Vibration Monitoring services include construction and blast vibration monitoring (short- to long-term), modal vibration measurement to determine natural frequencies and damping of structural systems, and dynamic soil/structure/machine measurement and analyses of vibrating machine foundations. We have conducted vibration-engineering studies for construction, blasting, mining, and electronics manufacturing clients.
The Impulse Response (IR) method is normally conducted in conjunction with the Sonic Echo (SE) method together as the SE/IR method. These methods are used for low strain integrity testing of piles and deep foundations for length and integrity determination. The SE/IR methods are used on both new and existing foundations and is performed by impacting the foundation top and recording echoes from a defect or the foundation bottom with a nearby receiver(s). The methods work best for columnal type foundations, such as piles and drilled shafts, but it has also been successfully used on mat foundations, abutment walls, and similar structures. The methods require access to either some small part of the foundation top, or part of the upper side of the foundation. The SE method performs the analysis of the data in the time domain, while the IR method performs the analysis in the frequency domain. The two analysis methods complement each other to allow the most accurate foundation length and defect analysis possible.
Ultraseismic (US) investigations are performed to evaluate the integrity and determine the length of shallow and deep foundations. This method is a variation of the more typical SE/IR test method, with the US method having a higher sensitivity to weak bottom echoes. The method is particularly useful in testing abutments and wall piers of bridges because of the relatively large exposed areas available for mounting instrumentation. The US method involves mounting transducers in multiple vertical and horizontal locations for the purpose of tracking the position of compressional, shear, and/or bending waves generated when the member is impacted with an instrumented hammer. This method was internally developed as a response to encountered difficulties with the SE/IR and SKM methods when many reflecting boundaries are present.
Pull-out of pre- or post installed inserts is perhaps, among the MDT tests, the most widely used technique to estimate the in situ compressive strength of concrete. It measures the force needed to pull out a metal insert (with enlarged head) embedded into concrete, deducing the material strength by means of experimental calibration curves related to the specific geometry of the insert. The pulling force is applied to the insert by a hydraulic jack counterbalanced by a reaction ring, the insert extracts a concrete cone whose dimensions depend on the geometry of the experimental setup. In this way, an Ultimate Limit State condition, with a mixed compressive-shear stress state, is obtained and makes this test highly reliable.
Infrared Thermography (IT) is a technique used for locating shallow delaminations, cracks, and voids in concrete and other pavements as well as air gaps and voids within concrete slabs, block walls, and other structures. Infrared Thermography senses the emission of thermal radiation from a member and can be used to produce a 2D image from the thermal signal. It is based on two basic principles from heat transfer: conduction and radiation. Sound concrete with no voids, gaps, or cracks is more thermally conductive than low density or delaminated concrete, which then creates small temperature differences on the concrete surface. These temperature differences can be detected and mapped by the sensitive Infrared Thermography equipment. This allows rapid areal mapping of internal conditions. It should be noted that the IT method is most useful for the detection of shallow defects and flaws. Deeper flaws produce in a structure smaller and more diffuse thermal effects on the surface.
The Acoustic Rebound Hammer (RH) method (also known as the Schmitt Hammer method) is used to quickly determine the uniformity of a concrete member with respect to surface strength and stiffness. In addition, from the data collected, an estimate concrete strength may be inferred using standard calibration curves.
Testing is carried out in accordance with ASTM standards, by taking the average of 10 measurements at each location after discarding the two extreme values. Note that the results of this test method are heavily influenced by local surface conditions and surface preparation, and thus the estimated strength is only a rough estimate. One typical application of this method is in locating areas of low strength for further evaluation with coring or other methods.
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