Concrete structure

Chloride ions are considered to be the major cause of premature corrosion of steel reinforcement. Chloride ions are common in nature and small amounts are usually unintentionally contained in the mix ingredients of concrete. 

Chloride ions also may be intentionally added, most often as a constituent of accelerating admixtures. Dissolved chloride ions also may penetrate unprotected hardened concrete in structures exposed to marine environments or to deicing salts. 

The corrosion rate of steel reinforcement embedded in concrete is strongly influenced by environmental factors. Both oxygen and moisture must be present if electrochemical corrosion is to occur. 

Reinforced concrete with significant gradients in chloride ion content is vulnerable to macrocell corrosion, especially if subjected to cycles of wetting and drying. 

Other factors that affect the rate and level of corrosion are heterogeneities in the concrete and the steel, pH of the concrete pore water, carbonation of the Portland cement paste, cracks in the concrete, stray currents and galvanic effects due to contact between dissimilar metals. 

In planning cathodic protection, the specific resistivity of the water, the size of the surfaces to be protected and the required protection current densities have to be determined. The protection current density depends on the type and quality of the coating. Thermosetting resins (e.g., tar-epoxy resin coatings) are particularly effective and are mostly used today on coastal structures. They are chemically 

The protection current densities for structures near the sea can amount to 60 to 100 m A for uncoated surfaces in the area in contact with water and 20% of that for parts driven into the soil. The land sides of retaining walls take so little current that they do not have to be taken into account in the calculation. With coated objects, the protection current density lies between 5 and 20 mA depending on the quality of the coating. About half this value must be expected for the part in die soil because either the coating is absent or is damaged by the driving. 

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Nesvijski, E.G., Nogin, S.I. Acoustic Emission Technics for Nondestructive Evaluation of Stress of Concrete and Reinforced Concrete Structures and Materials. Third Conference on Nondestructive Evaluation of Civil Structures and Materials, Boulder, CO, 1996. Nesvijski, E. G. Failure Forecast and the Acoustic Emission Silence Effect in Concrete. ASNT s Spring Conference, Houston, TX, 1997. 

The ultrasonic tomograph A1230 was developed to visualize in case of one side access the internal reinforced concrete structure at the depth of 1 m. This device uses 36-elements matrix array. 

A Practical Approach to Inspection of Concrete Structures using High Energy Radiography and other Advanced NDE-Methods. 

The failure of a concrete structure is of course not confined to catastrophic collapse. A concrete structure has failed or reached the end of its serviceability life when it is no longer capable of fulfilling its design functions, e.g. leak-tightness or as a barrier against deleterious elements which may cause corrosion. 

An experienced inspection engineer will attempt to identify the characteristics of a given structure to determine potential. specific critical damage mechanisms. No individual major concrete structure can be adequately analysed by simple mass-accumulation of data and using criteria, which are based on standard codes. This does not mean that either data-collection or available criteria are not useful, but they should be exercised with care and flexibility and the procedures for inspection customised for the given structure. 

The most challenging of these applications has been the location and characterisation of anomalies in thick concrete structures using seismic methods and the detection of reinforcing steel and pre-stressing cables in congested structures using radar. 

Lambert,P Predicting and achieving serviceability, Proceedings of International seminar Management of Concrete Structures for Long Term Serviceability University of Sheffield, 1997... 

Basic construction Wood, concrete, structural steel UKE50-500 USS75-750... 

Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than lO m ... 

Steel constructions and pipelines must either be electrically connected to the reinforcement of reinforced concrete structures or electrically separated. If they are connected, a current density of about 5 mA m should be applied to the external reinforcement and calculated on the total area of the concrete surface. 

Very often steel sheet pilings exist in conjunction with steel-reinforced concrete structures in harbors or locks. If cathodic protection is not necessary for the reinforced concrete structure, there is no hindrance to the ingress of the protection current due to the connection with the steel surfaces to be protected. The concrete surface has to be partly considered at the design stage. An example is the base of the ferry harbor at Puttgarden, which consists of reinforced concrete and is electrically connected to the uncoated steel sheet piling. 

Harbor structures are very accessible and can be investigated without the effects of wave motion. Grounding of steel pilings presents no problems and the work can be carried out from the quay (see the left-hand side of Fig. 16-13). With steel-reinforced concrete structures, measurements have to be made from a boat if no reliable contact has been provided in their eonstruction (see the right-hand side of Fig. 16-13). 

Cathodic Protection of Reinforcing Steel in Concrete Structures... 

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

The passivating action of an aqueous solution within porous concrete can be changed by various factors (see Section 5.3.2). The passive film can be destroyed by penetration of chloride ions to the reinforcing steel if a critical concentration of ions is reached. In damp concrete, local corrosion can occur even in the presence of the alkaline water absorbed in the porous concrete (see Section 2.3.2). The Cl content is limited to 0.4% of the cement mass in steel-concrete structures [6] and to 0.2% in prestressed concrete structures [7]. 

In practice, the current densities for protecting concrete structures are generally lower than the values in Table 19-1. The reason is that the cathode surfaces are not well aerated and areas of the anodes are dry. Practical experience and still-incomplete investigations [43] indicate that at even more positive potentials than those given in Table 19-1 with U = -0.35 V, noticeable protection can be achieved so that = -0.4 V can be regarded as the protection potential. In DIN 30676, t/jj5 = -0.43 V is given [44] (see also Section 2.4). 

Since cathodic protection of concrete structures in the United States has been very much advanced, protection criteria have been developed [46]. They correspond to the pragmatic criteria Nos. 3 and 4 in Table 3-3 (see Section 3.3.3.1). It is assumed that the protective effect is adequate if, upon switching off the protection current, the potential becomes more than 0.1 V more positive within 4 hours. The measurements are carried out in various parts of the protected object with built-in Ag-AgCl reference electrodes or with any electrodes on the external surface. 

The decision to cathodically protect reinforced concrete structures depends on technical and economic considerations. Cathodic protection is not an economic process for small area displacements of the concrete due to corrosion of the reinforcing steel arising from insufficient concrete covering. On the other hand, the... 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

There are different concrete replacement systems available for renovating reinforced concrete structures. They range from sprayed concrete without polymer additions to systems containing conducting polymers (PCC-mortar). Since with the latter alkalinity is lower, more rapid carbonization occurs on weathering [59] and the increased electrical resistivity has to be taken into account, so that with cathodic protection only sprayed concrete should be used as a repair mortar. 

Cathodic Protection of Reinforcing Steel in Concrete Structures, Proposed NACE-Standard, Committee T-3K-2 NACE, Houston 1985. 

Cathodic protection can be used as a renovation measure for steel-reinforced concrete structures (see Chapter 19). Although material costs of from 100 DM m" (particularly with preparation, erection, and spray coating costs) up to 300 DM m are quite high, they do not compare with the costs of demolition or partial replacement. ... 

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