IN-SITU MEASUREMENT OF RELATIVE HUMIDITY AND EXPANSION OF CRACKS IN STRUCTURES DAMAGED BY AAR

This paper demonstrates an in-situ system for measurement of relative humidity and expansion of cracks in structures damaged by alkali aggregate reaction (AAR). The concept of the system is use of inexpensive equipment and that structural owners themselves carry out measurements. Relative humidity is measured by use of wooden sticks mounted in plastic tubes drilled into the concrete. The moisture percent of the wooden stick is manually measured by use of a moisture probe (electrical conductance). Expansion of cracks is measured on 3 triangular arranged rosettes with equal distances on 50 mm. Measurements are manually carried out by a Demec gauge. Since 1995 the in-situ measurement system, has been in use in a principal road bridge in Trondheim and a dam structure in Southern Norway, both damaged by AAR. The same system has since 1998 been mounted on an important office building damaged by AAR in Oslo. Here, three types of impregnation/coating (two Silanes and one Elastic Acrylic Dispersion) as well as cladding by aluminum plates are assessed according to humidity and expansion. Relative humidity is measured several places on each structure as well as expansion of cracks. The paper gives documentation of the methods and some results of measurements. The following conclusions can be drawn: humidity measurements by wooden sticks are very stable over time and lies in the interval 80% to 100% relative humidity in structures with AAR. The precision of relative humidity measurements is about +/- 3% when calibration curves valid for only one wooden stick has been used. It can be shown that surfaces exposed to periodically rain obtain higher humidity than leeward surfaces. This are revealed as humidity profiles. For the dam structures no difference in humidity occur between the surface and 40 cm in the concrete. Measurements of cracks show that cracks are “alive” and expand yearly from zero to 0,25mm. Preliminary results of the coating systems suggest that all the systems reduce the relative humidity over time, 3 cm from the outer surface.