Daniel McPolin

Development and Longer Term In Situ Evaluation of Fiber-Optic Sensors for Monitoring of Structural Concrete

In this paper, new solutions to the problem of making measurements, of carbonation and chloride ingress, in particular, in concrete structures are considered. The approach has focused on the design, development, and use of fiber-optic sensors (FOSs), recognizing the need in that conventional devices are often either inaccurate, expensive, or unsuitable for encapsulation in the material. The sensors have been designed to monitor, in situ and nondestructively, relevant physical, and chemical changes in cementitious materials. Three different types of FOS were constructed, tested, and evaluated specifically for this application, these being a temperature sensor (based on the fluorescence decay) and pH and chloride sensors, based on sol-gel (solidified gel) technology with appropriate impregnated indicators. The sensors were all designed to be inserted into the structures and evaluated under the harshest conditions, i.e., being mounted when the mortar is poured and thus tested in situ, with the temperature and pH sensors successfully embedded in mortar. The outcomes of these tests have shown that both the temperature sensor and the pH sensor were able to function correctly for the duration of the work - for over 18 months after placement. The laboratory tests on the chloride sensor showed it was able to make measurements but was not reversible, limiting its potential utility for in situ environments. Research is ongoing to refine the sensor performance and extend the testing.

Fibre optic chemical sensor systems for internal concrete condition monitoring

Methods for estimating the lifetime of reinforced concrete structures are being investigated with a fiber optic chemical sensor system, using a sol-gel as the matrix for the active material, for embedment in concrete for evaluation and testing. Results are presented on recent work.

Fiber optic chemical sensor systems for monitoring pH changes in concrete

Carbonation-induced corrosion of steel is one of the principal causes of deterioration of reinforced concrete structures. When concrete carbonates, its pH decreases from a value in excess of 12.6 to less than 9 and, hence, a measure of the pH is an indicator of the degree of carbonation. This paper describes the development, testing and evaluation of two types of fibre optic sensors for the pH monitoring. One of these used a sol-gel based probe tip, into which an indicator dye has been introduced and the second used a disc containing an indicator operating over a narrower range of pH with shorter lifetime. Both were connected to a portable spectrometer system, which is used to monitor the spectral changes in optical absorption of the probe tip. A white light source to interrogate the active elements is used as the systems operate in the visible part of the spectrum. The two types of sensors have been found to be sensitive to the changes in pH due to carbonation, but the response time depended on the thickness of the coating material in the case of the sol-gel sensor. The durability of the sensors is still under investigation. The disc type sensor has a life span of approximately 1 month and, hence, it is not suitable for embedding in concrete for long-term monitoring of pH changes. However, it can be used for assessing the pH in vivo. The harder sol-gel is more durable and, hence, has a slower, but acceptable response time.

Shear Strength and Durability Testing of Adhesive Bonds in Cross-Laminated Timber

This paper addresses the quality of the interface- and edge-bonded joints in layers of cross-laminated timber (CLT) panels. The shear performance was studied to assess the suitability of two different adhesives, polyurethane (PUR) and phenol–resorcinol–formaldehyde (PRF), and to determine the optimum clamping pressure. Since there is no established testing procedure to determine the shear strength of the surface bonds between layers in a CLT panel, block shear tests of specimens in two different configurations were carried out, and further shear tests of edge-bonded specimen in two configurations were performed. Delamination tests were performed on samples which were subjected to accelerated aging to assess the durability of bonds in severe environmental conditions. Both tested adhesives produced boards with shear strength values within the edge-bonding requirements of prEN 16351 for all manufacturing pressures. While the PUR specimens had higher shear strength values, the PRF specimens demonstrated superior durability characteristics in the delamination tests. It seems that the test protocol introduced in this study for crosslam-bonded specimens, cut from a CLT panel, and placed in the shearing tool horizontally, accurately reflects the shearing strength of glue lines in CLT.

Preliminary Development and Evaluation of Fiber-Optic Chemical Sensors

Conventional methods of measuring carbonation and chloride ingress exist, but they are either inaccurate, expensive, or involve destructive testing. In seeking new solutions to this problem, the potential to use novel fiber-optic chemical sensors developed specifically for this application and designed to monitor chemical changes in cementitious materials, in situ and nondestructively, was explored in this study. Three types of fiber-optic sensors were thus constructed, tested, and evaluated, viz a temperature sensor, a pH sensor, and chloride sensors. The temperature sensor was based on the fluorescence decay of temperature-dependent materials, whereas the pH and chloride sensors were based on sol-gel technology, with pH and chloride sensitive indicators impregnated in the sol-gels. All the sensors were tested in situ, and subsequently, the temperature and the pH sensors were embedded in mortar and tested. It was found that both the temperature sensor and the pH sensor could function correctly for over 18 months after placement, but there was an issue with alignment of the sensor each time it was reconnected to the hardware. However, the laboratory tests showed that the chloride sensor was not reversible, and therefore further refinement was considered to be necessary before it could be used in situ in environments where the chloride content was known, from other measurements, to decrease. Research is ongoing to refine the sensor performance and expands the in situ testing program. The sensors themselves are inexpensive to fabricate, but the sensing hardware used in this work is costly because of its high versatility.

Influence of embedded length on strength of BFRP rods bonded parallel to the grain in low grade timber by pullout-bending tests

Bonded-in rod connections in timber possess many desirable attributes in terms of efficiency, manufacture, performance, aesthetics and cost. In recent years research has been conducted on such connections using fibre reinforced polymers (FRPs) as an alternative to steel. This research programme investigates the pull-out capacity of Basalt FRP rods bonded-in in low grade Irish Sitka Spruce. Embedded length is thought to be the most influential variable contributing to pullout capacity of bonded-in rods after rod diameter. Previous work has established an optimum embedded length of 15 times the hole diameter. However, this work only considered the effects of axial stress on the bond using a pull-compression testing system which may have given an artificially high pull out capacity as bending effects were neglected. A hinge system was utilised that allows the effects of bending force to be taken in to consideration along with axial forces in a pull-out test. This paper describes an experimental programme where such pull-bending tests were carried out on samples constructed of 12mm diameter BFRP bars with a 2mm glueline thickness and embedded lengths between 80mm and 280mm bonded-in to low-grade timber with an epoxy resin. Nine repetitions of each were tested. A clear increase in pull-out strength was found with increasing embedded length.