GB2603841A - A concrete composition and method of manufacture thereof - Google Patents

Abstract

A method of producing a pre-concrete mix with materials comprising gravel with a particle size between 4-10mm, sand with a particle size under 4mm, cement, ground granular blast-furnace slag (GGBS), the ratio between the GGBS and cement being between 36-50% by weight, and fresh water. The water to cement ratio is preferably maintained at around 0.3:10, and the moisture level between 68-74%. Further additives may include accelerators, plasticizers, workability extenders, microsilica and microfibres. The microsilica or silica fume may be added as a 50/50 mix with water as a slurry and may comprise 8-15% of the mix by weight. The microfibers may be of polypropylene or glass fibre, and may have an added amount corresponding to 6kg/ 3/4m3 of concrete.

Description

A Concrete Composition and Method of Manufacture Thereof

Field of the Invention

The present invention relates to a composition for a concrete material. Additionally, the invention relates to a method of forming a concrete composition.

Background to the Invention

The product now known as concrete has been used in construction for over 2000 years to produce dwellings, public buildings, bridges, aqueducts etc. In principle, the process of manufacture is relatively simple in that a cementitious binder material is typically mixed with an aggregate material to add strength. Other materials can be added to change particular properties such as viscosity, setting time, stability to heat/cold. The mix is then allowed to set.

During the setting process in particular, the conditions need to be relatively carefully controlled, although in outdoor settings this is not always easy to achieve. However, although the general process is well known, the product and the process of manufacture is still evolving. The composition and process of manufacture can be used to change the characteristics of the material produced, such as hardness, porosity, heat resistance etc However, the process by which a concrete material is formed is a complex one, and apparently small changes in the process used can result in a product which has radically different physical and chemical characteristics. It is important therefore that control is exerted where possible.

This is most easily accomplished when producing a centrally mixed concrete composition, which is intended for use on the same site as the mix is prepared or does not require a Long transportation journey to the site where it is to be set. The current invention therefore is directed to a central mix composition and process.

It is an object of the current invention, to provide a pre-mix concrete composition which produces a concrete having superior properties. Moreover, it is a further object of the invention to provide a method of manufacture of a concrete material which produces an improved concrete material.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of producing a pre-concrete mix, the method comprising the steps of: selecting a mixing vessel, said vessel including mixing means to mix together components within the vessel; adding a gravel material to the vessel, the gravel having a particle size of from 4-10 mm; adding a sand material to the vessel, the sand having a particle size of less than 4mm and mixing the gravel and sand together; adding a cement and a ground granular blast-furnace slag (GGBS) composition to the vessel and mixing the components in the vessel together; the GGBS being added such that the weight ratio of cement composition to GGBS is from 36 - 50% by weight; adding fresh water to the vessel whilst continuing to mix.

Preferably, the cement is added before the GGBS. Alternatively preferably, the GGBS is added before the cement Preferably, a plasticiser is added to the mix to improve the theology of the mix, and further preferably is added prior to addition of the water.

Preferably, the water: cement ratio is 0.3: 1.0.

Preferably, during the mixing the water: cement ratio is maintained at 0.26 -0.35v/w, further preferably 0.28 -0.32v/w.

Preferably, the moisture content of the mix is 68-74%, further preferably 70 -72%.

Preferably an accelerator is added to increase the speed of concrete formation.

Preferably a workability extender is added to improve the mixing characteristics.

Preferably a microsilica is added to the mix to the increase the resistance of the concrete to sulphate and chloride corrosion and also to increase the strength of the concrete produced. Optionally preferably, the microsilica is added as a slurry in water, which is further optionally, in a ratio of 50:50 w/w. Further preferably, the microsilica is added in an amount 8-15 %w/w of the mix.

Preferably, microfibres, which are further preferably a polypropylene microfibre are added to the mix. Further preferably, the fibre is added in an amount of 6kg/ 3!4m3 of concrete. Optionally, the fibres are glass fibre strands to produce a final concrete product of improved tensile strength.

Detailed Description of the Invention

The preparation of concrete is, as indicated above, on the surface a relatively simple one, but in practice quite complicated and one which can be sensitive to small changes in materials and process conditions. Although in its simplest embodiment, a concrete material comprises an aggregate material and a binder, additional ingredients can change the bulk properties of the pre-hardened material and the final concrete. Moreover, changes to the formation of the pre-concrete mix and the setting conditions can also have a Large effect on those bulk properties. The invention as disclosed herein, provides a new composition of pre-hardened mixture and also of the final material as well as a new method of manufacture.

With regard to the aggregate, this typically forms the bulk of a concrete material. The hardness of the aggregate is usually selected to be from around 2 -9 Moh depending upon the concrete's function and can comprise a mixture of aggregate materials. Common materials used are gravel, crushed stone, and sand. A range of particle size is typically selected in order for the aggregate to pack together more densely and to this end the sand is often used as a part of an aggregate mixture to fill in the interstices between the other larger components, due to the sand's relatively small particle size.

The binder is designed to be able to flow around the aggregate particles and to interact with the surface of the aggregate material, sometimes chemically, to hold the aggregate material together in a solid mass. The binder, referred to as cement, is usually a mixture of the calcium salts of silicates, aluminates and ferrites which have been treated to have a low water content. The addition of water back to the cement during the concrete manufacturing process causes hydration and chemical reactions to take place between the cement and the aggregate.

Often used as part of the aggregate material are other materials such as ground granular blast-furnace slag (GGBS) and fly ash. However, these are used as a minor component. In one optional aspect, the current invention has found that replacing a proportion of the cement component by GGBS at a level of from 36 -50 %w/w and preferably at a ratio of 55:45 %w/w cement: GGBS is advantageous.

As an example of this, a first process comprises the following steps: Table 1: First composition Material Moisture Mass (Kg) Addition (%w/w) Time (s) Gravel (4-10mm particle size) 0.00 571 32 Sand (0 -4mm particle size) 4.34 640 30 GEM TM 1 52,5N 227 40 GGBS 187 34 Fresh Water 83 53 14 137 CHRYSO PREMIA TM 540 1.800 18 0.360 8 SIKA RAPID-3 TM 1.980 17 In the above table, the following materials are utilised: CEM TM 1 52,5N: this is a finely ground Portland cement having around 5% w/w of minor additional constituents such as Limestone, fly ash or GGBS.

CHRYSO PREMIER TM 540: super-plasticiser which provides better workability, better concrete stability, better surface finish, reduction in the amount of water used, and higher compressive strength due to better compaction.

SIKA RAPID-3 TM: accelerator Because there is some variation in consistency and workability of the mix depending, for example on the temperature and also on the materials added, a plant computer adds a pre-set amount of the water and plasticiser. An operator can then visually inspect the consistency and workability of the mix and add additional_ water/plasticiser as required.

The means of producing the pre-hardened concrete mix are well known in the art. A mixing vessel is provided of the required size. The mixing vessel is typically formed of a steel material and has, optionally, heating and cooling means to control the temperature of the mix during its formation. Further optionally, the water supplied to the mixer can be heated, which is especially to be applied during cold weather and can aid in improving concrete setting times. The component parts of the concrete mix are then added sequentially, again by means known in the arts such as conveyor belts or hoppers, with mixing being undertaken. Where required, measurements on a component can be carried out to ensure that it is within specification. For example, the moisture content of sand used in some embodiments of the process can be determined, and the value obtained used to determine the mass of the sand and also of water added as part of the process. The pre-hardened concrete mix thus formed is then discharged either to a forming frame or transported to the forming site.

In a typical general method in accordance with the invention, the aggregate is added into a mixer, which is preferably a planetary mixer rather than a pan mixer to yield better homogenisation of the mix produced, the aggregate comprising sand and gravel. The sand and gravel are mixed together for around 2 seconds. The cement powder and GGBS is then added and mixed in with the aggregate. It has been found advantageously that adding, sequentially, the cement powder and GGBS at this point reduces the unwanted production of Lumps within the mixture. Further advantageously, the GGBS is added before the cement powder. A suitabie mixing time for the GGBS once added, is around 1 second and for the cement powder around 5 seconds.

Water is then added and mixed in for around 2 seconds. This produces a relatively high viscosity mixture which is quite stiff, which stiffness aids in blending the concrete produced to create a good consistency.

The next step of adding the plasticiser at this point, acts to reduce the viscosity of the mixture so aiding its flow. It has been found that although, within the art the plasticiser is added to the water prior to the water's addition, it has been found that adding the plasticiser after addition of the water facilitates the manufacturing process.

The above compares with a second generai process, not in accordance with the invention, shown

in Table 2.

Table 2: Second process Material Moisture Mass (Kg) Addition (%w/w) Time (s) Gravel (4-10mm particle size) 0.10 204 24 Sand (0 -4mm particle size) 4.57 367 35 CEM1 TM 52.5 N 152 53 Fresh Water 17 14 19 17 Recycled Water 5 35 1 24 1 26 1 707 DYNAMON SW 1.030 11 CHRYSO PREMIA TM 540 0.998 12 0.087 4 0.065 5 A third general process, not in accordance with the invention, is shown in Tabie 3 in which the aggregate material is solely sand.

Table 3: Third process Material Moisture Mass (Kg) Addition (%w/w) Time (s) Sand (0 -4mm particle size) 3.55 219 23 CEM1 TM 212 58 Fresh Water 4 21 2 329 Recycled Water 1 257 327 DYNAMON SW 0.687 9 CHRYSO PREMIA TM 540 1.042 12 Glass Fibre 14.700 0 In more detail and in a further fourth embodiment of a process, the following general process is disclosed. The process produces 0.75m3 of concrete. The volume produced is however chosen dependent on the size of the mixer, subject to heat transfer. Any thermal issues can optionally be dealt with utilising a suitable heating or cooling system.

Gravel (4 -10mm) (573kg) and sand (<4mm) (634kg) (which includes moisture included with the sand, the value of which is determined prior to its addition) are charged together into a mixing vessel and mixed together for 2s. In a typical process, sand and gravel is fed via conveyor belts into a hopper (that holds 2000kg). The hopper winches the aggregate up to the mixer. The concrete mixer is the mixing vessel, which has a typical size of approximately 2000mm diameter x 800mm high. However, to increase the volume of batches produced, a mixer having a diameter of 3000mm and a height of 1200mm can be used. To enable efficient transfer times for materials, this is served by a skip-hoist bucket of capacity 2200kg. The larger size of mixer can be used to produce 1.5m3 per batch.

Prior to the addition, the water content of the sand is measured and the mass of water added Later in the process adjusted to take this into account. GGBS (182kg) is charged to the gravel and sand mix, and mixed in for 1s once charging is complete. CEM TM 52,5N (223kg) is then charged to the vessel and, once charging finished, mixed in for Ss. The mains fresh water (82 litres) adjusted to take into account the moisture in the sand) is added and mixed in for 2s.

Further ingredients can be added by an operator depending on the mix required and on the operating conditions pertaining. The adjustments are made to achieve the finaI workabiIity and can include adding additional water, super-plasticiser such as Chryso Premia 540TM or 10 workability extender such as Mapei Dynamon SW TM.

The operator can make visual observations of the mixture and also monitor instrumentation read-outs to determine if any of the additional ingredients should be added. In general, the mix within the vessel after the above additions should have a moisture content of around 70% and a water: cement ratio of around 0.3, both of which values are dispIayed on a batching plant control system. Mixing of the ingredients is continued. The operator should observe a glossy sheen on the surface of the mixture. There should also be some visual evidence of some of the gravel sitting just beneath the mixture's surface. There should however be no evidence of separation of the sand. This can exhibit itself as stone settling to the bottom of the vessel and excessive yellow-coloured water on the surface of the mixture. A further sign to an operator that the mixing process is proceeding correctly is that if the mixer is switched off, then the mixer Wades should slow to a gradual stop. For exampIe, if the mixer blades come to a dead stop when turned off, this is an indication that more water and super-plasticiser need to be added. Moreover, if during operation, the mixer blades flick up small amounts of water and/or slurry the operator should take this as an indication that the mixture is too wet. Should super-plasticiser have been added, then the operator should observe bubbles popping on the surface of the mixture to indicate the super-plasticiser is working: this can be audibly determined on occasion.

During the process, the water: cement ratio should remain within the range of 0.26 -0.35 and preferably within the range of from 0.28 -0.32 v/w. Further, the moisture content should remain from 68% -74% and preferably from 70% -72%. This value is a measure of the fluidity of the mixture relative to that of water. Slump flow tests can be carried out on the mixture to ensure that the value is in excess of 700mm and preferably is greater than 780mm.

Within the basic process of the fourth embodiment described above, the following optionally preferable steps can be included. First, an accelerator such as Sika Rapid-3 TM can be added, the overall amount depending upon the temperature around the mix vessel. Should the temperature be between 5°C and 0°C then up to 1 Litre should be added. The operator uses the temperature reading to determine the precise volume to be added. In the event the temperature is between 0°C and -5°C, then up to 2 litres should be added, and if below -5°C, then up to 3 litres, which is the maximum value for any temperature, should be added.

Similarly, if the external temperature is too high, and also if the mixture becomes too hot, such as over 25°, a workability extender such as Mapei Dynamon SW can be added. The workability extender increases the time in which the concrete formed can be worked. Should the external temperature be within the range of 15°C -20°C then up to 1 litre of extender should be added to the mixture. Again, the temperature as measured, informs the operator with regard to how much extender to add. If the temperature is between 20°C and 25°C, up to 1.5 litres should be added. Should the temperature be between 25°C and 30°C then up to 2 litres should be added, and if above 30°C, then again, up to 2 litres should be added.

In a preferred embodiment, the process involves the step of adding in a component of small particle size compared to the other solid materials, which component is preferably a silica, referred to herein as a microsilica. The small particles easily fit into voids surrounding the larger particles and so contribute to increase the strength and density of the concrete material. It has been found in some mixtures that approximately 180 particles of particle size of the order of a few microns can surround a cement particle.

In particular, with reference to a micro-silica material, the material can be added either as a solid particulate to the mixture or as a slurry. Due to the small particle size, it is preferable to add a slurry of the micro-silica material. The slurry also blends in better in the pre-concrete mix producing a more uniform distribution in the final concrete product In one embodiment of a process a 50:50 by weight mixture of a micro-silica powder in fresh water was utilised. The silica can be either used in addition to the sand component or as a replacement for a portion thereof. Alternatively, the micro-silica can replace another aggregate material such as the GGBS. It has been found that a micro-silica material is preferably used in an amount of from 8 -15% w/w/ of the overall cement mixture. Using values within this range provided a concrete having good workability and also improved compressive strength results. Further preferable is a range of from 8 -12% w/w and especially further preferably a value of 10% w/w. It was noted that the use of a micro-silica imparted a slightly darker colour to the finished concrete material.

The use of a micro-silica material requires additional super-plasticiser to be added simultaneously with the micro-silica. For example in a typical example, the amount of a Chryso Premia 540 plasticiser needed to be doubled compared to a cement mix without the micro-silica.

To make the process of hydration work correctly when concrete is setting there is an optimum amount of water that is required. This is called the water / cement ratio. If this is too high (ratio of 0.4) then the mix strength will be reduced but workability improved. If the ratio is too low (below 0.28), there is insufficient water to initiate the hydration process, but the finished strength is usually improved. Where, for example, a sulphate resistant concrete is required, the ratio is held below 0.35. Incorporating micro-silica improves the flow characteristics, and so the flow characteristic needed for self-compacting concrete could be achieved whilst maintaining the water cement ratio below 0.3 and resulting in a 10-15% increase in finished strength. Additionally, it has been found that the incorporation of micro-silica materials imparts greater chloride and sulphate resistance to the finished concrete material.

As examples of the ratios of cement to aggregate + micro-silica material (other materials not shown), the following amounts can be utilised, each of which has a cement to GGBS weight ratio of 1.22.

Table 4: Examples of Portland Cement -GGBS ratios when including micro-silica % Micro-silica Portland GGBS (kg) Micro-silica; (w/w) Cement (kg) (kg) dry 0 223.0 182.0 0.0 ° 204.9 167.7 32.4 200.5 164.0 40.5 12 196.0 160.4 48.6 14 191.6 156.7 56.7

Example 1

An experiment was carried out utilising the components below, added in accordance with the abovedescribed method.

Gravel -573kg Sand -613kg Elkem Emsac 500E TM slurry -54kg (8%) (Micro-silica) Cement -225kg GGBS -182kg Water-SO litres Chryso Premia 540 TM-2.7201itres Sika Rapid 3 TM-2.00 litres The resulting wet pre-concrete mix showed no signs of segregation. The workability of the mix was good, and concrete products formed therefrom demoulded easily with good surface finish.

Example 2

A further experiment was carried out utilising a Lower water to cement ratio.

4mm-10mm Gravel -574kg 0-4mm Sand -614kg Elkem Emsac SOOE TM slurry -54kg (8%) CEMI 52,5N -224kg GGBS -180kg Chryso Premia 540 TM -3.5 litres Sika Rapid 3 TM -2Itrs (only used in cold weather) Water -54 litres + 28 litres in sand Water/cement Ratio: 0.269 Total Water/cement ratio: 0.269 It was observed that the pre-concrete mix had too little water, and possibly less than the minimum required to properly hydrate the cement powder.

It has been found that the inclusion of micro silica improves the strength of the concrete produced. Moreover, the resistance of the concrete to sulphate and chloride corrosion is improved.

In order to improve the characteristics of concrete under tension, the most common practice is to incorporate steel rods or bars (often referred to as rebar) into the body of the concrete material. Although this gives a long-Lasting structure, care needs to be taken that the concrete produced is not too porous to water, as otherwise the steel will corrode over time and the concrete lose its structural integrity. Also, the use of chloride-containing components in the manufacture of the concrete needs to be avoided as chloride ions accelerate any decomposition.

It has been found that the incorporation into the pre-concrete mix of fibres, preferably formed of a plastics material, can allow the proportion of steel to be reduced and yet still provide a concrete material of at least the same strength, as tests on headwall structures demonstrated.

Table 5 illustrates the improvement in strength on addition of fibres.

Table 5: comparison of concrete strength with and without fibre addition during manufacture Sample Differential Movement (mm) at Specific Loads (kPa) 0 5 10 15 20 25 30 Angled Wing Walled Headwalls 150 mm2 Steel 0 0.77 0.98 1.66 4.93 5.8 6.87 Reinforcement Fibres + 300mm2 0 0.06 0.22 0.48 0.74 0.78 0.83 Steel Reinforcement In a typical process a polypropylene fibre, such as that marketed under the trade name Durus 5500, having an average fibre length of 48mm, is added at an amount of 6kg of fibre material to 3A of a cubic metre of concrete. The fibre material can be added via the same means as that provided to add sand component material. Fibres should be added with the aggregates at the start of the mixing process to ensure they are well blended, as they can have a tendency to float and not blend if added later in the process. Adding the fibres later in the process extends the mixing time required. Extending the mixing time creates more heat and expedites the setting process thus reducing workability and creating a poor surface finish Similarly, a portion, which can include the totality, of the 4-10mm gravel can be replaced with glass-fibre strands, which produces a final concrete product of improved tensile strength. This allows thinner structures to be produced which have lower mass, but which still provide the necessary structural strength. For example, for certain smaller products such as a Series 300 Headwall a reduction in thickness from 75mm to 20mm can be achieved and a corresponding weight reduction of around 70%.

Claims (20)

1. Claims 1. A method of producing a pre-concrete mix, the method comprising the steps of: selecting a mixing vessel, said vessel including mixing means to mix together components within the vessel; adding a gravel material to the vessel, the gravel having a particle size of from 4 -10 mill; adding a sand material to the vessel, the sand having a particle size of less than 4mm and mixing the gravel and sand together; adding a cement and a ground granular blast-furnace slag (GGBS) composition to the vessel and mixing the components in the vessel together; the GGBS being added such that the weight ratio of cement composition to GGBS is from 36-50% by weight; adding fresh water to the vessel whilst continuing to mix.
2. 2. A method according to Claim 1, wherein the cement is added before the GGBS.
3. 3. A method according to Claim 1, wherein the GGBS is added before the cement.
4. 4. A method according to any preceding claim, wherein a plasticiser is added to the mix.
5. 5. A method according to Claim 4, wherein the plasticiser is added prior to addition of the water.
6. 6. A method according to any preceding claim, wherein the water: cement ratio is 0.3:1.0.
7. 7. A method according to any preceding claim, wherein during the mixing the water: cement ratio is maintained at 0.26-0.35v/w.
8. 8. A method according to Claim 7, wherein the water: cement ratio is maintained at 0.28 -0.32v/w.
9. 9. A method according to any preceding claim, wherein the moisture content of the mix is 68-74%.
10. 10. A method according to Claim 9, wherein the moisture content of the mix is 70 -72%.
11. 11. A method according to any preceding claim, wherein an accelerator is added to the mix.
12. 12. A method according to any preceding claim, wherein a workability extender is added to the mix.
13. 13. A method according to any preceding claim, wherein a microsilica is added to the mix.
14. 14. A method according to Claim 13, wherein the microsilica is added as a slurry in water.
15. 15. A method according to Claim 14, wherein the ratio of microsilica: water in the slurry is 50:50 by weight.
16. 16. A method according to Claims 13 -15, wherein the microsilica is added in an amount 8 -15 %w/w of the mix.
17. 17. A method according to any preceding claim, wherein microfibres are added to the mix.
18. 18. A method according to Claim 17, wherein the microfibres are formed of a polypropylene.
19. 19. A method according to Claim 17 or Claim 18, wherein the fibre is added in an amount of 6kg/ 3/4m3 of concrete.
20. 20. A method according to Claim 17, wherein the microfibres are glass fibre strands.