How To Measure Cracks In Concrete

The determination of concrete cracks includes location, shape, distribution, width, length, depth, quantity, etc.
The location, quantity and trend of cracks are commonly recorded by photos and drawing crack expansion maps, and the length is measured with a ruler and steel ruler. This post will introduce the determination method of crack width and depth in concrete.

Determination of crack width in concrete

1)Microscope is commonly used to measure crack width, an instrument composed of an optical lens and vernier scale, with a minimum scale value less than 0.05mm.

2)It can also use to compare and measure the crack with the standard width plate (crack card) printed with different width lines;

3)A set of gauges with different thickness are used for comparison.

Other common crack width methods:

Optical fiber crack sensor

Ansari measured the crack width in concrete by ring optical fiber.

The principle:

The light transmitted by ring optical fiber is a function of light propagation fluctuation caused by crack growth.

Paint coating method

It is a kind of paint that cracks under a certain tensile strain. The cracking direction of the coating is orthogonal to the principal strain direction to determine the specimen’s principal stress direction.

Pros：

It can use for any structure without being limited by structural material, shape and loading method.

Cons：

The cracking strength of the brittle paint layer is closely related to the tensile strain. Only when the cracking strain of the specimen is less than the minimum natural cracking strain of the coating can it be used to determine the specimen’s cracks.

In 1975, the BLH company developed a method to find cracks with conductive paint film. It is to apply an elastic conductive paint with a small resistance value on the cleaned concrete surface to produce a strip with a length of about 100-200mm and a width of 5-10mm, which is connected to the circuit after drying. When the concrete crack width reaches 0.001-0.004mm, the elongated conductive paint film will spark until it burns out due to the tension of the concrete. After the conductive paint film circuit is cut off, it can continue to observe with the naked eye.

Determination methods of crack depth

Sensor instrument monitoring

It is to use the instrument embedded in the concrete for crack monitoring by crack meter, with a range of 0.12 ~ 1, so it is a point detection method. Due to the spatial randomness of the cracks’ emergence, it is commonly missed. To monitor the cracks better, we must make large-scale, continuous and distributed monitoring-full distributed monitoring.

Optical fiber sensor network monitoring

There are many advantages for optical fiber sensing network monitoring.

Pros：

flexible, high precision, anti-electromagnetic interference, reliable and durable, and easy to transmit through optical fiber to produce an active telemetry system.

The generation of cracks can be determined by the light intensity change of the optical fiber embedded in the concrete. We can measure the position of cracks correctly through the sudden change point of crack loss on the attenuation curve.

Based on the optical time-domain reflection technique, an optical fiber crack sensing network can detect concrete structures’ dispersion and determine the width, location and orientation where the crack intersects with the optical fiber sensing network.

The thing to be considered before testing crack width in concrete

1)The crack width test requires that the accuracy of the crack width tester is better than 0.02mm.

2)The concrete surface at the measuring position should be clean and flat, and there should be no dust or mud inside the crack.

3)It had a better test when the crack was open.

4)it should be arranged in more than two width measuring positions on a continuous crack.

5)Mark the detection and maximum crack width positions in the crack distribution map.

Ultrasonic testing

Ultrasonic testing is a non-destructive detection method to get the internal information of the object with ultrasonic as the medium.

The principle :

Determine the crack depth by comparing the propagation time difference of sound waves in concrete with and without cracks under the same conditions of concrete material properties (including definite composition, mix proportion and age, etc.).

Due to the tiny particles and high and even density of concrete, the sound wave can propagate well and find its internal defects and parts correctly. Knowing the crack depth on the concrete surface is significant for durability diagnosis and exploring repair reinforcement.

How to measure the crack depth by ultrasonic tester

In general, the emission and absorption probes are arranged around the crack on the same surface of the concrete. However, there are many specific methods due to the different waveforms (longitudinal wave, transverse wave and surface wave) and acoustic parameters (sound velocity, frequency, phase, etc.)

Which crack width testers are good?

1. Crack microscope:

Observe the crack width with a microscope with an accuracy of 0.02mm-0.05mm directly. It needs manually adjust the focal length and read and record at a close distance.

Cons:

Slow test speed, high labour intensity

But crack microscope is the main method of crack testing at present. For example: wysk-40 crack width tester: measurement range: 0-4mm, accuracy: 0.05mm; Ele35-2520 \ 2505 crack width tester: measuring range: 0-4mm, accuracy: 0.02mm.

2. Feeler gauge or crack width comparison card:

Pros：

Simple

Cons：

Low accuracy

It can only be used for rough measurement.

Micrometre method

It is mainly used to measure the width and offset of cracks, including one-way punctuation and three-way punctuation.

Unidirectional point method: it is commonly used to measure the width of cracks. According to the distribution of cracks, choose a representative position of cracks, and bury a punctuation mark on both sides of the crack. Use a metal rod with a diameter of 20mm and a length of about 80mm embed in the concrete for 60mm at the point, and there is a protective cover on the point. The distance between the two points should be more than 150mm. We can measure the change value of the distance between two points regularly with a vernier calliper to know if they tend to crack. The measurement accuracy can reach 0.1mm.

Three-way point method: there are two kinds of three-way points: plate type and rod type. At present, plate type is mainly used, which is to make two metal plates with a width of 30mm and a thickness of 5-7mm into corners perpendicular to each other in three directions, weld three pairs of stainless steel triangular prisms on the plate to observe the changes in three directions of the crack, and fix the scale on the concrete with bolts.

We can get the three-dimensional relative shift by measuring the distance change between each pair of triangular prisms with an outer diameter vernier calliper.

3. Crack width tester with image display and manual

In recent years, there are crack testing instruments in the market that take crack images through the camera and enlarge them on the screen, and then manually read the crack width according to the scale on the screen, such as on-FK (o) crack width tester, SW-LW-101 surface crack width observer, Z 72, etc.

It does not need to adjust the focal length of the crack microscope closely and reduces the labour of the crack test, but it still needs to estimate and record the width manually. Therefore, there is a dispersion in manual reading.

4. Crack width tester with image display and automatic

This instrument can display the crack depth automatically.

First, the camera photographed the crack image and enlarged and displayed it on the screen. The crack image is processed and recognized, and specific algorithm programs automatically interpret the crack width. This measuring instrument can photo the crack image and automatic display, record and store it.

Pros：

real-time test, fast and accurate

For example, the test range is 0.05-2.0mm, and the accuracy is 0.03mm for the FCV-21 crack width tester.

Wrapping up

Crack is a common phenomenon in concrete structures, which will not only reduce the impermeability of buildings but also cause the corrosion of reinforcement, the carbonization of concrete, reduce the durability of materials and affect the bearing capacity of buildings. Therefore, it is essential to measure cracks in concrete.