Hydraulic and earth engineering – Earth treatment or control – Ground stabilization or reinforcement
Reexamination Certificate
2000-05-12
2001-11-06
Neuder, William (Department: 3673)
Hydraulic and earth engineering
Earth treatment or control
Ground stabilization or reinforcement
C405S302600, C405S258100
Reexamination Certificate
active
06312198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to a geogrid comprising drawn, polymeric longitudinal straps which run parallel or substantially parallel to each other and polymeric transverse straps bonded to the longitudinal straps.
2. Description of the Related Art
Grids as such are known. In GB 2266540 a grid is described which is made of fully stretched polymeric longitudinal and transverse straps bonded together by means of, e.g., partial fusion of the straps.
WO 94/26503 describes a grid of drawn, polymeric straps bonded together by melting the polymer in the area of contact between the longitudinal and the transverse straps. The melting of the polymer is accomplished by heating conductive particles situated directly underneath the surface of the straps in a high-frequency electromagnetic field. In this way it is ensured that only the portion of the polymer used to effect the bond will melt. The remaining polymer is hardly affected at all and so the strength of the drawn straps remains substantially unaffected. The grid according to WO 94/26503 can, in principle, be subjected to heavy loads.
However, in actual practice it was found that in the case of heavy loads, e.g., such as occur in civil engineering structures (i.e., structures to do with, int. al., hydraulic and road engineering), loaded longitudinal straps will break at a significantly lower load and exhibit a significantly wider breaking load distribution than might be expected on the basis of the specifications of these straps and the bonding technique applied.
SUMMARY OF THE INVENTION
The object of the instant invention is to provide a grid such as described in the first paragraph that is especially suited for use in civil engineering structures and that does not suffer the described premature failure. This is achieved by making use of transverse straps of which the crosswise elastic modulus is less than 15%, preferably less than 8%, of the lengthwise elastic modulus of the longitudinal straps. Preferably, the crosswise elastic modulus is also more than 0.1%, preferably more than 1%, of the lengthwise elastic modulus of the longitudinal straps
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In an especially advantageous embodiment it holds that the crosswise elastic modulus of the longitudinal straps is less than 15%, preferably less than 8%, of the lengthwise elastic modulus of the (drawn) transverse straps. It is further preferred that the crosswise elastic modulus of the longitudinal straps is more than 0.1%, preferably more than 1%, of the lengthwise elastic modulus of the (drawn) transverse straps.
It was found that premature failure probably results from an unfavourable interaction between the longitudinal and the transverse straps. An insight into this interaction will be provided with reference to the example below.
Use is made of a geogrid where drawn, polymeric longitudinal and transverse straps (having a width of 12 mm and always 30 mm apart) have been welded together at an angle of about 90 degrees over their entire contact area. Because the straps are drawn, their molecular chains are oriented essentially in the longitudinal direction. As a result of this orientation the straps have poorer mechanical properties (strength, elastic modulus, elongation at break) crosswise than lengthwise.
If a tensile force is exerted on a longitudinal strap, a certain lengthwise elongation will occur in said strap. In places where the longitudinal strap is bonded to a transverse strap, this elongation will result in a crosswise force being exerted on said transverse strap. As was stated, it is precisely in this direction that drawn straps are less strong. Hence when subjected to heavier loads, the transverse strap will split.
This splitting does not in itself constitute a major problem for the geogrid. However, because the transverse strap and the loaded strap are bonded together over the entire contact area, the transverse strap's splitting or cracking will lead to a crack and/or a load peak in the loaded longitudinal strap. This crack in its turn will lead to the premature failure of the loaded longitudinal strap.
Selecting transverse straps with a comparatively low crosswise elastic modulus means that the transverse straps will be deformed along with the longitudinal straps without splitting or cracking on the side where they are welded to the longitudinal strap, and that the unfavourable effect described will not occur.
Preferably, in the geogrids according to the invention use is made of transverse straps (or longitudinal straps) which even when a tensile strain is exerted on one or more of the longitudinal straps (or transverse straps) of at least 90%, or even at least 95%, of the specific strength of the longitudinal straps (or transverse straps) will co-deform without cracking or splitting. In this way optimum use is made of the strength of the straps.
Geogrids generally are made up of a “lattice” of longitudinal and transverse straps bonded together at an angle, preferably of between 80° and 100°. Especially preferred are geogrids where the straps are bonded together through the polymer of the straps themselves, since such grids can be made comparatively easily without recourse to glue or other adhesives. Moreover, because only a fraction of the polymer of the straps is melted, the strength of the straps is affected hardly if at all. Preferably, only 5 to 100 &mgr;m, or even only 5 to 30 &mgr;m, of the polymer is melted.
A highly suitable method for effecting the bonds in the grids according to the invention is the one where the straps are placed one on top of the other, pressed together, and heated using a radiation source emitting electromagnetic radiation, e.g., a laser, with the strap facing the radiation source being transparent to the radiation and the material at the point where the straps are bonded together absorbing said radiation (to a high degree).
It was found that this technique makes it possible to produce a very strong weld rapidly (e.g., in 10-20 milliseconds). The strength of this weld can be as high as the strength of the employed straps. In other words, two straps which are in the same straight line and have been welded together at a point where they overlap (which overlap, e.g., is at least twice the width of the straps) using this technique will have (substantially) the same strength as a single continuous, untreated strap.
Also, the aforesaid absorption of the radiation may be either by the polymer itself or by a pigment added to the polymer.
In a very simple embodiment the strap facing the radiation source is composed entirely of transparent material. In that case there are several alternatives. For instance, the strap facing away from the radiation source may be made of an absorbent material. Alternatively, the straps to be bonded will both be transparent and a (thin) layer, e.g., ink or a film or foil, of an absorbent material is provided between the straps.
It will be obvious that, in principle, any configuration is possible so long as there is a material absorbing the radiation at the point where the bond is to be effected and so long as the radiation is able to reach this material.
Another suitable embodiment is the one where the strap facing the radiation source is made up of more than one component. Use may be made, e.g., of a bicomponent strap (width 12 mm; thickness 0.55 mm) of transparent polyester (0.50 mm thick) and polyester (0.05 mm thick) to which a pigment has been added or of which the optical properties have been changed. This strap can be bonded to itself or to another strap in various ways, so long as the radiation is able to reach an absorbent section via a transparent section.
One advantage of using the multi-component strap is that this strap can function both as an exposed and as an unexposed strap. This means that during production there is no need to provide two or more supply lines for two or more different materials.
The thickness of both the absorbent section of the strap comprising two or more components and an intermedia
Peters Sebastian M.
Van Vliet Arie H. F.
Voskamp Wim
Akzo Nobel nv
Lagman Frederick L.
Neuder William
Oliff & Berridg,e PLC
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