CC-2 Single Slab NDT Plan and Data

CC2 Phase II NDT Procedure and Data

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NDT was conducted assess surface displacements in the PCC single slab due to curling. The NDT testing included FAA profiling equipment, the data collection process, the analysis of the response data measured by the profiling device and the comparison of the slab curling measurements obtained with the profiling device, and the in situ vertical displacement transducers installed in the slab. Specifically, the FAA inertial profiling device was initially assembled to measure the surface elevation profiles of airport pavements.   This profiling device was adapted to measure the transverse absolute elevation profile of the traffic lanes at the NAPTF.

The current configuration of the FAA pavement surface profiling device consists of a vertical displacement transducer, an incremental rotary encoder, a data acquisition box, and a laptop personal computer.   The profiling equipment is mounted on a trolley that rolls along the flange of an aluminum beam 24 ft (7.3 m) long with a steel top member for reinforcement and stability. The vertical displacement transducer is a Selcom infrared laser unit with a spot size of 0.04 inch (1 mm).  The unit is located approximately 6 inches (15.2 cm) from the PCC test slab surface and used to measure absolute elevation profile of the slab surface. The incremental rotary encoder is used to measure the distance on which the profiling device travels along the beam.  The resolution of the distance measurement is 0.02 inch.  The data acquisition box accepts data from the infrared laser unit, along with the pulse train from the encoder.   The laser unit samples at a 32 kHz rate but the data is stored at a 8 kHz rate. The laptop PC is used to collect and store the measurement data when the profiling device is running.

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Profile Measurement

The FAA profiling device was used to measure the profile of the test slab surface during the three month drying period, after placement of the concrete. The device was driven along the north edge of slab from the west to east twice a day, in the morning and afternoon. Each measurement is repeated three times. The data collected from the device is saved as a binary file by the name of date and time. The FAA developed its own software in Visual Basic, called ProFAA, for analyzing the profile data.   An adopted version of this software called “ReadSelTransverse” is used to match the latest profiling device configuration to make a comparison of all the profiles collected.

NDT Test Results

The in situ vertical displacement gages (VDs) are used to measure the vertical deflections of the slab.   Gage VD2 is located at the center of north edge of the slab.   The other four gages (VD1, VD3, VD4, and VD5) are positioned at the corners of the slab (i.e.,  Figure 4).  The FAA profiling device was used to measure the profile along the north edge of the slab from the west to east.   The locations of VD1, VD2, and VD3 are very close to the path of the profiling device.   For comparing the profiling device measurements with the in situ Vertical Displacement Transducers, we define an index  Rc = The reading at the center of profile with zero at the profile ends, as shown in  Figure 9. 

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Figure 9.  Idealized View of a Curled Slab.

Rc is compared with the difference between the average of the corner deflections and the center deflection on the north edges of the slab 0.5 *(V3+V1)-V2.   Note that sensor VD3 did not work past 6/16/03, therefore, 0.5*(V3+V1) is replaced by the average of the corner deflections of the slab, (V1+V4+V5)/3 or V3=V1 is assumed.   Figure 10 shows the time histories of the Rc and VD measurements with initial values = 0.   Figure 11 gives the relationship between the readings of VD2 and the corner deflections of the slab.   They show a strong linear relation.   Rc, is therefore, a good indication of curling of the slab corners with, in this case, a proportionality factor of slightly more than two.

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Date time

Figure 10.  Relationship of deflections at center of north edge vs. vertical displacement transducers.

The estimating method used was as follows:

1.      Measure the profile along the slab edge.

2.      Take Rc from the profile.

3.      Assume the slab corner curling Vsc = α * Rc in which the user can select α.   As an example a simple formula for evaluating the slab curling:






  = Upward curling of slab  

Sc = Difference in lineal unit shrinkage between top and bottom of slab

L  = Length of slab (Use diagonal length to get corner curling)

T = Slab thickness (Assumed un-reinforced)

The diagonal length of an  L x L slab is equal to √2*L. Assume  Sc is constant; the corner curling of the slab is twice the curling at the edge center.   Therefore, α = 2, which is close to the results in  Figure 11 .
The Relationship of Deflections at Corner V.S center of north edge

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Figure 12  shows the slab curling measurements and the results from the estimated method with α = 2.   It is evident that the estimated results closely approximate the true vertical deflection at the corners.   However, the slab edges were not constrained and the relationship will not necessarily hold when adjacent slabs constrain each other by aggregate interlock or by dowel or tie bar reaction.

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Figure 12.  Comparison of the slab curling measurements with the estimated curling

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