WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE

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1 Dramix WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE Stray Currents: Definition (1) Stray currents arise from rail transit systems, cathodic protection systems, or high-voltage power lines. Stray currents may be DC or AC depending upon the source. Stray DCs are known to be much more dangerous than stray ACs. In case of DC, a cathodic reaction (e.g. oxygen reduction or hydrogen evolution) takes place, where the current enters the structure (or, reinforcement), while an anodic reaction takes place (e.g. metal dissolution), where the current returns to the original path, through the soil (or, concrete). Effects of stray ACs are more complex. It has been shown that AC can influence the anodic behaviour of steel and, thus, may increase the corrosion rate of steel as well as galvanic effects. Stray Currents and RC (1) Stray currents can also flow through reinforced or prestressed concrete and produce an alteration of the electric field inside the concrete, which can influence the corrosion of the embedded steel. Several types of concrete structures may be subjected to stray currents, such as bridges and tunnels of the railway networks or structures placed in the neighbourhoods. The concrete is the electrolyte and the rebars or prestressing strands can pick up the stray current. AC stray currents are known to be much less dangerous then DC stray currents. For steel embedded in concrete, it was observed that current densities up to 50 A/m2 applied for 5 months to passive steel in concrete with up to 0,4 % chlorides did not lead to corrosion initiation. Hence, AC stray currents are not considered to be critical for SFRC durability and excluded from the further discussion in the present document. Henceforth, stray currents always refers to DC stray currents. INFO SHEET WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE 1

2 Corrosion Resistance of SFRC Fibres Compared to RC Rebar (2) The improved resistance against chloride ingress and carbonation of SFRC compared to RC is well understood. Investigations have shown that the corrosion resistance of SFRC fibres is considerably higher than the one of RC rebar based on identical exposure conditions as well as materials and compositions of concrete and steel under practical conditions, corrosion of fibres in uncracked SFRC is generally limited to the fibres protruding from the surface or, in case of carbonation, to the fibres in the associated layers close to the surface. Several research investigations have shown that the durability of steel fibre reinforced concrete (SFRC) under chloride exposure is superior to the one of steel bar reinforced concrete (RC). Among others, it has been demonstrated that the chloride threshold of fibres in SFRC is 5-10 times higher than the one of rebars in RC. Under practical conditions, corrosion in SFRC is generally limited to fibres protruding from the surface or to the surface layer that may be affected by leaching or carbonation; rust stains on the surface may occur, but chloride induced corrosion within a SFRC member is highly unlikely. Furthermore, even in the highly unlikely event of corrosion of internal fibres, spalling and cracking due to the formation of voluminous corrosion products (a common durability issue for RC) cannot take place for SFRC, because the individual cross-sections of the fibres are limited. TEST REPORT Determination of resistivity of Steel fibre Reinforced concrete (3) Electrical properties of concrete are of concern in specific applications such as cathodic protection of reinforced concrete, conductive concrete for bridge deck deicing, protection from stray currents in electrically powered transit systems and accommodation of electric equipment. This concrete can be reinforced with steel bars, but also with steel fibres. This type of reinforcement is widely used in concrete to improve specific mechanical properties of concrete. INFO SHEET WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE 2

3 Bekaert has the intention to deliver Dramix steel fibres to producers of prefab concrete low volt-age substations. One important issue is the influence on the electrical resistivity of the concrete because of safety. Therefore, Bekaert has requested KEMA to test the electrical resistivity of this fibre reinforced concrete. The objective of this study is to measure the electrical resistivity of steel fibre reinforced concrete samples with a composition specified by Bekaert and to quantify the effect of these fibres on electrical resistivity by comparing with samples without steel fibres. Fig 1 test method set up From the test results the following trends are seen: 1. The dry specimens with steel fibres have a significant lower resistivity the the dry specimen without steel fibres. 2. The wet specimens with steel fibres have a lower resistivity than the wet specimen without steel fibres. 3. All specimens which are measured under wet conditions have a significant lower resistivity than the dry specimens. 4. The resistivity increases in the same specimens which are dried an extra 30 days at 60 C. INFO SHEET WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE 3

4 Current State of the art for SFRC Based on the literature, we consider that there is no risk of stray current induced corrosion in non-carbonated and chloride-free concrete. Contrary to the usually long, orientated, and electrically interconnected layout of RC rebar, SFRC fibres are small sized and not interconnected. Under certain circumstances (structural detailing and exposure conditions) RC rebar might be in a situation that enables a sufficient increase in potential of the anodic site to allow current flow However, this situation is very unlikely to occur for fibres due to their limited size and missing interconnectivity. Furthermore, some of the additional aspects that grant improved resistance towards chlorideinduced corrosion of SRFC fibres are considered to increase resistance to stray currents as well. Consequently, stray currents, being unlikely to initiate corrosion for RC rebars in chloride-free and non-carbonated RC, will most likely not affect durability of chloride-free and non-carbonated SFRC. Stray currents have more serious consequences in chloride contaminated concrete. A similar, but much less emphasized effect is considered for SFRC fibres and the chloride content must generally be at a considerably higher level to initiate corrosion of SFRC fibres compared to RC rebar. The chloride content adjacent to a fibre must be much closer to the actual chloride threshold of the fibre than for RC rebar to initiate corrosion by stray currents. That means that the relative chloride content to chloride threshold must be considerably greater for SFRC fibres than for RC rebar for stray current to initiate corrosion. INFO SHEET WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE 4

5 Keep in mind => The relative chloride content to chloride threshold must be considerably greater for SFRC fibres than for RC rebar for stray current to initiate corrosion. => Several research investigations have shown that the durability of steel fibre reinforced concrete (SFRC) under chloride exposure is superior to the one of steel bar reinforced concrete (RC) (1) COWI Durability of SFRC Stray Currents (2) The consultant s view on service life design Carola Edvardsen COWI A/S (3) Determination of the resistivity of SFRC Kema report Octobre 2010 INFO SHEET WHAT ABOUT STRAY CURRENTS WITH STEEL FIBRE REINFORCED CONCRETE 5