[0001] The invention relates to a tendon comprising a tensionable core, an envelope formed
of a hardenable composition, and a shield, whereby the hardenable composition has
a lubricating action and hardens after that the core has been tensioned.
[0002] Such tendons are generally known. European Patent Application No. 83 810 431.3 (publication
number 0105 839) describes such tendons which are composed of a core around which
a hardenable composition is applied, which, in turn, is encased in a synthetic material
layer hardened by radiation. When the tendon has been put in its place, the core is
tensioned, and subsequently heated by passing an electric current through it so as
to harden the hardenable layer around the core. Such tendons are used for making,
e.g. concrete structures.
[0003] For making such concrete structures two methods of reinforcement can usually be used.
[0004] According to the first method, which we shall call the conventional method, a strand,
rope or rod is tensioned on a drawing bench ; subsequently, concrete mortar is applied
around this tensioned strand, rope or rod, and the concrete is allowed to harden.
The hardened concrete then contains a stressed tendon so that the resistance against
bending of e.g. the so manufactured beam, is considerably improved.
[0005] Another method deals with the so-called posttensioning of tendons. A rope encased
in a shield is placed in a mould to be filed with concrete mortar. Subsequently, the
rope is tensioned up to the required stress. In a further operation, cement mortar
is injected between the rope and the shield so as to obtain a bond between the shield
and the central rope as soon as the mortar is hardened. Such a method is particularly
suitable for on-site jobs.
[0006] With this kind of post-tensioning, the inside of the shield will preferably be profiled
in order to enhance anchorage between the rope and the shield. As soon as the concrete
mortar around the shield and the cement mortar inside the shield are hardened, the
anchors used for stressing the rope can be removed.
[0007] Another method of prestressing consists in using a stressing rope encased in a shield
or sheath, whereby the stressing rope is frequently movable inside the sheath (unbonded
tendons). On site the tendons are placed in a mould to be filled with concrete mortar
and subsequently tensioned. The concrete mortar is then cast and allowed to harden.
The tensioning anchors remain in position and provide the required stress in the tendons
throughout the life of the structure. This method offers the advantage that in certain
cases the stress can be adjusted afterwards when this should be desirable. An obvious
disadvantage is that, in the case an anchor breaks, the entire stress may disappear
with all its consequences.
[0008] It is an object of the present invention to provide an alternative to the method
of post-tensioning ropes ; however, without requiring for the complicated operation
of injecting cement mortar inside the sheath in which the rope is encased.
[0009] A similar improvement is also aimed in the aforementioned European Patent Application
No. 0 105 839. An important drawback of the tendons and their applications referred
to in said Patent Application is that the synthetic material must be hardened by means
of heat. According to said Application, this heat is provided by passing an electrical
current through the central tensionable core or through a resistance element specially
provided in the system.
[0010] However, electrical current heating for such tensioned constructions must be considered
as extremely dangerous as this may lead to strength reductions in some cases. It is
obvious that any risk of reduced strength in such structures is entirely unacceptable
so that heating of the cores of such elements must be avoided.
[0011] It is an object of this invention to provide a tendon of the abovementioned type,
which, however, does not require the supply of heat to the system, but, in which nevertheless,
excellent hardening of the hardenable composition between the stressed core and the
sheath is obtained.
[0012] According to the invention a tendon of the abovedescribed type is characterized in
that the hardenable composition is a hardenable material which does not require the
supply of heat and of which the components or preproducts are distributed over various
distinct phases ; that at least one continuous phase of the hardenable composition
is provided with lubricanting action which after the hardening process substantially
disappears and that there also is provided a barrier layer which regulates the speed
of the hardening reaction of the hardenable composition.
[0013] In a tendon according to the invention there is a combination of three important
properties :
(a) the components of the hardenable composition are distributed over various distinct
phases ;
(b) at least one phase of the composition has lubricating action ;
(c) the speed of the hardening reaction of the hardenable composition is regulated
by the presence of a barrier layer.
[0014] A stressing rope according to the invention may be submitted to considerable elongation
of the core during stressing ; e.g. 5 % elongation. As will be described further on
in greater detail, the rope is tensioned not before the concrete is hard so that during
this process the core moves relative to the surrounding concrete.
[0015] As a result of the profiled core surface and its movement relative to the concrete
and the barrier layer bonded thereto, there will be a blending of the phases of the
hardenable composition, so that, after hardening of the system, a homogeneously hardened
bond is obtained between the surrounding concrete and the core. The degree of blending
of the phases of the hardenable composition during the tensioning process may be influenced
by the nature of the core in terms of elongation and surface profile.
[0016] By the choice of the barrier layer, it is possible to regulate accurately the speed
of the hardening reaction of the hardenable composition, which is distributed over
several phases.
[0017] In a first preferred embodiment of the tensioning elements according to the invention,
the sheath constitutes the barrier layer which consists of a hydrolizable synthetic
material.
[0018] More specifically, this layer consists of a synthetic material that is hydrolizable
in an alkaline aqueous environment, e.g. 100 % esterified polyvinyl acetate. Preferably,
said material is as thin as possible to prevent lubricating action after hydrolysis,
but, on the other hand, it must be sufficiently thick to serve as a barrier layer
during the desired period of time.
[0019] It is advantageous that, in this case, the hardenable composition is formed by a
layer adjacent to the core which contains a suitable epoxy resin and an adjacent layer
containing polyisocyanate.
[0020] The action of such a system can be described as follows.
[0021] The tendon is placed in a casing, the concrete mortar is poured in the casing and
the concrete is allowed to harden. After the hardening process, the tensionable core
is tensioned by means of jacks and anchors. As known, during the hardening process,
the concrete contains, an amount of strongly alkaline water ; a pH value of 13 is
a normal value. The sheath, which is composed of a layer based on, say, polyvinylacetate,
will, in the coarse of time, hydrolyse in the alkaline water present in the concrete.
Subsequently the alkaline water comes into contact with the polyisocyanate-epoxy resin
system lying directly beneath the barrier layer. Under the influence of alkali and
water, the polyisocyanate will be converted into a compound containing amino-groups
such as, for example, the corresponding primary amine. In combination with the epoxy
resin in the system, this amine will now form an epoxy resin-hardening system, whereby
the duration of the hardening will depend on the thickness of the polyvinyl-acetate
layer and on the concentration of the polyisocyanate in the polyisocyanate-containing
layer.
[0022] It is evident that the epoxy resin contained in the epoxy-resin-containing layer
around the core must have a hydroxyl number 0 in order to avoid premature reaction
between the polyisocyanate component of the top layer and the epoxy resin.
[0023] Normally, the anchors will not be removed after the hardening process, although this
would be possible. It is, however, important to note that the anchors are no longer
essential for the action of the tendon. This means that, should the anchors be put
out of service owing to e.g. corrosion, the tendon would maintains its normal function
thanks to the presence of the hardened bonding synthetic material between the core
of the tendon and the hard concrete.
[0024] Instead of an epoxy resin, it is also possible to use a polyalcohol in the abovedescribed
embodiment so that, after hydrolysis of the barrier layer, the slow reaction between
the said polyalcohol and polyisocyanate, with the associated formation of a polyurethane
compound is accelerated under the influence of the catalytic action of the alkali
metalions, which are present in the water of the concrete.
[0025] With the application of such a polyurethane system it is evident that an ultimate
date of use must be specified.
[0026] In the abovedescribed embodiment of the tendon according to the invention, it is
advantageous to envelop the entire tendon in a gas- and liquid-tight jacket that can
be removed prior to use.
[0027] This measure makes it possible to prevent premature reation between the polyisocyanate
layer with, for example, water vapour from the atmosphere.
[0028] In another advantageous embodiment of the tendon according to the invention, the
barrier layer is located between the phases of the hardenable composition and the
shield or sheath is formed by an impenetrable inert profiled synthetic material layer.
[0029] In that case two situations must be distinguished :
(a) the phases separated by the barrier layer are present in the form of layers at
both sides of the barrier layer ;
(b) one of the phases is present in the form of areas surrounded by a barrier layer
in a continuous layer of the other phase.
[0030] In both abovedescribed cases it is advantageous that the barrier layer regulates
the diffusion speed of one or more components of the hardenable composition.
[0031] Very advantageously, in suh a case, one of the phases contains an epoxy resin, while
another phase may contain an amine compound. A suitable barrier layer may then be
composed of polyethylene oxide.
[0032] It is evident that the barrier layer may be composed, instead of of polyethylene
oxide, of another amine diffusible layer ; however, a requirement is that the barrier
layer contains no groups that trigger off a reaction between the barrier layer and
the epoxy resin.
[0033] It is advantageous that at least one of the phases of the hardenable composition
has lubricating action. This can be achieved by adding one or more lubricants or viscocity-reducing
agents, such as inert waxes, solvents, silicon products or polytetrafluorethylene
products, to the phase concerned.
[0034] Also with similar lubricants or viscosity-reducing agents care must be taken that
they cannot react with the components of one or more phases of the hardenable composition.
[0035] As indicated before in the scope of the invention, it is evident that with the hardenable
compositions, care must be taken that the hardening of the system does not go together
with a considerable reduction in volume. Indeed, many hardenable synthetic materials
have the characteristic of shrinking slightly when being hardened. Usually, such shrinkage
phenomena can effectively be counteracted by incorporating adding fillers to one or
more phases of the hardenable composition. Such fillers are known to anyone familiar
with the art : calcium carbonate, clay, diatomene earth, polyethylene, polypropylene
powder, gypsum, etc.
[0036] In certain cases however, adding fillers shall not be required. This, indeed, is
the case when the hardenable composition is formed by a polyisocyanate and an epoxy
resin. During activation of the polyisocyanate, whereby as already said an amine is
formed, a molecule of carbon dioxyde per isocyanate group is liberated. Under certain
conditions, carbon dioxide may remain trapped in the hardenable composition and so
lead to an increase in volume during the hardening process. In this way, it is evident
that an excellent filling of the cavity inside the sheath is obtained.
[0037] The invention also relates to a method for tensioning concrete under the application
of a tendon according to the invention. It is evident that the tendons according to
the invention can also be used for making a prestressed concrete element made in a
conventional manner in a workshop.
[0038] The invention will now be further clarified with reference to the drawing in which
:
Figure 1 is a cross-sectional view of a tendon according to claims 1 to 7 ;
Figure 2 is a cross-sectional view of a tendon according to claims 7 to 11, and
Figure 3 is a view of another embodiment of a tendon according to claims 8 to 11.
[0039] In Figure 1 a tensionable core is shown with reference number (1) ; reference number
(2) represents the first phase of the hardenable composition around the core, while
reference number (3) represents a second phase of the hardenable composition. Reference
number (4) refers to a barrier layer applied around both phases of the hardenable
composition, which layer (4), in this case, also has the function of a sheath or a
shield. Preferably, a very thin, additional jacket is applied on this barrier layer
(4).
[0040] In Figure 2, reference number (5) designates a tensionable core ; a first phase (6)
of the hardenable composition is applied around this core ; a barrier layer (7) is
applied around this first phase and a second phase (8) of a hardenable composition
is applied around this barrier layer (1) ; a shield (9) is applied at the outside.
[0041] In Figure 3, reference number (10) again shows the tensionable core ; reference number
(11) refers to a first phase of the hardenable composition and reference number (12)
to the second phase of the hardenable composition. Reference number (13) refers to
the barrier layer located between both phases of the hardenable composition.
[0042] With reference to Figure 1, the tendon is applied as follows. After having been freed
from the outermost thin inert jacket, if any, the tendon is placed in a mould to be
filled with concrete ; the concrete mortar is cast around the tendon and starts to
harden ; after complete hardening of the concrete the core is tensioned. During the
hardening process of the system, the latter possibly being mixed due to the tensioning,
the concrete mortar contains water with a high pH-value, e.g. a pH of about 13. At
these high pH-values the barrier layer 4, e.g. polyvinyl acetate, will hydrolyze.
During the hydrolysis of the hydrolizable layer, the alkaline water of the concrete
comes into contact with a polyisocyanate from the layer 3 so that the polyisocyanate
is converted into a mixture of amine compounds. In turn, these compounds can react
with the epoxy resin from the layer 2, so that, together with the amine formed from
the isocyanate, the epoxy resin forms a solid hard synthetic material. In Figure 2
the situation is slightly different. Here a core 5 is enveloped by, for example, an
amine-containing layer 6, which, in turn, is enveloped in a barrier layer 7 consisting
substantially of polyethylene oxide. Around that, an epoxy resin layer 8 is applied
; finally at the outer side of the element there is an inpenetrable inert synthetic
material layer 9 with profiled surface. After manufacturing the rope the diffusion
process of the amine over the barrier layer 7 will start and continue until all epoxy
resin has been converted. Under the application of such a rope it will be necessary,
through selecting the thickness of the barrier layer 7, to take account of both the
required period of time between tensioning the rope and the end of the hardening process,
and of the period of time between the manufacturing of the tendon and the applying
of the rope. Also in this case, it is possible to achieve, through a correct selection
of the profile of the core and the elongation during the tensioning of the core, that
a blending of the layers is carried on. In that case, a substantial acceleration of
the hardening process can be achieved if care is taken that, during the tensioning
operation, the barrier layer is broken down.
[0043] Finally, Figure 3 shows that the phase 12 of the hardenable composition is a continuous
phase, while the phase 11 is a discontinuous phase, which is distributed in the continuous
phase 12. Between both phases there always is a barrier layer in the form of a jacket
13. As concerns the period of time between the hardening of the so formed hardenable
system and the manufacturing of the tendon, the same time control is required as described
for Figure 2.
1. A tendon comprising a tensionable core, an envelope formed by a hardenable composition
and a shield, whereby the hardenable composition has a lubricating action and hardens
after the core has been tensioned, characterized in that the hardenable composition
hardens without the supply of heat whereby its components or preproducts are distributed
over various distinct phases (2, 3, 6, 8, 11, 12) ; that at least one continuous phase
of the hardenable composition is provided with lubricating action, which after hardening
substantially disappears, and that there is also provided a barrier layer (4, 7, 13)
regulating the speed of the hardening reaction of the hardenable composition.
2. A tendon according to claim 1, characterized in that the shield forms the barrier
layer (4) and is composed of an hydrolizable synthetic material.
3. A tendon according to claims 1 and 2, characterized in that the hydrolizable layer
(4) is a synthetic material which is hydrolizable in an alkaline aqueous environment.
4. A tendon according to claim 3, characterized in that the hydrolizable synthetic
material is polyvinyl acetate.
5. A tendon according to claims 1 through 4, characterized in that the hardenable
composition is formed by a layer (2) adjacent to the core and containing a suitable
epoxy resin and a surrounding layer (3) of polyisocyanate.
6. A tendon according to claims 1 through 4, characterized in that the hardenable
composition is formed by a layer (2) adjacent to the core and containing a suitable
polyalcohol and a surrounding layer (3) containing polyisocyanate.
7. A tendon according to any one or more of the claims 1 to 6, characterized in that
the tendon is entirely surrounded by a gas and liquid tight jacket which is removed
prior to use of the tendon.
8. A tendon according to claim 1, characterized in that a barrier layer (7, 13) is
present between the phases of the hardenable composition and that the shield is formed
by an inpenetrable inert profiled synthetic material layer (9, 14).
9. A tendon according to claim 8, characterized in that one or both phases of the
hardenable composition is (are) continuous.
10. A tendon according to claims 8 and 9, characterized in that the barrier layer
(7, 13) is a layer regulating the diffusion speed of one or more of the components
of the hardenable composition.
11. A tendon according to one or more of the claims 8 through 10, characterized in
that one of the phases contains epoxy resin, that another phase is an amine compound,
and that the barrier layer substantially contains polyethyleneoxide.
12. A tendon according to one or more of the claims 1 through 11, characterized in
that at least one of the phases is provided with lubricating action by blending this
phase with one or more lubricants or viscosity-reducing agents, such as inert waxes,
solvents, silicon products or polytetrafluorethylene products.
13. A tendon according to one or more of the claims 1 through 12, characterized in
that at least one of the phases forming the hardenable composition contains a filler
material.
14. A method for stressing concrete, characterized in that a tendon according to one
or more of the claims 1 through 13 is applied.
15. A prestressed concrete element obtained in a known manner under the application
of a tendon according to one of the claims 1 through 13.