US2892341A - Composite floor structure - Google Patents

Description

June 30, 1959 c. L. SAINTY 2,892,341

' COMPOSITE FLOOR STRUCTURE Filed June 4, 1954 2 Sheets-Sheet 1 i I W June 30, 1959 c. SAINTY 2,892,341

COMPOSITE FLOOR STRUCTURE.

Filed June 4, 1954 2 Sheets-Sheet 2 ports for the moulding operation.

Ue t t e P t COMPOSITE FLOOR STRUCTURE QApplication June 4, 1954, Serial No. 434,555

imam priority, application Great Britain December 16, 1953 1 Claim. 01. 72-68) invention is concerned with the provision of improvedreinfo'rced concrete structures or structural members whereby the weight of such structures or members can be conside'ralblyreduced for a given load carrying capacity. The use of lightweight concrete structural members which are formed with a light-weight aggregate or have an aerated or cellular structure has previously been proposed. Owing to the relative weakness of such concrete in both compression and shear as compared with conventionalconcrete and its low adhesion to steel it has been proposed to'employ reinforcement in the form of wires, Wire mesh or perforated or expanded sheet metal. A' disadvantage of wire or mesh reinforcement is that it offers a small surface for adhesion so that a large amount of wire is necessary, and also the wires tend to cut into the concrete under load. The

edges of expanded sheet metal when this is used also tend went into the concrete.

Concrete reinforced in the above manner cannot be satisfactorily sawn since the saw causes vibration of the reinforcement with a consequent loss of adhesion. Also reinforcement of the above character is not self-supporting except in short spans and usually requires a number of intermediate sup- :It is an Object of the invention to overcome the foregoing disadvantages in a simple and efiective manner.

.In conventional reinforced concrete structures the concrete in those parts which are under tension contributes nothing to the tensile strength of the structure. or member, this being provided entirely by the steel reinforcement. This is because when the steel is economically loaded, the concrete in which it is embedded is. stressed beyond its "ultimate tensile strength. The greater the ratio between the ultimate tensile strength of the concrete and its modulus of elasticity the more the steel reinforcement can be stressed'without risk of fracturing the concrete.

Many of the known light-weight concretes although comparatively weak under compression are proportionally stronger under tension than conventional or dense concrete. Furthermore, some light-weight concretes, have a low modulus of elasticity and the ratio of their ultimate tensile strength to it is from three to five times the correspending ratio for conventional or dense concrete. Also the ratio of the modulus of rupture to the modulus of elasticity of certain light-weight concretes is from four to sixtimes the corresponding ratio for dense concrete. It is accordingly a further object of the invention to provide an improved concrete structure or structural member based on the foregoing considerations. I

A further object of the invention is to provide an improved reinforced concrete floor, roof or like structure which is lighter for a given strength than conventional structures. Concrete floors where moulded in situ or made up of previously cast units such as beams or slabs, require a continuous covering layer usually a screed of sand and cement mixture which is applied after assembly of the units and affords a smooth and flat surface upon which the floor covering such as blocks or tiles 2,892,341 Patented June 30, 1959 is laid. This screed layer increases the dead load of the floor but does not enhance its load bearing capacity in any way owing to poor adhesion between layer and floor. This is due largely to the fact that the layer must be applied as a fairly dry mixture to a surface which is no longer in so-called green condition. Also it is difiicult to remove dust and dirt from the concrete under the usual conditions on site. It is accordingly a further, ob ject of the invention to utilise such applied layer to increase the strength of a floor, roof or like load bearing structure.

According to the invention a concrete structure or structural member is composed at least in part of lightweight concrete having embedded therein reinforcement comprising metal members of relatively thin sheet form shaped and arranged to afford a relatively large surface for adhesion per unit length as compared with their cross sectional area. I

According to a further feature of the invention a reinforced concrete structure or member for resisting bending loads is provided having a composite character in which the member containing the tension reinforcement lying on one side of the neutral axis comprises lightweight concrete of a character capable of carrying a' substantial proportion of the tension and the remainder of. the member or structure, i.e. that part lying on the other side of the neutral axis, is formed from a denser concrete suitable for resisting compression. p

According to a further feature of the invention the part of the said structure or member containing the ten sion reinforcement comprises a concrete of which the ratio between its modulus of rupture and its modulus of elasticity is substantially greater than that forconventional or dense concrete. ,7

According to a further feature of the invention a floor, roof or like structure comprises pre-cast units of reinforced concrete arranged side by side and bonded to an applied continuous layer of sand and cement or similar mixture to provide a load bearing combined structure of increased strength and stiffness. Preferably thesaid bonding comprises metal elements which are partially embedded in the upper part of the unit and are adapted to lie flat on the surface thereof for transport and storage purposes, said elements being adapted to be raised to provide a secure bond between the unit and a subsequently applied layer of cement mixture or the like.

The invention will be best understood from a consideration of the following examples.

In the accompanying drawings:

Figure 1 is a cross sectional view of part of a composite floor structure constructed in accordance with the invention;

Figure 2 is a longitudinal section of one of the elements of Figure 1;

Figure 3 is a cross section through a concrete beam having angle section reinforcing members in accordance with the invention;

Figure 4 is a cross section through a concrete girder having reinforcement of inverted trough section;

Figures 5, 6 and 7 are cross sectional views of suitable reinforcing members. l V

In carrying the invention into effect according to one convenient mode as applied by way of example to a floor structure comprising concrete beams laid side by side as shown in Figures 1 and 2, each of the beams is composed of a lower section 1 and an upper section 2. In this example the beam is about 1' in width and is intended to have an effective span of 14' and to carry a load of lb. per sq. ft. The lower section 1 of the beam which constitutes the tension zone constitutes a major portion of the depth of the beam 1, 2, e.g. about three-fourths of such depth in the form shown, and is w asel moulded from a light-weight concrete in which is embedded longitudinal tension reinforcement elements 3. Each element 3 consists of angle section steel of relatively thin gauge to afford a relatively large surface for adhesion to the concrete per unit length as compared with its cross sectional area and is disposed with the exterior angle of the section facing the direction of applied load. The light-weight concrete forming the sectign 1 consists of a mixture of light-weight aggregate and cement in the proportions of 7:1 by volume, the aggregate being relatively coarse, say to /2 screen size. Also the mixture may include reinforcing fibre of a suitable character to increase the strength under tension. The moulding of the beam is continued to form a section 2 above the neutral axis of the beam with a denser concrete consisting of a mixture of light-weight aggregate cement in the proportion of :1 by volume and also including about 30% of fine material to afford a relatively hi h c mpressive strength. Such fine material may conveniently comprise ground aggregate. The light-weight aggregate for the section 2 is less coarse than that of the section 1 say about A screen size. Any suitable lightweight aggregate may be employed for the sections 1 and 2 and these may comprise expanded clay or shale products, pumice or the like. Each of the beams is moulded with a recess 4 along one longitudinal side there of and a corresponding mating projection 5 along the opposite side thereof.

'The upper section 2 of the beam constitutes a minor portion of the depth of the beam, e.g. about one-fourth of such depth in the form shown, and is also provided with two (or more) metal or steel reinforcing rods which of relatively light section sufiicient to prevent possible breakages during handling and transport. Other forms of reinforcement may be employed if desired. Also, in the'rnoulding of this section or beam there are provided anumber of metal bonding elements comprising strips 7 of metal having holes by which they are threaded upon the rods 6. These bonding strips are bent substantially at right angles as shown on the left hand side of Figure 2 so that they lie flat on the upper surface of the composite beam and do not interefere with the storage, transport or handling thereof. The beams so formed are. laid side by side to form a floor with the adjacent recesses and projections 4 and 5 interengaged as shown in Figure 1 and thereby relative vertical movements of the units are prevented. By reason of the relatively soft character of the light-weight concrete as compared with ordinary concrete, the beams can be forced sideways into close engagement so that such relatively vertical movements are prevented in a particularly effective manner. After the beams have been placed in positionto form the floor, the bonding strips 7 are prized up as shown at 7a on the right hand side of Figure 2. A

screed or layer 8 composed of cement or sand mixed with water to givea fairly dry mixture is then applied to the upper surface of the assembled beams and is allowed to set. This mixture should be fairly dry to prevent contraction on setting and consequently the permissible compression stress in this section is limited to about 600 lbs. per sq. in. By reason of the bonding afforded by the strips 7 the assembled units and the applied layer constitute a monolithic or unitary structure. In the'ex ample given, the weight of the composite beam prior to the addition of the layer 8 would be in the neighborhood of 27 lbs. per sq. ft. It is found that by the use of a suitable light-weight concrete for the section 1 a substantial proportion for example up to 30% or more of the total resistance of the beam to bending is constituted by the light-weight concrete below the neutral axis. In such an arrangement the steel reinforcement 3ca'n be stressed economically i.e. the minimum amount of reinforcement is employed and a part of the tension load'is carried by the lightweight concrete so that a. beam of h m size n e t e an a mq ld l i 'we ventional dense concrete. In this example the heat con ductance of the beam would be about .2 British thermal units per degree Fahrenheit' per hour per sq. ft. Also, owing to the high insulating properties of the light concrete and its softness as compared with conventional concrete, the beam is highly'resistant to fire and the concrete does not crack or flake when exposed to high temperatures.

The effects of plastic yield considerably influence the distribution of stress in the component parts of the beam as described above. Initially, the stress in the tension reinforcement may be about 10,000 lbs. per sq. in. and the concrete under tension may provide about 55% of the resistance to bending. With continuous loading and in the course of time, the stress in the steel may rise to say 18,000 lbs. per sq. in. and the concrete under tension will then take about 28% of the bending moment. If

. the concrete below the neutral axis were not capable of taking a proportion of the tension load about 40%rnore steel reinforcement would be required and it will there fore be seen that very considerable economy in steel rein; forcernent is afforded by the invention.

It will be understood that the dimensions of the parts of the composite beam or unit described above maybe widely varied. If the applied layer 8 is from one half to one inch in thickness it is found that an adequate bond will be afforded by from one to four steel bondiiig strips 7 for each superficial square foot. A further ad}: vantage of bonding the applied layer 8 is that this prevents disturbance of the floor surface which might be caused by cracking or lifting of the applied layer.

Another mode of carrying the invention into effect applied to a simple or non-composite light-weight con; crete beam or the like is shown in Figure 3. The rein; forcement comprises a series of right-angle section steel strips 10 extending along the length of the beam forming a horizontal series across its width. It will be seen that by the inverted arrangement of the strips 1Q, vertical shear forces will not cause the strips to cut into; the concrete since the bearing area per unit length s large as compared with the cross-sectional area of the metal, thes'e'areas being related to the vertical loads. 'In Figure 4 there is shown a girder of light-Weight concrete reinforced in the upper zone by two inveited channel section 'steelstrips 12 and a lower seiies (in the tension zone) of three similar strips 12.

The reiiiforcing strips may have sections as shown in Figures 5, .6 and 7. The tensile strength of these: strips can be chosen over a wide range by changingthe gauge of the metal so that the strength can ue'm'tcnd' with the adhesion between the metal and the concrete. Thus it is iiot necessary to employ a greater Cross-see; tional 'ar'ea'of metal than necessary in order toob'taih sufficient adhesion surface.

The lightei 'the concrete, the more necessary it is to distribute the stresses between" it and the reinforcement; and hence the size of the reinforcing memb'ers shoud vary inversely as the density of the concrete. As" ample s, the size of right angle .sect ionsmay ibe ed from A by for use in light-weight concrete ha a density of 7.0 lbs. per cubic foot to sections of l fib y 21";- for concrete of 20 lbs. per cubic foot density. The thic ness of the steel from which the angles are made may. vary from 16 standard wire gauge (.064) to 28 (.0148"); In general the heavier sheet is used for the smallerangles and the lighter sheet for the larger angles. Thus'the /1 by A" angles for denser concrete may be from 16 to '20 standard wire gauge in thickness, whilst the 1" by l angles for. the, lightest type' of concretc may be of 24 and 28 gauge sheet. Similar considerations apply 'io rein orce t r in t fF ures nd AHih Q' l .r m r 1 iw i we l fl I. if m Q2 n iiii @fb to, the mould, C sg. Byh'ecesses in end members, and if necessary intermediate supports of suspension or cradle form may be employed.

The reinforcement may be variously shaped and arranged provided always that it affords a relatively large surface per unit length, as compared with its crosssectional area, for adhesion with the concrete. This invention accordingly enables the weight of reinforcement required to be reduced as compared with known practice without danger due to breakdown of its adhesion with the concrete. Also units having reinforcement of this kind can be cut with a saw without damage.

A particular advantage of structures or structural members according to the invention is their high thermal insulating capacity. This high thermal insulation arises from the possibility of using a very low density concrete in the tension zone which constitutes about two-thirds of the depth of a beam. Thus more particularly in roof and floor constructions, where the temperature difference above and below may be high, there is a very real advantage in a construction which is inherently a good heat insulator.

I claim:

A reinforced concrete floor comprising a plurality of pre-formed reinforced masonry slabs, each slab having a recess along one longitudinal side thereof and a corresponding mating projection along the opposite side thereof, said slabs being laid side by side with the slabs in abutting relationship and the projection of each slab fitting the recess of the adjacent slab, each slab composed of a lower layer and an upper layer, the said lower layer being formed of a lightweight concrete in which are embedded longitudinal tension reinforcement elements, each of said elements being of relatively thin gage steel of angular cross-section to afford a relatively large surface for adhesion to the concrete, the said upper layer being of substantially less thickness than the lower layer and formed of a denser concrete, said upper layer including a plurality of metal reinforcing rods and a plurality of metal bonding elements comprising strips of metal threaded upon said rods, and a continuous surface layer of concrete overlying the said edge abutting slabs, said surface layer being denser than said intermediate layer, the thickness of said surface layer being less than that of the adjacent underlying layer, said strips being embedded in said intermediate and said surface layer and bonding them together, the lower lightweight concrete layer constituting the major portion of the tensile zone of said floor and being capable of carrying a substantial proportion of the tensile stresses therein.

References Cited in the file of this patent UNITED STATES PATENTS 889,249 Loveley June 2, 1908 1,685,886 Speller Oct. 2, 1928 2,103,969 Davis et al Dec. 28, 1937 2,220,349 Plumb Nov. 5, 1940 2,303,544 Goss Dec. 1, 1942 2,310,442 Knudsen Feb. 9, 1943 2,431,104 Bright Nov. 18, 1947 FOREIGN PATENTS 12,523 Great Britain May 24, 1911 424,268 Great Britain Feb. 15, 1935 498,386 Great Britain Jan. 2, 1939 613,901 Great Britain Dec. 3, 1948 268,656 Italy Oct. 24, 1929

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