Cracks carry the same affinity towards concrete that wet cement holds towards solidifying, which can be summarised in a single word, inevitable. On 29th August 1907, The Quebec bridge situated in the western part of Quebec City, during the penultimate stages of its construction, suffered a collapse in the southern concrete beam arm and a portion of the central section, collapsing into the St. Lawrence River situated below, causing the whole bridge to collapse in under fifteen seconds. 

The unfortunate collapse took the lives of 75 workers, leaving its mark as the world’s worst Bridge construction disaster. This came to occur as the visual examination techniques available at the time allowed the engineers to issue a warning for no additional load to be applied but were limited from predicting the collapse. The section of the bridge suffered another constructional accident in 1916 due to negligence; the construction, which concluded in September 1917, had incurred a hefty $23 Million cost and the loss of the lives of 88 bridge workers.

1907 Quebec bridge disaster (image credits:  Holgate, Henry; Derry, John, G. G.; Galbraith, John. (1908). Royal Commission Quebec Bridge Inquiry Report. Sessional Paper No 154. S.E. Dawson printer to the King Ottawa. Appendix 19, figure 20)

Disasters of this kind stress the power of preventive and mitigating techniques in checking these instances and saving invaluable lives and capital. Through years of sophistication and technical refinement, these techniques have evolved into NDT (Non-destructive testing) methodologies that are followed in the 21st century, which involve methods of crack depth measurement as a parametric pivot for its procedures.

Concrete refers to a composite material fabricated using water, cement, and bonded together aggregates (coarse and fine) and stands as the second-most-used substance in the world only next to Water. It forms the backbone of modern industry and infrastructure, with widespread usage in reinforcing structures and being used as a layering material for myriad purposes.

 The construction industry has come a long way since the Quebec disaster regarding safety and damage prevention and the raw materials used in concrete. Still, the growing complexity of infrastructures and technologies has given way to new hurdles in the sector.

A Linear fracture that extends partially or completely through a concrete structure is a crack. Concrete structures can precipitate cracks throughout their life span because of both early-stage flaws and deterioration of the composite with time. Hence, imperative mitigation of the cracks gives importance to the methods involved in extensive evaluation and measurement of their dimensions, the width of the crack along the surface, and the methods for crack depth measurement in concrete.

Factors causing the cracking in concrete structures can range from flaws in its manufacturing process and transportation to errors during pouring, setting, or curing, along with deterioration from aging. Environmental factors play a crucial role in the above processes; hence cracks are quite prevalent when it comes to concrete structures.

Also Read, Pipe Line Isometric Drawings and P & ID Drawings

Factors that influence cracking in concrete structures (Image credits: Safiuddin, Md, et al. "Early-age cracking in concrete: Causes, consequences, remedial measures, and recommendations." Applied Sciences 8.10 (2018): 1730.)

Measurement of the depth of the cracks helps analyze the propagation of the fracture through the structure to determine the future action taken to curb further deterioration. Visual inspection and imagery methods discussed in the following sections help analyze the width, giving an accurate analysis of the crack if it has propagated to the foundations or the reinforced metal, depending upon the usage.

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Type of Cracks in concrete from various environmental and artificial causes (Image credits: gambrick.com)

Conventional methodologies, as used during the construction of the Quebec bridge in 1907, involved the evaluation of in-situ properties of concrete that would result in damage to the concrete structure during extractions of test specimens. These specimens could not be accumulated repeatedly under this constraint, nor did it show an accurate gauge of the effects of time, loading, and Ambient factors surrounding the concrete structure.

Non-destructive methods of testing (NDTs) are non-invasive and non-destructive techniques employed to examine the composition and integrity of materials without compromising their functionality. These techniques help analyze the cracks while preventing further damage and predicting any structural failures; as evident from numerous disasters since the 1907 Quebec bridge collapse, these failures, and even the most meager cracks, can be catastrophic economically to infrastructure and industry to precious Human-life. 

Further Reading, Safety Precautions That Must Be Taken While Performing Non-Destructive Methods

WHAT IS CRACK DEPTH?

The Crack depth refers to the dimension which indicates how deep the crack goes through the concrete structure, as indicated in the figure with the dimension h:  

Figure indicating the crack depth (Image credit: "Microprocessor application to concrete crack depth measurement." Journal of non-destructive Evaluation 6 (1987): 67-72.)

Methods for Crack depth measurement in concrete help calculate an important parameter to evaluate how further the crack has propagated through the concrete Added. That this method helps in deciding the costs of repair, the methodology for mitigating damage, and the methodology of repairing the cracks to prevent further dilapidation.

The crack depth is less; the Crack is not deep enough to reach the metal reinforcement.

Crack depth is greater, Giving way to corroding chloride ions to the metallic reinforcement of the beam (Image Credits: "Modeling of chloride ions penetration in cracked concrete structures exposed to marine environments." Structural Concrete 19.5 (2018): 1460-1471.

As shown in the figure, in a concrete beam, for instance, Epoxy resin can easily repair shallow cracks, depending on the usage. Still, suppose the core has been affected by the cracking. In that case, it requires a more extensive restoration as the cracks give access to the corrosive aggressors (Chloride ions) affecting the metallic assembly.

EVALUATION OF CONCRETE CRACKS

The preliminary steps involved in the evaluation of concrete cracks are visual inspection followed by monitoring, helping assess the properties of the crack, and evaluating the underlying causes causing the cracking of the concrete. Visual inspection plays an important role as the angle of the crack can relay the form of stress/load being applied on the concrete structure; for instance, shear stress in a concrete beam leads to incline cracks. 

Cracks in a concrete beam are angled differently depending upon the kind of strains applied (shear strain, bending strain; CFRP: Carbon-fibre reinforced polymer) (Image credits: "Mechanical behavior of RC beams reinforced by externally bonded CFRP sheets." Materials and Structures 36 (2003): 522-529.)

Visual inspection can also confirm the presence of corrosive elements in the crack, which can also indicate any corrosion in the metallic assembly. 

Also Read, Acceptance Criteria for Liquid Penetrant Testing

Cracks in a concrete beam with visible corrosive elements indicating metallic corrosion of the core (Image credits:  FRCM composites for strengthening corrosion-damaged structures: experimental and numerical investigations. Diss. Université Laval, 2018.)

The main parameters which help us in the evaluation of the cracks are:

Crack Width:

This parameter helps access the severity of the cracking in concrete and helps classify them based on the evaluated severity and depending on the size. A crack width ruler is used to get a manual surface measure of the parameter. 

Crack width ruler used for surface crack width measurement (Image credits: Fprimec Solutions)

Crack Depth:

As stated in the previous section, this parameter helps us evaluate the integrity of the concrete structure and carry out performance durability verifications. The methodology for repairs and the extent and cost can be determined using this parameter.

Different non-invasive and invasive techniques have been used through the years to measure these parameters. Still, with technological advancements, the non-destructive techniques have provided a safe and preventive measure for cracks without the need for disassembling or breaking down the concrete structure and will be covered in successive sections. 

CRACK DEPTH MEASUREMENT IN CONCRETE

The methods for crack depth measurement in concrete involve several NDTs to access the crack while keeping any further deterioration from the measurement process. These methods are stated as follows:

Visual Examination of Concrete cores:

One of the more popular approaches of the past decades, this method involves extracting the cores from their cracks and studying the cores and the core holes for the extent of the crack, the severity of the damage, and in turn, through physical measurements, calculating the crack depth. 

Visual examination of core holes for crack depth measurement (image credits: Fpimec.com)

A more specialized and non-invasive approach to visual examination involves injecting the cracks with dye using minimal pressure to check any degradation of the cracks. After settling, the concrete cores will be investigated under a microscope analyzing the sample and measuring the depth of the cracks. 

Further Reading, What is the defect size accepted in Ultrasonic Testing ?

Visual examination of the cracks after injecting the dye (Left: macroscopic view; Right: Microscopic view of the dye tip) (Image credits: Crack depth measurement in reinforced concrete using ultrasonic techniques. Diss. Georgia Institute of Technology, 2014.)

Impact-Echo Method:

The impact-echo method (IE) is stress-wave dependent, used in detecting flaws and determining the dimensions of the concrete member in various infrastructures along with specific locations of cracking, voids, and any surface delamination. This method involves monitoring the motion of the concrete surface after the stress pulse is generated, causing a short-duration mechanical impact. The test further measures the amplitude of the reflected shock waves, which helps detect flaws in the concrete sample. 

An electro-mechanical short pulse-generating transducer is used to produce ultrasonic stress waves that propagate into concrete structures. The stress pulse thus generated is reflected over the layers with changes in material density and elasticity. The resulting waves are analyzed by a similar high-precision receiving transducer in the frequency domain (using an oscillator) to measure wave speed and thickness and, in turn, the crack depth.

Impact-Echo Method for crack depth assessment (Image credits: ASTM C1383, “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method”)

In the above figure, L1  denotes the length between the horizontal impact point and the crack; L2 represents the length between the second sensor (High-precision Transducer) and the surface opening of the crack; L3 states the distance between the wave impact point and the first sensor (Pulse generating Transducer); VP represents the P-wave velocity; and Δt denotes the time traveled by the P-wave from the start of the impact to its arrival to the sensor 2. Industry guidelines on the impact-echo method are standardized as the ASTM C 1383 -04: “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.”

Ultrasonic Pulse Velocity Method (UPV):

The ultrasonic pulse velocity method is one of the oldest yet most effective NDT methods for quality control and damage detection of concrete structures.

                   It involves using multiple transducers on either side of the crack at a specified distance. This distance changes in the same trajectory while the UPV measurements are carried out for the different spacing scenarios. 

Ultrasonic testing of concrete (UPV) (Image credits: “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016)

UPV methods have traditional usage in the quality control of welded materials, usually metals. Still, with the advent of technology, UPV has proved to be an effective method for crack depth estimation along with uniformity and quality assessment in concrete materials. Industry guidelines on UPV methods are standardized as the “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016).

Read Further, Preheating in Welding Procedure Specification

Additional methods:

The Non-contact Video-based methodology is used not to obtain an absolute measure of the crack depth but to the change in the depth of the crack and stiffness of the beam, providing an accurate estimation of the progression of the deterioration and causal factors. 

This methodology involves using a high-speed video camera matched with appropriate lenses, the specific target attached or labeled on the object to be measured, and a data acquisition and processing system. Owing to the high-resolution cameras and multi-threading processors available, this method helps in precise data collection to study the rate of change of crack depth and the structures.

INSTRUMENTS INVOLVED IN CRACK DEPTH MEASUREMENT

Visual Inspection:

Visual inspection of concrete is one of the most commonplace but underrated methods of concrete inspection. For more precise inspection, magnifying glasses may be used to thoroughly understand the cracks within the concrete structure's size, depth, and shape.

Concrete core samples can also be extracted and visual inspection to deduce the cause of crack formation. If needed, the concrete is marked with dyes via pressure injection for better visibility of anomalies. Concrete cores can also be tested under a laboratory microscope for further analysis of the structure of cracks formed.

Common accessories used in this inspection method are :

• Measuring tapes, markers, and rulers
• Telescopes
• Binoculars
• Endoscopes
• Borescopes
• Fibre scopes
• Crack-width microscope and gauges
• Portable microscopes

To counteract the variation in the crack area as the dye is being injected, the coring bit is positioned so that the outer edge of the core corresponds with the location where the ultrasonic equipment was placed across the width of the beam, as shown in the figure:

Dye injected into concrete, used for Visual Inspection of cracks (Image credits: Kevin C Arne(2014))

Impact-Echo Test:

According to the industry standardization provided by the American Society of Testing and Materials C 1383, the Impact-echo test induces stress pulses to the concrete test subject, reflected by the anomalies, cracks, and other deformities within the concrete specimen. 

 The Impact-echo test apparatus (Image credits: Mohammed, Hussam. (2019). PRACTICAL METHODS FOR MEASURING CRACK DEPTH IN CONCRETE MEMBERS IN SITE.)

A highly sensitive transducer receives these resultant reflected waves. This P-wave is mathematically used to determine the properties of the cracks found within the specimen. This process is precise and gives detailed results, which provide a thorough analysis of the state of the structure being tested.

Set-up for the impact echo test (Image credits:  "Review of NDT methods in the assessment of concrete and masonry structures." Ndt & E International 34.2 (2001): 71-84.)

The data thus obtained in the frequency domain (owing to the Fast Fourier transform analyzer) is used further to calculate the depth of the flaw under observation. The apparatus commonly used in the impact-echo testing method includes:

• A signal processing unit that accepts incoming signals and digitizes them after processing, further sending the processed signals to the connected computer/display system.

• The number and kinds of transducers determine the instrument's configuration and may range between two to four in number. The types available are:
• Pistol transducer
  
• Cylindrical transducer
• WaveSpeed transducer
• Impactors, consisting of multiple steel balls held on spring rods, are used to create stress waves.
• Bayonet Neill-Concelman cables (Radio frequency connectors)
• Impact-Echo software

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 Ultrasonic Pulse Velocity (UPV) Test:

As per American Society of Testing Materials code C-597, Ultrasonic Pulse Velocity testing is an efficient method of crack depth analysis. Ultrasonic Pulse Velocity testing uses transducers that transmit pulses to the concrete in direct, semi-direct, or indirect methods. The time for wave transmission is hence accounted for concerning the properties of the concrete, and an assessment of the quality of the concrete is attained.

Ultrasonic Pulse Velocity Testing Apparatus (Image credits: Controls Group)

Lower transmission time accounts for better quality concrete, whereas higher travel time of the acoustic waves indicates the presence of irregularities and defects.

Set-up for UPV method (Image credits: "Detection of concrete damage using ultrasonic pulse velocity method." National Seminar on Non-Destructive Evaluation. Hyderabad: Hyderabad, 2006)

The modus operandi of the Ultrasonic Pulse Velocity test is to place transducers on both sides of the crack and carry out multiple iterations of the Ultrasonic Pulse velocity test at a varied spacing of the transducers for the calculation of the crack depth in the concrete.

The ultrasonic pulse velocity tester is a microprocessor-based apparatus with the following features. (Some features may be specific to the UT-3034 model)

It can be linked to a pc using an output (e.g., the RS 232)

It can also be affixed to an oscilloscope.

The range of transit time the instrument can measure is between 0.1µ to 1999.9µ with a resolution of 0.1µs.

It also has two transducers (54kHz), a connecting cable, a calibration rod, and a coupling agent.

FORMULA FOR CRACK DEPTH MEASUREMENT IN CONCRETE

Crack formation in concrete is an inevitable phenomenon. Some crack formations can be perilous, whereas some can be trivial. However, thorough monitoring, documentation, and analysis of these defects should be carried out to ensure safety. A difficulty in measuring crack depth is that the further the crack penetrates the concrete, the narrower its width gets, making it harder to determine the extent to which the crack has entered the concrete.

Crack defects in concrete can cause catastrophic failure and damage. To inhibit these cracks, non-destructive techniques are vital in locating and estimating the extent and type of cracks present. 

Different non-destructive techniques may have their formulae and methods of calculation, some of which are:

The formula for crack depth measurement in concrete for Ultrasonic Testing methods:

The Ultrasonic Testing method uses a transducer to induce impact on the surface of the concrete. Laws of refraction are hence taken into consideration, and various properties of the concrete are gauged. Crack depth is one of those measured features, the formula for which is as follows:

Where D denotes the Crack depth; Cp represents Longitudinal wave velocity; ∆t gives the time of flight of the wave; H is the distance from the crack to the receiving transducer.

Schematics of the wave generated on the impact of ultrasonic incident waves. (Image credit: Arne, Kevin C. Crack depth measurement in reinforced concrete using ultrasonic techniques. Diss. Georgia Institute of Technology, 2014.)

The formula for crack depth measurement using impact-echo methods.

The impact echo method uses the induction of a physical impact on the test surface; the resulting test waves generated comprise P-waves, S-waves, and R-waves. These waves are stress waves named Dilation waves, Distortion waves, and Rayleigh waves, respectively.

This procedure uses the Dilation waves or P-waves to measure the crack depth. The formula for this is as follows:

Where:   D- Crack depth 

Cp- Travel velocity of the P-wave

H1-Distance between the point of impact and crack

H2-Distance between the second sensor and crack

H3-Distance between the impact point and the first sensor

∆t-Travel time of the P-wave from the impact's start to the P-wave's destination point at the second sensor.

Schematics of impact-echo measurement methodology (Image credits: Sun, Yamin, et al. "Depth estimation of a surface-opening crack in concrete beams using impact-echo and non-contact video-based methods." EURASIP Journal on Image and Video Processing 2018.1 (2018): 1-10

FORMULA FOR CRACK WIDTH MEASUREMENT IN CONCRETE             

Crack defects occur within concrete structures during their service life and cause deterioration in performance ability. A thorough analysis of crack dimensions is essential, including the crack depth, width, shape, and nature. 

Crack width measurement is a vital testing process to be carried out while analyzing the integrity and quality of concrete. Various options are available to understand the width of the crack defect formed in the concrete specimen, some of which include crack comparator cards, digital vernier calipers, width gauges for cracks, microscopes for measuring crack width, and optical comparators.

In reinforced concrete, factors like the spacing of reinforcement bars, coding of concrete, the thickness of the structure, applied load, pre-existing stresses, and physical properties of the reinforcement bars affect the size and shape of the crack, often enlarging the width of the crack defect.

As per Indian Standards for plain and reinforced concrete-456 (2000), the crack width WCR can be calculated using the following formula:

Wherein, 

acr= displacement between the selected point on the surface of the concrete to the closest reinforcement bar

Cmin= minimum concrete cover over the longitudinal bar (Obtained from the section of the concrete sample)

h= overall depth of the member from the surface (Obtained from the section of the concrete sample)

X= neutral axis depth from the surface

ε= Average value of strain at the selected region

CONCLUSION

Concrete forms the backbone of the 21st century. As humanity advances in both technology and industry, non-destructive methodologies for quality control and damage regulation of concrete structures have proved to be an asset for the increasing demand for these structures, consequently implying a greater probability of catastrophe in case of structural failure. The latest advancements have enabled better damage evaluation, mitigation, and preventative methods against dilapidation and deterioration in concrete structures.

The methods of crack depth measurement in concrete have benefited from advances in technologies, such as using artificial intelligence instead of human labor where conditions are harsher, saving both Economic and Human capital. High-precision sensors and cameras have enabled us to have an accurate remote estimation of the deformities and damage, leading to a more durable infrastructure.

In the recent past, ignorance of these structural deformities has led to catastrophic outcomes, which are now easily prevented by timely evaluation and repair, only possible from the advancements in the NDT methods and related technologies showing Mankind’s commendable progress in this sector.