Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

CONDITION MONITORING OF THEREINFORCEMENT’S CORROSION IN

STRUCTURAL ELEMENTS

L. IUREŞ1, C. BOB1, R. CHENDES1, S. DAN1, R. POPA1

Abstract: Knowing the status of the steel reinforcements in existingstructural elements, represents an important issue in building rehabilitationand long-time behaviour. A quick non-destructive method of pin pointingareas, where active corrosion of re-bars is taking place on reinforcedconcrete structures can be done by use of half-cell measurements, using theM.C. Miller Concrete Corrosion Mapping System. The exchange of energywhich is the nature of corrosion in different sections of steel reinforcing, canbe measured by potential. Half-cell potential measurements allow acomprehensive survey of bridge deck/structures to be performed in a relativeshort period of time.Key words: condition monitoring of reinforcement, non-destructivemethod, Half-cell potentiometer.

1 Politehnica University of Timisoara, Faculty of Civil Engineering, Romania

1. Introduction

The periodical condition monitoring ofthe concrete reinforcement corrosion in theexisting structures it is mandatory. Thesuitable consolidation and strengthening ofthe damaged concrete elements it is basedon the non-destructive tests as well asvisual observations made by experts. Theassessments of structural concrete elementsdepends on factors such as: the experienceof the assessor, the time available for theassessment and the data available, anddoes not always reflect the hidden defectsproduced by corrosion, de bonding ofreinforcements due to atmosphericconditions, etc. For these reasons there is aneed for a reliable, safe, accurate and costeffective method of assessing concrete’sreinforcements integrity [7].

Inside a reinforced concrete element, or apre-stressed concrete element, steel

reinforcements are protected against thecorrosion process, by the concrete coatinglayer existent due to the hydration of thecement; that means there exists a denselayer containing calcium hydroxide-Ca(OH)2-a basic compound [2].

This steel bar protecting layer can beaffected by external factors, so that alongthe reinforced concrete bars appears adifferential potential electric which leadsto the possibility of an electrochemicalcorrosion of steel. From the technicalliterature the degradation process of anelement can be described as the initial timeperiod before the corrosion appearstogether with the corrosion time period.In a reinforced or pre-stressed concreteelement a so-called passivated layer isformed on the surface of reinforcing bars;it is a thin but very dense layer of Ca(OH)2 which prevents further corrosion.

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This film may, however, be attached by thesurrounding concrete environment so thatelectrical potential differences maydevelop along the bars: electrochemicalcorrosion will then take place. Theprogression of deterioration of an elementwith time is described by; initial periodand corrosion process period [2].

a) The initial period occurs in twodifferent ways: carbonation of the concretesurrounding the reinforcement andpresence of chloride.

The process of carbonation consists ofseveral stages. First, gaseous, CO2penetrates the surfaces of concrete,generally through the capillary pores.Second the carbon dioxide reacts with thecalcium hydroxide. As a result, thealkalinity of the concrete is lowered andthe passivated can no longer be preserved,so that the corrosion of the reinforcementis then possible. This neutralizationprocess continues as further CO2 enters thepore system.

The mechanism of concrete carbonationis very similar with chloride ingressbecause the main factors of influence arethe same. Main factors influencingcarbonation and chloride ingress are:carbon dioxide and chloride concentration.

b) Corrosion process periodAfter disruption of the passivated layer-

whether by carbonation or by chlorideattack-corrosion of the reinforcing steelmay occur, due to the presence of waterand oxygen.

The corrosion products take up aconsiderably larger volume than theoriginal iron (8 times), which may result infracturing of the concrete by expansivepressure. Parallel cracks with reinforcingsteel bars and spilling of the concrete coverwill take place.

Chloride based salts provide serious risksfor local pit corrosion of the bas when thechlorides reach the reinforcement.

In order to assess the service life of amember or a structure it is necessary to knowthe factors that affect it. Safety and durabilityof reinforced concrete structures are affectedby the corrosion process, which leads tocross section degradation and also affects thesteel-concrete bond [3,4].

2. Reinforcement Corrosion M by Useof Half-cell Potentiometer

Energy differences which appear in crosssection of steel reinforcement because ofthe corrosion process can be quantifiedusing the Half -cell potentiometer. Relativeenergy levels can be determined using abaseline electrode with a stableelectrochemical potential. A highimpedance voltmeter is connected betweenthe baseline reinforcement and thereference electrode located on the testedconcrete surface. The potential recordingsat the voltmeter represents the electrical-energetically levels - the corrosion process- of the steel reinforcement nearbyreference half-cell.

For a better accuracy of the test results,must be considered the following: anelectrical bond with the steel reinforcement(if there are no reinforcements out of theconcrete, at least one must be uncovered).The testing process follows next steps [8],[9]:1. Adding the concentrated CuSO4 on theconcrete surface, with a pre-wetting of themachine sponge and an appropriatepacking with electrical contact solution.2. Tracing a grid on the concrete surfaceand highlighting the hubs where are madethe potential readings.3. Connecting the reference cell to thebaseline reinforcement.4. Attaching the sponge to a network point.5. Measuring a recording of the potentialdifferences between reference cell andreinforcement.

L. IURES et al.: Condition Monitoring of the Reinforcement’s Corossion in Structural... 101

Using the same reference reinforcement,the steps 4 and 5 should be repeated for allthe network hubs.

The tested concrete surface should befree of any insulation or asphalt materialsin all the network hubs where the potentialmeasurements will be made; the concretehas to be dried.

For an accurate determination thenetwork mesh should be done having aneye dimension no greater than 2 m2.The half-cell potentiometer results areinfluenced by oxygen content of thereinforcement. If the out outcome potentialmeasured is greater than 0.20V than thereis no corrosion in the tested steelreinforcement. Possible obtained resultswhen the test it is done:1. Smaller potential (absolute value) than –0.20V: 90% or a bigger probability of thecorrosion absence in the cross section;2. Potentials inside the [– 0.20V, – 0.35V]area: inconclusive;3. Potentials greater (absolute values) than0.35V: 90% or a bigger probability ofcorrosion existence in the cross section;4. Potentials with positive values: nottaken into account because indicates thatthe concrete is not dried enough.

1. Half -cell potentiometer; 2. Referencere-bar; 3. Tested element

Fig. 1. Half -cell potential circuit

2.1. Experimental ControlMiniaturization of ReinforcementCorrosion

Two pre-stressed concrete beams (Fig.2)were tested to determine the corrosionstage of the reinforcement, using the Half -cell potentiometer – M.C. MILLER, madein New Jersey, SUA. The tested beamswere placed in the yard of CivilEngineering Faculty of Timisoara, eachbeams have 15.1 meters length [5], [6].

(a) Half – cell potentiometer

(b) Reference reinforcement

Fig. 2 (a, b). Using a Half -cellpotentiometer

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The chosen network for thedeterminations, in both cases, has the griddimensions of 1.5 meters between the lines(fig. 3 (a), (b)). In the network hubs,

CuSO4 was applied with a bonding role.Each of the two beams has a pre-cleanedreinforced zone, considered as being thereference steel bar.

(a) Beam no.1: pre-stressed concrete

(b) Beam no.2: pre-stressed concrete

Fig. 3 (a, b). Potentials obtained using a Half-cell potentiometer

After the measurements were made, thefollowing conclusions were drawn by theresearch team:

1) The pre-stressed concrete beam no.1,presents reinforcement corrosion to the leftside, on a 5 meters zone.

2) The pre-stressed concrete beam no.2,presents reinforcement corrosion on theentire length, with increased defectstowards beam's edges.

3) The Half -cell potentiometer cansuccessfully be used for:

- identifying the risk of corrosion(process that it is not apparent at thesurface);

- identifying the extent of corrosion(partially observable process at the surface

of the concrete elements);- assessing the integrity of a concrete

structure which may have experiencedcorrosion [10].4) The concrete functions as an electrolyteand the risk of corrosion of thereinforcement in the test region may beempirically related to the measuredpotential difference [10].5) Measurement of the potentials &magnitude of corrosion current flow mayenable assessment of existing & futurecorrosion behaviour to be made [10].

The obtained data are graphicallyrepresented in the Fig. 4.

L. IURES et al.: Condition Monitoring of the Reinforcement’s Corossion in Structural... 103

Fig. 4. Beams corrosion results

3. Conclusions

The main problem of steel reinforcementis their corrosion in time due toenvironmental conditions.

Corrosion of reinforcing steel andother embedded metals is the leadingcause of deterioration in concrete.When steel corrodes, the resulting rustoccupies a greater volume than the steel.This expansion creates tensile stresses inthe concrete, which can eventually causecracking and de-lamination[1]. This leadsto a big economical issue. Non-destructivetest will provide good and accurateinformation for choosing the suitablerepearing method.

Knowing that corrosion it is anelectrochemical process that it is producedby the flow of charges (electrons and ions),that being the case of reinforced concreteelements [1], the Half -cell potentiometertesting method, based on a detailedscanning of different electrical potentialvalues obtained on areas tested on the bothpre-stressed concrete beams tested, lead to

the following observations:1) The testing apparatus consists of half-

cell, electrical junction device, electricalcontact solution, voltmeter and electricallead wires. Electrochemical Half – cellpotentiometer test provides a relativelyquick method of assessing reinforcementcorrosion over a wide area without theneed of wholesale removal of the concretecover.

2) Quantitative measurements are madeso that a structure can be monitored over aperiod of time and deterioration can benoted. Areas of usage include marinestructures, bridge decks, and abutmentsand so on. [11]

3) The collected data reveals the fact thatthe second pre-stressed beam tested has anactive corrosion in its steel reinforcement.

4) The measurements made using theHalf – cell potentiometer can be performedin almost any exterior conditions, in arelative short time and the results obtainedare very accurate if the concrete surface iswell previously wet using CuSO4 solutionas a bond material.

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References

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