Division of Medical Physics, Groote Schuur Hospital and University of Cape Town, Cape Town 7925, South Africa.
Division of Medical Physics, Groote Schuur Hospital, Cape Town 7925, South Africa.
Phys Med. 2020 Sep;77:43-47. doi: 10.1016/j.ejmp.2020.08.001. Epub 2020 Aug 7.
To estimate the concrete density of a newly constructed bunker using impact-echo testing prior to the installation of the linear accelerator.
A newly constructed bunker showed visible honeycombing after the removal of the construction formwork. Impact-echo testing, which is based on the propagation and reflection of elastic waves in solids, was applied to confirm the bunker shielding integrity. A mechanical impact on the bunker wall generates a stress pulse, which propagates through the wall and is reflected or refracted by voids or changes in material characteristics such as density. Surface displacements caused by the reflected waves are recorded by a transducer, located near the impact point. The resulting displacement-time curves are analysed in the frequency domain for anomalies. The dominant frequencies are related to the depths from which stress waves are reflected within the structure. If the dynamic elastic modulus and Poisson ratio of the concrete are known, then the measured velocity of the so-called P-wave can be related to the concrete density.
Validation measurements on a wall with known concrete density gave an estimate within 3% of the true density. Measured velocities on the honeycombed wall ranged from 3750 m/s to 4300 m/s, corresponding to densities of 2894 kg/m and 2201 kg/m respectively, with the majority of estimated densities ranging from 2307 kg/m to 2544 kg/m. A radiation survey after the installation of the linear accelerator confirmed adequate shielding.
Impact-echo testing presents a viable solution to confirm bunker shielding integrity before the installation of a linac.
在安装直线加速器之前，使用冲击回波测试来估计新建造的碉堡的具体密度。
一个新建造的碉堡在拆除施工模板后出现明显的蜂窝状。冲击回波测试是基于弹性波在固体中的传播和反射，用于确认碉堡屏蔽完整性。在碉堡壁上进行机械冲击会产生一个应力脉冲，该脉冲通过墙壁传播，并被空隙或密度等材料特性的变化反射或折射。位于冲击点附近的换能器记录由反射波引起的表面位移。所得的位移-时间曲线在频域中进行分析，以查找异常。主要频率与结构内从何处反射应力波有关。如果已知混凝土的动态弹性模量和泊松比，则可以将测量的所谓 P 波速度与混凝土密度联系起来。
对具有已知混凝土密度的墙壁进行的验证测量，其估计值在真实密度的 3%以内。在蜂窝状墙壁上测量的速度范围为 3750 m/s 至 4300 m/s，分别对应于 2894 kg/m 和 2201 kg/m 的密度，大多数估计的密度范围为 2307 kg/m 至 2544 kg/m。在安装直线加速器后进行的辐射测量证实了足够的屏蔽。
冲击回波测试为在安装直线加速器之前确认碉堡屏蔽完整性提供了一种可行的解决方案。
