Observing crack width changes is one of the techniques used to monitor structural damages due to ground movements, and there are several instruments used to monitor crack width changes. These instruments are discussed below.
Methods to Monitor Crack Width Changes in Concrete
The following are the instruments used to monitor crack width variations:
- Steel ruler
- Magnified graticule
- Plastic tell tale
- Glass tell tale
- Brass screws and caliper
- Displacement transducer
Steel ruler is simple instrument used to monitor crack width variation. The width of the crack can be measured to the nearest 0.5mm provided that great care is practiced.
It should be bear in mind that steel rule measurements are subjective because it is not possible to measure crack width from the same point each time the measurement is taken.
That is why steel ruler measurements are used for assessing state of damage at the beginning of investigation.
Fig.1: Steel Ruler
Magnified graticule is used to monitor internal crack in smooth finishes. The monitoring is conducted by measuring the offset between two pencil marks utilizing magnified graticule.
The resolution of 0.1mm can be achieved with magnified graticule if necessary cautions are practiced.
Fig.2: Crack Width Measurement Magnifier
Plastic Tell Tale
It is the most famous system used to monitor crack width variation. Plastic tell tale consist of two overlapped plates.
The plate with scales marked in millimeter units of measurement is fixed on one side of the crack and the other plate marked with cursor is fixed on opposite side of the crack as it is shown in Figure 3.
The instrument is screwed on the wall in such a way that the cursor of one plate and the middle of the scale of opposite plate will be aligned. So, as the crack experiences movement (including shear or normal movement), the variation can be measured to the closet of millimeters by recording the position of the cursor with respect to the scale.
It should be known that, the reading can be taken any time by any individual and it does not require initial zero reading.
Fig.3: Details and Installation of Plastic Tell Tale
Glass Tell Tale
This technique used to measure crack width variation in the past, but it is not popular any more. It basically consists of strip of glass cemented on to the cracked structural element as shown in Figure 4.
As it may be observed from the figure, glass tell tale is neither show the direction of the movement nor the magnitude of the movement. That is why it is not used any longer.
Fig.4: Glass Tell Tale
Brass Screws and Caliper
In this technique of monitoring crack width variation, two screws are fixed on each side of the crack as shown in Figure 5.
Fig.5: Fixing screws on each side of the crack
After that, a caliper will be used to measure the width of the crack as illustrated in Figure 7. As it may be observed, the screws will guarantee the correct measurement of the crack width and prevent errors.
Fig.6: Measuring crack width using caliper by fixing its ends on the installed screws on each side of the crack
Moreover, the caliper can be employed in different modes to measure crack width at corners and other locations with similar difficulty in accessibility. Furthermore, it is possible to measure both normal and shear movements provided that three screws are installed at right angle triangle as shown Figure 7.
Fig.7: Installing three screws at right angle triangle to measure both shear and normal movement
Finally, this technique provides accuracy better than +/-0.1mm and resolution of 0.02mm using digital caliper
This instrument is used to continuously monitor the movement of cracks so as to provide warnings when abrupt movement or in the case where the location of the crack is not accessible like railway tunnel.
Linear variable displacement transformer (LVDTs) and potentiometric displacement transducer are the two commonly devices used to continuously monitor crack width variations. Both instruments can be read either manually using hand held unit or automatically employing data logger.
This method of monitoring crack width variation is expensive but the requirement for such monitoring would justify the utilization of these devices.
Fig.8: Installation of Linear variable displacement transformer instrument