The growth in the use of composite materials systems for structural applications either as the sole material component or for applications that involve concrete, metallic and timber materials highlighted the need for more guidance on the design, specification, application and approval processes. Classification and qualification schemes for composite materials systems enable engineering designers to select such systems based on system performance requirement.
The applications for composites, or FRP, within the civil infrastructure
can be divided into the categories of repair or strengthening and new build.
The most significant applications currently involve repair or strengthening
of existing concrete, metallic or timber structures using the following forms
of FRP:
• Pultruded rods, bars and plates (applied unstressed
or prestressed)
• Moulded bespoke beams, shells, hoops, etc.
• On-site laminated sheets
• On-site infused fibre pre-forms
The methods of joining of these forms of FRP, either to themselves or to other materials, include mechanical fastening, adhesive bonding, or a combination of both. For flexural strengthening applications, adhesive bonding and on-site lamination dominate, so that all of the aspects impinging upon these methods provided a focus for the project.
External reinforcement with FRP is suitable for many structural applications, such as:
• Capacity upgrade due to a change in
use
• Passive confinement to improve seismic resistance
• Crack control and crack growth prevention
• Strengthening around new openings in slabs.
The variety of ‘system’ components associated with the category of repair or strengthening can be significant. The thrust of the project was to develop a performance-based qualification scheme to enable designers to select FRP materials systems based on performance requirements. Throughout the project the following definitions were used:
• Composite Materials Systems
comprise FRP materials, primers, adhesives or laminating resins (excludes
substrate)
• Classification Schemes are for
individual components of the materials system, e.g. adhesives
• Qualification Schemes provide the
materials interface with the design process and installation of the composite
materials system to a specific structure.
The objectives of the work to be undertaken in this project
were three-fold:
To develop performance-based classification and qualification schemes,
in line with CEN strategies and Eurocodes dealing with the use and application
of materials associated with other forms of construction. This now enables
engineers and specifiers to select materials systems on the
basis of performance requirements, preventing substitution or use of inappropriate
materials components of the system. It also provides the impetus for manufacturers
and suppliers of FRP, resins, adhesives and related products to further develop
materials that satisfy a broad range of requirements. Methods for qualification
and certification of factory-made components and products are already covered.
A classification of such materials was developed and now
needs to be implemented by their producers. Such classifications can now feed
into the overall qualification scheme that was developed.
To identify, adapt and develop acceptance and quality control procedures
associated with on-site applications of materials systems.
Part of this process was associated with application issues, and part with
the development of techniques for the on-site fabrication of reliable and
consistent test pieces. Few procedures used currently provide quantitative
or meaningful data and, where good quality data is obtained, guidance on acceptance
is imprecise and open to wide interpretation. The performance classification
scheme developed automatically provides the framework, using a suite
of test procedures and QC acceptance criteria that have been validated.
To compile guidance on how to incorporate the new classification and
qualification schemes, and test procedures, into design and application.
Experience and case histories were used to further reinforce the guidance.
The project partners encountered a number of issues in practice that could
have been avoided through tighter specification related to testing associated
with application needs.
The overall project was split into 11 tasks:
Project flow diagram
A particular preoccupation throughout the project concerned materials testing for design and QC acceptance: what to do, how to do it, and how to interpret and use the data. This required a thorough description of the design process and the materials inputs.
Early discussions with the industrial partners involved in this project highlighted a number of common key issues. These included the need for:
• Clear and un-biased comparison of
material performance
• The availability of characteristic short and long-term
properties of materials
• Clearly defined material partial factors
• Methods to assess the adhesion and durability of
material systems
• Practical, reliable and reproducible QC tests
• Clear definition of acceptance criteria for QC testing
• Suitable training of on-site operatives.
DESIGN INPUTS AND MATERIALS QUALIFICATION
The four key steps involving the use of FRP materials in any project include material selection or specification, design, application (or installation) and acceptance. For flexural strengthening, the relevant components and definitions are:
• Substrate – parent material
comprising the existing structure
• Reinforcement – composite material (fibre
and resin)
• Adhesive/resin system – the bonding agent(s)
between the substrate and the reinforcement.
Even in the case of flexural strengthening, although the end result is a compound beam acting primarily in flexure, the adhesive and reinforcement act under very different and complex loading. For example:
• The substrate is acting predominately
in flexure
• The adhesive is acting primarily in shear
• The reinforcement is acting principally in tension,
but
• All are subject to localised shear and (tensile)
peel loading in the region of the composite plate end-anchorages.
The individual performance of each component must, therefore,
be assessed with the relevant loading conditions in mind. The purpose of developing
a qualification scheme is to identify the stages in the process of design
and installation where material properties are required to be either defined
or measured.
The design stage for flexural strengthening is summarised by the following
four steps:
• Material component selection (from
classification schemes), including adhesive system and composite reinforcement
• Material property determination (through appropriate
testing) required for design
• Design calculations: the output is the area of reinforcement
and stresses within the adhesive layer
• Design check: is the reinforcement strong enough
and will the reinforcement remain bonded to the substrate?
It is this final step that links the qualification scheme to the design process. For the Designer to check that the design conforms to the Original Specification, the appropriate performance-based material properties are required. The qualification scheme provides the route, i.e. through testing procedures and interpretation, to obtain the appropriate values.
The installation stage is summarised by the following three steps:
• Preparation for installation (including
substrate surface preparation)
• Application of adhesive and composite reinforcement
• Quality control checks: does the reinforcement and
adhesive conform to the Original Specification?
This last step links the qualification scheme to the installation process. The qualification scheme will provide the appropriate tests to perform both on-site and off-site, along with the appropriate acceptance criteria.
PROJECT PARTNERS AND INDUSTRIAL ADVISORY GROUP
Partners
Oxford Brookes University
AEA Technology (now ESR Technology)
MouchelParkman
Tony Gee and Partners
Industrial Advisory Group (IAG)
Around twenty organisations were represented under four categories:
Materials suppliers
BASF
Dupont
Exchem
Rotafix
Saint-Gobain Vetrotex
SBD Weber-Broutin
Sika
Toray Europe
Contractors
Balvac Whitley Moran
Concrete Repairs
Makers
Taylor Woodrow
VT Composites
Designers
DML Composites
Gifford & Partners
MouchelParkman
Parsons Brinckerhoff
Tony Gee and Partners
Specifiers and specifying authorities
BBA
Highways Agency
Network Rail
Oxford County Council
Tube Lines
Website designed and maintained by PC Net Services. This page last updated: Juy 16th, 2006
