Materiali da costruzione svanzati - Parte 2

INSPECTION AND DIAGNOSIS OF REINFORCED CONCRETE STRUCTURE

We have to evaluate the conditions of structure.

During design phase we have to define service life, we have to know what is the expected service life related to the time of inspection and the time

of end of service life. This evaluation is to understand haw many material we have to use during the maintenance and also the costs.

We have to evaluate the residual service life.

To design and schedule repair and maintenance works, to reach the expected service life, of a r.c. structure is necessary:

- to carry out an analysis of the condition of the structure (inspection)

- to perform a diagnosis of causes of deterioration

- to make a prediction of the future evolution of damage

- to choose the repair strategy (minimizing the amount of concrete removal in repair works)

- to evaluate the consequences of deterioration for structural safety and serviceability

We can consider that we can be in different conditions. If the structure is in corrected

design and we make inspection we have in the initiation time, in the oldest structure during

inspection we are in the propagation time and we can also are at the end of service life.

We have to do inspection during the initiation period if the service life of the structure is low,

for example we can have the end of initiation period after 20 tears of service life but the

client ask us what is the residual service life after 14 years we have to make inspection and

we are in the initiation period.

This approach give a lot of advantage because if we are in the initiation period we can do small and cheap maintenance to reach the service life

that we want.

Inspection

Preliminary survey :

- collection of information (exposure, design data, info. about the structure ...)

- visual observation (corrosion products, cracks, spalling, errors, extension ...)

Detailed survay:

- analysis on concrete and on corrosion of the reinforcement (destructive and non-destructive tests)

Have an initial idea about the structure and the area in which we have to do the analysis, analysis need time and have also am high cost, so we

have to select an area in which make the test.

Diagnosis

Initially questions about the general situation, after there are the stipulation of simple analysis to understand better the situation and after there

is the lecture of the results of these tests and the control about the previous hypothesis in order to confirm that.

The goal is related to the diagnosis, we have to obtain a diagnosis at the lower costs in the faster way.

Observation and information —> reasonable hypothesis —> Confirmed by specific analysis (sulfates, Alkali-Silica, Rc ...)

The necessary analysis must be required in sufficient numbers (costs)

Target —> reach to a correct diagnosis without waste of resources

If deterioration is due to corrosion —> future evolution of damage (residual life, repair).

Technique to evaluate the corrosion condition of reinforced concrete structure:

Non-destructive analysis

Can help to have a lot of important information without destroying anything.

- Measurement of the concrete cover, magnetic instrument that can detect the presence of the rebar and the position of concrete cover

- Ultrasonic tests, in order to evaluate the quality of concrete with the evaluation of the speed of the sound inside the concrete

- Measurement of the hardness (rebound value) of the concrete surface, this test can also give a general idea about the quality of concrete.

- Resistivity Measurements

- Potential mapping

- Corrosion rate

Concrete cover thickness

We can observe the presence of some numbers (indications of the results of the rests) we

can observe that we can measure that we have different thickness of concrete cover. This

can give an idea about the distribution of concrete cover and this is something that we can

use for the performance based approach. These information are fundamental in order to

predict the durability of the structure. This test is also fundamental to know the position of

the rebars for eventual maintenance.

Rebound hammer (Schmidt) test

An other instrument, inside there is a spring that is used to measure the rebound of the

structure to evaluate the mechanical properties.

This effect is affected by a lot of parameters but it is a very easy test that we can di in

very small time. The results is the schelometric resistance, we can find the correlation with

the compressive resistance in MPa.

We have to consider also the gravitational force if we make the test horizontally.

Measurement of the hardness (rebound value) of the concrete surface

The quality of concrete in 1995 and 1930. This tests give us an indication about the quality

of concrete. We can use this for the deterioration phenomena.

Ultrasonic Testing

Are measurement that use an instrument that create ultrasonic wave and the emitter and

the receiver. We can measure the time that we need to transport the sound from the

emitter to a receiver.

Direct measurement in which we have the emitter in front of the material but we also have

an indirect method.

We have the time and the thickness and we have to calculate the speed. We can consider

to use these datas also to have idea about the porosity because when we have voids

generally we need a long time for the transmission of the wave inside the concrete.

Sclerometer test and this give us a complete idea about the quality of concrete.

We can find also correlation between ultrasonic speed and compressive strength.

Resistivity Measurements

Is an electrochemical measurement, give the information about the

quality of concrete but also the information about the possibility of

corrosion. Resistivity is easy to measure, depend on the porosity and

on the saturation of the concrete.

r depends on concrete type (cement) and on the environment

In carbonated structure we have correlation between resistivity and corrosion of concrete (this is not valid for chloride induced corrosion).

This correlation is valid only if we measure the resistivity only directly in the concrete that is closed to the rebar. Because this measure the

humidity closed to the rebar and not on the surface.

Potential mapping

We have to measure the potential in different way in order to understand the real potential. This technique use

difference electrode on the surface of the material and we need an electrical connection with the rebars. Is not

important if the rebar is closed to the surface of the analysis, because all the rebars are connect together.

If we go in new structure is not so easy to have the connection between the rebars but there is an element

that goes outside. The standard give us the probability. When we measure a corrosion potential that is

<-350mV/m2 respect the cooper surface electrode (CSE) we have the probability of

corrosion that is higher than 90%. Why? Because we can not be sure.

The condition in which we can measure the potential that is lower the protection potential is

the active condition.

If the corrosion potential is higher than -200 the probability of corrosion is lower than 10%.

If potential is high we have in passive behavior.

With chlorine we measure low potential. With dry conditions we can have high potential and also the probability to have corrosion. Because in very

dry condition we have the problem related to the ohmic drop.

When we measure potential the circulation of current between anodic area and cathodic area affected the

measurement, and create problems rekated to the presence of ohmic drop, in the pit we can measure -600

but externally we can measure a value of -300.

Corrosion rate

We need to limit the area in which we perform the polarization.

When we measure the corrosion we have anodic behavior and cathodic behavior, when we reach the

corrosion potential the current goes to zero and we don’t see part if the diagram (we see only the green

one).

I

1 amodic To measure this we have to made polarization and we change a little bit the corrosion potential.

When we calculate this is important to have the dimensions of element in which we change the conditions. In real

cathodic structure when change potential we change the potential of all, and this is a problem. There are systems to reduce

the area in which we change the polarization but this is not so easy.

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Technique to make expection

During the inspection the results are effected by weather conditions, with monitoring we can have all the information about the structure.

Monitoring

In real structure we have rebars embedded in concrete cover, the structure is subject to the penetration

of aggressive pieces. We can make some tests related to the embedding in concrete different prove to help

us to obtain different measurement of different parameters.

Corrosion potential Reference electrode are embedded inside of concrete. The most important is magnesium

dioxide. We embed this closed to the rebars and when we have the penetration of

aggressive pieces we can have drop the potential when we have the initiation of

corrosion. This can give an idea about the initiation period and the period in time when

we have active corrosion. (In the period in which we measure low potential we have active

corrosion). An other problem of reference electrode is that is related to a single bar.

Concrete electrical resistivity For this we have to use soecial prove embedded in concrete that give us information about

the conditions oh humidity that we can have ad different (profondità).

If we embed this prove during construction we have the dry contact and so we have

information about the real resistivity of concrete. If we post pone this test after the

construction, the problem is that if we want to have the contact we need water. In this

way we do not measure only the resistivity of concrete but also the resistivity of the

mortar.

When we are in dry condition we measure an high resistivity.

Corrosion propagation

Info on setting and hardening (curing)

Resistivity used for the analysis of quality of concrete. After casting we have

the increase in time of resistivity because we have a decrease of porosity.

We have an idea about current setting and current curing.

Monitoring - Macrocell sensors Macrocell sensor use corrosion in order to have information. We can embed into

concrete during construction or during a strong maintenance work.

We put different pieces of carbon steel with different concrete cover, they need

external electrical connection. When aggressive pieces reach the first steel

element we have the initiation of corrosion.

We can measure the current in order to detect the presence of corrosion. When

we have a difference in potential between rebar one and rebar two we are sure

that on the rebar one we have corrosion.

We have also an idea about the corrosion condition and the propagation of

corrosion.

Destructive analysis

Take a sample

If we want a real answer about the corrosion we have to do destructive analysis, the problem is that we create

some boule and for some structure is not possible to remove material.