Patent Application
20210040739
The present invention relates to an improved constructive system for structural floors and its erection method. The structural floors are made out of improved structural precast concrete elongated elements and reinforced concrete placed in the job able to properly work together with the precast elements thanks to a proper bonding, being such precast floor elements fabricated thanks to improved industrial installations.
Known in the art are number of floor systems based on precast concrete elongated floor elements and reinforced concrete placed in the job. In sake of clarity, from here on all the term elongated floor element will be used exclusively to refer to a particular family of floor elements: those which span directly from end to end bearing at both ends exclusively in primary structural members (such as primary beams, girders or walls). Also those elements which work in cantilever are included as long as the mentioned structural floor elements are made of one sole piece. These mentioned structural elements typically have a continuous steel reinforcement from one end to the other. Excluded of this field are all those structural elements and/or formers which form structural floors only as a result of a juxtaposition of elements in the direction of the span. This sort of elements that work by addition, typically have their reinforcement interrupted in the direction of the span (and splices often have to be arranged), and also temporary props and/or formers are needed during the erection process, as these small structural elements are too small to span from one main bearing (wall, girder, etc.) to the next one.
In order to analyse the differences between the currently existing systems of structural floors, those can be studied seeing 5 main features:
A) CROSS SECTION of the precast floor elements, transverse to their longitudinal direction;
B) METHOD OF VOIDING THE CROSS SECTION to make lighter and more efficient elements;
C) AMOUNT OF CONCRETE POURED IN THE JOB and its relative position in relation to precast floor elements;
D) BONDING SYSTEM to keep together precast concrete to cast in situ concrete;
E) Existence of EFFECTIVE NEGATIVE REINFORCEMENT to enable the structural floors to resist negative moments over the linear supports where structural floor elements bear their ends.
For each of the 5 features, the main solutions are described, some examples are mentioned, and their main advantages and/or drawbacks are mentioned.
A) Cross Section
Two main sorts of cross section of the elements can be defined. Solid elements and light or voided elements.
Among solid elements, the most common are known as preslabs, predalles or half slabs, among other names. These are typically rectangular section flat solid elements intended to form solid slabs by pouring considerable amounts of concrete in the job. The precast elements normally have a height around ⅓ or ½ of the total height of the finished slabs. Their main advantages one can count that their prefabrication is generally easy. However, some examples of very complicated preslabs can be found: QIU ZEYOU (CN1975058), QIU ZEYOU (CN1944889) and QU, YUAN, ZHOU, LI, WEI (CN201924490). Among the main disadvantages of predalles (or preslabs) (apart from the expensive fabrications in some cases, as the mentioned examples) is the fact that precast elements may be heavy and finished solid floors are very heavy, and inefficient compared to light or voided floors.
Among precast floor elements with a light or voided, there is a considerable variety. Some of the more generally used are hollow core slabs, double-T slabs and voided preslabs (or predalles). All this elements' cross sections are specifically designed searching their optimization. This means a minimum consumption of concrete (and steel), and thus a minimum cost and weight, but also a maximum moment of inertia and a height as small as possible. Voided cross sections always have a bigger radius of gyration (i) than solid sections with the same depth. This means a higher ratio (Moment of Inertia)/(Area). This simply means light or voided section precast elements are more efficient than solid section precast elements.
B) Method of Voiding the Cross Section
This feature, obviously, is only applicable to precast elements with light or voided cross sections. There are two main strategies to void the section: using removable/reusable formers, and or embedding light permanent formers.
Using removable/reusable formers is typically used in elements such as hollow core slabs, double T slabs and similar sections. It is a cheap an efficient technique as formers are reusable for a very big number of elements. However, the floor elements obtained with this technique have one important drawback. Their low notional size leads to a rapid initial shrinkage of the precast element. This is because this elements have a cross section with a small area in relation to its perimeter.
Embedding light permanent formers is a solution used when using removable formers is not possible or too complicated. This is a solution used in voided preslabs (or predalles). A recently published example is JINLONG, JUNWEI, WANYUN (CN104032870). These precast elements are often prefabricated in two (sometimes tree) main steps. A first step consists in casting a flat thin solid slab. A second step consists in placing light permanent formers on the precast slab. And a third step (not always exists) is to cast vertical ribs (or stems) connected to the lower slab. This way of making light or voided cross sections is somewhat expensive, because the light permanent formers are often expensive, not only because of the material cost (normally polyestirene or tile) but also because of the cost of the handling during the placing operation.
C) Amount of Concrete Poured in the Job
We can find mainly four cases: 1) Those where the amount of concrete cast in the job is greater or similar to the amount of concrete of the precast element, and concrete is typically placed all over the precast element; 2) Those where the concrete placed in the job forms a relatively thin layer all over the precast elements, typically known as topping; 3) Those where the amount of concrete is minimum, and is typically placed only in the lateral joints along the sides or of the precast elements; 4) Those where no concrete at all is poured.
Those structural floors where the amount of concrete cast in the job is greater or similar to the amount of concrete of the precast elements, are of two sorts: solid preslabs (or predalles) (very usual) and hollow core slabs where some alveoli are open in the upper face (unusual in current practice). Using solid preslabs (or predalles) causes a typical dichotomy to solve. The thinner the precast solid preslab is, the more flexible it is, and the greater is the amount (and the weight) of the concrete paced in the job, so the more intense becomes the required shoring required during the erection (while cast in situ concrete is still fresh) to prevent the deflection of the thin preslab, thus the more expensive and slow the construction becomes. The thicker the precast solid preslab is, the less flexible it is and the lesser the mount of concrete cast in the field is required; so the lesser (or none) is the shoring required during the erection process. But, even if the shoring cost can be reduced or supressed for thicker solid slabs, the bigger amount of precast concrete very often increases the cost of the whole structure, as the precast concrete is often more expensive by m3 than the cast in situ concrete due (among other reasons) to the fact that precast concrete is typically richer in cement and richer in additives. In the case of hollow core slabs where some alveoli are open in the upper face, the moment of inertia is reduced by the superior openings (and become more flexible). So, slabs typically need shoring in the job in order to withstand the weight of the considerable amount of concrete cast in the job.
Those structural floors where only a topping is placed, can have virtually any cross section (hollow core, double tee, solid or voided high-depth preslabs, etc.), as long as their superior face is flat or almost flat. There is a number of advantages in placing only a thin topping on the precast elements. Firstly, the precast elements have almost the same depth as the definitive structural floor, thus they are very stiff and do not deflect easily and typically need very little or no shoring. Secondly, the relatively thin topping is not too heavy, so does not deflect too much the already stiff precast element. Finally, the topping, despite being thin is able to effective act as a horizontal diaphragm that properly guarantees a good behaviour of the floor versus seismic forces (typically great horizontal forces). One drawback must be mentioned: toppings cast in situ have typically a considerable shrinkage, due to their shallowness and big surface exposed to the air (low notional size). This often leads to considerable differential shrinkage. Further than all the aforementioned, it must be said that a considerable number of the precast floor elements (but not all of them) used in this sort of structural floors are designed so that when placing the topping in the field, a small amount of concrete enters and completely fills the lateral joints between precast floor elements. For example, hollow core slabs are typically designed to have this lateral joints filled with concrete; while double T slabs do not have this lateral joints designed to be filled with concrete. The main function of the filling of these lateral joints can be understood, by reading the following.
Those structural floors where concrete only is placed in the lateral joints along the sides of the precast elements, can have solid sections or voided sections. All these structural floors have two main advantages. On the one hand, the height of the precast element is the same as the height of the finished structural floor, thus the stiffness of the precast element is very high and shoring is typically unnecessary. On the second hand, the amount of concrete poured in the field is very low, so that its weight is almost neglectable, and it causes nearly no deflection to the precast floor elements. The combination of this two advantages means that this sort of structural floors are the more efficient of all during the construction process, because the deflection caused by the weight of the fresh concrete does not cause an important deflection nor does it “consume” a significant part of the positive moment strength of the precast floor element. However, these floors have two significant drawbacks. On the one hand, the small volume of cast in situ concrete may have a relatively important surface (the superior face) in contact with the atmosphere, and thus a considerable shrinkage, which is especially high for precast elements with a small depth (as the concrete volume is smaller). The transverse shrinkage of the concrete poured in the joint will, per se, open cracks in the contact with the precast element, but additionally, the longitudinal shrinkage will probably lead to differential shrinkage, and favour the breaking of the bonding. On the other hand, the precast floor elements without topping typically work as pinned-pinned (only resist positive moments), and when deflected under service loads, the ends of the precast elements tend to rotate considerably in relation to the linear supports where they bear. This typically causes long and wide cracks parallel to linear supports in the contact of linear supports and the ends of the precast floor elements. This sort of imperfections in the structure, which are normally hidden by finishings, are still not desirable, as such wide and deep cracks are bad for the durability of the structure.
Further than the aforementioned, it is important to highlight the main function of the filling of the lateral joints. This lateral joints have the mission to transfer vertical shear forces from one precast floor element to the precast floor element placed immediately beside it. This is achieved thanks to the shape of the lateral faces of the precast floor elements, which are typically designed to form shear keys when concrete is poured in the joints. This vertical shear keys are mainly achieved in two ways: or the lateral side of the precast element has an upper tab (in the longitudinal direction) protruding transversally from the side, or the lateral side of the precast floor element has a groove (parallel to the longitudinal direction). On the other hand, the filling of concrete also helps solving the imperfection of the joints, as concrete needs certain precasting and placing tolerances, not easily compatible with the avoiding of leakage of the concrete placed in the field. To reduce and try to avoid leaking, the mentioned lateral joints are closed in their lower parts thanks to tabs protruding from the lateral faces of the precast elements. Such tabs typically protrude more from the lateral faces of the precast element, than any other tab or element protruding from those faces. This is to guarantee the proper closing of the joint.
Those structural floors where no concrete at all is placed in the job, on top or at the sides of the precast element are not so usual, but there are some outstanding examples. Among the modern examples, maybe the most important are “pretopped” double tees. This is a sort of double T designed to work without topping, which have a superior slab thicker than usual double T elements designed to be covered by a topping cast in the job. In this category (no concrete at all) one may also mention some patents of the early twentieth century, now considered outdated and not feasible. Several decades ago not so much attention was payed to precasting and erecting necessary tolerances, now considered essential. At that time some inventors considered wrongly that perfect matching of precast elements was easy to achieve. This sort of structural floor construction by simply placing elements side by side is rapid and easy but has a number of drawbacks. Firstly, the transfer of vertical shear forces is not possible, or metallic inserts must be added to guarantee such an important structural feature. For instance, steel teeth or tabs protruding from the lateral faces of the precast elements (this sort of solutions are usual in pretopped double tees). Secondly, the transfer of horizontal forces (such as seismic forces) is not guaranteed. To solve this problem, the aforementioned protruding metallic inserts (or other equivalent means) must be able to fixely connect a precast element to the one beside it. Achieving this will require some work in the field (welding, screwing, small concrete pouring in pockets, etc.). So the “economies” achieved thanks to not pouring a topping, are in part payed in other sorts of tasks an material consumption in the job. Finally, this sort of floors have the same problem at the end of the precast elements as those where only the lateral joints are filled with concrete: wide and profound cracks appear parallel to the linear support elements.
D) Bonding System
The main mission of a bonding system able to make precast concrete and cast in situ concrete work together is to withstand shear forces parallel to the faces of the precast element (superior face, or lateral faces). To achieve such bonding, five main strategies may be described: 1) Reinforcement passing through the contact surface, say reinforcement embedded in the precast element and coming out of it, intended to be embedded in the cast in situ concrete; 2) Labyrinthine contact perimeter in the transverse cross section of the precast element with the cast in situ concrete 3) Flat contact surfaces between precast concrete and cast in situ concrete are made smooth o rugose; 4) Linear or isolated concrete protrusions coming out of the precast element faces which will be in contact with cast in situ concrete; 5) Grooves or holes on the precast element faces which will be in contact with cast in situ concrete.
Those structural floors where reinforcement is embedded in the precast element and protrudes out of it to embed in the cast in situ concrete are relatively common. This strategy is very usual in preslabs (or predalles). One example can be seen in the patent JILONG, JUNWEI, WANYUN (CN104032870) and in some embodiments of patents QIU ZEYOU (CN1975058) and QIU ZEYOU (CN1944889). In fact one can also find it in precast elements of other cross sections, such as in the patent BORI, FABRA (ES2130037). However, this solution—protuding steel—is unusual in most conventional floor elements such as hollow core slabs or double tees. This solution, which a priori may seem the more straightforward, has three main drawbacks. Firstly, steel is expensive per se (both the material and the placing). Secondly, placing protruding steel into precast concrete is often difficult, because protruding reinforcement cannot exist in faces in contact with a former or near to mobile parts of casting machines. Finally, embedded reinforcement will typically complicate compaction of precast concrete, which is why elements made of dry concrete (such as hollow core slabs) have very rarely protruding reinforcement elements.
Those structural floors with a labyrinthine contact perimeter in the transverse cross section are not too usual, but have been tested in a number of real buildings. The most outstanding example are hollow core slabs where some alveoli are open in the upper face. These openings are used to place negative reinforcement within at the job, and then pour concrete, which typically fills the open alveoli. This solution, which is even accepted in some national codes, is unusual in the practice due to four main disadvantages; 1) Opening the upper part of the alveoli of the slabs requires an additional work during the precasting process, which requires human workforce and leads to waste the removed concrete, or requires an investment in specific machinery able to do the openings and recover the removed concrete. 2) Openings are typically not made along all the hollow core length, but typically ⅔ of the length of each slab, which complicates precasting and makes it more costly to solve local defects on the slab occurred during the casting process (as bigger lengths of precast element must be rejected and wasted, when compared to very short rejected parts necessary when the cross section is totally uniform). 3) Eliminating a part of the upper flange of the slabs (to open the alveoli) reduces considerably the moment of inertia of the slab, and makes it more flexible and less efficient during the erection process, leading often to the need of shoring during the erection. 4) Around ⅔ of the length of open alveoli are filled with concrete cast in the job. As a result, the slab reduces considerably its lightness and becomes less efficient. As a whole, this solution is somewhat similar to voided preslabs,
Those structural floors where mainly flat contact surfaces are smooth or rugose, have the advantage that are very easy to cast. That is why most common use precast structural floor have this sort of surface. However, this has an important drawback: while a certain bonding often exists in the first weeks, months or years after the structural floor is finished, this bonding typically breaks completely as time passes, differential shrinkage occurs, and the structure has to go through the cyclic loading and unloading due to the normal use of any structure. This issue is one of the reasons why there is a certain trend in the last decades in trying to eliminate the topping in this sort of structural floors. As bonding breaks the topping is no more a part of the main structural section, and its contribution to structural strength versus flexure moments becomes neglectable. In the end it becomes mainly a dead load on the structure, with the sole function to act as a horizontal diaphragm in the case of earthquake.
Those structural floors where isolated or linear protrusions come out of the faces of the precast elements are very usual, but some outstanding examples exist. On the one hand there is a considerable variety of precast elements that include protrusions only in their lateral faces. Most of these solutions are thought to make the structural floors able to resist seismic forces. This is nowadays a usual solution in the practice for hollow core floors that do not have a topping and need to be seismic resistant. An example is CUYVERS (BE858167). Protrusions on the upper face of floor elements are more unusual, but a couple of examples are MING, WEIJIAN, ZHEZHE (CN102839773) and MING, WEIJIAN, YANTING, PEINAN (CN104727475). This sort of solution, in general, is a good solution to transfer shear forces, as long as this forces do not overcome the shear strength of the unreinforced concrete in the weakest sections. Among its advantages is the fact that no steel is needed to guarantee the connection of the two concretes (precast and cast in situ), which makes the fabrication of these bonding system easier and cheaper. One of its main drawbacks is that unreinforced concrete fails fragilely under shear forces, and shear strength of unreinforced concrete is not easy to predict (shear strength results of a same concrete typically show quite disperse statistical distributions, because shear strength depends on tension strength which is based in part on aleatory factors, such as aggregate distribution, cracking geometry due to shrinkage or tension forces, etc). As a consequence, a solution based on unreinforced concrete working under a shear force must be designed with a big security coefficient, much bigger than reinforced concrete under the same shear force. For example, a security coefficient of 2.0 (or even 2.5) for the material (or sort of ULS) and of 1.4 for the loads. Thus a global security coefficient of 2.8 (or even 3.5). That is one of the reasons why not all sorts and shapes of protrusions are appropriate. Some important details must be taken into account in their design:
Those structural floors where holes or grooves are made on the faces of the precast elements are quite rare in the conventional practice, but some examples can be found in patents. On the one hand one can find cases where holes or short grooves are placed only in the lateral faces of the precast elements. The intention is often the same as in solutions with protrusions: making the structures able to withstand seismic forces. Some examples (not all intended to withstand seismic forces) are MICHEL DE TRETAIGNE (FR2924451), LEGERAI (FR2625240) and BORI, FABRA (ES2130037). Even more rare are the solutions with holes or grooves in the upper face, but some examples are PRENSOLAND, S. A. (ES2368048), QIU ZEYOU (CN1975058), QIU ZEYOU (CN1944889) and QU, YUAN, ZHOU, LI, WEI (CN201924490). PRENSOLAND, S. A. (ES2368048) includes holes in the upper face and in the lateral faces; and the three next examples include transverse grooves all over the surface of the element, always cut by a central rib (or stem). The advantages and drawbacks of this bonding solution (holes or grooves) are very similar to that of protrusions. However, one of the main differences is that one has to take care in not weakening the faces of the precast elements where the holes or grooves are made. By reviewing the list of important details that one has to consider when designing protrusions, we will review next which of the aforementioned examples have issues in some or several of the details to take into account:
E) Effective Negative Reinforcement
The main mission of an effective negative moment reinforcement is to make the finished floor able to withstand such negative moments, which typically cause tension in the upper face of the structural floor and compression in the bottom face. Most of the most usual structural floors made out of precast floor elements and cast in situ reinforced concrete are floors only able to withstand positive moments. This is due to the fact that all modern precast floor elements are designed to resist positive moments, by means of including longitudinal reinforcement (which may be passive or prestressed). However, achieving this floor structures to properly resist negative moments is more difficult than it seems for two reasons. On the one hand, negative reinforcement (placed near the upper face of the structural floor) can only be embedded in cast in situ concrete. Thus a certain amount of cast in situ concrete is necessary. On the other hand, proper bonding between precast concrete and cast in situ concrete is essential for the negative reinforcement (under tension) to work together with the bottom face of the precast floor element (under compression) and resist the negative moment. Currently three main situations can be found in the existing technology: 1) Effective negative reinforcement is embedded in cast in situ concrete which is properly bonded to precast concrete; 2) Only crack control reinforcement is embedded in cast in situ concrete; 3) No reinforcement at all is placed.
Those structural floors where effective negative reinforcement is embeddded are usual, but are limited to only two sorts of structural elements: preslabs (or predalles) [much more usual] and hollow core slabs with superiorly open alveoli [unusual]. In preslabs there usually is plenty of place to embed negative reinforcement and there typically is reinforcement embedded in the precast element protruding from its superior face to properly guarantee the bonding with cast in situ concrete. Hollow core slabs with superiorly open alveoli have limited space to place reinforcement, so it has to be carefully placed to guarantee a proper wrapping with concrete cast in the job. Thanks to having negative reinforcement, preslabs (or predalles) and hollow core slabs with superiorly open alveoli are particularly efficient and can reduce their depth when compared to structural floors without negative reinforcement. However, as mentioned previously conventional preslabs (or predalles) get typically expensive due to the need of reinforcement to guarantee the bonding and due to their heavy and inefficient solid section or to their expensive embedded permanent forms (in the case of voided preslabs). Hollow core slabs with superiorly open alveoli are also expensive due to their very specific precasting process. So this two sorts of structural floors are typically thinner (structurally more efficient) but not necessarily less expensive than only positive-moment-resistant floors made with voided section floor elements, such as conventional hollow core slabs or double tees.
There is a considerable number of currently usual structural floors where negative moments are not intended to be resisted, and reinforcement is placed only to control the width of the cracks that typically appear at the end of precast floor elements, parallel to linear supporting elements—beams or walls. This solution (reinforcing to control cracking) is adopted is those cases where the structural system is not able to guarantee a proper bonding between precast concrete elements and cast in situ concrete, but there is still some place to embed the reinforcement. This is the case of all conventional floors made of voided section precast elements, where typically only small amounts of concrete are poured in the job. Be it mainly to form a topping or only to fill the lateral joints. This virtually occurs in all hollow core floors (with or without topping), all double tee floors with topping and a number of the most common structural floors.
For example, in the patent by CHAO, ZHAOXIN, GUOPENG, JIANFENG (CN203347077), the reinforcement embedded in the topping is aimed at controlling the crack width.
There are cases where no reinforcement is placed, as there is no cast in situ concrete where to embed such a reinforcement to control cracking. This is the case of structural floors made with “pretopped” double tees, which have not topping cast in the job.
As a summary, nowadays when erecting a structural floor made with precast floor elements and reinforced concrete cast in the job, one has to choose between the two following solutions:
Thus, nowadays one has to choose: or an easy-to-build but structurally less efficient solution (hollow core slabs, double T slabs, etc.); or a labour-costly and slower-to-build but structurally more efficient solution (preslabs, hollow core slabs with superiorly open alveoli)
For overcoming the mentioned drawbacks of the existing solutions, the present invention proposes a prefabricated floor element having an elongated shape wherein a longitudinal direction, a transversal direction, a height direction, two end faces which delimitate the element in the longitudinal direction, two lateral faces which delimitate the element in the transversal direction, a lower face and an upper planar face that delimitate the element in the height direction are defined, which comprises transversal continuous upper grooves on the upper planar face.
This prefabricated floor element is destined to be supported at its ends on two respective linear supporting elements, like walls or beams arranged in the transversal direction. Specifically, this element allows, by arranging an armature placed on the upper planar face and extended beyond the end faces and pouring a layer of concrete (also called topping) in which said armature is embedded, to transmit tension forces having the longitudinal direction, due to negative flexure moments, thanks to the continuous upper grooves on the floor element, while allowing to avoid the effects of differential shrinkage of the two concretes (that of the prefabricated floor element and that of the layer of concrete). These tension forces in the upper armatures, in combination with the compression forces on end faces of the floor element allow to transmit negative moments through said end faces, these moments being around the Y direction (or axis).
In some embodiments the upper grooves are present only on two end portions, each covering ⅓ of the length of the entire upper face, such that the central portion is devoid of grooves. In this way the grooves are only in the places where they are useful, leaving the floor element unchanged (and unweakened) at the central portion.
In some embodiments the prefabricated floor element has a lower tab on a lower edge of the lateral faces. The aim of this lower tab is to prevent the cast in situ concrete to leak between two floor elements, as a cast in situ rib forms when those are put side by side, parallel to the longitudinal direction.
In some embodiments the prefabricated floor element comprises an upper tab on an upper edge of the lateral faces, the lower tab being longer than the upper tab in the transversal direction. When a cast in situ concrete rib is formed between each two floor elements, the aim of the upper tab is to allow the cast in situ rib to transfer vertical shear forces. In this embodiments, the proper transfer of vertical shear forces, the upper tab works together with the lower tab from one precast floor element to the adjacent one.
In some embodiments, instead of an upper tab, a groove exists on lateral faces, which enables the cast in situ rib to transfer vertical shear forces.
In some embodiments the prefabricated floor element comprises vertical lateral grooves on the lateral faces. Like the upper grooves, these lateral grooves allow to transmit longitudinal forces between concrete poured in the cavity and an armature embedded therein.
In some embodiments the prefabricated floor elements has a light or voided cross section, such as that of a hollow core slab.
In some embodiments the prefabricated floor element is a double-T floor element, such that an upper planar plate and two vertical stems extending downwardly from the upper planar plate are defined.
The fact that double T slabs are provided with upper continuous transversal grooves has two main advantages, just as in other light floor elements (with low dimensionless thickness). On the one hand, the transversal grooves on the upper face enable the possibility to transfer forces in the longitudinal direction form the prefabricated slab to the armature by the means of the concrete of the topping. This ultimately enables the floor made with prefabricated slabs to be fixed (=negative moment−resistant) at one or both of its ends. On the other hand, the fact that the grooves are able to prevent the effects of differential shrinkage; which is particularly high in precast elements with a low dimensionless thickness (under 0.6). The effects of shrinkage in the longitudinal direction are blocked thanks to grooves of the proper depth and with faces perpendicular to longitudinal shear forces; so that differential shrinkage in this direction will only add to other flexure forces, acting as a positive or negative moment, depending on the fixity on the topping slab at its ends. Transversal differential shrinkage has no effect on the slabs, thanks to the fact that grooves are continuous, so that there is no edge or face parallel to longitudinal direction. Such edges and faces, parallel to the longitudinal direction tend to prevent a proper transverse shrinkage of the cast in situ topping, leading to a slight upward deflection of the topping, which leads to the detaching of the topping from the slab. Such a behavior is incompatible with the transmission of longitudinal forces, essential to this invention. That is why, upper grooves must be continuous, and neither edges nor faces parallel to the longitudinal direction should cut the upper grooves.
The two advantages aforementioned are common to double T slabs and other light slabs, such as hollow core slabs, however there is an additional advantage for double T slabs (and inverted-U slabs—similar to T slabs in cross section): making negative-moment-resistant floors leads to a considerable reduction of the height of the precast element (−30%). Double T slabs, and inverted-U slabs are typically elements with big heights (from 40 cm to 80 cm), and such reduction in the depth is very useful, as it enables this sort of elements to be used in a wider range of buildings, where heights of floors must be smaller. Currently, due to their considerable height, double T slabs are mainly used in parking buildings, warehouses and sports pavilions. However, a reduction of a −30% in their typical depths, would considerably increase the applicability of this sort of structural slabs.
The invention also relates to a prefabricated floor element having an elongated shape wherein a longitudinal direction, a transversal direction, a height direction, two end faces which delimitate the element in the longitudinal direction, two lateral faces which delimitate the element in the transversal direction, a lower face and an upper planar face that delimitate the element in the height direction are defined, which a lower tab on a lower edge of the lateral faces, which comprises vertical grooves on the lateral faces, the lateral grooves extending from the lower tab to the upper planar face.
This prefabricated floor element is destined to be arranged side by side to another floor element, along the longitudinal direction, and then both supported at their ends on two linear supporting elements, like walls or beams arranged in the transversal direction. Specifically, these elements allow, by arranging an armature in the upper part of the shear key formed by pouring concrete in the volume delimited by the lateral faces and the tabs and extending beyond the end faces, to transmit tension forces having the longitudinal direction thanks to the lateral grooves. These tension forces in the armature, in combination with the compression forces acting upon the lower part of the end faces of the prefabricated floor element allow to transmit negative flexure moments, these moments being around the Y direction.
In a preferred embodiment the vertical grooves on the lateral faces are present only on two end portions, each end portion covering ⅓ of the entire length of the lateral face, such that the central portion is devoid of grooves. In this way the grooves are only in the places where they are useful, leaving the floor element unchanged (and unweakened) at the central portion.
In some embodiments the lateral grooves have a minimum depth and width of 1 time and 1.5 times, respectively, the diameter of the biggest aggregate of the concrete poured in the job.
In some embodiments the upper grooves have a minimum depth and width of 1 time and 1.5 times, respectively, the diameter of the biggest aggregate of the concrete poured in the job .
This minimum size is aimed to effectively prevent the slipping of the concrete cast in the job from its place on the prefabricated element. This is achieved on the one hand by ensuring the correct filling of the grooves by the poured concrete; and on the other hand by ensuring that the shear forces act upon the aggregate that enters the grooves, and not only on the cement wrapping the aggregate; thus avoiding that the aggregate detaches from the cement. Typical diameter of biggest aggregate of cast in situ concrete ranges from 10 mm to 20 mm, but most often 20 mm. In accordance, the depth and width must be at least of 20 mm and 30 mm, respectively.
In some preferred embodiments the dimensionless thickness of the floor element cross section is below 0.6.
The dimensionless thickness is obtained from dividing what is known as a notional size (or fictitious thickness) by the real thickness (say height of the floor element). The notional size is a parameter defined by Eurocode EC-2 in the section devoted to shrinkage of concrete elements. The notional size (h0) is equal to twice the shape factor (Ac/u) of the cross section. That is, the notional size is equal to 2*Ac/u, where “Ac” is the area of the cross section and “u” is the perimeter of the concrete cross section in contact with the atmosphere. For elements with interior holes, such as hollow core floor elements, this perimeter includes the perimeter of the interior hollow channels.
Then the dimensionless thickness (h′) would be defined as h′=h0/h, where h is the real thickness, and h0 is the notional size.
The following table includes several cases studied. The first column corresponds to the name and the width of the prefabricated floor element. The second corresponds to the thickness or height (h). The third corresponds to the dimensionless thickness (h′). And the fourth is for the notional size (h0). In the cases analysed, at the beginning there are two groups of solid slabs, those with a wide of 1.2 m and those with a wide of a wide of 0.6 m. Notice in all cases h′ is equal or superior to 0.6. Notice also how the case with the lower dimensionless thickness h′ can barely be considered a solid slab, as its 40 cm×60 cm cross section more that of a column or beam than that of a floor element like a slab.
Next are studied two sorts of hollow core slabs, depending on the sort of interior holes. Finally three examples of American double T slabs are studied. All these precast floor elements are light elements, all taken from actual commercial products. Notice that all have dimensionless thickness clearly under 0.6 (the lesser the h′ is, the lighter the element is). In these light elements, the influence of modifying the wide of the element is neglectable, that is why, different widths are not displayed in the table.
Light elements (those with a low dimensionless thickness) have typically a bigger differential shrinkage between the concrete of the floor element and the concrete cast in the job than solid elements. This is because a smaller dimensionless thickness leads always to a bigger shrinkage. So, if the grooves described in the patent are good to properly resist the effects of a bigger differential shrinkage (in light elements), the same grooves will also withstand a lesser differential shrinkage of solid floor elements.
Differential shrinkage and its importance in floors made with prefabricated floor elements: Prefabricated floor elements are typically casted some days or some weeks before being placed in the job. After their being placed, some steel reinforcement is arranged on top of the precast elements and finally concrete is poured on the elements. This concrete may be poured only in the cavities between the floor elements, or may be poured all over the floor elements, as a topping. Therefore, the concrete placed in the job is at least a weak younger than the concrete of the precast elements, and it is not unusual that the difference in age is of several weeks. The two concretes are typically very different in their composition. The precast concrete is typically richer, and designed for a very fast hardening, which typically leads to a rapid initial shrinkage; so that after a week a very significant portion of the whole shrinkage of the precast floor element may have occurred. Early shrinkage is bigger in elements with a cross section with a smaller dimensionless thickness, such as all light prefabricated elements: hollow core slabs, double T slabs, inverted-U slabs, etc. When concrete is placed in the job in contact with precast floor elements, a considerable early shrinkage has already happened on the precast elements, so that shrinkage of the precast elements is decelerating. However, fresh concrete just placed in the job, experiences a rapid shrinkage, which is not synchronized with the shrinking rhythm of the precast. This causes what is known as differential shrinkage. This phenomenon tends to cause the slipping of the concrete cast in the job over the precast element. This slipping is initially (under small differential shrinkage) prevented by the adherence between the two concretes, but as differential shrinkage increases (as months pass) it weakens more and more the adherence, and may completely break it. This phenomenon typically leads, after some months or years, to a complete or nearly complete rupture of the connection of precast floor elements and concrete cast in situ (for example of the topping). This leads to two important drawbacks: a) on the one hand concrete placed in the job cannot work together with the precast floor elements; an thus it is pointless to try and put negative reinforcement embedded in the cast in situ concrete; b) concrete cast in the job ends as a dead load on the structure, with little or no structural function.
Trying to control the effects of differential shrinkage only by making efforts to synchronize the shrinkage speeds of the two concretes through a control of the concretes mixtures is extremely risky, as shrinkage is a phenomenon depending on a number of aleatory factors (temperature; humidity; wind; compaction of concrete; etc.) which are very difficult to control in a precasting plant, but even more in a job.
All the drawbacks caused by differential shrinkage are solved by the solution here presented: transverse and continuous grooves, be those placed on the superior surface or on the lateral faces.
The invention also relates to a structure comprising a prefabricated floor element having an elongated shape wherein a longitudinal direction, a transversal direction, a height direction, two end faces which delimitate the element in the longitudinal direction, two lateral faces which delimitate the element in the transversal direction, a lower face and an upper planar face that delimitate the element in the height direction are defined, which comprises transversal continuous upper grooves on the upper planar face, the structure further comprising:
a linear supporting element which supports one end of the prefabricated floor element such that in the linear supporting element a supporting surface is defined and:
a moment resisting system arranged on the linear supporting element and facing an end face of the prefabricated floor element,
an upper concrete layer (topping) poured all over of the precast floor element, and armatures arranged along the longitudinal direction, such that a portion of the armatures is embedded in the concrete layer (topping) and another portion of the armatures extends such that they are embedded in the moment resisting system, such that the armatures, when acted under tension forces, can transmit forces to the concrete layer, and the concrete layer can transmit forces to the prefabricated floor element through the upper grooves on the upper planar face, and then a negative moment is transmitted from the moment resisting system to the prefabricated floor element.
This invention enables that structural floors made out of precast floor elements, reinforcement (passive or post-tensioned) placed at the job, and a relatively small amount of concrete—under the shape of a topping—poured at the job, to become up to a 35% more efficient than similar conventional floors, say those were there is no negative reinforcement, or such reinforcement does not come to be effective.
The increase in efficiency is obtained thanks to the fixity obtained when negative reinforcement, which is properly anchored to a moment resisting system, works properly bonded to the cast in situ concrete, and the cast in situ concrete is properly bonded to the precast floor elements.
The proper bonding of reinforcement to concrete cast in situ is easy to get as long as concrete properly wraps reinforcement. The proper bonding of cast in situ concrete and precast concrete is usually broken by the effects of differential shrinkage when contact faces are flat and smooth and do not include protruding reinforcement, but with this invention, this drawbacks are avoided, and proper bond is maintained over time.
The increase in efficiency obtained thanks to properly fixing the ends of a precast floor element can be seen in that, a precast floor element with a certain depth but fixed at two ends deflects much less than the same floor element pinned at both ends. Moreover, floor elements fixed at their ends need much less reinforcement at their bottom face than elements pinned at their ends.
Precast floor elements fixed only at one end can act as a cantilever; which is a totally novel capacity. A precast floor element pinned at one end, and free at the other would collapse, that is why conventional precast floor elements are not suited for cantilevers.
All these achievements are reached without changing the way in which the precaster is used to fabricate, nor the way the structural designer is used to design, nor the way in which the contractor is used to erect the buildings. So this innovation has the additional advantage that it should be easy to accept by all trades involved in the structure design and the structure construction.
In some embodiments the moment resisting system includes an upper extension of the linear supporting element, a cast in situ concrete placed between the upper extension of the linear supporting element and the end face of the precast floor element.
In some embodiments the moment resisting system includes a cast in situ concrete placed on top of the linear supporting element and between the end faces of two prefabricated floor elements arranged facing their end faces.
In some embodiments the armature has a diameter comprised between 10 and 20 mm, and the concrete layer has a height of at least 50 mm.
In some embodiments the cavity defined between the tabs and the lateral faces comprises a post-tensioned element.
The invention further relates to a structure comprising two prefabricated floor elements, each element having an elongated shape wherein a longitudinal direction, a transversal direction, a height direction, two end faces which delimitate the element in the longitudinal direction, two lateral faces which delimitate the element in the transversal direction, a lower face and an upper planar face that delimitate the element in the height direction are defined, which includes a lower tab on a lower edge of the lateral faces, which comprises lateral vertical grooves on the lateral faces, the lateral grooves extending from the lower tab to the upper planar face, which includes either a longitudinal groove at a lateral face or an upper tab on an upper edge, the floor elements being arranged adjacent such that a volume is defined therebetween the volume being filled with concrete such that a shear key is defined, the structure further comprising:
a linear supporting element which supports one end of the prefabricated floor elements such that in the linear supporting element a supporting surface is defined and:
a moment resisting system arranged on the linear supporting element and facing an end face of the prefabricated floor elements,
the structure further comprising armatures arranged along the longitudinal direction, such that a portion of the armatures is embedded in the upper portion of the shear key and another portion of the armatures extends such that they are embedded in the moment resisting system, such that the armatures can transmit forces to the shear key, and the shear key can transmit forces to the prefabricated floor element through the lateral vertical grooves on the lateral face, and then a moment is transmitted from the moment resisting part to the prefabricated floor element.
This variant of the invention, where no topping is required is particularly efficient, because suppressing the topping reduces considerably the weight on the structure, and in particular the weight that has to withstand the structure under construction, when the concrete cast in situ has not hardened and prefabricated floor elements behave as elements pinned at their beings.
Floors made in this way are cheaper, lighter and more sustainable than any conventional similar floor (with the ends not fixed to linear supports).
In some embodiments the armature has a diameter comprised between 16 and 25 mm.
In some embodiments the structure comprises armatures placed in the shear key and extending from the upper part to the lower part thereof, such that it allows the concrete shear key to withstand higher vertical shear forces.
When prefabricated floor elements do not have a topping, negative reinforcement is placed at the sides of each floor element, in the relatively narrow cavities filled with concrete between floor elements, which forms a negative-moment-resistant rib. As a consequence most of the surface load applied all over the structural floor is applied directly on the prefabricated floor element, and only a small part is directly applied on the rib (cast in situ shear key). However, the prefabricated floor elements are not directly fixed at their ends, being not negative-moment-resistant. This situation tends to lead the floor elements (intensely loaded) to deflect as a pinned-pinned element, while the cast in situ rib deflects much less, just as a fixed-fixed element does, thanks to the negative-moment reinforcement embedded in the rib. As there is a key able to transmit vertical shear forces (longitudinal groove or tab) on the vertical faces of the precast floor element, the differential deflection between the cast in situ rib and the adjacent precast floor elements is prevented. As a result prefabricated floor elements equal their deflection to that of the cast in situ rib. But this happens thanks to the fact that the floor elements “hang” on the rib. This “hanging” means an important transfer of load form the floor element to the rib, leading this rib to withstand important vertical shear forces. Reinforcement is necessary for the rib not to break under this important vertical shear forces. Thus, if one ads negative reinforcement only in the ribs (as there is no topping to place those negative reinforcements placed all over the precast floor element), shear reinforcement is also required, in order to withstand the considerable vertical shear load transfer from the floor elements to the rib.
In some embodiments the structure comprises at least one duct which extends continuously in the shear key and a post-tensioned tendon inserted within the duct.
To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:
As shown for example in
Throughout the following description, these directions will always be used with the same meaning.
By ‘elongated’ it is meant that the length (dimension in the X direction) will be generally longer than the dimension in the transversal direction, i.e. the width, which in turn will be longer than the height (dimension in the Z direction). The height may also be referred to as depth, and in the context of shrinkage study, also as thickness.
Also two end faces 11 which delimitate the element 1 in the longitudinal direction X, two lateral faces 12 which delimitate the element 1 in the transversal direction Y, a lower face 13 and an upper planar face 14 that delimitate the element 1 in the height direction Z are defined.
An embodiment of this first variant where all the face 14 is covered with grooves 15 is advantageous, not for structural reasons, but for production reasons. It makes serial production more efficient as it allows an easy removing of the short segments of defective slab that occasionally appear during the casting process on the casting bed. The variant with grooves only at the ends may demand to reject bigger parts of the precast slab on the casting bed.
The prefabricated floor element 1 also comprises an upper tab TS on an upper edge of the lateral faces 12 the lower tab TL being longer than the upper tab TS in the transversal direction Y.
This element is advantageous when used in a structure as shown in
This arrangement gives rise to the moments as depicted in the following
Specifically,
In
The variants shown in
The armature has a diameter comprised between 10 and 20 mm, and the concrete layer LC has a height of at least 50 mm.
Therefore the difference with the first variant is that the grooves are lateral.
The prefabricated floor element comprises a lower tab TL on a lower edge of the lateral faces 22, the lower tab TL being longer than the upper tab TS in the transversal direction Y.
An alternative embodiment of this second variant can be seen in
Like in the first variant, and as shown in
As shown for example in
Description of the Flexure Strength Mechanism
When prefabricated floor elements do not have a topping, negative reinforcement is placed at the sides of each floor element, in the relatively narrow cavities CC filled with concrete between floor elements 2, which forms a negative-moment-resistant rib or shear key SK. This means most of the load of the floor is applied directly on the prefabricated floor element, and only a small part is directly applied on the rib of shear key SK. However, the prefabricated floor elements are not directly fixed, so not negative-moment-resistant. This situation tends to lead the floor elements more loaded to deflect as a pinned-pinned element, while the cast in situ rib or shear key SK deflects much less, just as a fixed-fixed element does. As there is a shear key, upper tab TS or longitudinal groove LG, transmitting vertical shear forces in the vertical faces 22 of the precast floor element, the differential deflection is prevented. As a result prefabricated floor elements equal their deflection to that of the cast in situ rib or shear key SK. This happens thanks to the fact that the floor elements “hang” on the rib or shear key SK. This “hanging” means an important transfer of load form the floor element to the rib or shear key SK, leading this rib to withstand important shear forces. Reinforcement is necessary for the rib not to break under this important shear forces. Thus, if negative reinforcement is added only within the ribs, as there is no topping to place those negative reinforcements, shear reinforcement is also required in order to withstand the considerable shear load transfer from the floor elements to the rib or shear key SK.
On the one hand, the fact that the neutral axis under Ultimate Limit State ULS is so low for negative moments, and on the other hand the fact that in the variant 2 the lateral faces 22 are the only contact surfaces between cast in situ and precast concrete able to transfer negative moments from floor elements 2 to the negative reinforcement, explain the importance that the lateral (vertical) grooves 26 are made as big as possible: extending them from the lower tab TL to the upper planar face 24.
Description of Unwanted Obliquus Forces and Their Remedy
Description of the Vertical Shear Strength Mechanism of the Rib or Shear Key SK
The variant shown in
In some case such as the depicted in
Placing post-tensioning PTT in the shear key SK has the additional advantage to prevent cracks in the upper planar surface 24, such as the ones depicted in
The minimum sizes mentioned above are aimed at effectively preventing the slipping of the concrete cast in the job from its place on the prefabricated element. This is achieved on the one hand by ensuring the correct filling of the grooves by the poured concrete; and on the other hand by ensuring that the shear forces act upon the aggregate, and not only on the cement matrix wrapping the aggregate; in order to avoid that the aggregate of the cast in situ concrete detaches from its cement matrix. Typical diameter of biggest aggregates ranges from 10 mm to 20 mm. Thus, the height and width must be at least of 10 mm and 15 mm, respectively; but 20 mm and 30 mm, respectively, are generally recommended in order to cover a bigger range of aggregate sizes with the same geometry of the grooves. Respecting these criteria, guarantees an ultimate mode of failure in which either the concrete of the cast in situ concrete or the precast member breaks under shear; but never a failure happens in the interface (separating both concretes). This second sort of failure is not desired, as it is very difficult to predict, as it depends on a number of aleatory factors (humidity history, temperature history, direct insolation, wind, dirt in the job, rain in the job) or of factors that are almost impossible to control from one job to another (formulation of cast and degree of compaction of cast in situ concrete; age of precast members when cast in situ concrete is poured, etc.). These factors will have a very strong influence in the differential shrinkage a differential stiffness of the two concretes. Moreover, the influence of a number of these factors on the interface shear strength of the junction is not even described in most common codes, which mainly base their formulas on principles of cohesion-adhesion of the interface. So, a proper prediction of the strength of this interface surfaces is extremely hard to achieve.
On the contrary, when deep grooves are available, that guarantee an ultimate mode of failure causing the rupture of one of the two concretes (rather than the interface) allows for a very good prediction of the actual strength of the junction. This is because the ultimate shear strength of concrete (one sole material) is very well known and well described in codes. It only depends on the tension strength of concrete, which in turn depends on its compression strength. Thus, none of the mentioned aleatory factors enter into play.
Spacing between grooves should preferably be proportional to the width of the groove. The relation of spacing of grooves to width of grooves must be similar to the relation of shear (or tension) strength of precast concrete to the shear (or tension) strength of cast in situ concrete. (Shear strength of plain concrete is considered here to be proportional to tension strength.) When this proportionality is respected both materials will break at the same time. This means, nor the precast concrete teeth (protrusions placed between each pair or grooves) nor the cast in situ concrete teeth (formed when filling in the grooves) are clearly weaker that its counterpart, avoiding weak points in the junction that would lead to lowering the horizontal shear strength of the junction.
Description of Experimental Results of Horizontal Shear Strength and its Relation with Differential Shrinkage
A series of tests have been performed to assess the horizontal shear strength of different geometries of the contact surface of a precast floor element and a topping cast on top of it. Three sort of tests have been performed: a) Tests with small specimens under pure horizontal shear (35 tests); b) Tests with midsize specimens under horizontal shear induced by bending (6 tests); c) Big size specimens under horizontal shear induced by bending (2 tests).
The different sorts of tests gave consistent results. Next are also described the results of tests with small specimens, as those are the more representative.
Five sorts of surfaces have been tested:
The two most studied cases are smooth surfaces (batch 1) and surfaces with appropriate transverse grooves (batch 5); also the case with holes (batch 3) has been studied. In all these cases, different concretes have been tested at different ages. These different concretes and ages have been designed to lead to different differential shrinkages, in order to assess the influence of this phenomenon on the horizontal shear strength.
Seeing all the results leads to the next conclusions:
The prefabricated elements according to the first variant were successfully tested as described in this section.
SG1, SG2 . . . are the strain gauges for measuring the elongations;
Upper Gauges SGA and SGB measure the upper surface elongations on the upper end portions of the slabs;
To make a valid comparison with the systems of the state of the art, the experimental arrangements of
A detail of the structure of
The photo of
In most common practice floors peak horizontal shear stress will be under 0.35 N/mm2. This corresponds to average stresses of 0.23 N/mm2 when grooves are only on the last ⅓ of floor elements, and to 0.175 N/mm2 when grooves cover the hole floor element. Only under. extremely severe conditions may the peak horizontal shear stress go up exceptionally to 0.50 N/mm2. In all these cases the safety coefficients are summarized in the next table.
Watching the results in the table, it can be seen that the solution with grooves is sufficiently secure in all cases, independently of the sort of concrete used for the topping.
The prefabricated elements according to the second variant were tested as described in this section, and showed a much better performance than a floor made with conventional precast floor elements.
The experimental arrangement to test the floor elements of the second variant is very similar to that of the first variant. So that the schematic experimental arrangement showed in
To make a valid comparison with the systems of the state of the art, the experiment was performed on the floors shown in
The plot of
The experimental arrangement of
Movable Formwork for Dry Concrete Precasts
As shown in
For imprinting the grooves, either lateral or upper, the installation comprises at least a rolling die I8, I9, I10 placed after the formwork I2, I3, I4 in the longitudinal direction X, there where the mould I7 extends, the rolling die I8, I9, I10 having continuous surface teeth I8T, I9T, I10T having axial direction of the die I8, I9, I10, the axis Γ8, Γ9, Γ10 of the die I8, I9, I10 being perpendicular to the longitudinal direction X, such that grooves 15, 26, 36 can be formed on the lateral 12, 22 or upper faces 14, 24 of the prefabricated floor elements 1, 2, 3.
According to an embodiment, shown in
According to another embodiment, shown in
A further embodiment is the result of combining the previous two embodiments. That is, an installation having two rolling dies having vertical axis and a rolling die having a horizontal axis, as shown in
A particular embodiment of the installation IM3 depicted in
A particular embodiment of installations IM1, IM2, and IM3 is one that includes a device for counting the length of produced slab including grooves.
A particular embodiment of installations IM1, IM2, and IM3 is one that includes at list a device able to cause vibration to at least one of the rolling dies I2, I3, I4, while the mentioned rolling die rolls around its axis. This vibration while rotating enables a more appropriate compaction of the concrete when passing through the dies.
Formwork for Self-Consolidating Concrete Precasts
As shown in
In this case, the removable upper part I24 is formed by a plurality of former structural profiles I241 perpendicular to the longitudinal direction X. The mentioned upper part I24 is removeable to allow for the demoulding of the precast member once it has hardened, but it typically stays stationary during the hardening process of the concrete.
The lower section L24 of the former structural profiles I24I defining a decreasing section that defines the section of the grooves 15, 26, 36, the upper section U24 of the former profiles I24I defining a constant section.
Therefore, to mold the floor elements 15, 26, 36 with self-consolidating concrete, the volume of the lower part of the mold must be filled up to the section change between the lower L24 and upper U24 section of the former profiles I24I.
The space G22 between each elongated former element I23 makes it easy to pour concrete, and avoids the formation of interior air bubbles, as the air can easily be evacuated by the multiple spaces.
The placing of the self-consolidating concrete may either be carried out once the upper part I22 is assembled to the rest of the installation Im11, or may the upper part I22 be put in place after the placing of concrete. In this second case, the upper part I22 must be placed right after placing the concrete, while this is still liquid, so that the elongated former elements can properly displace the liquid to form the grooves.
The upper part I24 further comprises joining profiles I24B having the longitudinal direction X and joined to an upper surface of the former profiles I24I, such that the former profiles I24I and the joining profiles I24B form a removable grid.
In the installation IM12, shown in
In this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements.
Thought the document one of the main features that characterizes the invention is the existence of “continuous grooves”. However one must understand that in the scope of this invention are also included “continuous protrusions”. I fact, grooves and protrusions are only two ways of referring to a same thing. One can understand that between each pair of grooves there is a protrusion or vice versa. Thus, defining groves is equivalent to indirectly defining protrusions.
The invention is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art within the general scope of the invention as defined in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18382159.4 | Mar 2018 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/056193 | 3/12/2019 | WO | 00 |
