Civil Engineering Reference
In-Depth Information
to modify physical properties, such as viscosity and heat generation, and to
increase volume. issa and Debs (2007) reported that, while cracks cause a
reduction in compressive strength of concrete of about 40%, the polymeric
adhesives, when properly applied, are able to restore the compressive strength
by decreasing the reduction down to less than 10%.
The selection of a polymeric adhesive must be done according to whether
it will harden to a rigid condition or to a flexible one. High stiff polymeric
adhesives of high shear and tensile strength (up to 30 MPa), such as epoxy
and acrylate, are usually preferred in bonding and repair applications. They
bond exceedingly well to dry substrate and some will bond to wet concrete,
can prevent all movement and restore the full strength of a cracked concrete
member. However, these materials have low ultimate strain (under 4%), thus
are inconvenient in bonding concrete when high deformability or dynamic
action are expected; moreover, if tensile or shear stresses exceed the capability
of the concrete, new cracks will appear in the concrete near, but generally
not at the adhesion interface. The liquid adhesives can penetrate cracks
somewhat finer than 0.05 mm, the penetration being dependent on viscosity,
injection pressure, temperature and set time.
Flexible polymers of elastomeric behavior, such as acrylamides and
polyurethanes, on the other hand, have ultimate strengths of 1-20 MPa but
much higher ultimate strains (up to 1000%) (Kwiecień et al., 2010). With
respect to rigid adhesives, they display higher values of deformation energy
and ductility, i.e. higher toughness. Flexible repair joints allow for local
slight deformation, absorbing or dissipating strain energy in this way, and
can tolerate repeated cyclic deformations (Kwiecień, 2010). Flexible-type
polymeric adhesives are mainly used to shut off or greatly reduce water
movement. They will not restore strength to a structure, but they generally
will maintain water tightness despite minimal movement across a crack. Most
flexible adhesives are water solutions and will, therefore, exhibit shrinkage
if allowed to dry, but they will recover when rewetted. Some of them can
be formulated at very low viscosity in order to be injected into any hole
through which water will flow. Others can be made to yield a foam that can
be used in cavities down to 100 mm wide.
one of the main characteristic of adhesives is the wettability, i.e. they
are able to wet the surface of structural elements that have to be joined. The
application of the adhesive, moreover, must be done to a properly prepared
substrate, in order to guarantee a satisfactory degree of adhesion. To this
aim, surface pre-treatment is often required to remove contaminations and
weak surface layers and to enhance the roughness of the surface. it is well
recognized, in fact, that a substantial roughness of the cementitious substrate
promotes mechanical anchoring of the polymeric adhesive and tends to
increase the contact surface area, both resulting in a higher bond strength
(Abu-Tair et al., 2000; Toutanji and ortiz, 2001).
