Proc. of the 10th fib International PhD Symposium in Civil Engineering July 21 to 23, 2014, Université Laval, Québec, Canada
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Measuring deflections of a corroded concrete beam
loaded dynamically by a four-point-bending test
René Veerman, Eddie KoendersFaculty of Civil Engineering and Geoscience, Delft University of Technology,
Stevinweg 1, 2628 CN Delft, The Netherlands Abstract
One of the key-elements in Structural Health Monitoring of reinforced concrete structures is the level of rebar corrosion and the way in which it affects the structural performance. A dynamic four-point-bending test was developed to help understanding the deflections of concrete structures under corrod-ed conditions. With this test, two equivalent beams are loadcorrod-ed under equal conditions. To generate corrosion, one beam was exposed to a chloride-water solution. The other beam was exposed to tap water only, in order to act as a reference. This paper reports and discusses seven weeks of exper i-mental data, including deflections and crack widths. Although there was not yet a significant effect of corrosion, some changes in crack widths and in deflections were observed. Furthermore, the influence of temperature and surface wetting exposure on the structural behaviour will be discussed.
1 Introduction
Corrosion of steel reinforcement is a key-issue for the durability assessment of reinforced concrete (RC) structures. For predicting the durability of RC structures, it is important to understand the tural response during the development of corrosion. The structural properties of corroded RC struc-tures were investigated by many researchers. However, the changes of these properties with time and as function of corrosion were not investigated yet. To measure the structural changes with time, a four-point-bending test was developed. With this experiment, two beams of similar configuration were loaded simultaneously, and with an equal dynamic load. A bath was mounted on top of both beams where one was filled with a chloride-water solution with the aim to stimulate corrosion, and the other only with tap water, to act as a reference. During a five days dry and two days wet cycle, corrosion was developing with time, to mimic real life situations.
During the test, deflection and some crack widths of both beams were measured by Linear Varia-ble Differential Transformers (LVDTs). As the crack patterns of the beams were not equal, the initial thought was that the crack width and the deflections of the beams could not be compared. However, the development of the cracks and the deflection of the beams could be compared once the results are expressed relatively to the initial crack and deflection situation at time t=0. From this, the develo p-ment of deflections and crack width measurep-ments of both beams show comparable results, which indicates that localized corrosion might have a limited impact on the structural performance of RC beams
2 Experimental programme
2.1 Concrete specimen
A formwork for two beams was casted in one go, from one batch and from one concrete composition. The hardened beams showed very accurate dimensions with a mass difference of less than 1%. Mix composition and beam dimensions were taken from Blagojevic [1]. In his IS2C research project Blagojevic developed the beams and embedded electrochemical needs to measure their potential resistance under static loading [2]. The research reported in this paper describes the chloride ingress under dynamic loading, using the same beams, embedded electrochemical configuration, and follo w-ing the same loadw-ing scheme (4 point bendw-ing, see Figure 1). This gives the opportunity to compare results of multiple experiments for different loading conditions, i.e. static versus dynamic, as repre-sented by two different IS2C projects [3]. The dimensions of the beams can be found in Table 1. The reinforcement properties can be found in Table 2.
10th fib International PhD Symposium in Civil Engineering
After three days of hardening, the beams were demolded and placed in a fog room (20 ºC, RH 100%). At an age of 21 days, the concrete beams were relocated in a climate room (20 ºC, RH 50%) for another 7 days, which was the same room as the actual tests were performed. After 28 days of hardening, the concrete compressive strength was measured from 3 cubic specimen with dimensions 150x150x150 mm3 (see Table 3), and also the two beams were prepared for testing and installed in
the test setup.
Table 1 Beam dimensions
Length Height Width Cover
1500 mm 150 mm 100 mm 30 mm
Table 2 Reinforcement properties
Diameter Steel quality Yield stress Modulus of elasticity
12 mm FEM 500 HKN 500 N mm-2 200 000 N mm-2
Table 3 Concrete properties
Concrete mass Concrete compressive strength
Beam 1: 54.68 kg Beam 2: 54.20 kg μ = 34.8 N mm-2 σ2 = 0.73 N mm-2
Fig. 1 Schematic impression of the experimental setup
2.2 Testing frame setup
In real life situations, de-icing salts on roads deposited during winter may cause corrosion of concrete bridges. The reinforcement bars, situated in the top part of construction elements suffer most, and will have a much higher probability of corrosion initiation. Particularly, in multiple span bridges, negative bending moments at the mid-supports are the reason that large amounts of reinforcement are exposed to enhanced chloride concentrations.
A four-point-bending test, loaded up-side-down, is mimicking a negative bending moment situa-tion and is generating cracks at the top side of the beam. In fact, it corresponds with the crack pattern that is expected at mid-supports of multiple span bridges. A triangular steel frame was installed to support the structure during the dynamic beam test. An oil-pressure cylinder with a pressure cell located below the framework was used to initiate and measure the force, acting in upwards direction. Since both beams were loaded simultaneously by one cylinder, the internal beam forces were the same for both action loads. There is only a slight load difference because of the mass of the upper beam, which increases the load on the lower beam. However, in this experimental configuration, the mass of the beam is less than 4% of the total load, and has, therefore, a limited impact.
Traffic loads acting on real bridges have a dynamic nature and are randomly distributed. To sim u-late these traffic loads in a lab-scale experiment, and to mimic real traffic situations, a dynamic load
Measuring deflections of a corroded concrete beam loaded dynamically by a four-point-bending test
René Veerman, Eddie Koenders 281
of 12 kN (approximately 50% of the failure load) was applied to the beams, with a frequency of 0.5 Hz. This load case corresponds to a situation where a vehicle passes the bridge every two seconds .
2.3 Generating corrosion
To understand the effect of corrosion on the structural performance of the beams, it is important to know the behaviour of the uncorroded situation as well. For this reason, two beams were applied which were loaded under the same conditions. One beam was exposed to a 10% chloride -water solu-tion, with the aim to accelerate the corrosion process. The second beam was exposed to tap water only, and acted as a reference.
Since a wetting-drying cycle is identified as the most unfavourable environmental condition [4], it was decided to apply similar cycle to the beams, and wetting and drying them for two days and for five days respectively. In previous publications [5], the authors showed by potential measurements that this cycle could be applied correctly to the specimens. Unfortunately, after three we eks of testing, some leakage was observed, which could not be stopped anymore. Because of this, the leakage influ-enced the wetting-drying and affected the corrosion rate. However, it did not influence the deflections of the beam in dry conditions. Moreover, it turned out that in many cases leakage did not influence the experimental results at all.
2.4 Measurements
During the experiments, deflections and crack widths of both beams were measured by Linear Varia-ble Differential Transformers (LVDTs). Due to irregularities in the crack patterns developed in the beams during loading, the deflections at midspan were measured at both sides of the beams. The exact location and morphology of the cracks strongly depended on the local concrete composition (tension strength) and on the internal stress distribution. Since concrete is a heterogeneous material, the co n-crete composition may slightly differ over the length of the beam, resulting in a variation of the local material properties. Because of this, the location of the cracks cannot be calculated precisely. In order to overcome this problem, the beams were statically pre-loaded, for one time, up to the testing load and prior to the actual dynamic test. During this static test, several cracks developed over the length of the beams. Three cracks developed in the continuous bending moment part of both beams, and LVDTs were placed over these cracks in the unloaded situation. As LVDTs are sensitive to water, they cannot be installed under water permanently. Because of this , the LVDTs were installed 30 mm above the concrete surface, which means that the actual crack opening was measured a little above the cracked surface, and the results, shown in this paper, are, therefore, slightly overestimated.
Besides deflections and crack widths, the potential resistance and other electrical properties were continuously measured as well [5]. Data from these measurements show that corrosion initiation started directly after the first chloride exposure and continued progressively with time. In the beam that is exposed to tap water the probability of corrosion is small, however, it could not be excluded. Future forensic engineering will be done to generate more knowledge on the average corrosion activ i-ty over the length of the beam. A detailed analysis on the results of these measurements is excluded from the scope this paper. Furthermore, the temperature and relative humidity of the area were mea s-ured by the control software as well as by a separate climate control unit.
3 Results
Results presented in this paper are based on a seven weeks testing period. During this period some interesting topics were observed which will be discussed in this section. The experiment is running 24/7 as an on-going process, which means that the test is producing a continuous data flow. However, due to technical problems, it was sometimes necessary to stop the experiment temporary for maintains reasons. Furthermore, electrical or oil-pressure problems have caused failure of the load,which means that the experiment was stopped temporary as well. These stops results in gaps of non- continuous data in the figures presented hereafter.
3.1 Crack locations
The locations of the cracks were measured after the initial static loading of the beams. Over the total length of the beam, nine cracks were observed for Beam 1 and eight cracks for Beam 2 (Table 4).
10th fib International PhD Symposium in Civil Engineering
Table 4 Crack locations. The width of the cracks marked with “(*)” were measured continuously during the dynamic loading
Beam 1 (beam with tap water) 36 cm 50 cm 58 cm 68 cm(*) 78 cm(*) 86 cm(*) 98 cm 108 cm 121 cm Beam 2 (beam with chloride-water solution)
32 cm 42 cm 54 cm(*) 69 cm(*) 87 cm(*) 97 cm 107 cm 122 cm 3.2 Temperature
The average temperature in the climate room, where the test setup was built, was recorded every 60 seconds. Generally, the fluctuation of the temperature was limited to one degree. In some cases, ho w-ever, the baseline of the temperature changed two degrees and continued fluctuating around this new baseline. On a more local scale, the influence of temperature can also be observed in the crack width and in the deflections. The correlation between the temperature and the deflection is displayed in Figure 2. Although the correlation looks clear, a straightforward correlation between the temperature and the deflection was not found in the data. The main reason for this is the retarding effect of the temperature penetrating through the concrete mass, which is not a linear function. It depends on mu l-tiple aspects like: the size of the concrete, the local concrete composition, the exposure time, the reference temperature, and the heat capacity of the ingredients. This was also observed by other r e-searchers [6].
Fig. 2 Correlation between room temperature and deflection of Beam 1
3.3 Wetting cycle
It was already noticed that temperature does have an influence on the deflection of the beam. As the initial temperature of the solution was lower than the temperature of concrete, it has direct influence on the behaviour of the structure because of the heat exchange necessary to balance this difference. Furthermore, the solution also penetrates into the concrete via the capillary pore system which might cause swelling that indirectly affects the structural behaviour in terms of fluid compressibility during dynamic loading. The effect of wetting on the deflection of the beam is displayed in Figure 3.
3.4 Crack width and deflection
The deflection strongly depends on the number of cracks, on the crack width and on the actual loca-tion where the crack has developed in the beam [7]. As discussed above, the number of cracks is not the same in both beams. Beam 1 has more cracks, which results in smaller crack widths and larger deflections. The deflections of both beams at midspan are displayed in Figure 4.
After initiating the cracks, their location is implicitly known, while the internal equilibrium b tween reinforcement steel and concrete has been established. The development of crack pattern d e-pends on the load and the structural properties. Since the impact of the dynamic load remains constant
Measuring deflections of a corroded concrete beam loaded dynamically by a four-point-bending test
René Veerman, Eddie Koenders 283
over time and both beams were loaded simultaneously and equally, the development of cracks of both beams should be comparable. The development of the deflection can be calculated by the normalized factor according to:
( ( )) (1)
In which δ(t) is the deflection as function of time, and δ(t0) is the initial deflection under the same
load.
Fig. 3 Effect of wetting the beam on the deflection
Fig. 4 Deflection at midspan
10th fib International PhD Symposium in Civil Engineering
Figure 5 shows the development of the midspan deflection of both beams. It can be concluded that the deflection of both beams increases with time. Furthermore, the average increase of the crack width over seven weeks of testing is comparable. The daily development of the difference in deflection between both beams changes only a little. The results show that during the first two weeks of loading, the deflection development of both beams is comparable. Between two and four weeks of loading, the deflection of Beam 1 increases faster than the deflection of Beam 2. After a temporary stop, the de-flection of Beam 2 catches up again and is increasing more rapidly in comparison with Beam 1. This might be because of a re-stabilization of the internal moisture state after a stop, or because of the initiation of chemical degradation, e.g. corrosion. Unfortunately, the testing time was limited and the presented and no further development of the data could be shown.
4 Conclusion and discussion
Deflections and crack widths were measured during a dynamic four-point-bending test. This test contained two reinforced concrete (RC) beams that were loaded simultaneously. Beam 1 was wet ted by tap water and was used as a reference; Beam 2 was exposed to a chloride -water solution to gener-ate and stimulgener-ate corrosion of the reinforcement bar. Although the experiment is still running, some interesting issues were observed and shared in this paper.
The number of cracks and the crack locations were not the same for both beams. Beam 1 has more cracks, which results in smaller crack widths and larger deformations. Because of this diffe r-ence, a relation between the absolute deflections of both beams could not be made. However, the relative development of the cracks and the deflections showed only a slight dependency on the nu m-ber of cracks and which makes them comparable.
It was observed that the environmental temperature influenced the behaviour of the beams. Since many properties respond to temperature changes differently, a straightforward relation between the temperature and the deflection could not be found. Due to leakage, extra attention was necessary once comparing the data in the wetting period.
It was concluded that over the testing period so far, the development of the deflection of both beams was almost comparable. Beam 2 showed a faster deflection development rather than Beam 1. This could be caused by a re-stabilization of the internal moisture state. However, it could be also the first signs of chemical degradation, i.e. corrosion. After seven weeks of testing, however, this could not be confirmed explicitly. At this moment, the experiment is still running and new results will be published soon.
References
[1] A. Blagojević, et al. "The influence of cracks on chloride-induced corrosion of reinforced concrete structures – development of the experimental set-up". Young Researchers' Forum II, London, 2014
[2] A. Blagojević, et al. "Corrosion resistance of reinforced concrete beams under the synergetic action of chloride ingress, loading and varying crack width: preliminary studies on the met h-ods suitability and derived parameters reliability through the application of electrochemical techniques (LPR, PDP and EIS)", Submitted to Materials & Corrosion (2013)
[3] E. A. B. Koenders "Integral Solutions for Sustainable Construction (IS2C) - A Structural Health Monitoring Program in The Netherlands". SMAR - First Middle East Conference on Smart Monitoring, Dubai, UAE, 2011
[4] H. Ye, et al. "Influence of cracking on chloride diffusivity and moisture influential depth in concrete subjected to simulated environmental conditions", Construction and Building Mate-rials (2013)
[5] R. P. Veerman and E. A. B. Koenders "Automatic Degradation Detection During a Dynamic Loaded Beam Test". CIC - Concrete Innovation Conference, Oslo, Norway, 2014
[6] C. Liu and J. T. DeWolf "Effect of Temperature on Modal Variability of a Curved Concrete Bridge under Ambient Loads", JOURNAL OF STRUCTURAL ENGINEERING (2007) [7] fib "Structural Concrete" (ISBN 2-88394-041-X) (1999)
