In this article, we will review an interesting category of ultrasonic test methods for concrete inspection and testing:Ultrasonic Pulse-Echo (UPE)广泛用于检查混凝土元件。The method has proven to be extremely useful in determining the thickness of concrete elements with one side access (i.e. tunnel linings, trunk sewer linings, abutment walls), detect sub-surface defects such as voids, honeycombing, and delamination, and to verify Location of grouting defects in tendon ducts.
Ultrasonic Pulse Echo
Ultrasonic Pulse Echo是用于扫描混凝土元件中的子表面目标的非破坏性测试（NDT）方法。UPE方法使用声应力波来研究亚表面层的性质，并通过识别与混凝土不同的声学阻抗的任何异常定位缺陷。开发了测试方法以解决一般的实际限制Ultrasonic Pulse Velocity测试，例如需要进入混凝土元件的两侧。
TheACI 228.2R第3.2.2节provides a comprehensive review on the evolution of ultrasonic pulse echo method, and instruments over the past few decades. While traditional UPE instruments were capable of providing A-Scans and B-Scans, modern Ultrasonic Pulse Echo Tomography devices are capable of providing real-time B-Scans that would enable engineers to see sub-surface targets with further clarity. Mobile-based Applications, along with Artificial Intelligence and Modern signal processing techniques have brought superior speed and clarity, with ease of use.
超声波脉冲回声如何工作？
As we discussed earlier, UPE uses stress waves. The principle concept behind the test is measuring the transit time of ultrasonic wave in concrete. A modern UPE instrument consists of an array of piezoelectric transducers that are capable of exciting concrete surface through short-burst high amplitude pulse-high voltage and high current- (see战略公路研究计划 - SHPR2，TRB，2013）。随着脉冲在混凝土内传播，它在空隙的界面或其他内部目标的界面中被反射和折射。声学阻抗中的任何异常导致发射的脉冲，并且在接收换能器中监测反射应力波。分析信号以计算波行程时间。
According to the SHRP2, “Based on the transit time or velocity, this technique can also be used to indirectly detect the presence of internal flaws, such as cracking, voids, delamination or horizontal cracking, or other damages.”
Applications of UPE Methods
超声波脉冲回波方法广泛用于混凝土检测和测试。以下部分介绍了主要应用程序和用例：
1. Estimate Thickness of Concrete Elements
超声波脉冲回波被工程师广泛使用，以评估混凝土元件的厚度。这在具有单面访问（单侧访问）的混凝土元件中是特别重要的，例如：
- Tunnel linings:厚度测量在隧道衬里的QC过程中至关重要。它也是结构评估目的的重要参数。
- 树干下水道：在行李箱下水道中，UPE可以帮助工程师估计现有衬里的厚度。这变得非常具有挑战性，因为涉及与核心钻井热工作的侵入式方法不是安全性也不是具有成本效益的解决方案。此外，浅截面始终存在具有高水力静压的浅剖面风险。
- 混凝土罐：Testing concrete tanks that are used in industrial chemical processes is often challenging. Maintenance managers of such facilities often have very short downtime windows, and permission to get inside the tank is not always practical (unless during essential maintenance cycles). UPE enables thickness measurement and quality assessment from exterior face.
2.肌腱管道中的灌浆缺陷
Along with地面穿透雷达（GPR）和Impact-Echo, UPE可以提供关键信息的空洞和defects that might have happened during grouting process of tendon ducts in post-tensioned concrete elements.
3.定位子表面缺陷
UPE tomography can be used to assess certain defects in concrete elements. UPE can pinpoint the following defects:
- Delamination:UPE methods can be used to assess the location and extent of delamination in concrete bridge decks, parking garage slabs, and concrete tanks
- 蜂窝：UPE is a great tool in the Quality Control and Quality Assurance of new construction. UPE can be used to localize honeycombs in concrete.
4. Quality Control and Quality Assurance
UPE can used as in-direct method to assess the overall quality of concrete. Through the measurement of pulse velocity, engineers can evaluate the quality of concrete materials after construction.
5. Evaluation of Fiber Reinforced Concrete
虽然GPR在评估纤维钢筋混凝土（FRC）元件中具有一定的实际限制，但UPE方法提供了可靠的替代品，用于元素的厚度测量和质量控制。这使得它们在隧道中的混凝土衬里检查和测试中是一个有趣的替代方案。
UPE和实际考虑的局限性
Like all other NDT methods, UPE comes with its practical challenges for certain field conditions.
- 关闭测试点的间距：为了产生亚表面缺陷的可靠和精确的地图，工程师需要在测试点之间使用近距离间隔。这可以对大型测试区域进行测试耗时。实际解决方案是使用另一种方法，例如GPR，用于快速筛选，并使用UPE进行高分辨率的缺陷成像。
- 耦合问题：声信号的质量依赖于换能器和混凝土表面的耦合。该驾驶室对粗糙表面非常具有挑战性。现代设备已经尝试在换能器的基础上使用弹簧支持机制解决问题，以允许在粗糙区域周围操纵。
- 未检测到的缺陷：Certain defects might remain undetected. This is specially true for very shallow flaws or when operators work with low frequencies.