GB2142619A - Self-levelling cementitious compositions - Google Patents

Abstract

Highly liquid self-levelling or semi-self-levelling cementitious compositions for use in forming concrete structures such as sub-flooring include hydraulic cement, a cement replacement which expands on hydration, sand, water, air-training admixtures and water-reducing admixtures, and optionally mono filament polypropylene fibres.

Description

SPECIFICATION Self-leveling cementitious compositions This invention relates to cementitious compositions. More particularly, it relates to liquid, self-leveling or semi-self-leveling cementitious compositions for use in forming non-shrinking concrete structures such as subflooring and the like for construction purposes and to methods for making same.

In most commercial structures, particularly multi-story structures, the structural flooring may be rough, uneven and/or include channels or conduits formed therein or of dissimilar materials to provide for electrical, communications or temperature controls. Accordingly, a subflooring is usually applied over the structural floor to provide a relatively smooth and level surface to which floor coverings such as carpet or tiles, may be directly applied. Such subflooring may take various forms. For example, a layer of plywood or the like may be applied as subflooring to provide a smooth, continuous subflooring surface. Alternatively or in addition thereto, depending on the nature of the structural floor support, a cementitious or gypsum subflooring may be applied.However, to provide a smooth, level, continuous subfloor of plywood or the like has proven economically impractical in terms of materials, manpower and construction time. Gypsum subflooring, while less expensive, is sorely lacking in structural strength and cannot be fully hydrated without the application of heat. Thus it tends to crack and rupture if if subjected to moderate stresses and may actually bleed free water unless special precautions are taken to fully hydrate a wet mixture applied in liquid form. Cementitious subflooring is thus preferred because of its structural strength and durability and because it may be fully hydrated at room temperature. Unfortunately, prior cementitious materials are not of sufficient liquidity to be a self-leveling and thus must be troweled into place. This process is obviously labour intensive and time consuming.Furthermore, great care must be taken to form smooth and level subfloor surfaces. Since hydraulic cement mixtures shrink as they cure, thin layers of concrete also often suffer from a severe tendency to shrink and crack. Fine cracks may not be disastrous but, because the subflooring is usually very thin the support therefor not totally rigid, flexing of the subflooring often causes uplifting action generally causes further cracking which ultimately results in wide-spead fracture, thus rendering conventional subflooring produced thereby less than fully satisfactory. Obviously, a high strength subflooring which obviates the above limitations is highly desirable and has long been sought by the construction industry.

In accordance with the present invention, a cementitious composition is provided which not only forms a concerete structure of high strength and relatively low weight, but the liquid mix is of sufficient liquidity and low enough viscosity that it may be pumped directly onto the structural floor and be self-leveling or at least semi-self-leveling to form a smooth, level surface without troweling or the like. Furthermore, the composition sets up to form a high strength subflooring structure without significant volumetric shrinkage during curing and which adheres to the structural flooring to form a hard subflooring surface particularly suitable for the application of floor covering materials directly thereto. The composition fully hydrates to set up and harden without bleeding of water therefrom.Therefore, it may be applied in most construction situations without fear of water damage to structural flooring or wall compositions. Since the cementitious composition of the present invention is at least semi-self-leveling, time consuming and labour intensive processes such as troweling and the like are virtually eliminated; thus resulting in vast savings in time and expense involved in providing a smooth, level subflooring. Furthermore, since shrinkage cracking is virtually eliminated, the resultant uplift cracking and fracturing is likewise virtually eliminated.

The following disclosure is offered for public dissemination in return for grant of a patent. Although it is sufficiently detailed to provide full understanding of the principles of the invention, this disclosure is not intended to prejudice the purpose of a patent which is to protect each new inventive concept therein no matter how others may later disguise it by variations in form, additions or further improvements.

For convenience and clarity of illustration, the invention is described herein with particular reference to batch forming of compositions particularly suitable for forming subflooring. It will be appreciated, however, that the invention is not so limited. Instead, the compositions as disclosed herein may find particular utility in other constructions and may, of course, be formed in batches of larger or smaller quantities of similar proportions or in continuous mixing processes. It will further be realized that the constituents of the composition may be pre-mixed in dry form and water added thereto at or near the job site in the proper proportions to form the ultimate liquid composition desired.

The compositions of the present invention may be formed using commercially available products even though the exact chemical compositions of some of the commercial products may not be readily available.

Thus the preferred compositions described herein are in terms of batch mixes employing representative commercially available component ingredients known for their specific characteristics. Other admixtures having similar characteristics manufactured by other suppliers may, of course, be substituted for those specified herein to produce similar results without departing from the inventive subject matter disclosed and claimed.

The basic composition of the invention may be generally described as a mixture of hydraulic cement, sand, water, a cement replacement whih hydrates and bonds with hydraulic cement in the hydrated form but which expands on hydration, a water-reducing admixture and an air-entraining admixture. As in conventional cementitious compositions, the compositions of the present invention may also include chloride or non-chloride additives to lower the temperature working range of the composition and may also include admixtures which accelerate hydration and thus accelerate set-up times.

Example 1 The preferred composition of a wet mix in accordance with the present invention is set forth in Table I. The amounts of ingredients specified in Table I are the amounts required to produce one cubic yard of liquid mix.

TABLE I Hydraulic cement 538 Ib. (244 Kg) Sand 2,036 Ib. (922 Kg) Water 57 gal. (US) 216 (dm3) Cement replacement (CHEM-COMP II) 73 Ib. (33 Kg) Air-entraining admixture (MB-VR) 19.5 oz. (553 gr) Water-reducing admixture (POZZOLITH 400-N) 104 oz. (2948 gr) Set-up accelerator (POZZOLITH 122-HE) 325 oz. (9214gr) The hydraulic cement used in the compositions of the invention may be Type I or, if high early strength is required, Type Ill may be used. The sand, of course, is dry sand which meets ASTM C-133 requirements. If the sand is wet, appropriate conventional calculations should be made to adjust both the sand and water proportions of the mix.

One of the essential ingredients of the compositions of the invention is a material to prevent shrinkage cracking. This is supplied in a cement replacement or substitute which meets ASTM C-150 requirements for cement and which directly substitutes for hydraulic cement in the hydration process but which expands on hydration. Since wet hydraulic cement mixes shrink on hydration, the volumetric reduction may cause the concrete to crack during or after curing, particularly where the cementitious mix is used to form a thin layer as in subflooring. However, by substituting a cement substitute as described above for a portion of the hydraulic cement, a wet mixture can be formed which suffers virtually no volumetric shrinkage when cured.

In fact, by appropriate replacement of the hydraulic cement, compositions may be produced which actually expand on curing. a commercially available cement replacement which meets all the above requirements is available from Master Builders Division of Martin Marietta, Cleveland, Ohio, under the trade names CHEM-COMP II and CHEM-COMP Ill. CHEMP-COMP III is a type K cement as described in United States letters Patent No. 4,419,136 which meets ASTM C-845 requirements. CHEM-COMP II is essentially the same as CHEM-COMP II but contains approximately five percent (5%) by weight Type II Portland cement.

It will be observed that the combined weight of hydraulic cement and CHEM-COMP II specified in Table I is six hundred and eleven pounds (277 Kg). Thus the wet composition of Table I is equivalent (in terms of cement composition) to a six and one-half sack mix with twelve percent (dry weight) of the hydraulic cement replaced by CHEM-COMP II. The precentage of hydraulic cement replaced by the CHEM-COMP II (or an equivalent cement replacement) may vary from as little as eight percent to as much as eighteen percent without serious detrimental effects. The major factors contributing to the amount of cement replacement used are cost and volumetric shrinkage permissible. Obviously, cost increases with increased percentage of cement replacement while volumetric shrinkage increases with decreasing percentage of cement replacement.Similarly, the cement (hydraulic cement plus cement replacement) concentration of the wet mix may also be varied as desired from the six and one-half sack mix set forth in Table I. Since the cement replacement meets ASTM C-150 requirements for cement, variations in total cement concentration will generally result in variations in final product properties, eg, strength, as in conventional concrete. In the six and one-half sack mix, twelve percent CHEM-COMP II has been found to produce a mix which suffers, essentially no volumetric shrinkage on curing and is thus the most preferred composition.

Air-entraining admixtures are conventionally used in cementitious compositions to reduce weight, to increase plasticity and workability of the wet mixture, and to impart other known desirable features to concrete. In the preferred composition of the invention, sufficient air-entraining admixutre is included to provide approximately fifteen percent (by volume) air in the cured concrete. This is accomplished in the composition of Table I using nineteen and one-half ounces (553 gr) of an air-entraining admixture commercially available from Master Builders Division of Martin Marietta, Cleveland, Ohio under the trade name MB-VR. The product known as MB-VR is a liquid solution of sodium dodecylbenzenesulphonate and is preferred because of its ease of handling. Other air-entraining compositions of similar characteristics may, of course, be used.However, MB-VR in the quantities specified in Table I has been found to be particularly satisfactory for producing a fifteen percent air composition. The air composition may be varied, of course, as desired from as little as about seven percent or less to as much as twenty-two percent or more. Since the amount of air entrainment is essentially proportional to the concentration of MB-VR in these ranges, this may be accomplished by varying the amount of MB-VR added from as little as ten ounces or less to as much as thirty ounces or more in a one cubic yard mix.

Water-reducing admixtures have conventionally been used in cementitious compositions to reduce the water content required at equal slump. Since one objective of the present invention is to produce a highly liquid composition which is at least semi-self-leveling (very high slump) without the addition of excess water which is not used in hydration, a water-reducing admixture is an essential ingrediant of the composition.

while various water reducers are commercially available, the admixture sold by Master Builders division of Martin Marietta, Cleveland, Ohio, under the trade name POZZOLITH 400-N has been found particularly suitable. The product known as POZZOLITH 400-N is essentially the sodium saltofformaldehyde- naphthalene H2SO4. Other compositions of equivalent characteristics may, of course, be substituted therefor. The amount of POZZOLITH 400-N in the composition of Table I may vary from as little as seventy-eight ounces (2211 gr) or less to as much as one hundred thirty ounces (3686 gr) or more per cubic yard (0.76 m3), depending on acceptable variations in water content and liquidity of the wet mix.

Since one of the objectives of the invention is to reduce over-all construction time, and further since the composition is to be applied in a highly liquid form, it is desirable that set-up (initial curing) be accelerated as much as practical without detrimental effects. Admixtures for accelerating cure time and providing high early strength are conventionally used in cementitious mixes for this purpose. In the composition of Table I a set-up accelerator commercially available from Master Builders Division of Martin Marietta, Cleveland, Ohio under the trade name POZZOLITH 122-HE is specified. The product known as POZZOLITH 122-HE is essentially a liquid solution of sodium resinate. The amount of set-up accelerator may, of course, be varied depending on set-up requirements and working temperatures.Three hundred and twenty-five ounces (9214 gr) of POZZOLITH 122-HE per cubic yard ( 0.76 m3) of mix in the six and one-half sack mix of Table I has been found suitable for producing a set-up time of about two to about five hours where the working temperature is from about room temperature to about 90"F (32"C).

Where the working ambient environement is both hot and dry, the set-up accelerator may be eliminated.

Slightly more accelerator may be used where the working ambient is colder and/or includes high humidity.

The amount of POZZOLITH 122-HE may be varied, therefore, from as little as none to as much as three hundred and seventy-five ounces (10631 gr) per cubic yard (0.76 m3) depending upon working conditions and set-up times required.

In the most preferred composition of the invention as specified in Table I, fifty-seven gallons (US) (215 dm3) of water are used to provide a water/cement (hydraulic cement plus cement replacement) ratio of about seventy-eight percent.

The composition provides a highly liquid mix which may be pumped directly onto the area where a subflooring is desired. Bulk spreading, as with a rake, broom, brush orthe like, may be required to rapidly disperse the bulk mix over the entire surface. However, after the initial bulk spreading, the mix is sufficiently liquid to be essentially self-leveling and fills voids, cracks, depressions and the like to form a smooth level surface over the entire area without troweling. Where the composition of Table I is used as a subflooring, a one cubic yard (0.76 m3) batch will, when properly spread, form a subflooring three-fourths inch (1.9 cm) thick over an area of approximately four hundred and thirty-two square feet (40 m2).

The water content can be varied within limits as desired, depending on the workability required of the mix and the working ambient environement. It is, of course, desirable that the water content of the mix not greatly exceed the proportion necessary to complete hydration of the cement after allowance for evaporation, etc. Where a more viscous mix is desired, the water content can be as low as fifty-five percent (approximately forty gallons (US) (151 dm3), per cubic yard (0.76 m3)) and for a highly liquid mix for use in dry, hot ambients the water content may be as high as eighty-seven percent (approximately sixty-four gallons (US) (227 dm3) per cubic yard (0.76 m3)).

As with other cementitious mixes, the working temperature of the composition of the invention can be substantially lowered by addition of calcium chloride in the range of about one to three percent of the cement (hydraulic cement plus cement replacement). Other non-chloride additives may, of course, be used as required.

From the foregoing it will be observed that cementitious compositions formed using the components described within the ranges specifid are particularly adapted for forming subflooring because of their unique highly liquid, self-leveling characteristics. Thus labor intensive and time consuming troweling into place is virtually eliminated. Furthermore, since the wet mix composition suffers virtually no volumetric shrinkage upon hydration, thin layers of concrete of high strength can be formed without cracking. Because of these unique features, the composition is particularly suitable for forming subflooring as described. This feature may likewise be particularly suitable for forming other concrete structures such as stairways, balconies and the like and even totally pre-fabricated structures for erection in panel form such as walls and roof sections.

Although the most preferred composition of the invention is particularly described using precise amounts of certain commercial admixtures, it will be readily appreciated that the invention is not limited to use of only those precise commercial preparations. Instead, other materials which are functionally equivalent to impart the same characteristics to the mix may be substituted therefor without departing from the inventive concepts disclosed. For example, where CHEM-COMP II is set forth in Table I, any cement replacement which replaces hydraulic cement in hydration but expands on hydration may be used. Accordingly, for purposes of this disclosure and the claimed invention, CHEM-COMP II and CHEM-COMP III are meant to include these particular commercial compositions and any substantial equivalents thereof in the appropriate proportions.

Likewise, the air-entraining admixture MB-VR, the water-reducing admixture POZZOLITH 400-N and the set-up accelerator POZZOLITH 122-HE are disclosed as the most preferred ingredients in the amounts thereof specified for best results. However, those terms as used in the claimed invention are meant to include those specific compositions as weil as any substantial equivalent thereof in the appropriate proportions to provide the same functional effects.

It should be particularly noted that some of the named commercial compositions are ordinarily supplied in liquid form. It may, of course, be advantageous to pre-mix the entire compositon of the invention (except water or water and sand) in dry form so that a liquid cementitious mix may be readily formed as required at or near the job site merely by the addition of water or water and sand and physical mixing. In this case dry equivalents of the commercial ingredients would be preferred and are intended to be within the scope of the invention disclosed and claimed.

Example 2 This is an example of a dry composition and the ingredients set forth in Table II respresentthe active ingredients of the mix of Table I excluding the first three items (hydraulic cement, sand and water) and may be packaged and sold as a dry mix to be added to water, hydraulic cement and sand at the job site.

TABLE II Approximate Weight Percent CHEM-COMP lil 11.589 calcium chloride 1.621 Na salt of formaldehyde naphthalene H2SO4 0.515 sodium resinate 0.024 Type Ill Portland cement 86.251 Example 3 The composition set forth in Table III is also a dry composition which may be added to appropriate amounts of water, sand and hydraulic cement at the job site to provide the mix of Table I or variations thereof.

TABLE III Approximate Weight Percent CHEM-COMP lil 71.58 calcium chloride 25.26 Na salt of formaldehyde napthalene H2SO4 2.63 Na dodecylbenzenesulphonate 0.53 It will be observed that the active indgredients in the above compositions are supplied in a dry mixture form so that the mixture may be added to appropriate compositions of sand, water and hydraulic cement at the job site and thus produce the final desired wet mix.

In each of the foregoing examples of compositions, the cement substitute which expands on hydration is included to cooperate with the other ingredients to provide a non-shrinking subflooring. The flexural strength of such subfloorings may be increased bythe addition of certain fibrous materials thereto. For example, the addition of monofilamentfibers of polypropylene in appropriate amounts has been found to increase the flexural strength of thin concrete layers by as much as twenty-five percent. In the preferred embodiment approximately twelve ounces (340 gr) of sixteen denier monofilament polypropylene fibers approximatley three4ourths inch (1.9cm) long are added to one cubic yard (0.76 m3) of wet mix. In some situations, a suitable flooring is formed using the polypropylene fibers in lieu of the cement substitute. Since the subflooring is applied in relatively thin layers, shrinkage cracking may still occur. However, the mixture formed is still self-leveling and the fibers add such flexural strength that uplift fragmentation caused by shrinkage cracking is eliminated. Accordingly, it is to be understood that while the invention has been described with particular reference to specific embodiments thereof, the forms of the invention shown and described in detail are to be taken as preferred embodiments of same. Various changes and modifications may be resorted to without departing from the spirit and scope of the invention as defined by the appended

Claims (1)

1. claims.

   1. A substantially self-levelling cementitious liquid composition comprising: (a) hydraulic cement; (b) a cement replacement which expands on hydration; (c) sand; (d) water; (e)an air-entraining admixture; and (f) a water-reducing admixture.

   2. A composition as claimed in claim 1 in which the said hydraulic cement and the said cement replacement combined are present in the said composition in an amount equivalent to approximately six hundred and eleven pounds (277 Kgs) per cubic yard (0.76 m3) of mix.

   3. A composition as claimed in Claim 1 or Claim 2 in which the said cement replacement comprises from about eight percent to about eighteen percent of the total combined dry weight of the said hydraulic cement and the said cement replacement.

   4. A composition as claimed in any one of Claims 1 to 3 in which the ratio by weight of water to the combined weight of cement and cement replacement is from about 0.55 to about 0.87:1.

   5. A composition as claimed in any one of Claims 1 to 4 in which the said air-entraining admixture is present in the said composition in an amount sufficient to produce cured concrete which is from about seven percent to about twenty percent air by volume.

   6. A composition as claimed in any one of Claims 1 to 5 in which the said air-entraining admixture is present in an amount equivalent to from about ten ounces (284 grs) to about thirty ounces (852 grs) of MB-VR per cubic yard (0.76 m3) of mix.

   7. A composition as claimed in any one of Claims 1 to 6 in which the said water-reducing admixture is present in an amount equivalent to from about seventy-eight ounces (2211 grs) to about one-hundred and thirty ounces (3686 grs) of POZZOLITH 400-N per cubic yard (0.76 m3) of mix.

   8. A composition as claimed in any one of Claims 1 to 7 further including a set-up accelerator in an amou nt equivalent to from a bout zero to about th ree hundred and seventy-five ounces (10631 grs) of POZZOLITH 122-HE per cubic yard (0.76 m3 of mix).

   9. A composition as claimed in any one of Claims 1 to 8 further including calcium chloride in an amount equal to about one percent to about three percent of the combined weight of the said hydraulic cement and the said cement replacement.

   10. A composition as claimed in any one of Claims 1 to 9 further including approximately twelve ounces (340 grs) of approximately sixteen denier monofilament polypropylene fibres per cubic yard (0.76 m3) of mix.

   11. A substantially self-levelling cementitious liquid composition comprising: (a) approximately five hundred and thirty-eight pounds (244 Kgs) of hydraulic cement per cubic yard (0.76 m3) of mix; (b) approximately two thousand and thirty-six pounds (1013 Kgs) of sand per cubic yard (0.76 m3) of mix; (c) approximately fifth-seven gallons (US) (216 dm3) of water per cubic yard (0.76 m3) of mix; (d) approximately seventy-three pounds (33 Kgs) of CHEM-COMP II per cubic yard (0.76 m3) of mix; (e) approximately nineteen and one-half ounces (553 grs) of MB-VR per cubic yard (0.76 m3) of mix; and (f) approximately one hundred and four ounces (2948 grs) of POZZOLITH 400-N per cubic yard (0.76 m3) of mix.

   12. A composition as claimed in Claim 11 including approximately three hundred and twenty-five ounces (9214 grs) of POZZOLITH 122-HE per cubic yard (0.76 m3) of mix.

   13. A composition as claimed in Claim 11 or Claim 12 including approximately twelve ounces (340 grs) of approximately sixteen denier monofilament polypropylene fibres approximately three-fourths inch long (1.9 cms) per cubic yard (0.76 m3) of mix.

   14. A substantially self-levelling cementitious composition comprising: (a) hydraulic cement and cement replacement in a combined amount equivalent to approximately six hundred and eleven pounds (277 Kgs) of hydraulic cement per cubic yard (0.76 m3) of mix; (b) from about forty gallons (US) (151 dm3) to about sixty gallons (US) (227 dm3) of water; (c) an air-entraining admixture in an amount equivalent to from about ten ounces (284 grs) to about thirty ounces (852 grs) of MB-VR per cubic yard (0.76 m3) of mix; (d) a water-reducing admixture in an amount equivalent to from about seventy-eight ounces (2211 grs) to about one-hundred and thirty ounces (3686 grs) of POZZOLITH 400-N per cubic yard (0.76 m3) of mix; and (e) sand.

   15. A composition as claimed in Claim 14 in which the said cement replacement is present in an amount equivalent to from about forty-nine pounds (22 Kgs) to about one hundred and ten pounds (50 Kgs) of CHEM-COMP II per cubic yard (0.76 m3) of mix.

   17. A composition as claimed in claim 1 substantially as specifically described in Example 1.

   18. A dry composition for use in forming a substantially self-levelling cementitious liquid composition when mixed with water, sand and hydraulic cement comprising: (a) a cement replacement which expands on hydration; (b) an air-entraining admixture; and (c) a water-reducing admixture.

   19. A composition as claimed in Claim 18 further including calcium chloride.

   20. A composition as claimed in Claim 18 or Claim 19 in which the said air-entraining admixture is sodium dodecylbenzenesulphonate and the said water-reducing admixture is the sodium salt of formaldehyde-naphthalene H2SO4.

   21. A composition as claimed in Claim 20 in which: (a) the said cement replacement is CHEM-COMP III and comprises approximately 71.6 percent of the said dry mixture by weight; (b) the said sodium dodecylbenzenesulphonate comprises approximately 0.5 percent of the said dry mixture by weight; (c) the said sodium salt of formaldehydenaphthalene H2SO4 comprises approximately 2.6 percent of the said dry mixture by weight; and (d) the said calcium chloride comprises approximately 25.3 percent of the said dry mixture by weight.

   22. A dry compsition for use in forming a substantially self-levelling cementitious liquid composition when mixed with water, sand and hydraulic cement comprising: (a) a cement replacement which expands on hydration; (b) calcium chloride; (c) a water-reducing admixture; and (d) a set-up accelerator.

   23. A composition as claimed in Claim 22 further including Type III Portland cement.

   24. A composition as claimed in Claim 23 in which: (a) the said cement replacement is CHEM-COMP III and comprises approximately 11.59 percent of the said dry mixture by weight; (b) the said calcium chloride comprises approximately 1.62 percent of the said dry mixture by weight; (c) the said water-reducing admixture is the sodium salt of formaldehyde-naphthalene H2SO4 and comprises approximately 0.52 percent of the said dry mixture by weight; (d) the said set-up accelerator is sodium resinate and comprises approximately 0.02 percent of the said dry mixture by weight; and (e) the said Type III Portland cement comprises approximately 86.25 percent of the said dry mixture by weight.

   25. A composition as claimed in Claim 18 or Claim 22 substantially as specifically described in Example 2 or Example 3.

   26. A substantially self-levelling cementitious liquid compositon comprising: (a) hydraulic cement; (b) monofilament polypropylene fibres; (c) sand; (d) water; (e) an air-entraining admixture; (f) a water-reducing admixture.

   27. A composition as claimed in Claim 26 in which the said air-entraining admixture is present in an amount equivalent to from about ten ounces (284 grs) to about thirty ounces (852 grs) of MB-VR per cubic yard (0.76 m3) of mix.

   28. A composition as claimed in Claim 26 or Claim 27 in which the said water-reducing admixture is present in an amount equivalent to from about seventy-eight ounces (2211 grs) to about one-hundred and thirty ounces (3686 grs) of POZZOLITH 400-N per cubic yard (0.76 m3) of mix.

   29. A composition as claimed Claim 26,27 or 28 further including a set-up accelerator in an amount equivalent to from about zero to about three hundred and seventy-five ounces (10631 grs) of POZZOLITH 122-HE per cubic yard (0.76 m3) of mix.

   30. A composition as claimed in Claim 26,27, 28 or 29 further including calcium chloride in an amount equal to about one percent to about three percent of the combined weight of the said hydraulic cement and the said cement replacement.

   31. A composition as claimed Claim 26,27,28,29 and 30 further including approximately twelve ounces (340 grs) of approximately sixteen denier monofilament polypropylene fibres per cubic yard (0.76 m3) of mix.

   32. A composition as claimed in Claim 31 in which the polypropylene fibres are approximately three-fourths inch (1.9 cms) long.