Concrete Society

Development of Structural Strengthening – Steel Plate Bonding

Posted on 05/11/2014 by islander350i

Steel plate bonding has been used in both buildings and civil structures in the UK since 1975 using first generation epoxy adheives. In 1994 the Highways Agency published BA 30/94 ‘Strengthening of Concrete Highway Structures Using Externally Bonded Plates’. This provided information on application, design and specification of the technique. The application of steel plates is still the best solution to some strengthening problems that occur today.

Steel plate bonding provided the basis for the establishment of strengthening using externally bonded reinforcement. The process involves the bonding of a mild steel plate with a minimum thickness of 4mm (for handling purposes) to a prepared concrete surface.

The steel plates are fabricated off site to the required dimensions and specification, including holes for anti-peel bolts.

To prevent any corrosion of the steel plate a primer system needs to be applied to the prepared steel surface during fabrication. This primer also provides the critical function of transferring forces from the structure to the steel plate and is hence a crucial part of the system.

Holes for anti-peel bolts also need to be inserted in the steel plate during fabrication. These bolts are required to provide additional resistance to peel forces applied to the bond line due to any residual force in the end of the plate. The bolts have to be positioned carefully to avoid damage to the existing reinforcement in the concrete surface.

Temporary works are required to support the heavy steel plates while the 2-part epoxy adhesive is curing. The curing period is dependant on ambient conditions but is likely to be a minimum of 3 days.

A fillet of adhesive is generally placed around the edge of the plate, this provides additional protection to the bond line but also allows the application of the final corrosion protection system to the steel plates to be lapped out onto the concrete surface. The corrosion protection system is likely to provide a life to first maintenance of 8 years and to major maintenance of 16 years in an exposed environment. However, the first project carried out in 1975 has only recently come to the end of its service life over 35 years after its first installation. Whilst the limited exposure conditons that these plates were exposed to may have extended the life span, current understanding of the performance of corrosion primers and adhesives could have possibily extended the life span.

Interestingly the steel plates have been replaced with a Carbon Fibre (CFRP) plate bonding solution.

Introduction to structural strengthening in the UK

Posted on 04/11/2014 by islander350i

The technique of bonding external reinforcement to structures was first used in the UK in 1975 on the M5 near Birmingham to strengthen the Quinton Interchange.

The method of strengthening using externally bonded reinforcement, structural strengthening, can be introduced by providing the answers to some simple questions.

What is Structural Strengthening?
Why do we need in?
What can be achieved by using the technique?
Where can we use it?

Structural strengthening involves the bonding of additional reinforcement to the external faces of a structural member. This additional reinforcement can incorporate steel plates, composite plates or composite wrapping systems. The method is attractive because it provides a cost effective solution to increasing load carrying capacity, especially when compared to demolition and rebuilding.

One of the main reasons for the use of the method in the UK is due to the change of use of a structure giving an increased load-carrying requirement. Other reasons such as, inadequate design, poor quality construction, structural damage, fire damage, seismic loading, reinforcement corrosion (If the cause is treated) and loss of prestress force are not uncommon.

Strengthening can improve the load carrying capacity of structures by;

Increasing flexural strength,
Shear strength,
impact resistance,
punching shear resistance,
redistribute loads around new openings.

Externally bonded reinforcement gives the opportunity to strengthen without having a significant visual impact on the structure. The installation process is fast and can minimise disruption to the function of the structure including the services attached to it.

Structures made from reinforced concrete, steel, cast iron, masonry and timber have all been strengthened to date using a form of the technique. Beams and slabs have been strengthened on both the top and bottom surface for flexural strength. Columns and beams have been strengthened on there side faces for shear. Slabs have been strengthened around columns to increase punching shear resistance. Various other types of structural elements have been strengthened for many different reasons.

The Development of Composite Materials for Strengthening Structures – Introduction

Posted on 02/11/2014 by islander350i

The technique of flexural strengthening of reinforced concrete structures using externally bonded composite reinforcement has been in use since the mid 1990’s. The method of strengthening by bonding steel plate, which had been used in the UK since 1975, has now generally been replaced by the use of composite plates.

The increased acceptance of the method has led to different types of structure and problems being considered for strengthening using composite materials. The flexural capacity of metallic structures such as cast iron and steel can be improved. The resistance of reinforced concrete or masonry against impact, blast and seismic loads can be increased. Reinforced concrete structures deficient in shear can be strengthened. The use of pre-stressed composite plates can relieved both dead and live loads at the same time as overcoming difficult detailing problems.

The success of any new techniques depends on detailed research and development and the long term durability of all the materials used in the strengthening system. The other main system components that should be included in this development are the adhesives used to bond the composite to the structure.