Diagnostics of Corrosion on a Real Bridge Structure

Real bridge structures are affected by environmental conditions.The environmental loads in time cause the degradation of concrete and reinforcement. The diagnostics of real state of existing bridges are very important due to actual degradation and corrosion. In the frame of research activities of Department of Structures and Bridges, Civil Engineering Faculty, University of Žilina, the real bridge structure was observed for a few years. It is girder reinforced concrete bridge near town of Žilina in Slovakia. The results of diagnostics which focused on reinforcement corrosion are presented. The paper deals with reinforcement corrosion and its influence on the moment resistance of the existing concrete structures.


Introduction
Roadway bridges together with other roadway structures, such as tunnels, are the most critical components of road infrastructures not only in Slovakia but also in all European countries.Throughout their life, they require regular maintenance actions whose costs are generally supported by the operators and owners.Accordingly, it becomes important to define strategies to maximize the society benefits, derived from the investment made in these assets, within a limited budget.This investment should be planned, effectively managed, and technically supported by appropriate management systems [1].To do this, the COST Action TU1406 entitled "Quality Specifications for Roadway Bridges, Standardization at a European Level (BridgeSpec)" was established [1].The main objective of this COST Action is to develop a guideline for the establishment of Quality Control (QC) plans in roadway bridges, by integrating the most recent knowledge on performance assessment procedures with the adoption of specific goals.This guideline will focus on bridge maintenance and life-cycle performance at two levels: (i) performance indicators and (ii) performance goals.By developing new approaches to quantify and assess the bridge performance, as well as quality specifications to assure an expected performance level, bridge management strategies will be significantly improved, enhancing asset management of ageing structures in Europe.
To fulfill the main objective of the project, it is needed to collect data about codes, standards, and provisions concerning the inspections, evaluation, environment loads and conditions [2,3], degradation processes [4][5][6], reliability [7][8][9], fatigue [10,11], monitoring, maintenance, and diagnostic [12][13][14] of road bridges in all countries included in the project.The Department of Structures and Bridges, University of Žilina, was included into the project to represent Slovakia.
Knowledge of real state of existing bridges, their loadcarrying capacity, and failures are very important parts of life-cycle performance [15][16][17].In the decision-making procedure, they help decide what to do with the existing bridge structure: use only maintenance, repair, reconstruct, and strengthen the bridge, or replace with new one [18][19][20].So, the diagnostic of bridges has significant influence on the bridge future.

Bridge Diagnostic Performed in 2005
In the frame of research activities of our department, numbers of bridges were diagnosed.In the paper, we focus on one bridge structure with denotation MO 244 in the village Kolárovice, part Škoruby, on the road I/10 (international road E442) between Bytča city and Makov village, over the Kolárovice river (Figure 1), which was for the first time diagnosed on 17 July 2005 in the frame of project APVT-20-012204 [21,22].Next diagnostics were performed on 13 November 2015 (after 10 years).The bridge was built and put into operation in 1937, so the age of the bridge is 79 years.It means that according to Eurocodes the remaining lifetime should be 21 years.
The bridge is the reinforced concrete single span beam bridge with theoretical span of 10.006 m (the length of the bridge superstructure is 10.824 m).The width of road is 7.51 m and the overall width of bridge is 9.51 m.The bridge skewness is 45.22 ∘ .The superstructure consists of bridge slab having thickness of 0.19 m and six main beams with dimensions of 0.325/0.84m.End floor beams with dimensions of 0.58/0.84m and three intermediate floor beams of dimensions of 0.20/0.74m ensure the transverse load distribution (Figure 2).Denotation of beams from B1 to B6 is from the left-hand side to the right-hand side.

Results from Diagnostics Performed in 2005
From the results of the bridge diagnostic it follows that the type of the concrete is C30/37 using nondestructive method (Schmidt hammer) and the beams are reinforced by rebar of type A (10210) (smooth reinforcement without ribs) in two layers (5A30 in the lower layer and 2A30 in the upper layer); see Figure 3.The measured values of geometric and material properties, concerning all the girders and slab, are shown in Table 1.
Accordingly, the reinforcement corrosion was indicated.The corrosion caused the rebar diameter loss from the assumed initial value of 30 mm to actual average value of 29.37 mm (the minimal measured value is 28.7 mm).Change of bar diameter was measured on reinforcement R1 of spandrel beam B1 (Figure 3).From visual inspection it can be seen that the insulation under asphalt is functional because the wet places and chloride dripstones between beams B2 and B5 were not found.The problem was on the edges between gadroons (cornices) and asphalt and behind edges of bridge, where wet places, chloride curtains, and corroded reinforcement were found, as well as the degraded concrete cover, which has dropped out at some places (Figure 1).

Results from Diagnostic Performed in 2015
The second diagnostics of the bridge were performed after 10.33 years (10 years and 4 months) on 13 November 2015 and were focused only on reinforcement corrosion.It means that only change of reinforcement diameter () due to corrosion was measured.At that time, reinforcement corrosion was measured on various bars of both spandrel beams (B1 and B6).Moreover, on both beams, change of stirrups diameter   () due to corrosion was also measured.The statistical characteristics of measures diameters are shown in Table 2.
Very important knowledge is that the bridge was in the worse state in comparison to the state in year 2005, reinforcement corrosion was larger, and the area of dropped out concrete cover was also bigger.Moreover, the concrete cover and the concrete layer between the reinforcements were expanding; thus it was possible to find another 3 bars inside the beam in the second layer.It means that the final number of reinforcement bars is 10, not only 7 (Figure 4).

Analysis of Results Obtained from Diagnostics
The histograms from measured values of bars' diameters are shown in Figures 5 and 6.For comparison, also histograms  from the first measurement (diagnostics) of reinforcement B1-R1 are shown.
In the case of geometric parameters of members and cross sections, Eurocode EN 1990 [23] recommends using normal distribution of a random variable.The probability of occurrence and the density functions of normal distribution of a random variable of reinforcement diameter are also shown in Figures 7 and 8.The mean values and the standard deviations given in Tables 1 and 2 were used.
Initial measurement of reinforcement diameter was not available due to irrelevant measurements at the start of the bridge lifetime.So, the measurements were done at places, where the minimal or no corrosion of bars was assumed.From this reason, the reinforcements' diameters R1 and R2 of the beam B2 (B2-R1 and B2-R2) and reinforcement R1 of the beam B3 (B3-R1) were also measured.Due to previous measurements, it was assumed that the initial reinforcement diameter was about 30 mm.The results of measurements are shown in Table 3 and Figure 9.
The diagnostics indicate significant influence of environmental loads on material degradation in the form of reinforcement corrosion.In 2005, the corrosion of longitudinal reinforcement was indicated only on beam B1 due to dropped concrete cover.Only the surface corrosion of stirrups and dropping out of stirrups concrete cover were indicated on beam B6.Due to 10 years of environment influence on reinforcement corrosion of bar B1-R1, the mean value of diameter changed from   = 29.37 mm to   = 26.68 mm (Figure 10).

Influence of Corrosion on the Beam Resistance
It is very important to take into account the reinforcement corrosion influence on change of the moment resistance of the beam in time  (Ultimate Limit States).The reinforcement diameter () change in time may be described according to Thoft-Christensen [24]:  The value of  corr = 260.4m/year is great, but it represents the corrosion of free reinforcement without any protection: reinforcement B1-R1 was cleaned from rust during the    first diagnostic and it was not guaranteed against corrosion with the new concrete cover (using repair mortar).The corrosion is due to chloride ions from the defrosting salt.
Corrosion rate  corr of other reinforcements was not possible to calculate because we had only one measurement.Measurement of diameter of reinforcement B2-R2 and B3-R1 shows that there is not corrosion or corrosion is very low and can be negligible: diameter is very close to assumed value of 30 mm.In the case of B2-R1, the small corrosion was found.
Using the reversed calculation, it is possible to calculate the length of the passive stage  0 = 65.6 years provided that  corr = 260.4m/year.Of course, the corrosion of free reinforcement (without concrete cover) and corrosion of protected reinforcement in concrete (under the concrete cover) are different, but the initial measurements were not known.
At the same time, using the obtained value of  corr , it was possible to calculate the moment resistance of the beam and how it changed with time during the remaining lifetime:   = 21 years.
The resistance () of the bending concrete element is given by formula, derived in [25] and based on Eurocodes [26]: where   is the concrete compressive strength [N⋅mm  The influence of the corrosion on changing the moment resistance of the RC flanged bridge beam in time is shown in Figure 11.
The basic value of the moment resistance at time  = 0 year is equal to  Rd ( = 0) = 977.391kNm.Hypothetically, the decrease of moment resistance during the remaining lifetime can be 49.9%.

Conclusions
The measurements of reinforcement corrosion on real reinforced concrete girder bridge are presented in this paper.Two diagnostics within the 10 years were performed.From the results it follows that the reinforcements in corners of the beams' cross sections are more incoherent due to degradation (corrosion) because the degradation factors enter into the beam not only from the bottom part but also from the side.Measurements indicate that the spandrel beams are mainly affected by chloride ions Cl − penetration and cause large degradation of concrete cover, corrosion of reinforcement, and consecutive concrete cover dropping out.Internal beams are affected by carbonization (ingress of CO 2 into concrete) if the insulation is functional.The importance of diagnostics shows the fact that not all the reinforcements may be found in cross section without using quality destructive or nondestructive diagnostic methods.
In addition, the influence of reinforcement corrosion on the moment resistance  Rd () in time of the concrete bridge element subjected to bending is presented.From the results it may be seen that the moment resistance may decrease about 49.9% during remaining lifetime.It is not a negligible value and it points to a huge influence of environmental loads on degradation of materials.However, it is necessary to mention that the value of the corrosion rate  corr was calculated for the free reinforcement without the concrete cover.

Figure 3 :Figure 4 :
Figure 3: Bridge cross section: identified number of bars after the first diagnostics in 2005.

Figure 7 :
Figure 7: Probability of occurrence and density functions of bars' diameters: longitudinal reinforcements.

Figure 8 :
Figure 8: Probability of occurrence and density functions of bars' diameters: stirrups.

MFigure 11 :
Figure 11: Change of moment resistance  Rd () in time of RC bridge girder due to the reinforcement corrosion.

Table 1 :
Measured values of geometric and material characteristics.

Table 2 :
Measured values of reinforcement diameters.
Figure 5: Histograms from measured values of bars' diameters: longitudinal reinforcements.s (t) (m) Science and Engineering Advances in Materials Science and Engineering
−2],   is the reinforcement yield strength [N⋅mm −2 ], ℎ is the cross section height [m],  is the cross section width [m], eff is the effective cross section width [m],  is the concrete cover [mm], () is the reinforcement diameter dependent on time [mm],   () is the reinforcement cross section area dependent on time [m 2 ], and  is number of bars: