JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 47
Example of CFRP applied through wet lay-up technique (Photo kindly supplied by
SIKA ITALIA Company, Italy)
(curing) conditions for the adhesive/matrix resin.
FRP composites currently used for the structural strengthening
of concrete, masonry and wood structures can be applied using
two different procedures: i) the pre-cured FRP prepregs are
adhesively bonded as prefabricated plates to the concrete/masonry substrate; and ii) the composite is applied through "wet
lay-up" of fabrics directly onto the substrate.
The application of prefabricated FRP plates ensures that the materials are produced by an industrial process in highly controlled
factory conditions, thus achieving a high level of uniformity for
the final product and better performance. On the negative side,
prefabricated FRP is less flexible and adaptable for unpredicted
configurations that can be found in the field application. In
addition, their elevated costs are not always justified for these
low-technology applications [Karbhari, 2002].
The application of a pre-cured FRP to a concrete, masonry or
wood substrate is carried out by means of a structural adhesive,
typically a "cold-cured" epoxy resin, i.e. an epoxy able to set
and harden at ambient temperature. In addition, this implies
the introduction of an adhesive interphase between the already
cured FRP and the substrate, which will represent the weakest
element of the reinforcement system.
The wet lay-up technique is more feasible as it provides enormous flexibility since the uncured fabric can closely follow the
geometrical configuration of the structure to be rehabilitated.
Moreover, the bond between the FRP and the concrete substrate is achieved with an adhesive resin, again cold-cured, which
is very similar to the matrix used for the composite. Therefore,
this adhesive interlayer is essentially part of the FRP itself by
forming a resin continuum between the FRP and the substrate.
The lack of careful control of curing, however, results in a significantly higher level of variation in performance.
Cold-cured resins
When correctly formulated and cured, epoxy resins provide a
high level of mechanical properties, an excellent resistance to
corrosion and chemicals and moderate costs. For economic
and practical reasons, the epoxy resins used in civil engineering
repair applications are "cold-curing" types, normally based on
bisphenol epoxy resins, cured with the addition of aliphatic
amines at ambient temperature [Hollaway, 2010]. In fact, applying any type of heat source over the large areas required for
the described applications is very difficult and prohibitively expensive. For such applications, epoxies are frequently preferred
to both vinyl ester and unsaturated polyester resins, which are
both characterized by excessive shrinkage during curing, with
the possible formation of micro-cracks or micro-gels in the bulk
resulting in micro-inhomogeneities and incomplete polymerization. Unsaturated polyester resins, in addition, display a high
susceptibility to moisture and low bonding efficiency in damp
or wet conditions, and when exposed to alkaline environments.
The main consequences of curing epoxy resins at ambient (often
uncontrolled) temperature are:
- long curing times (in the order of weeks) are necessary to
achieve sufficient mechanical properties, the lower the curing
temperature, the longer the curing time [Moussa et al., 2012];
- the curing (cross-linking) reactions taking place in epoxy
resins at ambient temperature are often not complete due to
kinetic restraints;
- a moderate glass transition temperature (Tg, as explained
below), in practice never greater than 65-70°C, is attainable by
these systems [Frigione et al., 2001, 2006a, 2006b; Sciolti et al.,
2010].
In addition, the absorption of external water (for instance, atmospheric moisture or rain) decreases the initial Tg of the resin,
which in turn affects the mechanical and adhesive properties
[Shin et al., 2011].
The chemical nature of the resin/hardener couple, the conditions in which the thermosetting system sets and hardens and,
above all, the environmental conditions in which it "operates"
(temperature, presence of water, moisture, chemicals, pollution,
etc.) determine the performance and the long-term behaviour
of the cold-cured resins. As a few examples of common environmental agent effects, the ingress of liquid/vapour water as
well as a moderate service temperature (even 40-50°C) can all
reduce appreciably the physical and mechanical properties of a
cold-cured (matrix and/or adhesive for FRP) epoxy [Frigione
et al., 2002, 2006a; Lettieri & Frigione, 2012; Sciolti et al.,
2010]. Moderate temperatures can even promote the post-cure
of the matrix, resulting in a stiffening of the whole composite.
The detrimental effects of moderate temperatures and of the
presence of water or moisture on the performance of a coldcured epoxy resin are also reproduced on its behaviour as an
adhesive: when analysing the adhesion strength developed
between an epoxy adhesive and concrete elements in relation
to the service temperature and in the presence of water, it was
found that both agents can reduce appreciably the bonding properties between adhesive and concrete [Aiello et al., 2002; Shin
et al., 2011]. In the presence of moisture, the failure mode shifts
No106 July 2016 / jec
composites magazine 47
Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #106 - July 2016
Cover
Edito
Point of view: Europe
Contents
COMPANY & BUSINESS
Development
Agenda of Events
APPLICATIONS
Thermoplastics
Automotive
MARKET
Aircraft engine
FEATURE AERONAUTICS
Aerostructures
Characterisation
Thermoplastics
Aero-engines
Process
CFRP
Semi-product
RESEARCH & DEVELOPMENT
Review
Core material
TECHNOLOGY & INNOVATIONS
NDT
Marine
Index
In the world
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Ad1
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Cover
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 2
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Edito
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Point of view: Europe
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 5
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Contents
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 7
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - COMPANY & BUSINESS
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 9
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Development
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 11
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Agenda of Events
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 13
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - AdHEC
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - AdCro
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Thermoplastics
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 15
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 16
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Automotive
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Aircraft engine
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 19
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 20
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 21
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 22
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Aerostructures
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 24
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 25
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Characterisation
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 27
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 28
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 29
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 30
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Thermoplastics
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 32
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Aero-engines
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 34
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 35
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 36
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Process
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 38
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 39
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - CFRP
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 41
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 42
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Semi-product
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 44
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 45
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Review
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 47
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 48
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 49
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Core material
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 51
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 52
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - NDT
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 54
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Marine
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 56
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - Index
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - In the world
JEC COMPOSITES MAGAZINE - Issue #106 - July 2016 - 59
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