Happy Friday everyone!
In this video, I am going to show you to use non-destructive testing for evaluation
of cracks in a concrete beam element.
But first, let’s see what a crack is?!
A crack is a linear fracture in concrete which extends partly or completely through the member.
Concrete is a relatively good material in compression; however, it can easily break
as a result of tensile stresses.
Several issues can result in cracks in concrete, including excessive external loads, external
restraint forces, internal restraint forces, differential movements, settlements, or corrosion
of reinforcement.
In a concrete beam, tensile stresses are initially carried by the concrete and reinforcement.
When the tensile stresses in the beam exceeds the tensile capacity, the concrete cracks.
After this point the tensile force is transferred completely to the steel reinforcement.
In a concrete beam, the crack widths and their distribution is mainly controlled by transverse
reinforcement, or simply the stirrups.
Cracks resulting from externally applied loads initially appear as hairline cracks and are
harmless.
However, as the reinforcement is further stressed the initial cracks open up and progressively
spread into numerous wider cracks.
Now that we have now why and how concrete cracks, let’s get back to our first question.
How can we evaluate cracks in concrete?
A systematic assessment often starts with detailed close-up visual inspection of the
element.
The cracks and other surface defects are identified and marked.
For each crack, two pieces of information are collected: The crack width, and crack
depth.
Crack widths can provide an indication of the severity of the cracks.
Crack widths can easily be measured using a simple crack width ruler.
Modern applications using image processing techniques, or laser scans can also help study
the distribution and width of cracks.
While hairline cracks might not impose serious risk on structural integrity, wider cracks
can be a sign of severe distress and require repair or retrofitting.
Crack depths on the other hand, can be used to study the structural integrity, and durability
performance of the concrete beam.
One simple NDT method for estimating the crack width is the ultrasonic pulse velocity or
simply UPV.
In this particular application, the sending transducer is placed on one side of the crack,
while the receiving transducer is placed on the other side.
First the sensors are placed at a distance D, and the transit time is measured.
Later, the distance is increased to 2xD, and the test is repeated.
Then the results are used to estimate the cracks depth.
Crack depth can show if the surface defects propagate deep into the element or are limited
to the surface, thus helping engineers in evaluating structural integrity.
Another great technology is ground penetrating radar or simply GPR.
We use GPR scans to locate the stirrups.
This will allow structural engineers to verify the adequacy of existing reinforcement for
the intended loading.
It will also provide information on the structural details such as the rebar spacing, and concrete
cover thickness.
How about concrete thickness in elements with one side access only?
Examples can be the beam I showed you earlier, retaining walls, and concrete tanks.
Ultrasonic Pulse Echo method can be used to evaluate the thickness of existing elements.
The method uses shear waves to estimate the depth and location of internal voids and anomalies,
as well as the thickness of the element.
And last but not least, the rebound hammer test.
Rebound hammer or Schmidt hammer can be used to identify weak spots in a concrete element.
It can also be used to get an estimate of the compressive strength if a reliable correlation
between compression test and rebound number is established.
Another approach would be to use combined NDT methods, such as the SonReb procedure
to get a more accurate estimate of concrete strength on spot.
This helps minimize the number of intrusive tests, and avoid unnecessary interventions.
Several researchers have developed mathematical models to determine the compressive strength
by combining the ultrasonic pulse velocity and rebound hammer number in an exponential
function.
Well that is for today.
To wrap up what I discussed in this presentation: Visual inspection is the first step towards
evaluating the condition of the concrete beams.
Crack width rulers can be used to identify the severity of cracks, while UPV method can
help evaluate the crack depths.
GPR can be used to determine the location, and spacing of stirrups, and get an estimate
of the concrete cover thickness.
UPE, on the other hand can provide critical information on the thickness of concrete element,
as well as identifying potential sub-surface defects.
And finally, rebound hammer test can be used to verify uniformity and strength in a concrete
element.
Thank you very much for your attention, and have a lovely weekend.
Stay Safe!