Strengthening of concrete structures in torsion with FRP

Repair or upgrading of existing concrete structures has become a large portion of society's budget all over the world, and the amount is increasing. The causes for this are several. Our cities are becoming larger with increasing traffic flow, while the existing infrastructure was built for other standards and traffic loads. Apart from this a large proportion of the infrastructure has reached its design life and consequently will need to be replaced. A further reason is that a large proportion of the buildings and infrastructure deteriorate. Repair and strengthening methods that solve some of these issues in a cost effective way are welcomed. One such method is composite plate bonding, which may be defined as a method where thin plates or fabrics are bonded to a structure's surface to enhance its stiffness and strength. The most common way to strengthen structures with advanced composites has been for bending but strengthening for shear, torsion and axial loads is also often needed. However, there are very few examples presented in the literature for concrete structures strengthened with CFRP in torsion, which may indicate that this is a minor problem. Nevertheless, the need still exists and therefore this paper presents work conducted to strengthen concrete structures in torsion. The paper presents mainly theory but also laboratory tests. It is shown that strengthening a structure for torsion is possible, but theoretically complicated and it is of utmost importance that the CFRP sheets or laminate are anchored property.