IE42535B1 - Building elements of hogh thermal resistance - Google Patents

Description

This invention relates to building elements, and more particularly but not exclusively, is concerned with building elements which can be used to construct the structural walls of buildings.

The structural walls of a building, especially the outside (or external) walls, are frequently constructed as'So-called cavity walls which comprise an outer leaf and at least one inner leaf separated from the outer leaf by a cavity (or air space). It is important that the thermal transmittance of the structural, wall should be below a predetermined maximum value and many attempts have been made to provide building elements of low thermal conductivity which can be used in the inner or outer leaf of a cavity wall so as to give a structural wall of relatively low thermal transmittance.

Thermal conductivity, (k), is a measure of a material's ability to transmit heat and can be expressed as the heat flow in watts per square metre of surface area for a temperature difference of 1°C. per metre thickness; it has the dimensions Wm/m^wC or W/m°C.

Thermal resistivity (1/k) is the reciprocal of thermal conductivity, and it has the dimensions m°C/W.

Thermal resistance, (R), is the product of thermal 2o resistivity and thickness, and it has the dimensions m C/W.

Thermal transmittance, (U), is the quantity of heat which will flow through unit area in unit time per unit difference of temperature between inside and outside 20 environment, and it has the dimensions W/m C.

If a cavity wall comprises an outer leaf and one inner leaf the thermal transmittance of the cavity wall is - 2 42535 calculated as being the reciprocal of the sum of the thermal resistances of the individual leaves of the cavity wall, the inner and outer surface coverings and the air space or cavity separating the leaves of the cavity wail. Thus, for a simple cavity wall comprising an inner and an outer leaf separated hy a cavity, the thermal transmittance U, is expressed as: U = 1/ (Rsi + Rso + R1 + R2 + R3 + R4 + Rc) where li . is the internal surface resistance, si R is the external surface resistance, so R^ is the resistance of the external rendering Rg is the resistance of the internal plaster, R3 and Rj are the resistances of the inner and outer leaves of the cavity wall, and R is the resistance of the cavity, c Concrete building elements in the form of blocks or slabs are widely used in the construction of external walls, but conventional concrete has a relatively high thermal conductivity which increases with the dry bulk density and moisture content of the concrete. A common type of external wall comprises an outer wall of relatively de.nse concrete building elements wiiich give good strength but relatively low thermal resistance, and an inner wall of lightweight concrete building elements which are relatively porous and have a higher thermal resistance but lower strength as compared with the dense concrete building elements. In order to manufacture lightweight concrete building elements it is generally necessary to use a lightweight aggregate. Conventional lightweight aggregates are either manufactured by expanding, bloating or sintering 42536 , . suitable clay, shale or slate or the ash obtained by burning pulverised fuel, or by expanding or exfoliating perlite or vermiculite. Alternatively, natural pumice is used as Large amounts of natural aggregates and aggregates obtained from industrial waste or by-products are available but these are generally of high, density.’ Examples of dense aggregates from industrial waste or by-products include blast furnace slag, furnace clinker, crushed waste bricks, bricks and blocks and gravel from the waste products of the china clay and other mining industries.

According to the first aspect of the present invention there is provided a building element which is manufactured from a cementitious composition having a density greater —3 than 1200 kg m and which is formed with one or more blind cavities which open on a planar face of the building element and have a depth perpendicular to said planar face which is not substantially less than the total depth of the element perpendicular to said planar face, the total area occupied by the cavity or cavities in a section of the element taken at right-angles to said planar face in which the cavity Or cavities open being from 60% to 90% of the total area of that section of the element, whereby said building element has a relatively high thermal resistance.

The building elements of the present invention are manufactured from cementitious compositions, such as concrete or calcium silicate mixes, having a density greater than 1200 kg in , preferably greater than 1300 kg m , and most preferably at least 1700 kg m The hulk density of the aggregate used in the concrete or calcium silicate mix is preferably at least 1200 Kg m for a fine aggregate (nominal size of particles not exceeding _3 mm) or at least 950 Kg m for a coarse aggregate (nominal size of particles greater than 5 mm). Suitable aggregates are foamed slag, natural aggregates complying with British Standard 882 Aggregates from Natural Sources for Concrete, air-cooled blast furnace slag, furnace clinker, crushed clay bricks and tiles of hand-picked brick rubble, china clay by-product sand, furnace bottom ash and granulated blast furnace slag.

Generally, l.lie overall shape of the building elements of the present invention will be substantially parallelepipedic and the following preferred dimensions are based on experiments carried out with building elements of this overall shape: The depth of the or each cavity perpendicular to said planar face in which said cavity opens is preferably from 75% to 98% of the total depth of the building element in the same direction. The total volume of the cavity or cavities should not. exceed 50% of the total volume of the building element in order to accord with British Standard 2028, 1864; 1968, Clause 7, and preferably ranges from 10% to 40% of the total volume of the building element. The total width of the or each cavity in a building element of the invention, measured across the width of said planar face of the element in which said cavity opens should not exceed 65% of the total width of the building element In order to accord with British Standard 2028, 1364: 1968,Clause 7, and preferably ranges from 10% to 60% of the total width of the building element. b 42535 Advantageously, the number of cavities formed in a section ι taken parallel to said planar face in which said cavity I or cavities open ranges from one to eight.

The sides of the or each cavity are preferably planar, and the or each cavity preferably has a cross-section, in planes parallel to said planar face of the element in which the cavity or cavities open, substantially in the shape of an oblong. However, the corners of the cavity or cavities may be rounded. Preferably, the oblongs have a length which decreases progressively in planes progressively further away from said planar face of the element in which the cavity or cavities j open. Preferably, the oblongs have a width of from 5 to 25 mm. It has been found that the thermal resistance of the cavities increases with the width of the cavity up to a width of about 20 mm, but beyond a width of 20 mm there is little or no change in thermal resistance with increasing width.

Usually, the longer dimension of the oblong cross-section of the cavity or cavities lies in a plane which is parallel to that face of the building element which is intended to form, part of the external surface of a wall. In a preferred embodiment'· of the invention, a building element is constructed with two pairs of parallel, oblong rectangular cavities formed along the horizontal axis of the face of the element as intended to be laid in a wall. The two pairs of cavities are of unequal length and the inner sides of the longer pair of cavities coincide with the plane which cuts the element parallel to, and midway between, the two end faces of the element as intended to be laid'in a wall.

This arrangement facilitates the accurate cutting of the - 6 42535 element Into two halves and ensures that tlie half element containing (he shorter pair of cavities has the cavities approximately centrally disposed. Another embodiment of the invention has one pair of cavities very much longer than the other pair so that the inner sides of the longer pair of cavities coincide with the plane which is parallel to the two end faces of the element and divides the element into one third and two thirds of its length. Such a block facilitates the construction of quoins (corners).

The building elements of the present invention can be manufactured using a mould which is provided with formers of appropriate number and shape to give the desired cavities.

It. may be advantageous to fill one or more of the cavities in a building element according to the present invention with a foam material, for example a plastics foam material such as a .urea-formaldehyde foam, a polystyrene foam, a polyurethane foam, a polyethylene foam or a poly (vinyl chloride) foam. The foam may be introduced into the cavities in a known manner.

According to the second aspect of the invention there is provided an exterior wall for a building, which wall comprises an inner and an outer leaf separated by an air space or cavity, both the inner and the outer leaf including building elements according to the first aspect of the invention.

The thermal transmittance of a wall constructed from building elements according to the invention and after a conventional rendering has been applied to the exterior face of the outer leaf and conventional plaster or plaster board - 7 42535 has been applied to the interior lace of the inner leaf is found to toe within acceptable limits. An exterior wall may be constructed in accordance with the second aspect of the invention at a lower cost as compared with the conventional type of exterior wall which comprises an outer lehf of solid building elements of relatively high density and low thermal resistance and an inner leif of lightweight elements to increase the total thermal resistance of the cavity wall thus produced. The cost of the lightweight elements is so high that a cavity wall comprising two leaVfes constructed with elements in accordance with the first aspect of the present invention is less expensive than a wall constructed in the conventional manner.

For a better understanding of the invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing in which: Figure 1 is a plan view of the bottom face of a building element in accordance with the first aspect of the invention; and Figure 2 is a sectional elevation on the line IIII of Figure 1.

A concrete building element 1 has a front face 2, a back face 3, a top face 4, a bottom face 5 and end faces 6 and 7. Two pairs of cavities 8 and 9,and 10 and 11 are formed in the block with their openings in the bottom face.

The cavities have tapering sides in order to facilitate withdrawal of the formers used in their manufacture. The cavities 8 and 9 are longer than the cavities 10 and 11, the 10 inner sides of the cavities 8 and 9 coinciding with the plane which cuts the block parallel to, arid midway between, the two end faces 6 and 7. Each cavity has a width A; cavities 8 and 9 have a mean length B;cavities 10 and 11 have a mean length C, and each cavity has a depth D. The building element has an overall length L, a depth M and a width, or thickness, N.

The invention is further illustrated by the following Example.

EXAMPLE Seven concrete building elements in accordance with the invention were manufactured to the general design shown in Figures 1 and 2 but with four different sets of dimensions and three different concrete densities. Each building element was pressed in a mould in which there were suspended two formers cut from flat steel plate, each former making two cavities. The specifications of the seven blocks are set forth in Table 1.

TABLE 1.

No: Width N (mm) Cavity Width A (mm) Concrete Density (kg.m-3) Composition Type 1 100 10 1700 A 2 100 12 1700 A 3 100 12 1750 B 4 100 20 1750 B 5 112 12 1750 B (1 100 20 1850 C 7 112 12 1850 C In each case the length L of the building element was 440mm. the depth M was 215mm., the cavity depth D was 200mm. and the mean cavity lengths B and C were 194.3mm and 164,3tnm. respectively. The compositions of the three concretes used 42335 were as follows: Concrete Dry Density (kg.m ) Parts by volume Cement Parts by volume Fi ne Aggregate (-5 mm) Parts by vol. Coarse Aggregate (+ 5mm) A 1700 1 5 3 B 1750 1 8 1 C 1850 1 7 3 ΙΟ The Dry Bulk density of the fine aggregate (i.e. smaller _3 than 5mm) was 1250 kg.m .

The Dry Bulk density of the coarse aggregate (i.e. larger _3 than 5mm) was 1450 kg.m The coarse and fine aggregate were both derived from china clay by-product sand. The effective thermal resistance (Rj^) for tho .inner leaf of a cavity wall, in ° C/W, may be calculated by means r* of the following formula:f IL Fs FE where tv is the width of a cavity in metres. t is the width N of a building element in the leaf in metres.

Nvis the number of cavities disposed across the width of the building element R is the thermal resistance of a cavity'in the v building element.

F Is the fraction of the cross-sectional area of the vbui.lding element on the line Tf-II of Figure 1 which is occupied by cavity. i.e. T'v - (cavity length B + cavity length C) x cavity depthD length L x depth M = (194.3 + 164.3) x 200 = 0.758 440 x 215 Fg is the fraction of the cross-sectional area of the building element on the line II-II of Figure 1 which is solid i.e. F = 1-F„ 1 - 0.758 _ 0.242 s v K is the corrected thermal conductivity of the concrete in W/m°C o Ryis the thermal resistance of one cavity in m C/w Rc is the thermal resistance of a normal cavity between the two leaves of a cavity wall and is constant at 0.18 m C/W R^ is the thermal resistance of a layer of dense plaster 2 θ of thickness 13 mm and is constant at 0.027 m C/W R . is the thermal resistance of the inner surface of the si wall and is constant, at 0.123 m2 ° C/W The values of K for concrete of density 1700, 1750 and 1850 kg -3 m when used in (1) the outer leaf of a cavity wall, i.e. exposed to rain (estimated moisture content 5% by volume), and (2) the inner leaf of a cavity wall, i.e. not exposed to rain (estimated moisture content 3% by volume) are set forth in Tahle If below.

TABLE II Concrete_gensity Thermal Conductivity (W/m °C) (kg. m )_ Outer Leaf_ Inner Leaf 1700 0.84 0.76 1750 0.90 0.82 1850 1.03 0.93 The values of Rv for cavities of width A equal to 20mm, mm. and 10mm. are respectively 0.18 m2 0 C/W, 0.16 m 0 c/W and 0.15 m2 ° C/W.

Similarly, the thermal resistance (¾^) for the outer leaf of a cavity wall, in m2 °C/W, may be calculated by means oi the above formula but substituting R for R , so si and Rj for R^ wherever they appear. R^ is the thermal 114 2 5 3 5; resistance of two coats or rendering having a total thickness of 16 mm, assumed Constant at .0.-025 m2 °C/W.

R is the thermal resistance of the outer surface of the so * 2 o wall and is constant,at 0.055 m C/W.

The values of RTT and R ' were calculated for ±Jj UL· the outer and inner leaves of a cavity wall according to the formula given above. Cavity walls were constructed using the building elements 1 to 7 in both the inner and outer leaves and the thermal transmittance (U values) was io calculated for each wall according to the formula: u = 1/ il + R0L) and the results are set forth ’ in Table III below TABLE III Building element Thermal Resistance Outer leaf rol R(m2 °C/W) Inner .leaf ril Thermal Transmittance U (W/mz °C) . 1 0.459 0.548 0.993 2 0.467 0.556 0.97820f 3 0.459 0.547 0.995 4 0.480 0.557 0.965 5 0.474 0.562 0.965 6 0.456 0.544 1.000 7 0.458 0.547 0.996 25 By comparison, a cavity wall with identical renderin and dense plaster but constructed with an outer leaf of conventional concrete blocks having a length L of 440mm, a depth M of 215mm, and a width N of 100mm and manufactured from —3 concrete of density 1750 kg m and an inner leaf of conventional lightweight concrete bldcks having the same dimensions as the dense concrete blocks but manufactured from -3 i concrete of density 900 kg.m had a value for RQL of 0.281 | m20C/W, a value for R^ of 0.610 m2<3C/W and a value for 0 of 1.122 W/m20 C.A wall with a thermal transmittance not greater than 1 is considered to have acceptable insulating properties under recent codes of practice in the ϋ,Κ. for building. _ 12 _

Claims (15)

CLAIMS:

1. A building element which is manufactured from a cementitious composition having a density greater than 1200 kg m and which is formed with one or more blind cavities which open on 5 a planar face of the building element and have a depth perpendicular to said planar face which is not substantially less than the total depth of the element perpendicular to said planar face, the total area occupied by the cavity or cavities in a section of the element taken at right10 angles to said planar face in which the cavity or cavities open being from 60% to 90% of the total area of that section of the element, whereby said building element has a relatively high thermal resistance.

2. A building element according to claim 1, wherein said 15 cementitious composition has a density of at least 1700 kg m \

3. A building element according to claim 1 or 2, having an overall shape which is substantially a parallelepiped.

4. A building element according to claim 3, having an overall shape which is substantially a rectangular parallelepiped. 20

5. A building element according to any of claims 1 to 4, wherein the sides of the or each cavity are planar, and wherein the or each cavity has a cross-section) in planes parallel to said planar face in which the cavity or cavities open, substantially in the shape of an oblong. 25

6. A building element according to claim 5, wherein the oblongs have a length which decreases progressively in planes progressively further away from said planar face in which the cavity or cavities open. 13

7. A building element according to claim 5 or 6, wherein the oblongs have a width of from 5 to 25 mm.

8. A building element according to any of claims 4 to 7, wherein the total width of the cavity or cavities, measured across the width of said planar face in which the cavity or cavities open, does not exceed 65% of the total width of the element.

9. A modification of the building element according to any of claims 4 to 8, wherein the corners of the cavity or cavities are rounded.

10. A building element according to any of claims 1 to 9, Wherein the depth of the or each cavity, perpendicular to said planar face in Which the cavity or cavities open, is from 75% to 98% of the total depth of the element in the same direction.

11. A building element according to any of claims 1 to 10, wherein the total volume of the cavity or cavities does not exceed 50% of the total volume of the element.

12. A building element according to any of claims 1 to 11, wherein the number of cavities formed in a section taken parallel to said planar face in which the cavity or cavities open does not exceed eight.

13. A building element according to any of claims 1 to 12, wherein one or more of the cavities are filled with a foam material.

14. A building element substantially as hereinbefore described with reference to the accompanying drawings and/or in the foregoing Examples.

15. An exterior wail of a building, which wall comprises an inner and an outer leaf separated by an air space or cavity, both the inner and the outer leaf including building elements according to any of claims 1 to 14. '