GB2182324A - Solid composition for airplaced concrete - Google Patents

Abstract

Solid composition for airplaced concrete, which contains hydraulic binders, aggregate and fibres (and, optionally, solidification aids and other conventional auxiliaries and additives), is characterised in that for reduction of concrete rebound, the fibres have a staple length of at most 20 mm and thickness of at most 3 mm and are present in an amount of 0.5 to 5% relative to the weight of binder. The fibres may be of mineral, glass metal, natural organic or preferably synthetic organic origin.

Description

SPECIFICATION Solid composition for airplaced concrete The present invention relates two a solid composition for airplaced concrete, which contains hydraulic binders, aggregate and fibres and, optionally, solidification aids and other conventional auxiliaries and additives.

Such compositions can, in a manner known perse, be premixed with the mixing water or have the mixing water added to them in the spray nozzle.

A known process for applying airplaced concrete is, for example, the so-called Torkret process which is extensively used in foundation engineering, underground civil engineering and mining engineering, and for the refurbishing and repair of structures in above-ground constructions, roadmaking, tunnel construction and bridge building.

In this process, a mixture of aggregates and binders is conveyed by an airplaced concrete machine through a pipeline to the spray nozzle on site. The mixing water is fed via a separate hose to the nozzle, mixed therein with the aggregate/binder composition and sprayed (by a dry spraying process) under pressure onto shutter- ing, constructions or native surfaces, optionally reinforced with structural steel grids or a reinforcing cage.

Depending on local conditions it is also frequently the case that the mix is completely premixed, namely aggregate plus binderwetspraying process.

It is also prior artto add solidification aids, namely accelerators, which accelerate the process ofsolidifica- tion ofthe concrete mix. These solidification accelerators, which are mostly based on sodium aluminate, are added, as an approx. 30% solution, to the mixing water via a metering pump. An addition of solidification accelerators in powder form is chosen for special applications (high inrush of water) because addition in powder form is not tied to the saturation limit of the liquid solution and overdosing, as a measure against additionally issuing water, becomes possible. In that case, however, a significant decrease in strength ofthe hardened concrete must be expected.

Equally, admixture offibres to the mixture of aggregate and binder, with our without mixing water, is prior art. These admixtures are employed in orderto improve theflexural tensile strength and the modulusof elasticity of the airplaced concrete and, where appropriate, to replace the usual incorporation of structural steel grids by "co-spraying of a reinforcement".

Glass fibres having a staple length of 20-30 mm and madefrom alkali-resistant glass such as aluminium borosilicate glass, orsoda-zirconia glass are employed, since normal glass is not stable in the alkaline cement mixture and is attacked by the hardened cement paste. These glasses are substantially more expensive than normal glass.

Further, steel fibres are used, these having either been milled in a staple length of about 30 mm from steel blocks, or being steel fibres of bright drawn steel having a staple length of about 25 mm or steel fibres made of bright wire with angled ends and a staple length ofabout30 mm.

The staple length ofthefibres hitherto employed in genral varies between 20 and 30 mm, and this is also true of plastics fibres which have been experimentally admixed. When using the fibres it has hitherto proved necessary to interpose, for the admixing process, a disentangling apparatus in order to avoid the tangling of the fibres and resulting inhomogeneity of the concrete consistency.

The values shown in Table 1 below provide the proof, from statics measurements, that the set objective (namely replacement of the constructional steel grid) can be achieved.

Table 1 and the attached drawing show the strength and modulus of elasticity values for known steel fibre airplaced concretes, the starting material being a concrete of the following composition: Aggregate Particle size fraction 0/4 1170 kg/m3 Particle size fraction 4/8 300 kg/m3 Particle size fraction 8/16 450 kg/m3 Aggregate content 1920 kg/m3 Binder Cement PZ 375 365kg/m3 Bin der content 365 kg/m3 Solidification accelerator Liquid 20 kg/m3 Mixing water 80 kg/m3 TABLE 1 Steel fibre type Aggregate Sample age Apparent Compressive Static modulus about 60 kg of days density strength ofelasticity fibre materiafima kgim3 Nlmm2 Nlmm2 Withoutfibres Particle size 3 2,324 32.1 22,448 Oto16 7 2,315 35.6 23,800 28 2,329 41.5 26,400 Steel fibres milled Particle size 3 2,369 34.4 23,179 fromsteelblocks; otto16 7 2,367 36.5 23,145 staple length 28 2,361 47.7 23,919 about 30 mm Steel fibres drawn Particle size 3 2,362 34.6 24,400 from bright wires; Oto 16 7 2,371 36.9 27,683 staple length 28 2,351 50.7 25,414 about 25 mm Steelfibresfrom Particle size 3 2,373 36.6 23,700 bright wire, with Oto 16 7 2,362 32.0 27,090 angled end; staple 28 2,335 45.0 26,560 length about 30 mm Adisadvantage in the application of airplaced concrete is considered to be the generally very high amount of rebound which may be as much as 45% of the total airplaced concrete and which, because the setting process has started, can no longer be utilized.

For outputs of installations on medium and large sites of 20 m3 of sprayed material/hour, which means 160 m3offresh concrete in an 8-hour shift, up to 70 m3 of concrete per shift would be lost in thisway.

The rebound figures are determined on site immediately after the spray process. They are calculated from the weight of the rebounding sprayed material relative to the concrete which remains in place, and are quoted in percent. The weight of concrete is functionally dependent on the feed rate and the spraying time.

The hitherto customary addition - for the purpose of increasing (reinforcing) the mechanical strength - of fibres with staple lengths of 20 to 30 mm in an amount of about 15% of the weight of binder (50to 60kg/rn3 of fresh concrete) produes no reduction in rebound.

More recently, latent hydraulic puzzolanes (electrostatic filter ash from power stations or natural puzzol anes, such as trass, etc.) are added to cement as a binder supplement. The advantage of admixing latent hydraulic puzzolanes is that the density of the concrete consistency is increased, strength decreased is redu ced, dust creation during the spraying process is reduced and the conveying characteristics of the dry mix are improved.

Accordingly, in the present case hydraulic binders are, where appropriate, also to be understood as a mixture with latent hydraulic binders.

Because of the specific adhesive action of the puzzolanes in the spraying mixture, a reduction in there- bound was found with puzzolane-containing airplaced concretes. Most recent experience in site operation in the presence of puzzolanes gives mean rebound values of 23-25% by weight; around 16% by weight atthe side walls of a tunnei and about 33% in the roof section of thetunnel.

This still means a loss of around one quarterofthe airplaced concrete.

It is the object of the present invention to provide a composition for airplaced concrete in which these rebound figures are substantially reduced.

This object is achieved, according to the invention, if the solid composition orthe production of airplaced concrete contains fibres having a staple length of at most 20 mm and a thickness of at most3 mm in an amount of 0.5 to 5%, relative to the weight of binder.

It has been found, surprisingly, that a marked reduction in rebound can be achieved with a great variety of types of fibres employed, providing these have the stated dimensions. As a rule, there is a reduction byat least one-third,forexample to 10% atthe side walls, compared to the previous values of 16% atthe sidewalls.

The length ofthefibres employed is preferably less than 10 mm and in particular 2-5 mm. Their thickness is preferably 5 Fm to 1 mm, especially 5to 20 iim.

Preferably, the fibres are employed in an amount of 0.5 to 2%, relative to the weight of binder.

As fibre material it is in principle possible to use any available type offibre, for example mineral glass or asbestos fibres and metal fibres, but it is preferred to use fibres which upon friction at their boundary surfaces generate contact electricity, especially fibres which in the triboelectrical voltage series precede collodion.

Organic polymer fibres of natural of synthetic origin are best suited to this purpose. The fibres ofnatural origin include all types of cellulose fibres, regenerated cellulose, cellulose ethers or esters and also alginate and protein fibres.

A particularly strong triboelectrical charge is shown by synthetic polymer fibres, also referred to as chemi cal fibres. These include a great variety of polymers, preferablythermoplastics, such as polyolefin, poiy amide, polyester, polyurethane or polycarbonatefibres. Accordingly, polyethylene, polypropylene, poly amide, polyacrylate and acrylonitrile polymers are just as suitable as methyl cellulose or ethyl cellulose and acetyl cellulose.

Similartypes offibres, fibre blends and fibres of mixtures may be employed.

Thus use of polyacrylonitrile and/or polyesterfibres is preferred.

Waste fibres, fibres of regenerated material or fibres of recycled plastics material may also be used.

The fibre material having the length and thickness defined above ensures excellent uniform fine distribution in the sprayed material and three dimensional interconnection in the mix, with crystal formation inthe concrete microdomain being unhindered by this point reinforcement, and so-called hedgehog formation being avoided.

The fibres, after introduction into a running mixing drum, and during conveyance ofthe mixed material in the pipeline, build up a high positive electrical contactvoltagethrough friction against the aggregate and the conveying pipe wall, and this voltage, after introduction of the mixing water on site, brings out strong cohesion of the fresh concrete in the spray nozzle.

Illustrative embodiments An airplaced concrete of quality 225 was processed, its composition being shown in Table 2 below: TABLE2 Aggregate Particle fraction 014 1,170 kg/m3 Particlefraction 4/8 300 kg/m3 Particlefraction8/16 450 kg/m3 Aggregate content 1,920 kg/m3 Binder Cement PZ 375 330 kg/m3 Fly ash 35 kg/m3 Bin der con tent 365 kg/m3 Solidification accelerator, liquid 20 kg/m3 Mixing water 80 kg/m3 Fresh concrete weight 2,385 kg/m3 Mean apparent density, drilling core, 28days 2,340 kg/m3 Mean modulus of elasticity drilling core,28days 25,000 N/mm3 Mean compressive strength drilling core, 28days 38.0 N/mm2 The rebound valuesforthis concrete are about 23-25%, being 16% atthe side walls and 33% attheroof.

Upon addition offibres ofabout3 mm staple length and fibre thickness 10Fm in the amounts shown below, the rebound decreases as follows: Addition: 5% of binderweight; rebound3.7%1) 3% of binderweight; rebound 8.2% 2) 2% of binderweight; rebound 9.0%3) 1 % of binderweight; rebound 9.45% 4) 1) polymerfibres of thermoplastic polymers 2) mixture of acrylic and polyesterfibres 3) polyester fibres 4) acrylicfibres With slightly modified composition and addition of 1 or 2% per weight offibres (3 mm staple length,10,am thickness, mixture of polyacrylonitrile and polyesterfibres) thefollowing values are obtained: TABLE3 Fibres Aggregate 2% 1% Particle fraction 0/4 1,086 kg/m3 1,086 kg/m3 Particle fraction 4/8 285 kg/m3 285 kg/m3 Particle fraction 8/16 420 kg/m3 420 kg/m3 Aggregate content 1,791 kg/m3 1,791 kg/m3 Binder Cement PZ 375 300 kg/m3 300 kg/m3 TABLE3 (continued) Fibres Fly ash 45 kg/m3 45 kg/m3 Bin der content 345 kg/m3 345 kg/m3 Solidification accelerator.

liquid 15 kg/m3 15 kg/m3 Mixing water 135 kg/m3 120 kg/m3 2,286 kg/m3 2,271 kg/m3 Fibres 6.9 kg/m3 3.45 kg/m3 Fresh concrete weight 2,292.9 kg/m3 2,274.45 kg/m3 Mean apparent density, drilling core, 28 days 2,242 kg/m3 2,240 kg/m3 Mean modulus of elasticity drilling core, 28 days 16,500 N/mm3 18,100 N/mm2 Mean compressive strength drilling core, 28 days 36.0 N/mm2 37.5 N/mm2 Reboundatthewalls 9.0 % 9.4 Rebound atthe roof 16.0 % 19.6 When using plastics fibres, in particular, it has been found that upward of about 5% perweightfibre content, relative to the total binder, processing difficulties manifest themselves, consisting in a non-uniform distribution of the plastics fibres, in particular in caking ofthese.

Further, it has been found that there is a dependence of rebound on staple length of the fibres inasmuch as, for example, for addition of 1 to 3% perweight of fibres there is optimal reduction of rebound at a staple length of about 3 to 6 mm, with the rebound values increasing again at shorter and longer staple lengths.

Further, a certain dependence of mean compressive strength (at 28 days) on the staple length ofthe admixed fibres was observed, namely in the sense that this mean compressive strength, for instance for addition of 1 to 3% of fibres, decreases with increasing staple length.

The present invention is not restricted to a concrete according to Table 2 and is equally applicable to so-called base concretes, which solely contain cement as the binder. Thus, with a base concrete of which the components corresponded to those given in Table 2 except that the binder used was 365kg/rn3 ofContragress cement PZ 375, it was found that comparable percentage reduction in rebound relative to the rebound values of the concrete free from fibres were achievable.

Claims (10)

1. Solid composition for airplaced concrete, which contains hydraulic binders, aggregate and fibres and, optionally, solidification aids and other conventional auxiliaries and additives, characterized in that it containsfibres having a staple length of at most 20 mm and a thickness of at most 3 mm in an amount of 0.5 to 5%, relative to the weight of binder.

2. Composition according to Claim 1, characterized in that it contains fibres having a staple length of less than 10 mm, preferably of 2 to 5 mm.

3. Composition according to Claim 1 or 2, characterized in that it contains fibres having a thickness of 5 Fm to 1 mm, preferably of 0.5- 20 pm.

4. Composition according one of Claims 1 to 3, characterized in that it containsthefibres in an amount of 0.5to 2%, relative to the weight of binder.

5. Composition according to one of Claims 1 to 4, characterized in that it contains fibres or fibre mixtures of a single material or mixture of materials which in the triboelectric voltage series precedes collodion.

6. Composition according to one of Claims 1 to 5, characterized in that it contains, as fibres, organic polymer fibres or fibre blends, of natural or synthetic origin.

7. Composition according to Claim 6, characterized in that it contains thermoplastic fibres orfibre blends, such as polyolefin, polyamide, polyester, polyurethane or polycarbonate fibres.

8. Composition according to Claim 7, characterized in that it contains fibres or blends offibres based on cellulose, such as regenerated cellulose, cellulose ethers or cellulose esters, as well as protein fibres or alginatefibres.

9. Composition according to one of Claims 1 to 8, characterized in that it contains waste fibres, fibres of regenerated material orfibres of recycled plastics material.

10. Composition according to one of Claims 1 to 9, characterized in that it contains polyacrylonitrile and/or polyesterfibres.