Patent Description:
The issues of sliding mandrels for cement concrete covers is addressed and defined by the standard ČSN-EN <NUM>-<NUM> and with more detailed conditions for the use of these elements according to ISO <NUM>-<NUM>. This standard specifies a tensile strength of at least <NUM> MPa as a requirement for the sliding mandrel. Steel must be flat, free of sharp projections and other unevenness, the surface of steel must be covered with a layer of plastic.

Sliding mandrels are specially designed exclusively for cement-concrete covers, and according to these standards are made of hot-rolled steel grade S235JR. The diameter in terms of dimension tolerance shall meet the requirements of EN <NUM> and shall be at least <NUM>, with a maximum length tolerance of +/-<NUM>.

Before installation in a cement-concrete cover, at least <NUM>/<NUM> of the sliding mandrel surface should be coated with bitumen or with a thin layer of plastic with a minimum thickness of <NUM> microns. The coating must guarantee corrosion protection while allowing slippage in concrete.

Conformity of the product to the relevant standard shall be demonstrated by an initial type test carried out by an independent body.

Previously known sliding mandrels used in the transverse joints of cement-concrete covers are made of plain rolled steel of quality standard S235JR, usually with a diameter of <NUM> and a length of at least <NUM>. Sliding mandrels are plastic coated with a minimum thickness of <NUM> over their entire surface, which guarantees protection of the steel against corrosion while allowing the mandrel to slip in concrete. The ends of the mandrels must not be deformed, they are ground to allow free movement of the mandrels in the concrete. Sliding mandrels are placed at H/<NUM>, which is ½ the thickness of the concrete slab minus half the diameter of the sliding mandrel.

The disadvantage of known solutions is the constantly rising price of steel and the unaddressed green deal of the product end-of-life.

For materials, these are mainly irregular deliveries and constantly increasing delivery times. Another disadvantage is the dimensional instability of steel semifinished products. Due to the absence of the requirements of the current standard for flatness of metallurgical rolled material, there is a disproportionate increase in waste already during mandrel production. Existing solutions are vary laborious in cutting and preparing the material for plastic coating.

When used on a construction site, the plastic coating of mandrels is regularly damaged in handling. This significantly reduces service life and increases corrosion within the building structure.

These types of sliding mandrels are described in accordance with the applicable standards, for example, in <NPL>, and in Ing. Birmbaumová, Ing. Grošek "Research in the field of structural elements and their dimensioning in cement-concrete covers", issue <NUM>.

As for the process of manufacturing the sliding mandrels produced so far, it is described by the following steps.

Smooth rolled steel grade S235JR is cut to required lengths. The ends of the cut material are ground to remove sharp edges resulting from the cut. Subsequently, the surface of the material is properly cleaned and degreased for better adhesion of the coating to the iron. After this step, the material is inserted into the inductor and heated to the necessary temperature, at least <NUM>, to apply the coating. The coating is applied using two methods, in a fluidized bed or electrostatically. Copolymer powders are used, from e.g. ICOSA Pulron or Dupont. Subsequently, the sliding mandrel is cooled and packed in appropriate transport materials.

This existing coating technology does not ensure a sufficiently resistant surface of the coating and therefore it is often damaged on construction sites, giving room to corrosion.

Document <CIT> discloses a sliding mandrel according to the preamble of claim <NUM>.

The abovementioned shortcomings are removed by a new design solution of the composition of sliding mandrels for cement concrete covers in the present solution. The essence of the new solution is that the structural core part of the mandrel is made of ribbed steel with a minimum tensile strength of <NUM> MPa. This core is provided with a composite surface consisting of a fiberglass filler having a diameter in the range of <NUM>µ to <NUM>µ and a minimum length of <NUM>, which are uniformly dispersed in the polyamide binder. Glass fibers in this composite surface are represented in an amount of <NUM> to <NUM> % by weight. The ratio between the diameter of the structural core part and the thickness of the composite surface is in the range of <NUM>-<NUM>%.

It is therefore a replacement for existing sliding mandrels consisting of smooth structural steel coated with plastic coating by a sliding mandrel solution with a ribbed steel core coated with a composite surface, wherein a smaller diameter core than plain steel is used to meet applicable standards. The final product is identical in shape and dimensions to the original solution.

However, it exhibits better properties and, using the newly created technology, enables the cost of production and final product to be significantly reduced to competitive prices. At the same time, the carbon footprint is also reduced.

The base of the composite is a core of ribbed steel of significantly smaller diameter, on which a composite material composed of polyamide, reinforced with fiberglass, is deposited. Both materials used are hot applied to a smaller diameter steel core using available technology. The resulting composite product has the same diameter as conventionally used steel reinforcement, saving roughly <NUM>% of steel raw materials. At the same time, the production of these sliding mandrels results in significant energy savings, at least <NUM>%, reducing labor and thus the price of the product.

Due to the reduced weight, the new solution brings further savings in terms of both transport and application of products on site. GREEN DEAL of the new solution consists in increasing the service life of the concerned building elements as well as possible recycling, based on the separation of composite components.

Another benefit of the new solution is the substantial extension of the service life of the cement-concrete cover elements. The prediction of increased service life is based on significantly better surface properties of the proposed solution.

An example of a sliding mandrel according to the presented solution is shown in the longitudinal section in the attached drawing.

The essence of the proposed solution is a partial replacement of structural steel in the tensile element, in sliding mandrels, by a composite solution consisting of ribbed steel reinforcement with a minimum tensile strength of <NUM> MPa forming the core <NUM> of the given building element, which is provided with a composite surface <NUM> of glass fiber<NUM> and polyamide <NUM>, which connects glass fiber <NUM> with steel. Glass fibers <NUM> form the composite filler and polyamide <NUM> is the binder. The final product has the same shape and diameter as conventional reinforcement. However, it exhibits significantly better properties and with the help of the newly created technology it also allows the production cost of the final product to be significantly reduced to competitive prices.

Core <NUM> is made of ribbed steel for the adhesion and tensile strength of the entire mandrel. Its diameter ranges from <NUM> to <NUM>. The glass fibers <NUM> forming the filler have a diameter in the range of <NUM>µ to <NUM>µ and a minimum length of <NUM> and are uniformly dispersed in the polyamide <NUM> binder. In the composite surface thus formed, <NUM> to <NUM>% by weight is the glass fiber <NUM> and the remainder up to <NUM>% is polyamide <NUM>. The ratio between the diameter of the structural core part <NUM> and the thickness of the composite surface <NUM> is in the range of <NUM>-<NUM>%.

In the manufacture of sliding mandrels, when the most commonly used diameter is currently <NUM>, that is, with a diameter of <NUM> core <NUM> made of ribbed steel, the thickness of the composite surface <NUM> is <NUM> and the overall diameter of the sliding mandrel is therefore <NUM> + (2x4. <NUM>) = <NUM>. Analogically, with core diameter <NUM> of <NUM>, the thickness of the layer is <NUM> and therefore the overall diameter of the sliding mandrel is18 + (2x3. <NUM>) = <NUM>.

Tensile and shear strength tests were carried out on several types of newly created sliding mandrels. These were sliding mandrels whose core <NUM> was ribbed steel with a composite surface <NUM> consisting of polyamide <NUM> and glass fibers <NUM>. For the production of sliding mandrels, ribbed steel grade B500B with a tensile strength of <NUM> MPa with a diameter of <NUM>, <NUM> and <NUM> was used. Tecamid <NUM> GF30 black, which is a PA6 plastic filled with <NUM>% by weight of glass fiber <NUM>, was used for comparison in the tests with the applied composite material.

The overall diameter of the mandrels tested was <NUM> or <NUM>. The overall length of the samples was <NUM>. The tested length Lc was first selected at a value of <NUM>, which is based on standard requirements. Then the tested length was reduced to <NUM>, which corresponds more closely to the real use of sliding mandrels. The length tested in this case corresponds to the distance of the specimen clamping jaws. In this case, the moment when the composite surface of the <NUM> sliding mandrels was broken was taken as the maximum force at breach Fm. Core <NUM>, made of ribbed steel, always remained intact at this point.

The different types of sliding mandrels tested with ribbed steel core <NUM> and composite surface <NUM> are listed below in Table <NUM>.

The samples marked 1_1 to 1_6 are sliding mandrels with a diameter of <NUM>, where core <NUM> consists of ribbed steel with a diameter of <NUM> and composite surface <NUM> l consists of polyamide <NUM> with glass fibers <NUM>, where the proportion of glass fibers <NUM> in polyamide <NUM> is <NUM>% by weight.

Samples marked 2_1 to 2_6 are sliding mandrels with a diameter of <NUM> - core <NUM> consists of ribbed steel with a diameter of <NUM> and composite surface <NUM> consists of polyamide <NUM> with glass fibers <NUM>, where the proportion of glass fibers <NUM> in polyamide <NUM> is <NUM>% by weight.

The samples marked 3_1 to 3_6 are sliding mandrels with a diameter of <NUM> - core <NUM> consists of ribbed steel with a diameter of <NUM> and composite surface <NUM> consists of polyamide <NUM> with glass fibers <NUM>, where the proportion of glass fibers <NUM> in polyamide <NUM> is <NUM>% by weight.

In the examples shown, PA6 polimid B <NUM> GF black was used for composite surface <NUM>.

The amount of glass fibers <NUM> can range from <NUM>% by weight to <NUM>% by weight, in order to optimize costs and utility properties, a proportion of <NUM>% by weight was selected for the tests. Glass fibers <NUM> correctly interacts with polyamide <NUM> throughout the entire range to form a composite envelope <NUM> of the required quality.

Table <NUM> shows that the sliding mandrels with ribbed steel core and composite surface comply with the standard required break stress value Rm. This standard specifies a tensile strength of at least <NUM> MPa as a requirement for the sliding mandrel.

The following Table <NUM> shows a comparison of the sliding mandrels currently used, where the core is made of smooth steel and covered with a layer of plastic, with the newly designed sliding mandrels. The current state is for rod diameters of <NUM>, <NUM> and <NUM>, which are most commonly used.

Testing of ribbed steel diameters of <NUM> and <NUM> and <NUM> and <NUM>, respectively, was carried out to see how far it was possible to go with saving steel in core <NUM>. These tests show that smaller diameters of ribbed steel can also be used.

It follows from Table <NUM> that a ribbed steel core of significantly smaller diameter can now be used to meet the tensile strength specified by the standard ČSN-EN <NUM>-<NUM>.

The standard sliding mandrel has a diameter of <NUM> and a length of <NUM> and the weight of the material is <NUM>/pc. If the standard break stress values of <NUM> MPa are met, the newly designed sliding mandrel with core <NUM> made of ribbed steel, with the same diameter of the resulting mandrel <NUM> and a length of <NUM>, can be used with a core diameter of <NUM>, saving steel and thus reducing the weight of the sliding mandrel. The weight of the new sliding mandrel made of ribbed steel and with a composite surface is <NUM>/pc, which is a saving on the total weight of the product of about <NUM>%.

In Table <NUM>, the overall diameter of the mandrel is <NUM> for the sliding mandrel, where <NUM> is for plasticizing. However, this was of no importance in the tests, because the thin layer of plastic on the currently produced mandrels serves only as a protective layer and does not affect the mechanical properties of the sliding mandrel.

From the technical data sheets of the materials used, that is the surface of the Pulron 101ES plasticizing of standard sliding mandrels, or in the new version of the composite surface Polimid, clearly results in a significantly higher surface hardness of the new sliding mandrel, which clearly increases the resistance to damage during handling on the construction site and thus prevents the possibility of occurrence of corrosion spots. This results in a substantial extension of the service life of the sliding mandrel.

Claim 1:
A sliding mandrel for cement-concrete enclosures, wherein its structural core (<NUM>) part is of ribbed steel with a minimum tensile strength of <NUM> MPa, and wherein the core (<NUM>) is provided with a composite surface (<NUM>) consisting of glass fibers (<NUM>) filler uniformly distributed in a polyamide (<NUM>) binder, wherein the glass fibers (<NUM>) in that composite surface (<NUM>) is represented in an amount of <NUM> to <NUM> % by weight and the ratio between the diameter of the structural core (<NUM>) part and the thickness of the composite surface (<NUM>) is in the range of <NUM>-<NUM> %, characterized in that the filler consists of glass fibers with a diameter in the range of 8µ to 12µ and a minimum length of <NUM>.