Evaluation of existing surface cracks is normally done during the visual inspection of structures. Crack opening on the surface of concrete is normally measured using a crack gauge. Depending on the opening of the cracks on the surface, cracks can be described (as tiny as hairline, or cracks with a few millimeters opening); however, this procedure does not provide enough information about the depth of these cracks. Different techniques have been developed and used over time for the purpose of crack depth measurement in concrete. But first, lets see why concrete cracks, and why crack depth measurement in concrete matters?
Why Does Concrete Crack?
A crack is a linear fracture in concrete which extends partly or completely through the member (OSIM, 2008). Some people believe that concrete is born with cracks; that its ingredients, and how it is produced and transported – from the batching plant to pouring, setting, and curing – is influenced by so many factors that cracking of concrete does not come as a big surprise; and to a great extent, that might be true. Surface cracks are the most commonly seen kind of defects in concrete structures (Shiotani and Aggelis, 2007). Cracking of concrete can happen in different stages: It can happen before hardening of concrete, and it can happen in an old concrete structure:
Crack in concrete can occur before hardening stage; this is normally due to settlement within concrete mass, or cracks due to plastic shrinkage. But cracks can develop after hardening. For example, drying shrinkage can occur due to the loss of moisture. Thermal contraction and sub-grade settlement can also cause cracking in hardened concrete. Overloading, and weathering of concrete can also result in cracks.
Crack Depth Measurement in Concrete
But why crack depth measurement in concrete matters? cracks can ease the access for aggressive agents (i.e. chloride ion) to reach to steel reinforcement, which will eventually cause corrosion. It is important to evaluate the depth of cracks, to make sure if surface cracking is well propagated into concrete or not. The effectiveness of repair methods such as epoxy injection relies on accurate prediction of crack depth. Different methods have been developed over years to evaluate the depth of cracks in concrete.
I. Visual Examination of Concrete Cores
In this method, dye is injected (using pressure) into surface cracks. Later, concrete cores will be taken from the area under investigation. The sample is studied under microscope for determining the depth of cracks in concrete. Photo adapted from Kevic C Arne (2014). “Crack depth measurement in reinforced concrete using ultrasonic techniques” MSc. Thesis.
II – Impact Echo
In Impact-Echo test, a stress pulse is generated at the surface of the element. The pulse spreads into the test object and is reflected by cracks, flaws or interfaces, and boundaries. The surface response caused by the arrival of reflected waves, is monitored using a high precision receiving transducer (Malhotra and Carino, 2004). When stress waves travel within the concrete element, a part of emitted acoustic waves by the stress pulse on the surface is reflected over the boundary layers, where different the material stiffness changes. The data recieved by the transducer is normally analyzed in the frequency domain to measure the wave speed and the thickness. This procedure has been standardized as the ASTM C1383, “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method”. However, Impact-Echo can be used to assess the depth of surface cracks.
III – Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) is an effective non-destructive testing (NDT) method for quality control of concrete materials, and detecting damages in structural components. The UPV methods have traditionally been used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. Ultrasonic testing of concrete is an effective way for quality assessment and uniformity, and crack depth estimation. The test procedure has been standardized as “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016). To learn more about UPV method, and its applications, click here.
UPV method can be used for estimating the depth of surface cracks. To do so, each transducers should be place on one side of the crack, for a given distance. Then, the distance between transducers is changed in the same trajectory. the UPV measurement will be repeated for different spacing of transducers.
Ministry of Transportation Ontario MTO, 2008, “Ontario Structure Inspection Manual”. pp 394
Shiotani, T., and D. G. Aggelis. “Determination of surface crack depth and repair effectiveness using Rayleigh waves.” Fracture mechanics of concrete and concrete structures—design, assessment and retrofitting of RC structures. Taylor & Francis, London, UK (2007): 1011-1018.
Kevic C Arne (2014). “Crack depth measurement in reinforced concrete using ultrasonic techniques” MSc. Thesis, Georgia Institute of Technology