s’MASH

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s’MASH

Purpose

mash_1 For a long time, users of NDT systems have wished for a rapid, easy to use method for rapid screening of the integrity of structures. The s´MASH impulse-response test system fulfills this wish. The idea is to quickly screen a structure for flaws and identify suspect areas for subsequent detailed investigation, e.g. by the impact-echo test (using DOCter or MIRA d’Or), ultrasonic-echo testing (using MIRA or EyeCon), or by invasive inspection with drilled cores (using CORECASE).

With the s´MASH, rapid evaluation can be conducted for:

  • Detecting voids beneath concrete slabs in highways, spillways, and floors
  • Detecting curling of slabs on ground
  • Evaluating the integrity of anchoring systems of wall panels
  • Locating delaminations and honeycombing in bridge decks, slabs, walls and large structures such as dams, chimney stacks, and silos
  • Detecting the presence of damage due to freezing and thawing
  • Detecting the presence of alkali-silica reaction (ASR)
  • Detecting debonding of asphalt or concrete overlays and repair patches from concrete substrates
  • Evaluating the effectiveness of the load transfer system in transmitting frorces across joints in concrete structures

The application of impulse-response to plate-like structures is governed by ASTM C1740, “Standard Practice for Evaluating the Condition of Concrete Plates Using the Impulse-Response Method.”

Principle

The s´MASH uses a low-strain impact, produced by an instrumented rubber tipped hammer, to send stress waves through the tested element. The impact causes the element to vibrate in a bending mode and a velocity transducer, placed adjacent to the impact point, measures the amplitude of the response. The hammer load cell and the velocity transducer are connected to a portable field computer with s´MASH software for data acquisition, signal processing, data display, and data storage.

As shown below, the time histories of the hammer force and the measured response velocity are transformed into the frequency domain using the fast Fourier transform (FFT) technique. The resultant velocity spectrum is divided by the force spectrum, to obtain the mobility as a function of frequency. An example of a mobility plot for a solid concrete plate-like member is given in the plot on the right. Mobility is expressed in units of velocity per unit force, such as (m/s)/N.
mash_2The parameters from the mobility plot that are used for integrity evaluation are:

  • The dynamic stiffness (the inverse of initial the slope of the mobility plot, the blue line in previous figure;
  • The average mobility (dotted blue line in previous figure);
  • The mobility slope between 100 to 800 Hz from fitting a straight line; and
  • The voids ratio (the ratio of the amplitude of the low frequency peak to the average mobility)

mash_3Examples of mobility plots for different types of flaws in plate-like structures are shown above. The figure on the left shows the mobility plot for a slab-on-ground with a void below the slab at the test location and the mobility plot for a slab with uniform support at the test point. The voids ratio is the ratio of the amplitude of the low frequency peak to the average mobility of the slab with good support. The figure on the right figure compares the mobility plot of a honeycombed region in a silo wall with the mobility plot of properly consolidated concrete. Honeycombed concrete is typically associated with a high mobility slope (the dashed lines).

Testing is performed on a grid marked on the surface of the structure. After testing at grid points is completed, the s’MASH software constructs color contour plots of the various parameters, from which it is easy to identify anomalous regions of the structure that merit further investigation. This is done on-site after the testing has been completed, producing immediate information on the presence of anomalies.

Testing Examples

mash_4 Shown on the right is the result of one test as displayed on the computer with the s’MASH software. The top left window is the force-time curve obtained from the impact of the instrumented hammer. The top right window shows the velocity-time curve obtained from the geophone in contact with the concrete surface. The plot in the middle window is the mobility plot obtained from the previous two waveforms. The upper right quadrant shows the various parameters calculated from the mobility plot. The bottom of the window shows the defined testing grid: the green points represent points that have been tested, the yellow point is the current test point, and red points are locations to be tested.

Below is the contour plot of the average mobility from s’MASH tests performed on the soffit of a bridge slab that was suspected of containing delaminations. Tests were performed on a 1 × 1 m grid. Based on the contour plot, cores were drilled at three locations: (1) a region of low mobility, (2) a region of intermediate mobility, and (3) a region of high mobility. The cores confirmed that low mobility (rigid response) corresponded to a sound slab and higher mobility (flexible response) corresponded to the presence of delaminations.
mash_5

Testing Applications

mash_6 Testing for voids behind tunnel lining
mash_7 Testing for delaminations in bride deck
mash_8 Testing for delaminations in dam spillway
mash_9 Testing for tightness of joints of concrete tank
mash_10 Testing for honeycombing and delaminations in bridge piers
mash_11 Testing for voids below industrial floor slab
mash_12 Testing for anchor integrity for granite panels in high-rise building
mash_13 Testing for delaminations in asphalt covered concrete pavement

s´MASH Ordering Numbers

s´MASH-4000 Instrument Case
mash_14

Item Order #
Instrumented hammer s´MASH-4000-10
Calibration certificate for hammer s´MASH-4000-20
Rubber tip for regular testing s´MASH-4000-30
Aluminum tip for pile testing s´MASH-4000-40
Transducer for 360º testing s´MASH-4000-50
Calibration certificate for horizontal transducer s´MASH-4000-60
Belt box with 3 m cable s´MASH-4000-90
Manual s´MASH-4000-100
Attaché case s´MASH-4000-110

The instrumented hammer and the transducers need to be calibrated once a year.

s´MASH-4000 Computer Case
mash_15

Item Order #
Computer with Windows®, Excel®, and s´MASH software installed s´MASH-4000-200
s´MASH software, CD-ROM s´MASH-4000-210
Amplifier box s´MASH-4000-220
Protection shield for connecting cable s´MASH-4000-230
110-220V AC adaptor with cords s´MASH-4000-240
Attaché case s´MASH-4000-250

The amplifier box can also be used for DOCter impact-echo testing, for spectral analysis of surface waves (SASW) testing, as well as for parallel seismic testing.

A two-day training course is offered separately. The course covers the theoretical background of impulse-response testing, the testing methodology, testing cases from a variety of structures and hands-on training on testing with the s´MASH.

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