Analysis of Fiber-Optic Strain-Monitoring Data from a Prestressed Concrete Bridge

The ability to collect data is not sufficient for any monitoring system to yield valuable information. Webb et al. (2014b) presented a classification system for SHM deployments, which can be classified as (1) sensor deployment studies, (2) anomaly detection, (3) model validation, (4) threshold check, or (5) damage detection. This classification system will be used to describe aspects of the deployment in this paper.

In addition to trialing a relatively new sensing technique (sensor deployment study), the monitoring installation also aimed to provide value to the designers/owners of the bridge. Measuring strains would allow the long-term behavior of the structure to be analyzed. There was interest in comparing these measurements to various empirical creep and shrinkage models (model validation). Finally, the ways in which this system could potentially be used for damage detection were considered.

Six beams in the western-most span, denoted B1–B6, were instrumented as illustrated in Fig. 2. The fiber-optic cables were installed in each beam at the precasting yard after the prestressing strands had been pretensioned but before steel reinforcing stirrups had been tied in place in the mold. Two types of fiber-optic cables (Fig. 6) were installed in the six beams, one bonded to the concrete (hereafter referred to as the total strain fiber) and one in which the sensing core was not bonded to the concrete (hereafter referred to as the temperature fiber). The cables used to measure temperature strain consisted of optical fibers suspended in a gel, which prevented shear transfer between the concrete and the sensing core, encased in a protective coating. These cables [Fig. 7(a)] were 6 mm in diameter but were only glued to the prestressing strands at the ends of the beam and supported every few meters by tape. As such there was room for the concrete to penetrate between the cable and the prestressing strand reducing any potential effects the fiber-optic cable would have on the prestressing bond. This allows the effects of temperature on the Brillouin frequency shift to be removed, such that the true strain in the total strain fiber (due to applied loading as well as time-dependent effects and the thermal response of the structure) can be calculated. The fiber-optic cables used to measure total strain were low-cost (regular telecommunications grade) cables except for one beam (Specimen B4). The low-cost cable was a 12-core ribbon cable (∼0.3 × 3.2 mm) as shown in Figs. 6 and 7(b). This provided sufficient surface area to bond with the concrete while at the same time not interfering with the bond between the prestressing strands and the concrete. In Specimen B4, a single, more robust, fiber-optic ribbon cable [Fig. 7(c)] with a reinforced protective sheath was installed along one of the top strands to compare the performance of these more expensive cables with the low-cost telecommunications grade cables. This cable consisted of 4 glass cores with cladding embedded in a nylon sleeve with steel wires running down each edge of the sleeve either side of the four cores. This cable was approximately 5 mm wide and 1 mm thick and, as with the other two cables, it was only glued to the strand at the ends and supported intermittently to minimize the influence the cable would have on the bond. The bottom cables in all beams were low-cost telecommunications grade 12-core ribbon.

The fibers were installed as a loop within each beam, as illustrated in Fig. 3. Each beam had a single fiber-optic cable for temperature measurement installed in this way. In Beams B1, B2, B5, and B6 two fiber-optic cables for total strain measurement were installed on either side of the cross section, hereafter referred to as the north and south fibers (Fig. 3). Because of time constraints, only one total strain measurement cable was installed in Beam B3. In Beam B4, as previously mentioned, a more robust fiber-optic cable was used along the top strand with a telecommunications grade 12-core ribbon cable used along the bottom. Each total strain measurement cable was installed with an initial pretension to ensure that the cable would not go into compression due to the applied loading or creep in the concrete.

Table 1 gives the dates on which measurements were taken from Beams B1 and B6, as well as key dates in the bridge’s construction process. Readings were taken from the north fiber along the bottom of Beam B1 and along both the bottom and top of Beam B6. Fibers in the remaining beams were unable to provide any data due to the robustness issues discussed next.

One of the aims of this monitoring campaign was to determine whether standard low-cost telecommunications grade fiber-optic cables could reliably be used to measure strains in concrete beams. Unfortunately, a number of these cables were damaged, both during the casting and installation of the beams. Fig. 8 shows the location of the fiber-optic cables in the mold prior to casting and Fig. 9 shows the completed beams. The cables proved to be highly susceptible to breakage during the pouring and vibration of the concrete, and as a result a number of cables were snapped during the casting process. Other fibers were damaged while the beams were transported from the precasting yard to the bridge site and lifted into position. After the beams had been cast, it was found that only half of the total number of fiber-optic cables that had originally been installed (11 of 21) remained intact. However, following transport and erection on site, this figure had fallen to just under one-third (6 of 21), meaning several beams were left with no functioning fibers at all. This figure had reduced to 14% (3 of 21) 314 days after installation, although readings were still possible along the bottom of Beam B1 and along the top and bottom of Beam B6.

At the bridge site, additional fiber-optic cables were spliced to those entering each beam and the fibers fed down through metal protective ducts from the ends of the beams to metal boxes mounted on the face of the bridge abutment at ground level to provide access to the fibers when taking readings. Standard telecommunications grade fibers were used for all beams because the metal ducts and boxes were intended to provide sufficient protection; however, the fibers were susceptible to damage during the construction process. The metal boxes were also vandalized on a number of occasions, damaging more of the fibers. By 2009 all the remaining fibers within Beam B6 had become unusable. Clearly, the inexpensive telecommunications grade fibers are not sufficiently robust to be installed in concrete structures. However, even if the more expensive, durable fibers had been used throughout, the total cost of installing the monitoring system would still have only been of the order of 1% of the total cost of the bridge. If the system could provide useful information to the owners of the bridge, then this cost might be considered a worthwhile investment.

The BOTDR analyzer produces, for each fiber connected to it, a data file consisting of raw strain values (positive values indicate tension) averaged over a 1-m gauge length, taken every 50 mm along the fiber. Postprocessing is required to turn these raw values into usable strain readings. Fig. 10 shows some plots of the raw strain output for Beam B1. [Raw strain data is presented for Beam B1 and Beam B6 in beam-1-data.xlsx and beam-6-data.xlsx (Webb et al. 2016)]. Each line shows data taken from a different measurement time and shows several features. In each data set there is a large strain change at the point in which the fibers enter the beam. Because the length of the fiber outside the beam was not known beforehand, this allows the spatial location of readings inside the beam to be determined. This fiber also had to be repaired following damage due to vandalism. The resulting change in fiber length can clearly be observed in the plot (Fig. 10). A shift can be applied to the data sets following the repair to ensure that this length change does not affect results. It is important to note that any individual set of readings cannot be interpreted to give the strain in the concrete because they depend on the original reference values of strain taken when the fiber was attached to the prestressing tendon prior to construction of the beam. Instead, the relative changes in values between readings taken at different times are related to the changes in concrete strain. The apparent changes in strain visible at a distance of around 36 m do not, therefore, represent a variation in strain in the concrete.

Raw strain readings from the analyzer (denoted ζ in the following discussion) are affected by temperature in two ways. When the glass fiber changes temperature the total strain will vary due to thermal expansion, which can be estimated using the thermal coefficient of the fiber, α_f. In addition there is a coefficient of temperature-induced apparent strain, α_a, which allows for the temperature dependency of the Brillouin effect. Adding these two effects gives a combined thermal expansion coefficient for the temperature fibers, which relates the change in temperature, ΔT, to the change in the raw strain as measured by the analyzer, Δζ_t, given in

For the cables used in this installation, α_f = 4.2 × 10⁻⁶/°C and α_a = 19.471 × 10⁻⁶/°C (values from Mohamad 2008). Table 2 shows the average change in temperature since the initial set of readings, for each reading from Beam B1. The initial set of readings was taken shortly after the concrete had been poured, when significant temperature rises are caused by the exothermic hydration reaction of the setting cement. This explains why subsequent data sets have large negative differential temperatures.

Because the strain fiber is bonded to the concrete beam, it is assumed that it will experience the same mechanical strain changes as the surrounding concrete, Δε_c, in addition to a temperature-induced apparent strain, giving a change in raw strain as measured by the analyzer, Δζ_s, given in

Eqs. (1) and (2) can be combined to give an expression for the actual change in the mechanical strain in the concrete as calculated using

This concrete strain will be a combination of strains due to thermal expansion of the concrete, creep and shrinkage effects, and any loads applied to the beam after the initial set of baseline readings, such as prestressing force and the addition of the in situ deck slab. Live loads are usually temporary loads, but they can be measured using this system if applied for more than the 20 min required to take a reading (e.g., a truck parked on a bridge). The strain change due to any load that is applied for significantly less than the scan time will not be measured correctly.

There are a number of important factors to be considered when characterizing the performance of a new sensing technique. In addition to system reliability and robustness, which were discussed in the previous section, it is also important to consider the achievable accuracy.

Fig. 11 shows the strain readings obtained from the BOTDR analyzer, which exhibit a significant degree of apparent noise (scatter of up to 100 με). This is a characteristic of the BOTDR method. Whether this level of accuracy is acceptable or not depends on the intended application of the data, as will be discussed later. In addition to limiting the accuracy of measurements, this also means that the temperature correction calculation cannot simply be performed point by point along the beam, because the error due to scatter would be amplified in the compensated readings. Fig. 12 shows a moving average line (with a period of 2 m) applied to each set of data to represent the important features of each data set, while compensating for the noise. The period of the moving average line needs to be chosen manually depending on the expected strain distribution being measured; if strains are expected to vary over a short length scale, then a moving average line with a small period will be required.

Because one of the primary aims of this monitoring was to investigate the long-term behavior of the bridge due to creep and shrinkage, it would be desirable to completely remove the effects of temperature from each reading. During the earlier stages of the bridge’s construction the concrete beams could be treated as determinate structures; hence, the strains due to thermal expansions could be predicted. However, when the bridge becomes fully composite and continuous over all three spans, the indeterminacy of the structure makes it harder to predict strains. This therefore makes it difficult to separate the effects of temperature from those due to creep and shrinkage when analyzing the measured strains. This will be discussed in more detail later in the paper.

Because one of the aims of this research was to investigate the long-term behavior of the structure and validate the structural analysis models used for design, it was necessary to predict the expected development of strain over time to allow comparisons to be made with each set of readings. There were various changes to the loading applied to the beams during construction, and these had to be taken into consideration. Because the precast bridge beams were constructed from prestressed concrete, creep and shrinkage of the concrete were important factors to consider. Once the in situ deck slab was cast, the beams and deck began acting as a composite section and the bridge became continuous over the three spans, changing the behavior. The behavior of the concrete and steel tendons led to losses of prestressing force, which resulted in both short- and long-term effects. The material and beam properties used in the analysis are listed in Table 3.

A commonly cited long-term aim for monitoring systems is to provide information to assist with the detection of damage or deterioration more effectively than relying solely on visual inspections. If using a discrete strain measurement system, then there is the possibility of damage occurring between individual point sensors and not being detected. A distributed strain monitoring system, such as BOTDR, does not have this limitation and therefore is particularly attractive.

One potential aim of this monitoring system could be to detect wire breaks within the prestressing tendons. Individual wire breaks within prestressing tendons cause minute changes in strain (less than 1 με) (Webb et al. 2014a). Because this magnitude of strain is far smaller than the accuracy and precision of BOTDR measurements, individual wire breaks will not be detected. However, Hoult et al. (2009) proposed that if an entire prestressing strand were to rupture, causing a loss of compression in the bottom of the concrete beam, then the resulting strains would be around 185 με, which should be detectable by this system. If continuous monitoring was to be used then the sudden change would likely be noticed, assuming that acceptable data interpretation protocols are followed. It is worth noting that other fiber-optic distributed strain measurement technologies have better accuracy and may be more appropriate for damage detection and assessment depending on the application. For example, Regier and Hoult (2014a) used a Rayleigh backscatter system to measure strains during a load test of a reinforced concrete bridge and obtained the same level of accuracy as electrical resistance strain gauges. However, even with increased measurement accuracy, long-term damage detection is still a challenge because Regier and Hoult (2014c) demonstrated that a temperature change of just 3°C created the same level of strain change in a bridge as a fully loaded truck. Additionally, one limitation of the Rayleigh system is the length of fiber that can be scanned (maximum 70 m), which may not be enough for some applications. As such, the choice of sensor system must be based on a consideration of many parameters to determine the most appropriate system, which may involve some level of compromise.

This monitoring system could also potentially have been used during the construction process to ensure that the prestressing forces applied to the beams were as intended by the designer. Because the technique can provide strain readings along the whole length of a beam, it would have been possible to detect the change in strain distribution, which would have occurred should any of the tendons not have successfully bonded to the concrete.