ABSTRACT
Experimental investigations of creep of epoxy adhesive at the concrete-FRP interfaces was made
1 INTRODUCTION
The strengthening of existing civil concrete structures or their rehabilitation is necessary when elements are damaged. The same situation occurs when an increase in load capacity is required. The most popular strengthening technology for reinforced concrete beams was steel plates. This technique consists in externally bonding plates to concrete using epoxy adhesive. This method is effective and convenient but also has some disadvantages, which could be eliminated using FRP laminates. Steel can corrode in contrast to the FRP materials. Carbon Fiber Reinforced Polymer strips are lighter than steel plates and easier in manipulating at the construction site. In the case of the upgrading of very long elements steel plates may generate some problems with transport. The steel plates have limits in their delivery lengths as opposed to the composite materials. This is the reason why the strengthening of building structures with the external composites, bonded to the concrete surface with an epoxy resin adhesive, is more and more often chosen as the way of increasing the load bearing capacity of concrete elements subjected to bending, shearing and compression. The results of experimental tests proved that the concrete elements strengthened in flexure with the externally bonded composites demonstrated a limit of crack development and width, as well as an increase of the beam’s stiffness
by Meshgin et al. (2009). This paper focused on time-before-loading, which is defined as the time period between the time of application of epoxy and the loading time. Nine specimens were tested under laboratory conditions controlled to constant temperature. Studies have been conducted for about nine months and have shown that as time-before-loading decreases, ultimate creep deformation increases. More recently, the creep behaviour of adhesives has also been considered by Ferrier et al. (2010). In that paper, short and long-term creep tests were performed on FRP/concrete bonded interfaces, using concrete specimens strengthened by four different CFRP systems. They noticed that epoxy systems used in construction exhibit low glass transition temperature values as opposed to epoxy formulations used for aeronautical applications. Their work was focused on this characteristic and proved that creep behaviour is correlated to the glass transition temperature value of the polymer. The value of this temperature should be higher than 55 °C in order to limit creep effects in the adhesive joint. Another solution is to maintain the service temperature of the structure to at least 15 °C less than than the glass transition temperature. Mazzotti and Savoia (2005) in their work about long-term properties of bond between concrete and FRP, which lasted for about 200 days, found that due to very high shear stresses at the adhesive and concrete cover level, creep phenomena may be highly non linear close to the loaded end. Hence, compliance increase at the beginning of the anchorage is much higher than for from it.
