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We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime. Concrete repair to en diagnosis, design, principles and practice. Upcoming SlideShare. Like this document? Why not share! Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Full Name Comment goes here. Are you sure you want to Yes No. Be the first to like this. Over the pits layers of rust are formed that separate the pit with low pH values from the concrete with high pH values. Upon removal from the con- crete the steel was cleaned by an acid solution containing corrosion inhibitors to remove the rust products.
After cleaning the steel loss becomes visible. It can be seen that a hole with a diameter of about 3 mm and a depth of about 3 mm has developed. In practice such pits are covered with rust and are only visible after removal of the rust. Usually macrocells with large cathodes and small anodes occur, resulting in high corro- sion rates. As the initial diameter of the stirrup was 8 mm, the corrosion rate must have been higher than 1 mm every year.
From Page, C. Such high corrosion rates can only be explained by the formation of macrocells with localised anodes and large cathodes. However, the corrosion products have not been found close to the anodes, but significantly above the corroding areas in the ground. In the electrical field between the anode and the cathode positive ions migrate toward the cathode, explaining the presence of ruse in the soil above the corroding areas.
One consequence of the formation of macrocells is that high corrosion rates can occur where they are not expected. It could be assumed that the possible corrosion rates are small because the availability of oxygen near the anode is small. However, the reinforcement of the opposite side of the bridge deck is well aerated and will act as a large cathode accelerating corrosion at the upper reinforcement depassivated by chlorides.
The electrical connection between all reinforcing bars is usually given within one concrete structure by the connections between the different layers of the steel reinforcement. When the inspection intervals are quite long, such damages may be undetected until they are in a progressed state. In certain cases it can be necessary to close the structure until a safe use can be stated. Often strengthening measures are required. The weak parts of concrete have to be removed immediately to ensure safety. Furthermore, in such cases the consequences of the defects on the structural safety need to be checked.
Concrete Repair According to the new European Standard EN
Typically, roughly about 20 to 50 cm is affected by this problem. Often it is not possible to remove the entire damaged concrete because the cross section of the column cannot be reduced too much, as it is required to bear the vertical loads from the structure. Therefore, the static reserves have to be proved in the phase of repair design before execution of any works on site. Usually a limited loss of cross section is possible. However, it has to be controlled carefully during removal of the concrete, that the condi- tions are kept. However, in some cases, a temporary additional column has to be installed to ensure safety during the repair works.
When concrete is directly exposed to seawater or close to the sea it is also attacked by chlorides from the seawater or winds carrying chlorides from the seawater. To prevent dam- ages high quality and thickness of the concrete cover are essential. Often additional mea- sures like coatings are used for protection. For older structures, often quality and thickness of the concrete cover are not sufficient, and corrosion problems can be found.
Experience has shown that reinforcement corrosion starts quickly and with high rates, especially when wide cracks reach down to the level of the reinforcement or cross the reinforcement.
Concrete Repair to EN 1504: Diagnosis, Design, Principles and Practice
The most unfavourable situations are when a crack runs through the whole cross section of a concrete structure. All pieces of steel have been connected to ampere meters to allow a measure- ment of the electrical currents flowing along the segmented bar. That means the level of electrical currents indicates the corrosion rate.
To generate a crack in concrete, the concrete beam has been clamped against a steel girder. By threads at the ends of the beam and a steel cylinder in the centre, bending forces have been applied to the concrete, resulting in high tensile stresses on top on the beam in the centre area. The loads have been increased until the concrete cracked with a defined crack width of 0.
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The electrical currents between the steel bars have been continuously measured. After generating the crack a chloride solution has been applied to the crack to induce corrosion. Immediately the electrical currents increased from insignificantly low values to quite high levels. The results clearly demonstrate the presence of a large macrocell.
The sign of the electrical currents shows the direction of the current flow. A positive sign means that more electrons are moving out of the piece of steel than into it; i. The sum of positive and negative currents must be zero. As to be expected, the cathodic reaction rates decrease with the distance from the anode. In a distance of about 30 cm from the crack, a still rather high cathodic action has been measured.
This result means that the reinforcement between the cracks acts as large cathode acceler- ating the corrosion rate of the reinforcement in the area of the crack in concrete. They are also visible on the top side of the deck.
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Water contaminated with chlorides can run through the cracks, causing corrosion of the reinforcement. To evaluate the condition of the reinforcement in the area of the crack a piece of steel has been removed and cleaned from rust by acid treatment. The crack width in concrete is 0. This is significantly higher than usual.
In most cases the calculated crack width limitation of the concrete by the reinforcement is about 0. The age of the structure was roughly 10 years. This example demonstrates again the detrimental effect of cracks in concrete on the cor- rosion behaviour. However, to estimate the corrosion behaviour in the areas of cracks, an individual assessment of the special conditions of every structure is required taking all rel- evant parameters from the materials, construction, and environment into account.
If no information on the history of the crack is available, carbonation depth and chloride content of the concrete within the cracks should be checked to enable evaluation of the corrosion risk. This can lead to a local reduction of the pH value where the alkali ions are transported away with the water. Due to water flowing through the crack for many years, corrosion has been initiated in the area of the crack and proceeded so far that most of the reinforcing bars have been torn apart after about 40 a. The chloride contents in the con- crete are so low that chlorides can be excluded as a reason for depassivation.
Photo taken by Consulting Engineer, Raupach, Bruns. If the pH value is reduced so much that corrosion can start, the colour will not turn to pink, which would indicate high pH values as for young concrete, but no colour change will be visible. Typical sources for electrical currents in the ground are electrical railroads or trams, electricity cables, and facilities for welding or electrolysis. However, to prevent problems with stray currents, electrical installations have to be installed in a way that the amount of possible stray currents is limited.
Nevertheless, if high stray currents are present in the ground, they can induce corrosion of the reinforcement of concrete structures within the electrical field. The area where the current enters into the metal is cathodically protected, but the area where it runs out of the metal and back to the ground is anodically loaded and subject to corrosion.
If corrosion problems at the reinforcement in the ground are difficult to explain and when strong AC or DC sources are present, it should be checked whether stray currents may be the reason for reinforcement corrosion. However, this requires special instruments and knowl- edge and should only be executed by experts.
These corrosion mechanisms are influenced not only by the environmental conditions, but also by the level of tensile stresses within the steel. A detailed description of the mechanisms would go beyond the constraints of this book. The quality of the grout or mortar around the prestressing steel plays an important role for the risk of stress corrosion cracking. To assess this risk, usually the prestressing steels are opened, e. After opening of the ducts the degree of compaction of the mortar can be seen and evaluated. Usually the mortars are well compacted, but also areas may be found where the compaction is bad.
In the latter case rust is already visible. Due to the uncontrollable risk of sudden failures by stress corrosion cracking, the structure had been demolished. The assessment of the condition regard- ing the risk of stress corrosion cracking shall only be carried out by experienced experts taking the individual conditions of the structure into account, e.
Chapter 3 How to assess the status of a structure 3.
The decision to what extent the assess- ment has to be carried out will usually be made on the basis of a preliminary visual inspec- tion, as well as the experience of the engineer.