the specially conceived
device
4.6 Testing the performances of the gunited concretes
There were designed and experimented standard concrete
compositions with water super reducer additive poured concrete
compared with the gunited concrete which, at the same dosage of
cement differentiated between themselves by the W/C ratio, by the
quantity of mixing water and by the maximum diameter of the
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compacted by vibration, tables 4.2 and 4.4.
Table 4.2 Resistance
Resistance/Concretetype
(MPa)/code
Standard
pouring
BS
Gunitedwith
additive
BAF
GunitedBT1
GunitedBT2
R28N 36.0 36.0 36.5 36.7
R90N 42.0 44.6 44.0 45.0
R28 gN 42.8 44.3 45.0 45.5
R28 gN g 38.7 41.1 42.2 44.1
R90 gN 44.0 46.1 47.5 48.5
R90 gN g 41.0 44.2 46.3 47.4
Table 4.3 - Permeability
Perm./Concretetype
(cm)/cod
Standard
pouring
BS
Pouringwith
additives
BAF
GunitedBT1
Gunited
BT2
P8-28N 4.03 2.96 2.03 1.83
P8-28 g10-N g 5.30 4.72 2.76 2.10
P8-90 g10-N g 4.72 2.25 2.10 1.98
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- N represents the medium maintaining temperature of the results bm
= 200C, in laboratory;
- g represents the period of time, expressed in days, of the gelation
test G50.
The compression strengths at 28 days, respectively 90 days
are superior (with 12.5 15%) to the minimum values imposed by
the standards.
This fact is because of the impermeability requirement
which limits the value of the W/C ratio to a maximum 0.50 for the
standard concrete (W/C 0.53 in order to realize the class condition)
and the W/C ratio = 0.40 0.44 for the gunited concrete or those with
additives (4).
10
8P
The compression strengths of the concrete submitted to the
gelation test G50 diminish with 7 10 % in the case of standard
concrete, with 4 8 % in the case of the concrete with water super
reducer additive and 3 7 % in the case of the gunited concrete.
The last category is the one that behaves best at this type of
test. The important thing is that the compression strengths of all the
types of concrete submitted to the frosting defrosting test G50are
superior to the minimum strengths admitted by the standard NE
012/99.
The gunited micro concretes as well as the concretes with
super plasticizers additives, behave very well in what concerns thepermeability the value of permeability is approximately 5 6 times
smaller than the admitted one.
Even after the frosting defrosting test G50was performed
the value of permeability of the gunited micro concretes are
approximately 4 5 times smaller than the permitted limit [5].
At the permeability test the gunited micro concrete with
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aggregate 0 7mm at the inferior limit of granularity, it behaves best.
The permeability of the micro concretes submitted to the
frosting defrosting test increases with 6% until 20%, but it stays 4
5 times smaller in the case of the gunited micro concrete, than the
admitted limit.
The concrete gunited with the help of the newly conceived
devices, composition BT2, table 4.1, it lead to optimum
performances, table 4.2.
Fig. 4.18 Gunited concrete immediately after application
The result was a well compacted concrete having a firm
consistency and do not get deform after pressure.
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