US3217075A - Method for making stressed lightweight concrete products - Google Patents

Description

Nov. 9, 1965 o. KJELL-BERGER 3,217,075

METHOD FOR MAKING STRESSED LIGHTWEIGHT CONCRETE PRODUCTS Filed Sept. 6, 1962 MOISTURE (PERCENT) I l I 0.? 0.2 0.36.405 0.6 0.8 L0 LL25 L6 2.0 2.5 3J5 4 s SHRINKAGE(MM/METER) INVENT OR OLOF KJELL- BERGER ATTORNEYS United States Patent METHOD FOR MAKING STRESSED LIGHT- WEIGHT CONCRETE PRODUCTS Olof Kjell-Berger, Skovde, Sweden, assignor to Skiivde Gasbetong Aktiebolag, Skovde, Sweden, a corporation of Sweden Filed Sept. 6, 1962, Ser. No. 221,913 Claims priority, application Sweden, Sept. 14, 1961,

9,148/61; Aug. 17, 1962, 8,975/62 2 Claims. (Cl. 264-228) The present invention refers to a method for the production of a re-inforced light concrete plate. The invention also refers to a light concrete plate, produced according to said method.

In the production of products, especially plates of light concrete, which have been re-inforced, the prior art shows placing in the mould before the ready-making and before the raising of the light concrete mass a re-inforcement crate or re-inforcement mat. As a rule the raising takes place by adding to the light concrete mass a gas development means, for instance aluminum, said metal reacting with the lime contained in the raw material for forming the light concrete thereby forming hydrogen. Thereby the bulk volume of the mass has increased, so that its upper surface has raised as by dough raising, but the re-inforceinent construction has still remained in its initial place in the mould. In other words, there has been a movement during the raising between the different particles of the concrete forming mass, on the one hand, and the reinforcement on the other hand, thereby displacing the concrete forming mass upwardly relative to the re-inforcement.

The concrete mass is thin fluid, when it is first poured into the mould, but simultaneously with its raising a thickening or plastification of the mass starts, and at least during the later phase of the abovementioned displacement movement between the mass and the re-inforcement, the mass is plastic and thickened to such a degree, that it has no longer the same power of filling out the space. The consequence thereof has been, that an empty space is formed above the re-inforcement means running in a horizontal direction, and the mass thereby only adheres to the lower side portions of the re-inforcement bars, but not to the upper side, where a moon-sickle like space forms between the upper side of the re-inforcement bar and the corresponding lower side of the mass. In tests with re-inforced light concrete plates one could immediately observe that this re-inforcing means did not give the rigidity which was expected according to theoretical calculations.

It appeared that these actions were dependent exclusively on the above indicated phenomenon of lack of contact between the concrete forming mass and the re-inforcement irons, especially the upper side of the horizontally running irons. This lack of contact with the re-inforcing means has been observed in light concrete plates, which have been sectioned. One has therefore also tried to compensate for the lack of structural strength capacity by increasing the re-inforcement. As an example by calculations it will be found, that a section area of the longitudinal re-inforcement irons of 3.1 cm. would be suflicient in order that a light concrete plate of standard dimensions, said plate being 3.46 m. long, should be able to stand for an evenly divided load of 1,800 kg. per longitudinal meter, but in reality this load capacity required about three times as much re-inforcement or iron area, i.e. about 9 cm.

It is obvious, that this is a very serious disadvantage. The great iron mass in the re-inforcement makes the production of the plate more expensive. The light concrete is very advantageous in its high insulation power both for sound and heat, but this will be lost to an essential degree as the iron is a good conductor both for sound and heat, and this degree of conduction will increase directly proportional to the amount of iron. The plate, due to the great amount of iron, will be heavier and part of its power of loading will therefore be consumed by its increased weight. The work of mounting the structure or positioning it in construction of a building is also made more difficult due to the heavier plate which will be more diflicult to handle. If for some reason the plate should be sectioned, a greater number of re-inforcement irons or thicker re-inforcement irons must be cut through. If holes should be provided for carrying through conductors, then the risk will increase that the drill will hit a re-inforcement iron directly proportional to the increased horizontal area assumed by the re-inforcement irons.

Many methods for avoiding or at least decreasing the consequences of the above mentioned phenomenon have been examined. Through experimentation, it has been readily realized that the possibility close at hand of working the concrete forming mass during the raising, so that also the upper side of the re-inforcement irons should stick to them will not provide the desired structure. The mass must move undisturbed, otherwise its cellular structure will be damaged.

It is known in the prior art with respect to other materials than light concrete, i.e. brick work, that smaller units are joined together to a bigger unit by connecting the different parts by using re-inforcement irons, which are in some suitable way inserted into the brick work material, usually by means of cement. In the tests mentioned above, we have tried to use the same method in the production for re-inforced light concrete plates, but these tests have not met with success. Tests, which were made immediately after the production of the plates proved that they were acceptable and that they fully corresponded to the expectations, but when they were later on used in practice, they did not give the desired power of loading. In the mounting of the plates in the building we have been able to observe an immediate bending down under load. This has as a rule been so appreciable that the stability and thereby the power of carrying by the plate was almost fully lost.

The tests have therefore been extended to an investigation of reasons for loss of load capacity. It was found, that the light concrete, when it is removed from the autoclave after steam curing, has a content of moisture of between 25% and 30%. However, under normal conditions this moisture is readily evaporated, i.e. the surface moisture, but due to the light concrete having another highly estimated property, i.e. an utterly low hygroscopicity, the evaporation of the interior moisture takes place only slowly. When delivered to buildings therefore the light concrete had often a remaining moisture, which could amount to approximately 25 When the piece of light concrete is thereafter mounted and will get into contact with mortar, plaster and so on, the surface moisture is re-instated, and the total moisture again increases. Only thereafter the moisture slowly decreases during the drying out of the building, so that it will after a normal drying period have decreased to 4-8%. Further moisture is thereafter as a rule not consumed, because, as mentioned above, the light concrete is strongly non-hygroscopic. The investigations have now shown that during the drying a shrinkage takes place of the light concrete. This shrinkage decreases with moisture removal up to 0.5% from its moist state to its dry state or in other words up to 5 mm. per longitudinal meter, and the shrinkage is essentially increased at moisture contents below 810%.

The invention is based upon the understanding that the said shrinkage causes a relaxation of the tension of the re-inforceinent in the re-inforcement irons which have been mounted in the moist state of the plate, this is believed to be the reason for the loss of strength in the above mentioned prior art structure.

According to the present invention, the light concrete is dried out before the joining of the light concrete structure or its parts with the re-inforcement bars, whereafter the re-inforcement bars are applied under a suitable bias tension in the dried out light concrete. This may per se be rather weak, and during mounting, transportation and so on the light concrete may thereafter assume moisture and swell, which will only increase the bias tension, be-

cause of shrinkage thereafter due to natural curing the re-inforcement will still have its initial bias tension left.

The invention can be used successfully on an undivided piece in the form of a plate of light concrete, in which one drills holes or previously arranges holes during molding, for inserting the re-inforcement means. However, the invention is not dependent upon the plate being undivided or composed by a plurality of pieces, the reinforcement can be applied in the plate after it has been dried out, and it is tensioned in such a way, that the plate in the building, were it is also subjected to a drying out of eventually added moisture after the re-inforcement is applied, will remain as stably re-inforced as at the production.

The invention is further described in connection with the attached drawing, in which FIG. 1 shows a diagram of the shrinkage of the light concrete as a function of the contents of moisture, and FIG. 2 shows a re-inforced light concrete plate according to the present invention, partly in section.

In FIG. 1 the horizontal axis indicates in logarithmic division the shrinkage of a light concrete plate in millimeters per meter of length dependent upon the content of moisture indicated along the vertical axis. The content of moisture is indicated along the vertical axis in percent.

The diagram according to FIG. 1 is based from the assumption that normal moisture immediately after steam curing of the piece of light concrete is about 30%. It is then evident from the diagram, that when the moisture has decreased to half of normal moisture after steam curing, that means to only minute shrinkage has taken place, i.e. 0.15 millimeter per meter. Thereafter, however, the rate and magnitude of the shrinkage rather quickly increases. At a remaining moisture of 5% a shrinkage from the initial state has taken place of 0.4 millimeter per meter, and when the content of moisture has decreased to zero value the shrinkage has increased to 5 millimeters per meter. If the environmental climate change takes place changing from dry to moist, the effect will be the contrary one expanding the block.

FIG. 2 shows a perspective drawing of a light concrete body, which has been re-inforced according to the present invention. The proper light concrete body 10 has either already when moulded been provided with channels or after moulding by drilling or in another suitable way been provided with channels, intended to house the re-inforcement irons 11. These re-inforcement irons are inserted through a pressure distribution plate 12 or 13 respectively at each end of the body 10, said plate being provided with holes, corresponding to the channels 14 in the light concrete body. The ends of the re-inforcement irons 14 are secured by means 15 so that a suitable bias tension is obtained to lock the re-inforcement iron in position.

As already mentioned above, the light concrete body should be dried out before securing the re-inforcement irons so that none or only unessential remaining moisture exists, and only thereafter the re-inforcement irons 11 should be introduced through the pressure plates 12 and 13 and the channels 14 provided in the light concrete body 10. The bias tension in the re-inforcement irons is thereby adapted by means of the nuts 15 in such a way, that the bias tension will still remain at a given value, after the light concrete has been fully dried out with the shrinkage following therefrom.

Even if iron is the natural material for a re-inforcement of the kind here concerned, iron is nevertheless for several different reasons less desirable. On the other hand, prior art showed that only iron could be used as material for the re-inforcement with regard for the demand for an economically obtainable material and also for a sufficiently strong material.

Even if iron has certain advantages as a re-inforcement, which are not to be disputed, the disadvantages of using iron may nevertheless in several cases be so serious, that it is economical to use another material instead of it.

The iron used for the re-inforcement as a rule is hot drawn or cold drawn iron with a lower allowable stretching limit between 13 and 20 kg./mm This re-inforcement iron takes up practically all of the strain, to which the light concrete plate is subjected, and in order that it shall get the desirable rigidity in many a case a rather essential quantity or iron is required. Thereby the product will be inconveniently heavy. Further it will conduct sound as well as heat to a rather high degree. Also that iron has a very great tendency of rusting, after it has been applied as a re-inforcement in concrete, especially light concrete. The tendency of the re-inforcement irons to rust has been decreased by protection by some suitable means, which makes the production of the final product more expensive.

A further developement of the present invention is based upon an investigation of the suitability of other materials concerned. It has been found that plastic bars, preferably glass-fiber re-inforced bars of plastics, are preferred over an iron-re-inforcement.

The specific properties of glass-fiber re-inforced plastic bars, are that the bars have a very high security against breakage when subjected to drawing as there is no stretching limit. Therefore, one would assume that the glassfiber re-inforcement plastic bars would, as a re-inforcement in light concrete products be too dumb, and would not allow for normal extension procedures before they break, which the iron will do. It has also been found, that light concrete products, which have been re-inforced by means of iron bars, have different characteristics than those which have been re-inforced by glass-fibers.

The advantages, when using glass-fiber re-inforced plastic bars as a re-inforcernent in light concrete products, could, according to investigations be summarized in the following way:

In mass production the re-inforcement with glass-fiber plastic is not more expensive per unit of volume of the light concrete product than would have been the case with iron re-inforcement. Certainly, the proper material in the re-inforcement will be somewhat more expensive, but this is compensated for by no rust-protection treatment being required, and that the product will get a longer life time, because it is not attacked by any kind of corrosion. On the other hand, iron practically without exception, even if it has been protected against rust in a very good way, will sooner or later be attacked by rust. Glass-fiber re-inforced plastic thus is both weather-proof and agingproof.

The fact that there is no stretching limit for glass-fiber re-inforced plastic makes possible the calculation of the required amount of re-inforcement material exclusively with respect to the security against breakage. Surprisingly it has therefore proved, that one can use a smaller total area of re-inforcement when using glass-fiber reinforced plastic in the re-inforcement bars than with iron in these bars.

Plastic, a material which forms the main part of glassfiber re-inforced plastic bars, which should according to the present invention be used for re-inforcement in light concrete products, has a very low power of conducting heat, whereas iron has a high power of conducting heat. The heat resistance of the thermal conductively value of light concrete products, re-inforced according to the present invention, will therefore be very good.

The property of the glass-fiber re-inforced plastic bars to be rather dumb from the point of view of extension causes that they are also very bad conductors for sound. A plate of light concrete, which has been re-inforced according to the present invention, therefore will be a much better sound insulator than the light concrete product reinforced in the traditional way by means of iron bars.

In this connection it should be mentioned, that the known or traditional method of re-inforcing light concrete products with iron, has been, that the iron bars were placed in the product when it was moulded, the light con crete product, usually a plate, thereafter being subjected to raising and steam curing. For other kinds of artificial stone plates, preferably of brick work or similar material, it has been proposed to divide the product in a plurality of mechanically different layers, whereafter one has in the upper side of a lower layer and in the lower side of an upper layer provided channels, in which one has applied the re-inforcement irons, and thereafter one has in one way or another secured together the two layers, for example by means of cement slurry. This last mentioned method cannot be used with light concrete products.

The glass-fiber reinforced plastic is not normally substituted, per se, in the process for making the concrete plate because then the plastic would be together with the light concrete product at the high temperature of between 180 and 200 C., which exists in the autoclave during the steam curing. Plastic cannot withstand this temperature, because the plastic will at this temperature be subjected to interior atomic changes, decreasing the rigidity, and the glass-fiber will melt at this temperature, and therefore its re-inforcing activity will get fully lost.

The use of glass-fiber re-inforced plastic in the form of bars therefore is subject to the method in which the moulded light concrete product after having been steam cured in its non-re-inforced state, is dried to none or only unessential remaining moisture, and the re-inforcement is thereafter introduced under a mechanical bias strain so adapted that it will increase to a suitable value after the light concrete has been moisture-conditioned and consequently has expanded.

When it is mentioned above that one should use glassfiber re-inforced plastic bars as re-inforcement material, it must not be understood that these bars must necessarily have such a form as the traditional re-inforcement irons, that means substantially circular cross-section area, but any extended form of the re-inforcement should be regarded as a bar. These bars are thus mounted into the light concrete products after they have been steam cured, in holes or channels, which have been provided in ad- Vance, for instance by cutting, drilling or already at the moulding.

The tensioning preferably takes place by securing the extending ends of the glass-fiber re-inforced plastic bar by providing a greater cross-section area than the cross-section area of the hole through which the re-inforcement extends. In other words, the thickness of the bar should be at the ends be greater than the thickness of the bar at its middle part. The thickened part could be conically made or it could be split end formed. One can also provide the said thickened parts of the re-inforcement bars at their ends in any conventional manner. It also is possible, before the chemical curing of the plastic to introduce anchoring means of metal, even of iron, in the plastic at their bar ends, or when mounting the re-inforcement to provide the plastic bars with split ends which may either be allowed to solidify in the usual manner or which may be driven apart by means of wedges, i.e. so called bracing locks.

When manufacturing the glass-fiber re-inforced plastic bars, which should form the re-inforcement in the light concrete products, it is preferable to add plastic to the bundle of glass fibers at the ends of the bars after the mounting of the re-inforcement bar into the light concrete product, so that one can connect to these ends the anchoring means which are suitably placed. One could even place all of the bar directly in the light concrete material, first drawing in the bundle of glass fibers in the holes or grooves in which the re-inforcement should be situated and thereafter pouring in the fluid plastic, which includes the required curing means and also acceleration means, whereafter this is allowed to solidify in place. In connection with the positioning of these plastic bars directly in the light concrete product, one can of course also take care of required anchoring means simultaneously. For instance, one can when drilling of holes for the plastic re-inforcement make the ends of said holes funnel formed, so that the plastic re-inforcement will be finished with cones, formed according to these funnels.

It is not necessary to bring the moisture of the light concrete down to 0%, when the re-inforcement takes place, because the expansion which the contained moisture would cause, could be easily compensated for by the bias tensioning of the re-inforcement iron at its mounting. In a corresponding way one can provide the proper reinforcement procedure in an aggregate for, automatically or semi-automatically bias tensioning the re-inforcement irons by machine to a constant value, and controlling the effective bias tension by choice of moisture of the plate at the securing of the re-inforcement.

It is advantageous to condition the concrete to a very low moisture when re-inforcing, but this may meet with economical difficulties. It is substantially for this reason that one can also according to this invention effect the re-inforcement at a certainly decreased but not maximally decreased moisture, and thereby determine the bias tension in the re-inforcement in such a way. Therefore, even a fully de-moisturized plate will have a certain amount of bias tension in the re-inforcement irons. If the plate should thereafter during storing, transportation or mounting assume a higher moisture the bias tension will increase proportional to the expansion caused thereby. The bias tension will, of course, decrease to its normal value, after the plate has been dried to normal humidity conditions in the building.

The drying of the light concrete plate or the parts forming the light concrete plate can take place either by storing in a dry atmosphere or in artificial Way, for example in a drying oven.

What I claim is:

1. A method for making light weight, reinforced concrete structures comprising the steps of providing longitudinal, through openings in the concrete structure for the insertion of reinforcing means, curing the structure with steam under pressure; drying the cured structure until substantially all of the free moisture is removed from the concrete, inserting reinforcing means in said openings, locking the ends of said reinforcing means to hold said reinforcing means stationary relative to movement of said structure and placing said structure in a normal humidity environment, whereby the reinforcing means is subjected to a bias tension when the light concrete structure expands under normal humidity conditions.

2. The method of claim 1 wherein the reinforcing means are glass-fiber-reinforced plastic formed by inserting glass fibers into the openings and inserting a mould- References Cited by the Examiner UNITED STATES PATENTS 10/33 Hutteman et a1.

3/41 Anderegg 264228 8 Muntz 264-228 XR Freyssinet 25118 XR Siegfried 264228 Goldfein 264--228 XR ROBERT F. WHITE, Primary Examiner.

ALEXANDER H. BRODMERKEL, Examiner.

Claims (1)

1. A METHOD FOR MAKING LIGHT WEIGHT, REINFORCED CONCRETE STRUCTURES COMPRISING THE STEPS OF PROVIDING LONGITUDINAL, THROUGH OPENINGS IN THE CONCRETE STRUCTURE FOR THE INSERTION OF REINFORCING MEANS, CURING THE STRUCTURE WITH STEAM UNDER PRESSURE; DRYING THE CURED STRUCTURE UNTIL SUBSTANTIALLY ALL OF THE FREE MOISTURE IS REMOVED FROM THE CONCRETE, INSERTING REINFORCING MEANS IN SAID OPENINGS, LOCKING THE ENDS OF SAID REINFORCING MEANS TO HOLD SAID REINFORCING MEANS STATIONARY RELATIVE TO MOVEMENT OF SAID STRUCTURE AND PLACING SAID STRUCTURE IN A NORMAL HUMIDITY ENVIRONMENT, WHEREBY THE REINFORCING MEANS IS SUBJECTED TO A BIAS TENSION WHEN THE LIGHT CONCRETE STRUCTURE EXPANDS UNDER NORMAL HUMIDITY CONDITIONS.

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