Crack Monitoring
Need for Dual Monitoring of Foundations and Cracks
There is often still a requirement to monitor relative movements across cracks. The ideal way of proving whether or not distress is related to foundations is to monitor both vertical movements and cracking. If new cracking occurs or there is movement across existing cracks while the Movement Gauges indicate no relative vertical movements then damage is not related to the foundations.
In most circumstances the most inconspicuous form of crack monitoring is the use of three studs or screws which are read by a vernier callipers. However, cracks often have to be monitored at high level and ladder access increases costs significantly.
Temperature should be recorded as movement at cracks can be related to thermal effects.
Moiré Gauges for Easy Viewing of Movement
While there are several high-tech crack gauges on the market, the most exciting advance has been the development of Moiré Patterns by Bill Harvey Associates Ltd (patent GB 2 386 189).
Moiré patterns are created when two regular grids or nets are superimposed. Any displacement of one net relative to the other changes the moiré pattern. These allow a camera to read movements of a fraction of a millimetre in any direction. An immediate and permanent record can be safely made from several meters away. They can even be read from considerable distances using binoculars or telescopes.
Reading the Moiré Gauges
The application here will allow you to experiment with the sensitivity of the gauges and see how the gauges are read. Moving the scroll bars shows the effect of displacement. The units are millimetres. Note how the pattern shows both magnitude and direction of movement. A photograph of the pattern from site can be compared with this diagram back in the office to indicate the vertical and horizontal movement to 0.1mm.
Dynamic Monitoring
Dynamic responses can be recorded with video cameras. The video below shows small dynamic movements across a crack as live loading crosses a bridge.