REPAIR OF CONCRETE STRUCTURES CASE STUDY No 1

UNDERGROUND PARKING SLAB

Redacted by: NÉSTOR TORREGO JON GARCÍA (ERASMUS STUDENTS)

CIVIL ENGINEERING

REPAIR OF CONCRETE STRUCTURES Case No1: Underground parking slab Page 1

INDEX 1. INTRODUCTION. .................................................................................................................... 2

2. ANALISYS POSIBLE CAUSE OF THE CRACKS: .......................................................................... 3

3. CHLORIDE IONS CORROSION .............................................................................................. 5

3.1. CHLORIDE CORROSION ANALYSIS ................................................................................. 6

3.1.1. Study of possible source of chloride ions. ............................................................. 7

3.1.2. Determination of presence of chloride ion and measurement ............................ 7

4. REPAIR METHODS ................................................................................................................. 9

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1. INTRODUCTION. Project Phases in the Process of Concrete Repair and Protection 1. Information about the structure:

At the beginning of a project carried out a survey to collect information about the

structure. This includes:

• General condition and history

• Documentation such as calculations, drawings and specifications, etc.

• Repair and Maintenance Program

• This information will provide data to understand the current state of the structure.

2. Evaluation Process:

There must be a thorough study for visible and not visible damage of the structure, and

identify, if possible, the root causes of such damage. This is used to assess the ability of

the structure to perform its function.

The study and evaluation will be undertaken by qualified and experienced staff. Failure to carry out repairs to the concrete structure, a qualified engineer can make an estimate of the remaining service life of the structure. The objective of a study is to identify defects concrete:

• Types of defects in concrete

- Mechanical

- Chemicals

- Physical

• Defects in the concrete due to reinforcement corrosion

3. Design Repair Work:

The principles of redress and protection will be defined from EN 1504 and repair options

management strategy.

4. Repair Work:

Select the most appropriate principle of EN 1504, the most appropriate repair methods

depend on:

• Access to the workplace

• Conditions of the working area

• Safety and health in the workplace

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2. ANALISYS POSIBLE CAUSE OF THE CRACKS: It has been determined that the circular cracks observed in the slab are produced by action of the temperature gradient. In the concrete impact the temperature gradient which occurs in the Concrete by its low conductivity. Solution: A good cured of concrete. Other thermal effects:

• Strong temperature variations.

• Point sources of heat such as fireplaces or furnaces.

• Thrusts produced by freezing water, etc.

Its symptoms are very similar to the retraction, which sometimes very difficult to distinguish. This process happens because the concrete used for the construction of columns is rigid because they need to sustain all the loads of the structure, however, the concrete used in the construction of the slab is flexible because it is used to distribute the load to the abutments and is the area in which the cars will move. The tensions produced by the union of the slab to the column are bigger than the maximum crack tension due to this appears cracks on the slab. The shrinkage caused by the concrete of the columns produced by the concrete slab is different due to the stiffness of the columns and the flexibility of the slab, this process can also create cracks in the concrete. We can see that the cracks are wet so the hydro isolation process has not been successful because this requirement has not been fulfilled. If the draining process is completed successfully cracks were produced by the process temperature gradient had been less affected by the action of water. When we are in a cold environment the action that can produce water to penetrate a crack is more dangerous because it freezes and expands, making the crack bigger. If the water freezes and thaws inside the crack can produce a wear and this make the crack bigger than the beginning. Different types of cracks: Strain in the concrete mass that manifests on the outside as a line. We can classify the cracks through two ways:

1. Depending on the size:

• Micro cracks (e <0.05mm): Overall unimportant.

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• Cracks (0.1 <e <0.2mm): In general they are very dangerous.

• Macro cracks (e> 0.2mm): may have important structural implications.

2. According to their behaviour:

• Active cracks: if they move, opening or closing.

• Inactive cracks: if you are already stabilized in its final state.

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3. CHLORIDE IONS CORROSION The aggression that chloride ion (Cl-) represents for the steel is known, and we know perfectly the risks that the oxidation of the reinforced steel carries. The steel that is in the concrete also can be exposed to oxidation, but the concrete cover gives it protection. The strongly basic pH of the concrete (approx. 12,5) causes that a protective layer is formed on the reinforcement, that is called passivating layer, and these imply that even with oxygen and wetness the steel doesn’t oxidize. The chloride ions are able to break this passivating layer and remove the steel protection, because they formed a soluble compound, the ferrous chloride (FeCl1), with the iron oxides of the protective layer. When the reinforcement hasn’t this layer, the steel is susceptible to be corroded if the necessaries elements are present. The chloride ions have different forms to enter in the concrete:

The chloride ions entered when the concrete mix was made. In this case, the risk is bigger but it is easier to be controlled if we are more careful and have a perfect control of mix components. The most common mistakes are that the chloride ions are in the water, in the fine aggregate, or in the additives.

The chloride ions entered during the service life. The chloride ions entered slowly from outside and when they reach the reinforcement, the process of corrosion start. The fall of the ions, is linked with the transport of fluid in a porous environment. These are capillary suction, diffusion, permeability and a combination between suction and permeability.

Corrosion process associated with chloride ions. The corrosion is an electrochemical process in which a steel section works like an anode and other random like a cathode. When the chloride ions break the passive layer, the oxide of ion occurs in the anode, and the reduction of oxygen in the cathode. The wet concrete is the environment in which de ions Fe2+ and OH- move to form Fe(OH)2, that has this brown characteristic colour. The free electrons move in the metal.

Fe(OH)2

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Analysis of potential risk of chloride corrosion. The potential risk of corrosion with chloride ions depends of a lot of factors, but the standards have established a maximum value of chloride concentration in the concrete. How the cement is the protector agent of reinforcements, because it gives the strong basic pH that does the passive layer on the steel, the maximum value of chloride concentration is voiced in percent of cement weight. According to the standards the chloride concentration mustn’t be bigger than 0.4g / 100g of cement, although this value must be only indicative.

3.1. CHLORIDE CORROSION ANALYSIS It’s important make a good report concerning the repair of concrete corroded by the effect of chloride ions. To do this we first try to find out what the source of ions. Then do an inspection of the penetration of chlorides in the concrete, in order to know the level of concentration which has. In this way we can study the following concepts are important:

- Time chloride ions take to reach the steel, so we must find the depth at which the chloride ions have penetrated at the time of the inspection. In well-designed case, the coating must be greater than the depth that the chlorides have at the end of its shelf life.

- The speed of propagation of chloride ions through the concrete. It is important to know the shelf life that our structure has.

Lifetime calculation of a reinforcement concrete which has suffered corrosion by chloride ion. If we can establish the penetration outline of chloride ion, and we suppose that the exposure condition won’t be very different from actual, is possible to calculate the time when the process of corrosion starts. For that we use the next graphic.

We can establish the next equation system:

( ) √ ( )

√

For example: if we did a control study in year 10 (t1=10), and we found the critical threshold at a depth of 18mm (X1(10years) = 18mm) and the structure has a concrete cover of 25mm (X2 = 25mm). We can obtain the time when the critical threshold will be at a depth similar that a cover.

( ⁄ ) ( ⁄ )

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3.1.1. Study of possible source of chloride ions. In this case, the structure is in a country where winter temperatures are very low, and for the safety of road we use salts that content chlorides. This salts whit chloride reach to our structure on the car wheels, and they accumulate on the ground and slowly penetrate in the concrete. Our structure had cracks, and they do the concentration and the penetration of the chlorides easier.

3.1.2. Determination of presence of chloride ion and measurement When we are clear that there are factors that favour and allow the presence of chloride ions, we proceed to perform the measurements and analyse results, and in this way can choose the best solution for repair. Next is explained one method that we can use to measure the chloride contents in the concrete, but there are some more. The chloride contents are studied on concrete samples of 20mg which we can obtain with a drill. Is recommended, in order to obtain a correct profile of chloride, catch samples of different depths. These samples are taken to the laboratory where they are crushed and treated in order to obtain the global chloride content. The laboratory process according to standard is the next. Laboratory process to obtain the chloride concentration We catch some different depths samples of 20mg each one. For each one, we use the following process.

Crush the sample until it can pass through sieve #20. Make it homogenous and only work with on 10mg. Dilute it in 75mg of water. Add 25mg of nitric acid. Add the indicator of orange methyl and remove. Cover it and let stand during 2-3 minutes. Heat until ebullition and leave it until it become cold. Rate it by potentiometry with silver nitrate.

And by this way we obtain the values of chloride ions measurements.

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Now, we can do the outlines of chloride measurements for each crack tested.

As we can see, there are cracks that surpass the maximum value at the steel depth. In these cases there may be corrosion in the steel, it appears if the chlorides ions was be able to break the passive layer of the steel. The corrosion caused by chloride produces bites in the steel, and theses bites may be able to completely destroy the corroded steel bar. To test if the passive layer has been broken, we can the method of overage cell voltage measurement. With this method we can test the level of corrosion and the places where it is.

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4. REPAIR METHODS First of all we are going to focus on the repair of the areas that have exceeded the maximum limit of chlorides can have. We believe the best option to repair these areas is the removal of the concrete by pneumatic hammer. The pneumatic hammers are difficult operation but the cost is low and they are very versatile. They can cut behind and between the steel bars. Once we have removed the concrete must decide which is the right method for repairing the steel bars that are affected by corrosion. We have come to the conclusion that the best method would be the cathode protection. The cathode protection is considered the only true method of corrosion control to allow the structure to behave as cathode reaching slow the corrosion. There are two ways to perform cathode protection:

Impressed current

Sacrificial anodes

We have determined that this structure is better impressed current. This method works by passing a small direct current from a permanent anode positioned within the concrete. Hydroxyl ions appear to chemical equations increase alkalinity and help regenerate the passive layer where chlorides have attacked. When we have made this method we put the new concrete. After the repair of the areas where they have exceeded the permitted levels of chlorides must analyze areas where chlorides are present but have not exceeded the maximum value. We have made a scheme where we can see the different option: ESQUEMA After seeing the last scheme we have decided that the best option to rehabilitate these areas is the electrochemical extraction of chlorides. We have taken this solution because the slab has a useful life of 50 years, found active steel and steel is continuous so we take that solution. Electrochemical extraction of chlorides is a method to repel the ion chloride from the steel surface and move towards an external anode. This process uses a temporary anode and an electric charge. Chloride ions, having negative charge, migrate in the field formed by the electric current from the steel to the concrete surface where we located outside anode. As a result of chemical reactions produced hydrogen ions are chloride ions with. In this method, the electric field is involved because the chloride ion is displaced by it. When we have reduced the concentration of chloride at low levels removed the external anode. After making all the methods for the extraction of chlorides and have reached low levels must repair cracks in concrete with patches. Once we have repaired the cracks and have extracted all the chlorides that damaged the structure must choose a method of prevention so that in the future we do not find these problems. For prevention we will differentiate between areas that have had to replace the concrete and we fixed with patches. For areas that have had to replace the concrete we believe that the steel rods should be coated with anti-corrosive elements.

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This coating should be done with elements against chlorides and penetrating sealers, these sealants have been used to stop the penetration of chloride creates a hydrophobic layer that stops water penetration. For areas that we have repaired the cracks with patches we believe the best solution is the waterproofing of concrete because the first defence against corrosion is that the concrete has a good quality and a sufficient coating. For waterproofing of concrete we use membranes that are generally a cover, a spray or a liquid applied to the new ant surface, in this case we think a good solution would be that of a cover over the concrete. These membranes have a useful life of 10 to 15 years and then must be replaced.