Crack Gauges / Monitors
Installed across surface cracks in either a vertical or horizontal position. This gauge measures movement within a specified period of time. Photographic documentation is done, and readings are made by comparing the position of a crosshair over a calibrated grid, from a baseline (when first installed) and the most recent readings at a specified time interval (after 1 day, 2 or 1 week). Photos are taken preferably at the same angle or vantage at a predetermined time lapse.
An electronic version which can be installed and remotely monitored could be placed in hard to reach or inaccessible areas or where it would be hazardous to send a technical crew to take photos of cracks.
Manual crack monitors currently in use are two strips of rigid plastics, placed one on top of the of the other. The lower strip usually opaque white has a grid-like pattern that is graduated in millimeters and can be accurately read to 0.5 of a millimeter and could be inferred up to 0.25 mm. The upper strip is a clear strip of plastic with a cross hair usually colored red to differentiate it from the black lines of the grid like pattern on the lower strip. When put together the crosshair should lie approximately on the center and align with the grid-like pattern of the lower plastic strip.
Manual crack monitors are installed across a surface visible crack and either glued or screwed by its end portions on opposite side, in either a vertical or horizontal position. The two plastic strips, the top with crosshair; and, below, the grid pattern aligned, would be free to move independent of each other. Movements are then determined by the position of the crosshair against the grid pattern.
After installation and when the glue or screw is determined to be stable or relatively immovable, pictures are taken and these are considered as the baseline reading. Succeeding pictures taken after a pre-determined period of time; usually one week or 7 days, are then compared with the baseline picture and the difference or the absence thereof, are submitted as the movement report for that period. The frequency of picture taking may be increased if movements are detected and upon the order of the Engineer of Record. Photos are taken preferably at the same angle or vantage at a predetermined time lapse.
An electronic version which can be installed and remotely monitored could be placed in hard to reach or inaccessible areas or where it would be hazardous to regularly send a technical crew to take photos of cracks. This set-up is composed of a String Potentiometer, stretched over a crack, and connected to a transmitting device, usually a WASP transmitter. Any movement on the crack would be recorded and graphed, and a periodic email sent to pre-chosen recipients. This whole set-up can be powered by small built-in battery that could last up to 5 years. This electronic crack monitor may also be rigged to sound an alarm or send an email to the pre-determined recipients when limits are reached or approached.