Non-destructive measurement of air voids at the reinforcing steel/concrete interface

Air voids are known to be sites for the initiation of corrosion in reinforcing steel bar in concrete structures. The development of suitable non-destructive methods for measuring the void content at the reinforcing steel/concrete interface would therefore provide a predictive measure of the susceptibility of a structure to future reinforcing steel corrosion, and also allow the concrete composition to be adjusted during subsequent pouring. Of particular interest here are air voids from 1 to 10 mm in size. Normal hardened concrete contains from 1 - 2 % air voids by volume. A literature survey of existing NDT methods showed that they did not possess sufficient resolution to detect small voids less than 10 mm in size in concrete. A drawback to existing methods is excessive signal attenuation by the concrete, mainly because they are applied from outside the concrete. It was considered that the only approach to overcome signal attenuation was to develop methods to measure voids from inside the concrete. This is the approach is adopted in the thesis. New methods and approaches to measuring small voids in concrete were proposed. A new sensor was developed, referred to as a voidmeter, and optical fibre and tactile pressure mapping techniques applied in unique ways for measuring small voids. The principle of the voidmeter is simple: a rubber sleeve is stretched over a perforated steel tube, cast in concrete and inflated using water. The volume of water in excess of that needed to fill the sensor itself equals the volume of voids at the sensor/concrete interface. Testing in concrete showed that the voidmeter could measure total void volume with good accuracy in the range covered of 0.2 - 1.2 ml, equivalent to a measured void volume content of 1 - 5%. Resolution was estimated at 0.1 ml. Void sizes at the concrete interface surface ranged from less than one to over 15 mm. The fibre optic sensor consists of a plastic fibre from which the cladding has been removed. A fibre embedded in concrete measures void content by sensing the different refractive index of air compared to other concrete materials at the fibre surface. The light response of a fibre embedded in mortar, using plastic sleeves on the fibre and aluminium powder to generate voids, was found to be proportional to air void contact over the range 0-100%. The tactile pressure sensor consists of a pressure sensitive pad, and would appear to have potential for measuring void contect by attachment to rebar in concrete. Three types of pressure sensor were tested and found to measure air voids as small as 2mm in size on concrete surfaces. Experimental work was also carried out to understand air void morphology at the steel/mortar surface and the effect of air voids on water transport in mortar. Air voids were found to collect preferentially next to solid objects such as mould surfaces. It was speculated that this increased porosity may result in rebar being conduits for increased water migration in concrete.EThOS - Electronic Theses Online ServiceGBUnited Kingdo