Detection of Voids in Prestressed Concrete Bridges using Thermal Imaging and Ground-Penetrating Radar
Detection of Voids in Prestressed Concrete Bridges using Thermal Imaging and Ground-Penetrating Radar
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发表时间：
2008-12
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通讯作者：
David G. Pollock;K. Dupuis;B. Lacour;K. Olsen
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作者：
David G. Pollock;K. Dupuis;B. Lacour;K. Olsen
对具有模拟空气空隙的混凝土样本进行了热成像和探地雷达测试。为了进行热成像检查，2007 年 6 月建造了 6 个混凝土试件来模拟后张箱梁桥的墙体。目的是检测灌浆后张法管道内的模拟气孔，从而定位后张法钢绞线易受腐蚀的区域。从这些检查中得出的最重要的结论是，20 厘米（8 英寸）厚的样本中的 PT 管道和模拟空隙比 30 厘米（12 英寸）厚的样本中更容易检测到。虽然对 20 厘米（8 英寸）厚样本的检查显示了大部分模拟空隙，但只有一份较厚的样本检查 (12c) 表明存在模拟空隙（两个管道中有四个空隙）。此外，PT 管道在较薄样品的热图像中更加清晰可见。 2007 年 8 月至 10 月期间，对 14 个混凝土样本进行了探地雷达 (GPR) 检查。根据本研究中进行的探地雷达调查，很明显，使用 1.5 GHz 探地雷达系统可以检测嵌入混凝土中的灌浆管道内的后张预应力钢绞线和模拟空隙。每个混凝土样本中钢筋顶层的布局在探地雷达图像中都很明显，但底层钢筋并未被清晰地检测到，因为它实际上“隐藏”在钢筋顶层下方。尽管在灌浆钢管内没有检测到后张拉钢绞线和模拟空气空隙，但在探地雷达图像中通常可以检测到塑料管道内的模拟空隙。钢管道的高介电常数不允许微波穿过管道表面并到达模拟的空隙。然而，使用探地雷达可以准确确定管道的大致位置、其方向及其在混凝土中的深度。因此可以推断，空隙方向对于探地雷达图像中的检测至关重要。
Thermal imaging and ground-penetrating radar was conducted on concrete specimens with simulated air voids. For the thermal imaging inspections, six concrete specimens were constructed during the month of June 2007 to simulate the walls of post-tensioned box girder bridges. The objective was to detect simulated air voids within grouted post-tensioning ducts, thus locating areas where the post-tensioning steel strands are vulnerable to corrosion. The most important deduction taken from these inspections was that PT-ducts and simulated voids were more detectable in the 20 cm (8 in.) thick specimens than in the 30 cm (12 in.) thick specimens. While inspections of the 20 cm (8 in.) thick specimens revealed the majority of their simulated voids, only one thicker specimen inspection (12c) indicated the presence of simulated voids (four voids in two ducts). Also, PT-ducts were much clearer and visible in the thermal images of the thinner specimens. Ground-penetrating radar (GPR) inspection was conducted on fourteen concrete specimens between August and October 2007. Based on the GPR surveys conducted in this study, it is apparent that the detection of post-tensioning strands and simulated voids within grouted ducts embedded in concrete is possible with a 1.5 GHz GPR system. The layout of the top layer of steel reinforcement in each concrete specimen was evident in the GPR images, but the bottom layer of reinforcement was not clearly detected since it was effectively “hidden” beneath the top layer of rebar. Although none of the post-tensioning strands and simulated air voids within the grouted steel ducts was detectable, simulated voids within plastic ducts were generally detectable in GPR images. The high dielectric constant of the steel ducts did not allow the microwaves to transmit through the surface of the duct and reach the simulated voids. However, the general location of the duct, its orientation and its depth in the concrete were accurately determined using GPR. Thus it can be inferred that the void orientation is critical for detection in GPR images.