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Construction of the California Precast Concrete Pavement Demonstration Project
Chapter 7. Instrumentation and Monitoring
As part of the demonstration project, it was necessary to monitor the behavior and performance of the test section. Monitoring of the behavior provides an indication of the accuracy of the assumptions made during the design process and the accuracy of the PSCP2 program. Monitoring of pavement performance also helps in identifying design details that need improvement and provides an indication of the true long-term durability of prestressed precast pavement.
Chapter 4 presented many of the variables that were part of the pavement design procedure. Temperature was one of the most important variables because it affects both horizontal (expansion and contraction) and vertical (curling) slab movements and their associated stresses. In addition to temperature, actual slab movements will be monitored over time.
Project-Level Condition Survey
Following construction, a project-level condition survey was performed to establish the initial as-constructed condition of the precast pavement prior to opening to traffic. This initial project-level condition survey will be used for comparison with future condition surveys to identify any new distresses that occur over time. For the condition survey, a distress map was developed, noting any distresses that occurred during construction, including longitudinal and transverse cracking, random shrinkage cracking, spalling, and corner breaks.
The distress map for the as-constructed condition is shown in figure 43 at the end of this chapter. To summarize the as-constructed condition:
The corner break was required to be repaired, but repairs to spalling were not required. Most of the minor spalls were removed by diamond grinding, and most of the moderate spalls were reduced to minor spalls during grinding.
Temperature instrumentation was embedded in the precast panels to monitor slab temperature during fabrication and over the life of the pavement. The temperature data are used to correlate measured slab movements to slab temperature. Temperature sensors were embedded at 25 mm (1 in.) from the top surface of the panel, at mid-depth, and at 25 mm (1 in.) from the bottom of the panel. Because panel thickness varied from 250 mm (10 in.) to 330 mm (13.1 in.), separate sets of sensors were positioned at those two panel thicknesses. The sensors were located at least 0.3 m (1 ft) from the panel edges to prevent “edge effects” in the temperature measurements.
The sensors used for measuring and recoding slab temperature are Thermochron iButtons®. These sensors, which are roughly 16 mm (5/8 in.) in diameter and 6 mm (1/4 in.) thick, measure and log the temperature internally. They are capable of logging over 2,000 temperature measurements at prescribed intervals between 1 minute and 4 hours. A single twisted-pair wire attached to the sensor is used to download the temperature data to a personal computer or handheld device.(19) Each temperature data point is recorded with its own date/time stamp. To ensure that the data from each sensor are kept separate from other sensor data, each sensor has a unique serial number that is recognized by the computer software.
In total, 15 temperature sensors were embedded in the precast panels in sets of three (top, middle, bottom). Two sets were embedded where the panel thickness is 250 mm (10 in.), and the remaining three sets were embedded where the panel thickness is 330 mm (13.1 in.). The sets of sensors were embedded in the joint panels, where slab movement is measured.
Each set of sensors was tied to a wood dowel to ensure proper spacing of the sensors, as shown in figure 37. The wood dowel with the sensors attached was tied to reinforcement in the panels to keep it in place during casting. The wires from each sensor were extended to junction boxes at the edge of the slab. The lids on the boxes are removable to permit access to the sensor leads over the life of the pavement. Figure 38 shows the general location of the sensors and junction boxes in the precast panels. (The location of the temperature sensors in the overall precast pavement test section is shown in the condition survey in figure 43 at the end of this chapter, with sensor identification and pavement thickness in accompanying table 8.)
Accurate prediction of horizontal slab movement is important for determining the maximum permissible slab length during the design process. Measurement of horizontal slab movement provides an indication of both the thermal expansion characteristics of the concrete as well as the amount of restraint from frictional resistance at the slab-support interface.
Horizontal slab movement is monitored by measuring the width of the expansion joints using dial calipers. Demec points epoxied to the surface of the joint panels on either side and at either end of the expansion joint provide a reference location for the measurements. Measurements are taken at various times of the day and night to obtain a thorough correlation of joint width and slab temperature.
Preliminary monitoring was conducted during panel fabrication and installation and soon after construction, prior to opening the pavement to traffic, as described below.
Figure 39 shows the temperature history from one set of temperature sensors during panel fabrication. The sensors revealed that the internal slab temperature during the peak heat of hydration (during steam curing) for this particular panel reached nearly 52 ºC (125 °F). The top–bottom slab temperature differential became much more evident after casting, while the panels were stored at the precast plant. A top–bottom temperature differential of nearly 17 ºC (30 °F) was recorded for several days during storage.
Figure 40 shows the temperature history during a 24-hour period after panel installation, prior to opening the pavement to traffic. The slab temperature shown is the mid-depth temperature from two different sensors (sensors 1-M and 5-M). This plot clearly demonstrates the lag between ambient temperature and slab temperature that is typical of concrete pavements. The plot also shows the insulating effect of the base on the slab temperature. In contrast to the slab temperature during storage (figure 39), the on-grade slab temperature is almost always higher than ambient temperature.
Horizontal slab movement (expansion joint width) was monitored for a 24-hour period approximately 3 months after panel installation. Dial calipers were used to measure the width of each of the three expansion joints (see figure 43 for joint numbers) using stainless steel Demec points as gage points for the measurements (figure 41). Each joint was measured at each end (north and south) and at the middle. These measurements were then correlated to mid-depth slab temperature. Figure 42 shows a normalized plot of the relative movement of the expansion joints with change in mid-depth slab temperature. The mid-depth slab temperature and ambient temperature for the measurement period were shown in figure 40 (August 26–27, 2004).
As figure 42 shows, roughly the same relative movement was measured for joints 1 and 3 at the north end and middle of each joint. Because joints 1 and 3 abut the cast-in-place pavement at either end of the test section, this relative movement represents the movement of one end of each of the 37.8 m (124 ft) slabs. The relative movement of joint 2, however, represents the movement of the two slabs on either side of joint 2, or 75.6 m (248 ft) of pavement. It would be expected, therefore, that the relative movement of joint 2 would be approximately twice that of joints 1 and 3. As figure 42 indicates, however, this was not the case. It is not known why joint 2 experienced so little movement, but it was most likely due to a temporary precast concrete median barrier resting across the joint during the period of measurement. The barrier, as well as a significant amount of debris in the joint, probably restrained joint movement during the measurement period.
Figure 41. Photo. Dial calipers were used to measure expansion joint movement using Demec points on either side of the joint as gage points.
Figure 42 also shows less relative movement at the south end of joints 1 and 3 than at the middle and north end. This was most likely caused by a shadow cast over the south side of the pavement by an abutting sound wall. The sound wall shades the south side of the pavement throughout the day, preventing slab temperatures from reaching as high as at the middle and north side of the slab, resulting in less movement at that end of the joints. Table 7 summarizes relative slab movement based on a linear regression of the field data, shown in figure 42, along with the degree of correlation (R2 value) of the data to the regression. A high degree of correlation was observed for the north end and middle of joints 1 and 3, but a lower degree of correlation at the south end, most likely due to the shading effect mentioned above. Only the south end of joint 2 showed good correlation.
Measured Versus Predicted Movement
To check the predictions made by the PSCP2 program during the design (chapter 4), the actual slab temperatures during construction (April 14, 2004) and the slab temperatures during the measurement period (August 26–27, 2004), were input into the PSCP2 program. (The analysis period input for comparison was 134 days after construction.) Based on the temperature inputs, and the design inputs discussed in chapter 4, the relative movement predicted by the PSCP2 program was 0.150 mm/°C (0.0033 in/°F). This value for relative movement is very close to that measured on site (for joints 1 and 3), indicating that the assumptions used during the design were accurate for predicting expansion joint movement.
Long-term monitoring is important for evaluating the behavior and performance of precast prestressed pavement. While pavement performance is the more critical aspect, monitoring slab movements will help to improve the design process through better prediction of pavement behavior.
Ideally, long-term monitoring should be performed at intervals approximately 1, 5, 10, and 30 years after construction. As a minimum, a condition survey should be completed to document changes to existing pavement distresses and any new distresses that may have appeared. If traffic conditions permit, horizontal and vertical slab movements and slab temperature should be measured also. Measurement of horizontal and vertical slab movements should be completed over a minimum of 24 hours each time. The measurement period should be during a season when large differences in daily high and low temperatures are expected (usually fall or spring) and preferably on days with a clear sky.