the strengthening procedures of concrete structural walls

8/13/2019 The Strengthening Procedures of Concrete Structural Walls

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Rzvan GIUC

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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Rehabilitation of Structural Wallsreinforced with dispense fibers.

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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Rehabilitation of Structural WallsNote:

- 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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the strengthening procedures of concrete structural walls

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