A METHOD OF PRODUCING A REINFORCED CONCRETE STRUCTURE

Abstract: The invention relates to a method of producing a reinforced concrete structure, more precisely a reinforced concrete structure, the crack formation of which must be controlled by means of reinforcement, such as submerged tunnels, pipes for remote heating systems and caissons for large structures, in particular reinforced concrete pavements for bridges, harbours, roads, sport areas and flight landing decks. By pre-initiating a macro or micro crack system in the concrete, the hardening process being not yet completed, viz. at a particular point in time before the appearance of contraction cracks "on their own" due to temperature, etc., it is possible to reduce the degree of reinforcement to around 10 % of that which is traditionally considered necessary. Methods of pre-initiating macro as well as micro cracks in the concrete are disclosed. The invention also relates to special concrete and binder compositions as well as the concrete structures as produced using the method of the invention. In particular, the invention relates to the use of a thin wearing course of a reinforced concrete layer on any load-bearing layer as an independent load-bearing layer.

Description: A METHOD OF PRODUCING A REINFORCED CONCRETE STRUCTURE Technical field This invention relates to a method of producing a reinforced concrete structure, more precisely a reinforced concrete structure, the crack formation of which is controlled by means of the degree of reinforcement. The method of the invention can be used for instance in the construc¬ tion of submerged tunnels, ducts, caissons for transmission pipes for heating systems and box caissons for large marine structures, but also for the construction of reinforced concrete pave¬ ments for bridges, container terminals, road pavements, sports arenas and airport pavements. The invention also relates to special concrete compositions and the resulting concrete struc- tures as such as produced by this method. In particular, the invention relates to the use on any bearing course of a reinforced concrete pavement as the base for a thin wearing course, and the use of a reinforced concrete pavement as an individual load-bearing layer. Background information and background art Concrete is a composite material comprising a hydraulicaliy hardening binder, aggregate and in some cases additives. During mixing the required amount of water is added and fresh concrete is obtained. The mix of the hydraulicaliy hardening binder and water is known as paste. The addition of water initiates a number of different hydration reactions, which of course are de¬ pending on the chemical composition of the actual hydraulic binders. The hydration reactions generate heat and consume water, and the reaction products cause a hardening of the con¬ crete. The hydration takes place in several phases, the length of which depends on the composition and the amount of the binder. The mixing process itself generates a large amount of heat due to saturation of the powder and transformation of mechanical energy. After mixing, the con¬ crete is characterized by a dormant period where only moderate heat is generated. The con¬ crete can be placed and worked on or compacted during this period, which normally lasts from 0 to 3 hours. In the next phase (3-24 hours) which includes the hardening (setting) of the con¬ crete, one third of the total hydration takes place. The third and final phase includes the slow part of the hydration, which normally, for a Portland cement concrete, is considered to be completed after 28 days. The hydration of the largest cement particles can however continue for several years; likewise, the use of pozzolans, blended cements and/or other binders can cause the hydration process to continue for a long period. The term "maturity" defines the relative extent of hydration at 20°C. Maturity is determined on the basis of the generation of heat in the concrete. The precise course and extent of the hy¬ dration depends on the chemical composition of the binder and the amount of binder in the concrete. During the hardening period the maturity is calculated as the actual isothermal heat generation in relation to the overall possible heat generation. The overall heat generation of the concrete can be determined by means of adiabatic calorimetry. Since the hydration processes consume water, the relative maturity during the course of the hardening process can also be determined as the amount of bound water (non-evaporable below 105°C) in relation to the amount of bound water at full hydration. This heating of the concrete material as a result of the hydration reactions is usually in- desirable, and for this reason the temperature may be controlled by cooling of the structures. This would typically be the case in connection with the casting of tunnel elements, caissons, bridge decks, etc. But during the laying of concrete pavements there is normally no temperature control. Accordingly, in the course of the hardening process, the temperature in the concrete will rise depending on the initial temperature, the amount and type of binder and the ambient tempera¬ ture. The concrete temperature usually rises during the first one or two days of the hardening pro- cess, and then it declines under adjustment to the surrounding climate. The subsequent drop of temperature is the most important cause of contraction of concrete structures, but the chemical reactions of the hydration process also cause a volume reduction of the paste. Thus the concrete will shrink or contract. In such cases where contraction is prevented, stress will be accumulated in the structure. The hydration reactions are described in "Haerdeteknologi" by P. Freiesleben Hansen (1978) and in the "Beton Bogen" (Aalborg Portland, 1985) where various types of concrete and cement are discussed. These handbooks also give a guidance in the calculation of the necessary amount of water in Portland cement and other types of cement and binder combinations. In this respect, also "Concrete Technology", Neville & Brooks, New York, 1993, is worth men¬ tioning. As mentioned above, the hydraulic binder in concrete can be cement, e.g. Portland cement, Pyrament, or calcium aluminate cement; natural pozzolans, such as volcanic ash, typically of basalt origin (described first in antiquity, from Pozzuoli in Italy), liparite (volcanic ash from Iceland), paganite (volcanic ash from Pagan Island), or moler; or artificial pozzolans such as blast furnace slag, fly ash or microsilica. The expression "microsilica" in the present application is intended to mean a waste product from the ferro-silicon production, stemming from the cleansing of flue gases and consisting primarily of Si0 2 of spherical shape with an average particle size of approx. 0.1 μm. Any combination of these binders can be used. However, a sufficient amount of a cement hydration product must be available to react with the pozzolans, otherwise the necessary amount of alkalis must be added separately. Accordingly, it is not vital that cement is used, since for example, alkalis in the form of seawater can be used together with pozzolans. Under normal circumstances, however, concrete typically comprises cement. The aggregate is typically concrete gravel, consisting of sand (0/4 mm) and stone (4/8 mm, 8/16 mm, 16/32 mm and larger). These fractions, which also can have other grading intervals, can be combined according to the purpose. Not all fractions have to be present, and within individual fractions size gaps may occur. Amongst the conventional additives, mineral additives should be mentioned, i.e. fillers such as crushed quartz or limestone, chemical additives such as air entraining agents, plasticizing agents and agents for the regulation of the setting and/or the hardening. Finally, polymer bin¬ ders, viscosity influencing materials (e.g. tylose) and colouring agents can be included. Typical concrete compositions are mentioned in the experimental part. Because of increased traffic loads and the use of higher tyre pressures and axle loads, more than 90% of the European roads are already overloaded. As a result thereof most of the asphaltic pavements show clear rutting. This problem cannot be solved by traditionally re¬ pairing the asphalt. Accordingly, the development of cheaper pavements with higher durability, longer lifetime and using environmentally more friendly materials is highly desirable. So it has been decided to base this development work on cement based materials (concrete). Therefore, one specific main object of the invention is the development of a cement-based road construction material for use as an independent, flexible bearing layer or as a wearing course on top of existing flexible pavements. In road construction, a concrete pavement is traditionally placed on top of a layer of bitumen stabilized sand/gravel (asphalt) or cement stabilized sand/gravel, which again is placed on top of a layer of sub-base material. On top of the concrete layer a wearing course, if desired, can be placed, for example a thin concrete layer or an asphaltic wearing course, which normally would have a thickness greater than 10 mm, or a surface dressing which typically would be less than 10 mm thick. Such surface dressings are usually produced using cut-back bitumen binders in combination with application of chippings (small stones), or artificial binders such as epoxy resins may be used. A number of requirements must be fulfilled by road pavements of concrete should - as well as by other types of road pavement. These requirements relate to evenness, skid resistance, service life, and adhesion to the layer below. Finally, a very important requirement relates to the ensurance against or the control of the crack formation. Cracks result from loading of the concrete. The load stems from traffic, moisture or tempera¬ ture changes, which are all of a reversible nature. This load gives rise to a tensile and/or com- pressive stress or strain in the concrete. A compressive strain for instance occurs by uniform heating or when the humidity increases, provided a limitation is imposed on the change of volume of the concrete, which exactly is the case in road pavements. Tensile strains result when there is a uniform cooling or drying. Combined tensile and compressive stresses and strains result from the variation of the temperature and humidity conditions, for example from the top to the bottom of a concrete layer. However, stress also arises in the concrete, as men¬ tioned above, due to its irreversible shrinkage, which is due to the physical/chemical hardening processes. Because of these stresses, cracks so to speak arise "on their own" in a road pave¬ ment at a particular point in time of the hardening process, viz. after the setting, at the begin¬ ning of the temperature decrease, typically after 12-20 maturity hours (at 20°C), but depending on the relative temperature of the surroundings. In the northern temperate zone the air humidity varies greatly and periods of frost are frequent, during which the pavement is exposed to low temperature. Therefore stresses result in the concrete, these stresses empirically corresponding to a maximum strain (in the longitudinal direction) of max. 3%o (0.3 %). The limit of deformation of cement concretes is less than 10% thereof, typically 0.1-0.2%o. In warmer climates the problem is primarily that concrete structures generally dry out or dehy¬ drate, resulting in a contraction or tension of the concrete material. An even bigger problem arises in road structures comprising a concrete paving material for wearing courses on top of an asphaltic pavement, since a high temperature has a marked influence on the E-modulus of asphaltic layers and accordingly on the bearing capacity. The deflection corresponding to a given load will increase, as will the strain or deformation of cer¬ tain parts of the structure. This problem of avoiding or preferably controlling the crack formation in concrete pavements on roads in the past has been dealt with as follows (refs. (1) to (3) below): (1) Cutting contraction/expansion joints in non-reinforced concrete This method is based on the cutting of joints with a diamond saw every 5 metres at a point in time corresponding to 12-18 hours of maturity, the joint width is typically 3-5 mm and the joint depth corresponds to 1/3 of the thickness of the concrete layer. The joint is later sealed with an elastic joint filler. Typically, steel dowels are placed transversely to the joints in the middle of the pavement at the time of paving. The distance between the dowels is typically 30 cm for pavements up to 25 cm and the dowel diameter is typically 25 mm. The dowel length is set by local practice but would normally be 50 cm. The dimensioning of the dowels is determined on the basis of he traffic load (Beton Teknik 6/08/1983). The dowels are coated with a suitable covering layer, for example bitumen, epoxy or grease. When during its hardening the concrete contracts as a result of hydration shrinkage, drying shrinkage or cooling, cracks are formed in a controlled manner at these places where the above-mentioned joints have been cut, the dowels being placed so as to allow lengthwise or longitudinal movements. Accordingly, the dowels absorbe the vertical loads when the width of the joint, due to contraction, reduces the natural aggregate interlock load-transfer properties of the concrete. (Aggregate interlock refers to the phenomenon of individual particles in the con¬ crete on each side of the crack, such as stones, locking together to some extent despite the existence of the crack. Because of aggregate interlock some load is transferable across the crack. This applies only to cracks under a certain size.) When using this method, the typical thickness of the concrete layer is approx. 25 cm (20- 27 cm). For the sake of good order it must be stressed that a non-reinforced concrete layer, as de¬ scribed above under (1), under no circumstances is applicable as a thin wearing course (thickness < 100 mm) on existing asphaltic layers. (2) Use of a suitably reinforced concrete The use of CRCP (Continuously Reinforced Concrete Pavements), i.e. concrete continuously reinforced with longitudinal steel bars being at appropriate intervals interconnected with trans¬ verse bars, is described in a report from PIARC (Permanent International Association of Road Congresses), Technical Committee on Concrete Roads, Subcommittee Continuously Rein¬ forced Concrete Pavements, dated May 1993. Such pavements are extensively used and have excellent properties; frequently lifetimes of 35-40 years are reported. Their lifetime depends, however, strongly on how well the crack formation can be controlled. In the above-mentioned report, methods of calculating the necessary percentage of reinforcement are also described. CRCP pavements are normally placed on top of a layer of cement-stabilised or hydraulicaliy bound gravel of varying thicknesses depending on the sub-base, the thickness of the CRC- layer varying between 20 and 25 cm, depending on the traffic load. FRC (Fibre Reinforced Concrete) is also used in road pavements. However, FRC is no real reinforced concrete material in the traditional sense, but rather a con¬ crete material of an improved ductility. Thereby the energy required for the formation of cracks is increased. The use of fibre reinforced concrete has been used for concrete pavements, is well known; for example such use has been reported in Canada (National Research Council Report, "The use of fibre reinforced concrete for Highway Rehabilitation," 1984-85), in the USA ("Performance of fiber-reinforced concrete pavement", Packard & Day, Detroit 1982 and "Field Performance of fiber-reinforced concrete Airfield Pavements", FAA 1986), in Denmark (Henrichsen, "Overlays of thin fiber-reinforced concrete on old concrete pavements", Transportation Research Board, Washington DC, 1986), in Sweden (Thorsen, Wuopio, "FRC pavement on bridge decks" Umea, 1991), in Belgium ("Construction and Maintenance of Rigid Pavements", PIARC 18th World Congress, Brussels, 1987) and in Japan (Ibukigama, Seta et al, "Steel fibre-reinforced concrete overlays on asphalt pavements", ACI Int. Symp. 1981). Several references to the use of fibres in road construction are found. In the following type of pavements, the use of fibres has been successful: In the repair of surface damage on existing concrete pavements, in the construction of concrete pavements of traditional thickness and with contraction joints as described above under (1); in continuous pavements with a high percentage reinforcement of fibres, typically 1.5-2 % by volume corresponding to 120-200 kg steel fibres per m 3 concrete. Generally, for continuously reinforced road pavements, whatever the kind of reinforcing ma¬ terial (e.g. steel, glass, polypropylene or aramid), the degree of reinforcement, the orientation and the dimension of the pavement play an important role in crack formation and in the transfer of load. Following the influences of loads such as those mentioned above, a continuous reinforcement will primarily lead to the formation of macro cracks, while the fibre reinforcement will lead to the formation of smaller cracks, so-called micro cracks. When as a result of the hardening and any cooling the concrete contracts, the cracks will expand or open. Here the general rule applies that the higher the number of cracks, the less will each individual crack open. In the references mentioned above, especially in the PIARC report and in the articles written by Stang and Aarre (Cement and Concrete Composites vol. 14, 1992, pp 143-154), semi-empirical calculation methods are provided for the determination of the degree of reinforcement which is required in order to control the crack formation. DS 411 (Danish Standard no. 411) and Eurocode 2 provide similar calculation methods for the necessary degree of reinforcement with a view to ensuring that the cracks do not exceed a certain upper limit of width. Certainly, these standards relate to concrete beams, and calcu¬ lations relating to road pavements necessitate taking into account the effect from friction against the basecourse, but nevertheless this technical basis generally seems correct with a view to calculating crack widths in concrete pavements. The required amount of reinforcing material depends basically on the cross-section dimension of the concrete layer, the type of concrete, friction against the basecourse, the temperature at the time of construction versus the potential minimum temperature, and of the properties of the reinforcement material (E-modulus and yield stress). In a vertical cut through a concrete pavement with continuous reinforcement (a cut without transverse reinforcement), the horizontal degree of reinforcement (for short: the degree of rein¬ forcement) is defined as that part of the cross section area which is made up of reinforcement material. This is due to the fact that the reinforcement in the longitudinal direction is the most important - the transverse reinforcement acts mainly to assure the correct location of the longi¬ tudinal steel. Obviously, the greater the yield stress and the higher the tensile strength and tensile modulus of a reinforcement material, the smaller is the necessary degree of reinforcement. In road pavements, steel is generally used as continuous reinforcement material, e.g. High Ferrite Steel having a yield stress of 550 MPa. Using this reinforcement material, experience supported by theoretical considerations (see PIARC) has shown that the required degree of reinforcement is 0.7% at least, with a view to establishing a satisfactory crack structure with crack widths of less than 0.2 mm and with a preferred spacing of (distance between) the cracks of between 0.8 and 1.2 m. Experience shows that this applies to climatic conditions which are prevalent in temperate zones. If a type of steel with a higher tensile strength/yield stress is used, such as 1000 MPa, the degree of reinforcement required is typically reduced to 0.3%. It is therefore important to specify precisely the reinforcement used. As regards fibre reinforcement there is no literature references which directly indicates a basis for design. In order to control the crack formation, experience shows that at least 2% (vol/vol) steel fibres of for instance Dramix 30 mm fibres are required. Continuously reinforced concrete pavements on roads typically have a layer thickness of ap- prox. 22 cm, whereas fibre reinforced concrete pavements will normally vary from 10 to more than 100 mm when used as a repair layer on old concrete pavements. For the present, method (2) is preferred over method (1), because of a longer lifetime, less maintenance and the fact that it constitutes a more comfortable pavement for the drivers. However, the reinforcement method is disadvantageous in being somewhat more expensive than method (1) (cut joints); about 20% more expensive. (3) Use of non-reinforced high strength concrete with pre-initiated cracks This method has been used experimentally recently and involves the initiation of pre-cracks every 2 m in the longitudinal direction of a hardened non-reinforced concrete (with a minimum compressive strength of 60 MPa). This method is called ECOPAVE (Economical Pavement) and is described in a research report to the European Commission, General Directorate XII, March 1993, prepared by the applicant. The report is not publicly available, but the method is described briefly in an information brochure called "ECOPAVE, Composite Pavement System: Combining the Benefits of Concrete and Asphalt". The pre-cracks of the hardened concrete are established mechanically with a view to pre-de- termining the outbread of cracks resulting from the loading. But since this initiation of pre- cracks takes place in the hardened concrete, it is not always possible to avoid the crack forma¬ tion resulting from thermal fluctuations and the shrinkage during hardening. Experience shows that there is a risk of such pavement in time acting in the same manner as a pavement with joints as mentioned above (1), main cracks breaking out approx. every 5 m resulting in insuffi¬ cient load transfer because of a too great width of these cracks. Experience shows that such instability in the crack system never occurs in CRCP as described above under method (2). Disclosure of the invention The present invention is based on the surprising observation that the necessary level of rein¬ forcement can be reduced drastically by introducing cracks in the concrete before completion of the hardening process. This applies to concrete structures such as concrete pavements being load bearing per se and to wearing courses on other primary load bearing structures such as asphalt and old concrete, and it applies to continuous reinforcement as well as fibre reinfor- cement or combinations thereof. US Patent no. 3,810,708 discloses a method of anchoring a concrete pavement on a base- course, said method comprising the pushing of clamps through the paved, non-hardened con¬ crete pavement and into the basecourse with a view to create holes, to be filled up with con- crete. Allegedly, this constitutes an attempt to influence the crack formation in connection with the hardening in such a way that a finely dispersed crack system results. However, this prin¬ ciple has proved not to work in practice, since the required load transfer ability is not provided simply by creating these cement-filled holes in the basecourse. The method has primarily been developed in order to prevent so-called "Paketreisen", which describes the phenomenon that the concrete plates so to speak move in "packets", since very large cracks arise every 20-30 m. More specifically, according to the invention as specified in claim 1 , it has surprisingly proved possible to considerably reduce the degree of reinforcement with a view to controlling the crack formation as compared to the degree of reinforcement traditionally considered necessary, by pre-initiating in the concrete a crack pattern at an early age, before its hardening, potential drying shrinkage and temperature changes have built up stresses exceeding the actual tensile strength of the concrete. Under all circumstances, the cracks absolutely must be pre-initiated, before the concrete has already cracked "by itself under the influence of the hardening conditions and the surrounding climate, because of the stress generated by its hampered or restrained contraction. The definition above and in claim 1 of the critical point in time, within which the crack system must have been pre-initiated is a functional requirement, but it can be used without problems in practice by the skilled man. As rule of thumb for a non-expert it can be formulated in such a way that the cracks must be pre-initiated before the temperature of the concrete has reached a maximum as a result of the hydration process and under the influence of the surrounding climate. However, the definition of claim 1 is the most correct. This is because there are so many factors which influence the determination of this point in time, that it is not possible to indicate a more precise method to determine it. Among other things it is dependent on the type of concrete and its composition - especially the development of the strain capacity, on the actual time of the day of casting the concrete and on the climate of the locality in its widest sense, both in general and specifically on the day in question (the development of temperature, wind speed, angle of sun, etc.). The relevant point in time, how- ever, will be specific for each casting and in practice it is not exactly predictable in relation to the development of strength of the concrete (maturity). In other words, the crack system must be pre-initiated before contraction cracks are formed under the influence of the normal hardening and thermal strains. That is to say before the point in time at which the temperature has decreased below that level which was prevailing when the concrete acquired an E-modulus of a certain magnitude, which typically occurs 4 hours after setting. Cracks are formed inherently when the stresses in the concrete resulting from the setting, a potential drying out and the temperature influence exceed the actual tensile strength of the concrete, and then it is too late to pre-initiate a crack system. Claim 1 relates to various reinforced concrete structures, examples of which are mentioned i.a. in the introduction. So the method of casting the concrete referred to in the pre-characterizing portion of claim 1 includes all conventional methods relating to the various structures, such as, for instance for road constructions, a slip form paver, laying it between forms, laying it using asphalt pavers or other types of suitable road construction equipment. Typically, under Danish climatic conditions and with Danish road construction traditions, the cracks will have to be pre-initiated either in connection with the mixing/placing of the concrete or at the very latest within 24 hours of maturity, cfr. claim 2. It is true that the main concept of pre-initiating cracks has been disclosed in the above-men¬ tioned ECOPAVE system and in US 3,810,708; but the solutions proposed according to these references has shown up not to function satisfactorily, primarily due to lack of control of the stability of the crack systems. Hence, the novel and surprising fact of the invention is not that it is possible to some extent to control the crack formation, but that it is possible to control the formation of acceptable crack patterns by stabilizing a pre-cracked pattern with a drastically reduced reinforcement degree as compared to the one which is traditionally considered ne¬ cessary. According to the invention, since cracks are pre-initiated - they form before the concrete cracks on its own - that part of the reinforcement which is otherwise required to control the formation of an acceptable crack pattern can be omitted. A certain degree of reinforcement, however, is indispensable, since it is still necessary primarily to stabilise the crack system as mentioned above. In the known structures the reinforcement has served the purpose of controlling the crack formation by overcoming the tensile strength of the concrete material as well as the frictional force which is established against the basecourse, but, according to the invention, with pre-initiated cracks the tensile strength of the material is reduced in the crack zone as is the reinforcement degree required, viz. to a level at which the tensile strength in the material is comparable to the mechanical strength of the crack itself. Accordingly, the reinforcement degree required can be related to a force consisting of the above-mentioned crack strength plus the friction against the basecourse of no more than 1 element, i.e. no more than the friction of a plate with a length equal to the maximum crack distance or spacing. Therefore, the necessary reinforcement degree depends on the crack strength, the crack spacing and the coefficient of friction to the basecourse, and it can be calculated at a given load as that degree which is sufficient to prevent yield of the reinforcement. In the experimental part below a theoretical teaching on the calculation of the necessary degree of reinforcement is given for a construction with pre-initiated so-called macro cracks, on the basis of the above considerations, said teaching more precisely relating to a continuously reinforced structure (see also the analogous considerations regarding fibre reinforcement and micro cracks below). As shown in the experimental part, with macro cracks present in continuously reinforced con¬ crete structures, the degree of reinforcement can be reduced to approximately 10% of the reinforcement degree traditionally considered necessary. In other words, considerable savings are obtained. The invention is not limited to concrete pavements but applies to all concrete structures which are traditionally reinforced in order to control the cracking. Examples of such structures are mentioned in the introduction. In the introduction typical concrete compositions are also men¬ tioned. The principles of the invention apply to continuously reinforced structures as well as to fibre- reinforced structures and to structures reinforced with a combination thereof. The pre-initiated cracks can be so-called macro cracks, easily observable with the naked eye, or so-called micro cracks of a considerably reduced size. The cracks can be established in many ways, some of which are described below. Claim 3 discloses a first special aspect of the invention, viz. the pre-initiation of macro cracks in continuously reinforced concrete structures A second aspect relating to the introduction in a fibre reinforced concrete structure of an inter¬ connected micro crack system is disclosed in claim 4. According to the invention, in continuously reinforced structures as well as fibre reinforced structures the degree of reinforcement will be reduced considerably as compared to the ex¬ isting, experimentally determined degrees of reinforcement, cfr. above. As regards fibre reinforcement, it is assumed that the reinforcement alone should be able to overcome the crack strength in the micro crack system with no yield in the fibres. The overco¬ ming of the friction against the basecourse will be negligible due to the small segment size. As shown in the experimental part below the reinforcement degree can also be reduced drastically in this aspect. However, till now the reinforcement degree has only been reduced to approx. 20% of the reinforcement degree traditionally considered necessary in initially uncracked FRC, since the tensile strength in a micro crack system has been shown experimentally to exceed the tensile strength of the macro crack system. A concrete pavement constructed in accordance with the principles of this aspect is particularly well suited to accommodate the deformations which occur in connection with a wearing course on asphalt pavements (10-100 mm, preferably 30-50 mm), cfr. claim 26. A third aspect of the invention (cfr. claim 5), relates to the formation of pre-initiated macro cracks and/or micro cracks in a concrete with a combination of continuous reinforcement (using reinforcing bars) and fibre reinforcement. Experimental data from tests with this combination in a traditional, un-cracked concrete show that the combination of continuous reinforcement and fibres is more promising as compared to traditional continuously reinforced concrete products, as well as to fibre reinforced products, taken alone. This combination is very interesting from an economical commercial point of view. A preferred crack system is obtained with closer crack spacing (smaller crack width), as com¬ pared to the two types taken alone. Results of preliminary tests with pre-initiated micro cracks in connection with such a combined reinforcement suggest that the degree of reinforcement in this case can be reduced to 10% of the traditional reinforcement, that is, for example, 0.5%o continuous reinforcement and 1% 0 fibre reinforcement. Such a combined reinforced concrete structure with pre-initiated micro cracks can advan¬ tageously be used in the wearing course on asphaltic pavements (30-100 mm), cfr. claim 26. In a fourth aspect of the invention (claims 6 and 7), the principle of the invention is applied in connection with a composite pavement consisting of at least one layer of concrete with a pre- initiated crack system in relation to the above-mentioned aspects 1 to 3, and with correspon- ding reinforcement and at least one layer of fibre reinforced concrete with a pre-initiated crack system and reinforcement according to the second aspect. As a reinforcing maerial one can use any material to which loads can be transferred and which - on account of its adhesion to the concrete - is capable of accommodating and transfer¬ ring loads in connection with the pre-initiated cracks. Examples of such materials are iron and steel characterised by yield stresses from 200 MPa to 2500 MPa, representing all traditional ferrous reinforcements according to DS 411 , DS 13080, DS 13081 and DS 13082 (DS = Danish Standard) relating to weak reinforcement, stress rein¬ forcement and welded nets. For a more detailed description of the individual, traditional steel types, reference is had to "Betonbogen," Aalborg Portland, 2nd ed., 1985, p. 345-349. Still further, other types of material can be used to substitute materials for traditional ferrous reinforcement. The use thereof, however, is still at a developmental and experimental stage, but the materials are well-known in connection with fibres. Such materials are, for example, polypropylene, glass, nylon, carbon, aramid, polyaramid, mineral wool, cellulose and asbestos, but as mentioned above all materials that fulfil the functional demands can be used. For continuous reinforcement typically iron and steel of all strength qualities, spun and weldless polypropylene wires, spun glass cables, spun and pressed polyaramid thread (Kevlar) etc. can be used. For fibre reinforcement typically steel, carbon, glass, asbestos, cellulose, polypropylene, polya¬ ramid, aramid, nylon, etc. can be used. The following materials are preferred: • Ribbed bar from Det danske Stalvalsevasrk (DK), dimension 6-12 mm; • Steel 600 from Nordisk Kabel- og Tradvarefabrik (DK) in the dimension 2-4 mm, called Ribbed Spring Wire, Piano Wire; Nordlocktrέd. • Steel fibres type Dramix, Bekaert, Belgium; • Polypropylene fibres, type Krenit, Danaklon A/S, Varde (DK). The concrete composition is mixed in quite conventional manner; problems of correct dosage and mixing of the fibres is generally a matter for the technology of the 1980's. The transportation of the concrete also takes place in quite conventional manner, i.e. either in rotating drum vehicles or in lorries with covers (tarpaulins). As regards concrete pavements, laying can be carried out by several different types of pavers, the most commonly used in the development of the product and for testing being cited in the experimental part below. The pre-initiation of cracks can be performed in several ways according to the invention. Macro cracks can for example be introduced as indicated in claims 8-11 and micro cracks as indicated in claims 12-15. In a special embodiment of the invention (cfr. claim 17), the pre- initiated cracks are formed by means of a preliminary crack formation in a lower layer. These methods are described in more detail in the experimental part. Some elaborations there¬ on are listed below. In one way of carrying out the mechanical impact as mentioned in claims 8 and 9 a groove (a joint) is made across the layer, e.g. using a Soff-cut diamond cutting machine, immediately following the setting of the concrete, the groove width being less than 3 mm. Preferably, the depth of the groove corresponds to about 10% of the layer thickness. Immediately following this groove cutting, a piece of hard wood, for instance of the dimensions 100 x 100 mm, is placed over the groove, and a falling weight or a falling sword is released so as to hit the wood beam with a drop or fall energy of minimum t x 50,000 J/m 2 (per unity of length of the groove, the expression t being the thickness in meter of the layer). Using as an example a layer thickness of 20 cm and a layer width of 4 m, the drop impact on the wood beam corresponds to a drop energy of 0,2 X 4 X 50,000 = 40,000 J, corresponding to an 80 cm drop of a falling weight of 5 tonnes, in this way, a very fine crack is formed top-to-bottom of the layer. At present, the use of a detonating thread, as mentioned in claims 8 and 10, is only possible for layers of a thickness above a certain minimum (about 10 cm), since detonating threads having such low charge as would be required for less thick layers are for the moment not marketed (not yet). Using this method a detonating thread, e.g. Pantrit, 3 g/m supplied by Dansk Spraengstofselskab A/S, is placed on the sub base before the laying of the concrete. When the concrete has reached its setting point, the detonating thread is ignited, for instance using an electrical detonator supplied by the above mentioned firm. By the detonation, a fine crack is established bottom-to-top with no damage of the surface. In one embodiment of the mechanical crack forming method as mentioned in claims 8 and 11 a tool comprising cutting organs of the ploughshare type is used. The spraying with a release agent takes place in order to prevent the disappearance of the cracks because of the sides fusing together. Care must be taken to ensure that the release agent is not applied at the top of the groove, since otherwise surface failures might occur involving a risk of shelling or peeling. Now, when the concrete contracts in connection with its hardening, a crack appears at the pre- cut position. This method is likewise preferred for layers of a greater thickness than 10 cm. Regarding the pre-initiating of micro cracks, as mentioned in claim 12, this is possible (for laboratory specimens only) by cooling, e.g. from a temperature of 20 to -20°C by dipping into a cooling or freezing liquid. In another method the hydration shrinkage of the cement paste, said shrinkage being a result of the chemical reactions, is used in connection with the formation of micro cracks, said shrinkage in a usual freely shrinking paste amounting to about 16% of the initial volume. In the method of pre-initiating micro cracks as mentioned in claims 13-15, an aggregate or stone skeleton is established, in which point contact is established at each larger stone. Because of this point contact, volume changes are prevented. Accordingly, during the shrinkage of the cement paste micro cracks are formed, which have shown up to establish a continuous network of inter-connected cracks, separating the structure in a lot of parts, being inter-locked in a sort of a three-dimensional jigsaw puzzle, deformation of the micro cracks however being allowed, and the tensile strength of the material being reduced. The desired point contact can for instance be obtained by using an aggregate mixture in which a heavy jump or discontinuity exists in the particle size distribution curve, typically so as to bring about lack of particles of a full range of order, e.g. a jump between 1 and 10 mm, between 2 and 20 mm, between 4 and 40 mm etc. The concrete then has to be composed so as to allow the filling mixture to flow so easily as to ensure contact between the larger particles, while not completely filling all those cavities of the stone skeleteon which are defined by the coarse aggregates. Following these instructions, micro cracks of the desired character will be formed in connection with the chemical shrinkage of the paste. The particles smaller than 0,1 μ as mentioned in claim 12 is for instance micro silica. The shrinkage of the paste depends on the chemical reaction type of the cement. Clay of the lllit type usually forms dehydration cracks in connection with the water consumption during the hydration. In the method of creating micro cracks of claim 16, the concrete is laid as usual, and at a given point in time, typically in the interval of between the setting time and before a compressive strength of 20 MPa is achieved, viz. usually within 1-3 days after laying, a vibration roller, a vibration plate compacting device, a rubber wheel roller or other static compaction devices are put upon and/or driven upon the layer, following which a nice pattern of micro cracks is formed in the layer. A deflection (deflection bowl) is obtained of the sub base layer, whether asphalt or something else, and when the concrete layer deflects correspondingly, appropriately spaced cracks are formed. Claims 18-21 relate to fresh concrete, binder compositions and paste being particularly useful for the formation of micro cracks, cfr. the corresponding method claims 12-15. Claim 12 dis¬ close one example of a paste of poor strain capacity at an early age, viz. a paste of a large hydration shrinkage and/or a low water content etc. Typically, but not exclusively, this is a paste as defined in claim 14. The method of claim 17 is well known, i.a. from Germany, for the positioning of transverse cracks every five meter in the basecourse below non-reinforced concrete layers. Using this method, the cracks of the cement bound gravel basecourse are positioned immediately below the joints of the concrete pavement. If cracks are made in the basecourse every meter instead of every five meter, cracks are formed in the concrete layer exactly according to this pre-de- termined pattern. The cracks are only formed, however, at a certain difference of about 10°C between the day and the night temperature. If this difference is not obtained, the cracks may be artificially provoked with a knock at the crack zone using a so-called Whip-hammer or using a mechanical impact as mentioned in claim 9. The structures as produced by the method of the invention, being characterized by a reduced degree of reinforcement, are made the object of claim 22. Claims 23-25 relate to preferred embodiments thereof, the structure being either more precisely defined and/or the degree of reinforcement being indicated in percentages of that usually considered necessary, and/or by way of intervals. Finally, claim 27 relate to the use of a reinforced concrete structure as produced by the method of the invention as an independently load-bearing layer. The invention will now be described in greater detail by way of examples, partly relating to laboratory tests, partly to full scale tests and finally, in Example 4, a method of calculating the required degree of reinforcement in systems of pre-initiated cracks is presented. Experimental part Laboratory tests Example 1 Tensile strength of test specimens with and without pre-initiated cracks Tensile tests are carried out on test specimens of the following dimensions: A: 500 x 800 x 40 mm (b x I x h) B: 440 x 5000 x 100 mm (b x l x h) The smaller specimens of type A are used in connection with the methods IV-VI (see below) in tests with pre-initiated micro cracks, and also because the specimens for micro crack testing do not need to be so large. Five different concrete compositions are used: Material, kg/m 3 1 2 3 4 5 Cement - low alkali 300 300 334 325 325 Microsilica 80 80 16 32 32 Fly ash, Danaske 27 27 0 0 0 Water 110 100 141 155 155 Superplasticizer (Sika) 4,5 4,5 2,0 3,0 4,0 Air entraining agent (Sika) 0 0 1 ,25 0 0 Sand 0/4 mm 743 796 666 800 300 Gravel 4/25 mm 1200 1170 1200 0 0 Gravel 4/8 mm 0 0 0 885 0 Gravel 8/25 mm 0 0 0 0 1600 Regarding the types and suppliers of raw materials here and in the following examples, the following applies: The cement is a low-alkali cement from Aalborg Portland, micro-silica is obtained from Elkem, sand 0/4 mm from Frederikssund Sten & Grus, gravel 4/8, 4/25 and 8/25 from Superfos Construction, Rønne, Sika air-15 and Sika plastiment A40 from Sika, fly ash is supplied by Danaske AS in Aalborg. The test specimens are divided into two groups. In one group a crack system is pre-initiated by means of the methods stated below, the other group being a control group with no pre-initiated cracks. Methods of pre-initiating cracks Macrocracks I A reflective crack is established at 12 hours of maturity by sawing with a diamond saw a 3mm lead groove through 10% of the slab thickness and afterwards loading with a Falling Weight on an ordinary railway sleeper placed across the test specimen. The drop energy should correspond to a 3 tons weight falling 20 cm per m beam width. II A reflective crack is established at 14 hours of maturity by setting off a detonating thread placed below the beam, transversely to the longitudinal direction. The detonating thread is supplied by Dansk Spraengstofselskab and has a charge or loading of 3 g/m. III A partially reflective crack is established at zero hours of maturity by scraping across the test beam using a brick jointer and spraying bitumen emulsion in the scraped joint. The joint is then carefully closed by manually vibrating both sides of the crack. Micro cracks IV Sudden cooling of the concrete specimen from 20°C to -20°C at 20 hours of maturity by dipping into a freezing liquid such as glycol from Søborg Køleindustri A/S. V Use of concrete with a surplus stone skeleton and an easily flowing, high shrinkage mortar. VI Creation of plastic shrinkage cracks caused by drying. At 1 hour of maturity, after laying, concrete with at least 10% microsilica is exposed, in the laboratory at approx. 20°C and 60% relative humidity, to an air current in a period of 60 minutes, the flowing velocity of the air being 2 m/sec. At 7 days of maturity, the test specimens are exposed to uni-axial longitudinal tension. The test method is no standard method. The results obtained are shown in the Table below: From the table it is apparent that the tensile strength of those specimens in which a crack system has been pre-initiated is drastically reduced as compared to the control test specimens. The example shows that in practice it is possible by means of all the above-mentioned methods to pre-initiate a crack system which leads to a reduction in the tensile strength of the material. Popularly speaking the result might also be interpreted as follows: In the specimens with pre- initiated cracks, due to their low tensile strength, a considerably lower load or energy is required to control the cracks, and accordingly a lower degree of reinforcement is required. Method IV seems to be less effective than methods V and VI, but a single less successful test should not absolutely lead to the exclusion of the method as a potential crack-inducing method. Example 2 Establishing the reguired degree of conventional, continuous reinforcement for slab cross sec¬ tions with and without pre-initiated cracks The following concrete composition is used: Components kg/m 3 Cement 334 Microsilica 16 Sand (0/4 mm) 666 Gravel (8/25 mm) 1202 Water 139 Sika Air-15 1 Sika Plastiment A40 2 The compressive strength of the concrete is: • at 3 days of maturity 26 MPa • at 28 days of maturity 55 MPa. Eight continuously reinforced slabs of this concrete with varying amounts of longitudinal rein¬ forcement material are produced; with and without pre-initiated cracks, as shown in Table 1 below. The dimensions of the slabs are: 100 x 440 x 5000 mm. Table 1 Slab no. Degree of reinforcement Φ (%) Pre-initiated cracks 1 0.06 + 2 0.13 + 3 0.19 - 4 0.19 + 5 0.32 - 6 0.32 + 7 0.69 - 8 0.69 + The cracks are pre-initiated as follows: The first meter of the slab is cast against a plywood board. The board is removed and the lower two thirds of the casting joint is painted with liquid asphalt. The plywood board is placed 1 m further down and the next section of slab is cast between the asphalt surface and the plywood board. In this way, four weak lines (pre-initiated cracks) are made in the 5 m long slab, see fig. 1. The reinforcement used is a high-strength steel (High Tensile Steel, rib bar, from Lemvigh- Mϋller & Munch, Copenhagen) with a diameter of 6 and 8 mm. The yield stress of the steel is 550 MPa. The number and diameter of the reinforcement bars and the resulting degree of rein¬ forcement Φ is shown in Table 2 below: Table 2 The slabs are tested at an age of three days of maturity in uniaxial tension. The stress-strain- relation and the distribution of cracks is studied for different amounts of reinforcement. The slabs are exposed to an overall elongation of 3 %o. The test arrangement is shown in Fig. 1. The load is applied by means of two hydraulic load cells. A manometer is attached to each load cell in order to record the load applied. Before the experiment is started, the surface of the slabs is painted white in order to make the cracks visible. The strain (the relative elongation) is measured in two ways: (1) By means of two 500 mm slide gauges (the approximate placing of the slide gauges is shown at Fig. 1); and (2) the overall elongation of the slab is measured by recording the distance between the movable traverse and the fixed traverse, to which the load cell are attached (see Fig. 1). The cracks patterns of the different slabs are shown at Figs. 2-9 and the stress-strain relations are shown at Figs. 10-17. At Figs. 2-9, L indicates the placing of the slide gauges and the numbering indicates the order in which the cracks were formed. In Fig. 2, crack no. 1 occurred at one of the pre-initiated cracks. In Figs. 3, 5, 7 and 9, cracks no. 1-4 occurred at the pre-initiated cracks. At Figs. 10-15, the development of cracks no. 1 , 2, 3 etc. is shown with a "Φ" . In Fig. 17, a "Δ" indicates the development of cracks no. 5-10, and in Fig. 16, a Δ indicates the development of cracks no. 1-9. Results A degree of reinforcement of more than 0.7% will ensure satisfactorily distributed cracks, even if no cracks are pre-initiated in the fresh concrete (slabs no. 7 and 8). The distance between cracks and accordingly also the crack width, is inversely proportional to the degree of reinforce- ment (i.e., the greater the degree of reinforcement, the smaller the crack spacing and the nar¬ rower the crack width). In a slab without pre-initiated cracks, a degree of reinforcement of 0.3% or less (slabs no. 3 and 5) do not lead to a satisfactory system of well-distributed cracks. When a degree of reinforcement of between 0.1 and 0.3% is applied, cracks are formed only at the pre-initiated crack positions (slabs no. 4, 6, 2 and 1). A degree of reinforcement of 0.06% (slab no. 1) does not suffice in order to activate the pre- initiated cracks and thus only one crack is formed. Accordingly, the results indicate that by pre-initiating a crack system in the slab, the reinforcing degree can be reduced from 0.7% to 0.1%, perhaps even more, to 0,07%, i.e. about 10%. It has to be noted that the amount of reinforcement required to obtain a certain crack distribu¬ tion among other things is dependant on the tensile strength of concrete. Consequently, if a concrete with different mechanical properties is used, the reinforcing degrees will have to be adjusted. The desired minimum distance betweeen the pre-initiated cracks of the structure depends on the maximum crack width, which can be allowed in the final structure and on the expected maximum load. Example 3 Establishment of the required degree of reinforcement in fibre-reinforced concrete slabs with or without pre-initiated micro crack systems Two types of concrete (Mix I and II) are used having the following compositions: Melment L-10 is a superplasficizer, supplied by NBK, Rødekro. The steel fibres used are of the type Dramix ZL 30/.50 from Bekaert. The fibres have hooked ends at a length-diameter ratio If/df = 30mm/0.5mm = 60. The equation in the last row of the Table corresponds to the Danish Standard calculation of the equivalent water/cement ratio. Five fibre-concrete slabs are made, as shown in Table 1 below. The size of the slabs are 40 x 500 x 800 mm. In some of the slabs, a micro crack system is pre-initiated, the rest being control slabs with no micro cracks. Table 1 Micro cracks are pre-initiated as follows: gravel is mixed with steel fibre and cement, and then water and the superplasticizer (SPT) is added. The viscosity of the paste is sufficiently low to ensure that there is contact between individual gravel particles. Thus a stiff stone skeleton is formed being surrounded by fibre-reinforced paste. The cement paste as such during the set- ting shows high shrinkage. Due to the stiff stone skeleton, however, the shrinkage of the paste is hindered and this leads to the formation of a well-dispersed micro crack pattern. The fibres ensure that stress can be transferred across the shrinkage cracks. The slabs are tested at a maturity of three days in uniaxial tension. The distribution of the cracks formed in the slabs during loading is studied at various fibre reinforcement percentages. The slabs are exposed to a total elongation of 3%o. The maximum crack width at 3%o is measured. The test setup is shown in principle in Fig. 1. The load is applied by means of two hydraulic load cells. A manometer is attached to each load cell in order to record the load applied. Before the test is started, the surface of the slabs is painted white in order to ensure that the cracks become visible. The strain is measured with two 500 mm slide gauges. Results The crack patterns of the slabs are shown at Figs. 18-22. The numbering indicates the order in which the cracks appeared. When tensile loading slab no. 1 , only one discrete crack appears. In slab no. 2 several cracks appear. The average spacing of the cracks is approximately 20 cm and the maximum crack width at 3%o elongation is 0.64 mm. In slab no. 3 a well distributed crack system appears, the average spacing of the cracks being around 12 cm and the maximum crack width at 3%. elon¬ gation 0,25 mm. In slabs no. 4 and 5 a well distributed crack system shows up. For slab no. 4, the average spacing of the cracks is approximately 15 cm and the maximum crack width at 3%o elongation is 0.32 mm. For slab no. 5 the average spacing of the cracks created is 10 cm and the maxi- mum crack width at 3% 0 elongation is 0.19 mm. A fibre reinforcement of 2 to 3 volume percent is generally assumed to be sufficient to obtain a well-dispersed crack system and to control the crack widths in a typical concrete pavement exposed to tensile strain (with no pre-initiated crack system). In the present tensile loading test an average crack spacing of maximum 10 cm was aimed at. Then, as the maximum elongation of the slabs is estimated at 3%o, it can be ensured that the maximum crack width will be kept at an acceptable level, i.e. around 0.2 mm. This test confirms the general assumption referred to above regarding the degree of fibre rein¬ forcement usually considered sufficient, since slabs no. 2 and 3 show that a fibre reinforcement of more than 2.0% would ensure a well-dispersed system of cracks, the spacing between the cracks - and also the crack widths - being inversely proportional to the degree of reinforcement. The test also shows that a degree of reinforcement of 1% in a conventional fibre concrete slab (slab no. 1) is not sufficient to ensure a well-dispersed crack system, since only one discrete crack is formed. However, in a fibre-reinforced slab having a pre-initiated finely dispersed micro crack system, a fibre percentage reinforcement of 0.2 to 0.5% by volume is sufficient to ensure that a well-dis¬ persed crack system is formed, when the slab is exposed to the influence of tensile strain. The spacing between the cracks created and the crack widths are inversely proportional to the degree of reinforcement. Accordingly, the results of this test series indicate that - aiming at a maximum crack width of 0.2 mm at an elongation of 3%o - the fibre reinforcement can be reduced to 17%, viz. from 3% to 0.5%, by pre-initiating a finely dispersed micro crack system in the slab. It has to be noted that the required degree of fibre reinforcement in a sample without pre-ini- tiated cracks depends on the tensile strength of the concrete; whereas it, in a sample with pre- initiated micro cracks, depends on the crack strength of the micro crack system. Accordingly it is necessary to adjust the above-mentioned degrees of reinforcement, if a concrete with diffe¬ rent mechanical properties is used. Example 4 Proposed interim dimensioning rules for concrete pavements with a pre-initiated crack system. The following notation is applied: b the width of pavement (m) f y the yield strength of the reinforcement (Pa) g the acceleration due to gravity (m/s 2 ) / the distance between pre-induced cracks (m) t the thickness of the pavement (m) A a the minimum area of the reinforcement perpendicular to the crack surface (m 2 ) A,^ the cross-sectional area of the slab (pavement) (m 2 ) F cr the force necessary to activate the pre-induced cracks (N) F r the force in the reinforcement (N) F f the frictional force Vf the fibre volume fraction Φ the reinforcing degree μ the coefficient of friction p the density of the concrete (kg/m* ) a cr the strength of the pre-initiated crack (Pa) Calculation of the reinforcing degree of longitudinally reinforced concrete pavements with pre- initiated macro cracks. Imagine a pre-cracked concrete pavement with a distance / between the cracks in which tensile strains are introduced due to, e.g., temperature changes, see Fig. 23. The force of the reinforcement F r must exceed the force necessary to activate the pre-initiated crack F cr plus the force F f generated by the friction between the sub-grade and a section of the pavement of the length /, see Eq. (3.1): (3.1) F r > F f +F CT = mgμ + A slab σ cr = btlpgμ +btσ,.. If the maximum allowable stress in the reinforcement is two thirds of the yield strength f y of the reinforcement, the minimum area A g of the longitudinal reinforcement is given by the expres¬ sion (3.2): (3.2) A r > - F ' btl ^ + btσ CT (2 /3)f y (2 / 3)f y Accordingly, the required reinforcing degree can be estimated from eq. (3.3): A r 3(lpgμ + σ cr ) (3.3) φ : Slab 2f , Calculation procedure of the reinforcing degree of fibre reinforced concrete pavements with pre-initiated microcracks. For a pre-cracked fibre reinforced concrete pavement, the fibre reinforcement necessary to ac¬ tivate the micro cracks can be calculated using the same principles as applied above. The main difference between the two cases is the distance between pre-initiated cracks. The distance between the macro cracks in the longitudinally reinforced pavement is of the order of 27 magnitude of 1 m, whereas the distance between the micro cracks in the fibre reinforced pa¬ vement is a few millimetres only. Accordingly, / in Eqs. (3.1) to (3.3) is getting very small and those parts of the equations origi- nating from the friction between pavement and sub-grade can be eliminated. Furthermore, the widths of the pre-initiated cracks are smaller for the same overall deformation and accordingly the strength of the pre-initiated micro cracks (σ C ) is higher than that of the pre- initiated macro cracks. Accordinglly, in a fibre reinforced pavement with pre-initiated micro cracks, the necessary force of the fibre reinforcement F.can be expressed as (4-1) F r ≥ F cr = A sl3b σ cr = bt σcr The maximum allowable stress in the fibre reinforcement is fixed at two thirds of the yield strength f y of the fibre material. Then the minimum area of fibres crossing or bridging a crack A r can be expressed according to eq. (4.2): (4.2) A r > - F ' btσ" (2 /3)f y (2 /3)f y and the minimum required reinforcing degree perpendicular to the crack surface can be ex¬ pressed according to eq. (4.3): (4.3) Φ = - A r - 3 ° a lab 2f„ The fibres are assumed to be randomly 3-D distributed. According to Aare, T., "Tensile Charac¬ teristics of Concrete with Special Emphasis on its Applicability in a Continuous Pavement," Ph.D. thesis, Institute of Supporting Structures at The Technical University of Denmark, the number of fibres bridging a crack per unit area of crack surface N 0 can be determined in accor¬ dance with eq. (4.4): 2V f (4.4) N 0 = 172 - πr f πd f Vindicating the fibre volume fraction and df the diameter of the fibres. Finally by combination of Eqs. (4.3) and (4.4), the fibre volume fraction necessary to activate the pre-initiated micro cracks can be expressed by eq. (4.5): 3σ (4.5) V f = — s- Full scale tests Example 5 Test pavement on E45/M20 motorway, west-bound lane, east of Ringsted, in Denmark The emergency lane in question was constructed as follows from the top downwards: 4 cm fibre reinforced concrete, 10 cm reinforced concrete with micro cracks, 10 cm stabilized gravel with micro cracks, 10 cm granular sub-base 0/32 mm, 60 cm frost-proof capping layer. The upper 14 cm, which is of relevance for the present test, was constructed as follows: A gravel layer was laid using a standard asphalt paver consisting of crushed Rønne granite, 45 mm rail track ballast from Superfos Construction, Rønne. The layer thickness was 10 cm fol- lowing compacting with a light vibrating roller, type Bomag 90. The width of the pavement was 240 cm. On top of this layer, the following continuous steel reinforcement was placed: 0 to 80 m: ribbed bar, 10 mm. 0.2% 80 to 160 m: ribbed bar, 10 mm, 0.1% 160 to 240 m: ribbed bar, 10 mm, 0.05% 240 to 300 mm: no reinforcement. On top of this a cement mortar with the composition as shown below was vibrated with a vibra- ting plate operated at 50 Hz. The mortar was produced at GH-beton A S Readymix in Ringsted and was delivered in a traditional rotary truck mixer to the construction site. The composition of the mortar (2.5 m 3 ): Cement, low alkali 2000 kg Microsilica, Elkem 200 kg Water 900 kg Superplasficizer, Peramin F 20 kg Sand 0/2 mm 2000 kg Peramin F is a suiphonated melamine formaldehyde in a 35% aqueous solution supplied by Perstorp Chemitech AB, Sweden. Immediately after vibrating of the cement mortar, a 4 cm thick layer of fibre reinforced concrete was laid with a CMI Bid-Well concrete paver. The fibre concrete was also produced by GH-beton and had the following composition per m 3 : Cement, low alkali 300 kg Microsilica 30 kg Fly ash 30 kg Water 130 kg Superplasficizer, Peramin F 6 kg Sand 0/4 mm 750 kg Gravel 4/11 mm, Rønne Granite 1100 kg Polypropylene fibre, Krenit 12 mm 5 kg. After laying and brushing (in order to create friction), the concrete pavement was covered with 0.1 mm plastic foil for 5 days (a normal procedure in order to reduce or eliminate water evapo¬ ration). The pavement surface was then inspected and it was found that a crack system had been established with a crack width of less than 0.1 mm and with a crack spacing between 0.4 and 0.8 m in the reinforced sections, whereas the non-reinforced control section had cracks in four places with cracks widths of > 2 mm. From the inspection of drilled cores, it was revealed that the lower concrete layer of 10 cm was permeated by a system of inter-connected micro cracks. The upper 4 cm were not micro cracked, whereas very fine, discrete cracks could be observed every 10 cm and having a crack width of between 10 and 80 μm. Accordingly, the conclusion of this test is that the principles of the invention are confirmed in full scale testing. In those pavement areas having a degree of reinforcement of 0.2%, a crack pattern was estab¬ lished with a crack spacing of approximately 40 cm, whereas at a reinforcement degree of 0.05% an acceptable crack distance of 80-90 cm was established. The moderate reinforcement of the top layer (0.5 vol-% polypropylene fibre) as expected, did not show up to have any effect on the primary crack formation. Very wide cracks appeared in those sections having no traditional reinforcement, despite the well-established microcrack system in the lower concrete. Accordingly, the reinforcement degree can not be reduced to zero. Example 6 Test pavement on a local asphalt road at Superfos Construction's gravel pit at Reerslev In 1992, a test pavement was constructed on an existing asphalt pavement on a local road at the above-mentioned gravel pit. The pavement was 2 m wide and had a thickness of 4 cm. The following concrete composition was used for the pavement: Ordinary Portland Cement 360 kg Microsilica 60 kg Water 170 kg Superplasficizer, Peramin F 12 kg Sand 0/1 mm (pure marine sand) 500 kg Granite gravel 8/12 mm 1450 kg Steel fibres, Dramix 30 mm 25 kg The concrete was mixed at Dansk Beton Teknik's concrete mixing plant at the gravel pit (of the type Rex Chainbe't Porto Paver, 9 cu. yd). The concrete was transported to the paver in a Volvo 860 open dumper. The wearing course was laid by first milling the asphalt surface slightly so as to establish an even base. Then a surfacing of fibre concrete with a thickness of 4 cm was laid with an asphalt paver of the type Vogele 2000, High Compaction, Superscreed. The concrete was compacted by by means of the vibrating beam of the asphalt paver only. Immediately after paving the concrete was protected against the evaporation of water by spray¬ ing with a 50 % bitumen emulsion at the rate of 100 g/m 2 . No discrete cracks (visible cracks) have ever appeared in the paving since laying. From the inspection of drilled cores a distinct micro crack system is observed in the concrete. The actual degree of reinforcement is about 0.3 vol-%, i.e. 10-15 % of the degree traditionally recommen¬ ded to obtain crack-free pavements. The above micro crack system was pre-initiated using the method of establishing point contact in the gravel layer together with a paste of high shrinkage. Example 7 Test pavement at Hannover airport With a CMI 2000 slipform paver a fibre reinforced concrete with 1 vol-% fibres was laid at Han¬ nover airport. This test was carried out solely for demonstrating that fibre concrete can be laid with a slipform paver. As the pavement was laid as fibre concrete on top of concrete, no at- tempts were made ed to pre-initiate micro cracks. Joints were cut every 5 m according to method (1). Example 8 Test pavement at the production area at Aalborg Portland With a Vogele 2000, High Compaction, Superscreed, Aalborg Portland A S has laid various pavements of fibre concrete in widths up to 8 m at their production area. These concretes contained steel and polypropylene fibres, concrete with traditional continuous reinforcement and a combination of both types. No tests comprising the pre-initiation of micro cracks were made. The tests were carried out solely to demonstrate the suitability of these paving materials.

Claims: PATENT CLAIMS 1. A method of producing a reinforced concrete structure, said method including the steps of mixing a hydraulicaliy hardening binder with water and aggregate and, if desired, additives and casting this fresh concrete in the desired form, said concrete structure being reinforced, characterized in that a crack system is pre-initiated in the concrete at a point in time before a level of stress is established in the concrete caused by such events as its hardening, any drying-out thereof and any temperature influences thereon, which exceeds the actual ten¬ sile strength of the concrete. 2. A method according to claim ^characterized in that the crack system is pre- initiated within 0-24 hours of maturity. 3. A method according to anyone of the claims 1 and 2, of producing a continuously reinforced concrete structure, said method including the steps of mixing a hydraulicaliy harde¬ ning binder with water and aggregate and, if desired, additives and casting this fresh concrete in the desired form including a continuous reinforcement, characterized in that a macro crack system is pre-initiated in the concrete at a point in time before a level of stress is estab¬ lished in the concrete caused by such events as its hardening, any drying-out thereof and any temperature influences thereon, which exceeds the actual tensile strength of the concrete. 4. A method according to anyone of the claims 1 and 2, of producing a fibre reinforced concrete structure, said method including the steps of mixing a hydraulicaliy hardening binder with water and aggregate and, if desired, additives and a fibre reinforcement material and cast- ing this fresh concrete in the desired form, characterized in that a micro crack system is pre-initiated in the concrete at a point in time before a level of stress is established in the concrete caused by such events as its hardening, any drying-out thereof and any temperature influences thereon, which exceeds the actual tensile strength of the concrete. 5. A method according to anyone of the claims 1 and 2, of producing a reinforced con¬ crete structure, said method including the steps of mixing a hydraulicaliy hardening binder with water and aggregate and, if desired, additives and a fibre reinforcement material and casting this fresh concrete in the desired form including a continuous reinforcement, characterized in that a macro and/or micro crack system is pre-initiated in the concrete at a point in time before a level of stress is established in the concrete caused by such events as its hardening, any drying-out thereof and any temperature influences thereon, which ex¬ ceeds the actual tensile strength of the concrete. 6. A method according to anyone of the claims 1-5, characterized in that the con¬ crete structure is a concrete pavement. 7. A method according to claim 6 for producing a reinforced concrete pavement, characterized in that at least one layer of a pavement produced in accordance with claim 4 is laid on top of at least one layer of a pavement produced in accordance with any one of the claims 3 - 5. 8. A method according to anyone of the claims 3, 5, 6 or 7, c h a r a c t e r i z e d in that a macro crack system is pre-initiated using one or more of the following methods: Mechanical impact, ignition of detonating thread, mechanical crack forming. 9. A method according to claim 8, characterized in that a macro crack system is pre-initiated by mechanical impact as follows: a groove is formed across the pavement, a suit- able support for the impact is placed on top of the groove and a falling object is released, so as to hit the impact support with an energy of at least t x 50,000 J/m 2 per groove length, t indica¬ ting the thickness of the pavement and being expressed in m, the groove preferably being formed using a diamond cutting machine, being of a width of 0-3 mm and a depth correspon¬ ding to 10% of the pavement thickness. 10. A method according to claim 8, characterized in that a macro crack system is pre-initiated by igniting a detonating thread, which is placed before casting and is ignited at the desired point in time using an electronic detonator. 11. A method according to claim 8, characterized in that a macro crack system is pre-initiated by means of mechanical crack-forming as follows: A transverse groove is formed by using a cutting or forming instrument, the sides of the grooves are treated with a coating or releasing agent, preferably consisting of one or more agents selected from amongst those containing bitumen and/or silicone and/or oil, following which the groove is re-closed, especially using surface vibration. 12. A method according to anyone of the claims 4, 5, 6 or 7, c h a r a ct e r i z e d in that a micro crack system is pre-initiated using one or more of the following methods: mechanical impact, especially vibrational impact, sudden cooling, use of an aggregate skeleton with point contact together with a shrinking cement paste, use of a paste with high hydration shrinkage and/or low water content and/or high content of particles smaller than 0.1 μm and/or containing clay of the lllite type, drying, the aggregate being preferably coatet. 13. A method according to claim 12, characterized in that an aggregate skeleton with point contact is used in connection with a shrinking cement paste, said aggregate skeleton generally lacking particles of an entire order of magnitude and the paste being composed so as to acquire a low strain capacity at an eariy age. 14. A method according to claim 13, characterized in that the strain capacity of the paste is less than 100 microstrain. 15. A method according to anyone of the claims 13-14, c h a r a c t e r i z e d in that the largest particles of the aggregate make up at least 2/3 of the volume of the concrete. 16. A method according to claims 6 or 7 and 12, cha ra cterized in that the method of mechanical impact is used for the creation of micro cracks, said method comprising the steps of laying the concrete and at the relevant point in time as defined in claim 1 establishing the desired micro crack pattern using a roller, a compacting device or the like, especially a vibrational device. 17. A method according to anyone of the claims 3, 5, 6 or 7, characterized in that the crack system is primarily formed in a hydraulicaliy bound gravel basecourse, from where it is transferred as a pre-initiated crack system onto a concrete pavement, which is subsequently laid on top of the gravel basecourse, the primary cracks being in particular formed by pushing a blade, knife or a similar cutting tool transversely across the gravel basecourse after its com¬ paction, following which the gravel basecourse and the concrete layer are combined, in particu¬ lar by slightly milling the surface of the gravel basecourse before the laying out of the concrete layer. 18. A fresh concrete composition comprising a hydraulicaliy hardening binder, water, ag- gregate and, if desired, additives, characterized by the fact that it is well suited for the formation of microcracks, because of the aggregate lacking particles through a full range of magnitude and the paste being composed so as to have a poor strain capacity at an early age. 19. A fresh concrete composition according to claim 18, characterized in that the fraction of the largest aggregate particles makes up at least 2/3 of the volume of the concrete. 20. A binder composition for the preparation of a fresh concrete composition in accordance with any one of the claims 18 and 19, characterized in that it is converted, following the addition of the calculated amount of water, into a paste of low strain capacity at an early age. 21. A paste composition for the preparation of a fresh concrete composition in accordance with anyone of the claims 18 and 19, characterized in that it has a low strain capacity at an early age. 22. A reinforced concrete structure produceable in accordance with any of the claims 1-17, characterized in that the degree of reinforcement is reduced in relation to the amount traditionally considered necessary. 23. A continuously reinforced concrete structure produceable in accordance with any one of the claims 3, 5, 6, 7-11 and 17, characterized in that the degree of reinforcement is 5- 50%, preferably 7-30%, more particularly 10-20% of that traditionally considered necessary. 24. A fibre-reinforced concrete pavement produceable in accordance with any one of the claims 4-6 and 12-16, characterized in that the degree of reinforcement is 5-50%, preferably 7-25%, more particularly 10-20% of that traditionally considered necessary. 25. A combined continuously reinforced and fibre reinforced concrete pavement produce- able in accordance with any one of the claims 5-17, c h a r a c t e r i z e d in that the degree of reinforcement is 5-50%, preferably 7-25%, more particularly 10-20% of that traditionally consi- dered necessary. 26. The use of a thin wearing course of a reinforced concrete pavement, produceable ac¬ cording to any of the methods of anyone of the claims 4-6 or 12-16, on top of any load-bearing layer, preferably an asphaltic layer and/or for rehabilitation of a concrete pavement and/or as a wearing course on a bridge deck. 27. The use of a reinforced concrete pavement, produceable according to any of the methods of anyone of the claims 1 -17, as an independently load-bearing layer.

Inventors: HENRICHSEN ANDERS, HENRICHSEN, ANDERS
Applicants: CEMENT & BETONLABORATORIET, DANSK BETON TEKNIK APS, ENGENHARIA LAB NACIONAL, HENRICHSEN ANDERS, STANG HENRIK, TRAFALGAR HOUSE TECHNOLOGY, TRANSPORT RESEARCH LAB, VEJTEKNISK INST, DANSK BETON TEKNIK A/S, HENRICHSEN, ANDERS, LABORATORIO NACIONAL DE ENGENHARIA CIVIL, STANG, HENRIK, TRANSPORT RESEARCH LABORATORY, VEJTEKNISK INSTITUT
Application year: 1995
Main classification: E01C19
IPC codes: B28B1, B28B11, B28B23, C04B40, E01C7, E01C11, E04C5