WO2001087794A1

WO2001087794A1 - Method for producing cement-based composite structures

Info

Publication number: WO2001087794A1
Application number: PCT/DK2001/000330
Authority: WO
Inventors: Eigil Verner SØRENSEN; Bjarne Nissen Roursgaard
Original assignee: Densit A/S
Priority date: 2000-05-12
Filing date: 2001-05-10
Publication date: 2001-11-22
Also published as: AU2001258236A1

Abstract

A method for producing cement-based composite structures comprising providing a mixture comprising fine particles of a cement binder having a size in the range of from about 0.5 to 100 νm, ultrafine particles having a size in the range of from about 5 nm to about 0.5 νm, e.g. silica fume, and water, and casting the mixture followed by subjecting the cast mixture to a high temperature curing sequence comprising (a) heating the cast mixture to a temperature of not more than about 300 °C over a time period sufficiently long to avoid steam explosions and heat-induced crack formation in the structure, (b) maintaining the cast mixture at an elevated temperature of not more than about 300 °C for a period of at least about 2 hours, and (c) allowing the cast mixture to cool to ambient temperature over a time period sufficiently long to avoid heat-induced crack formation to result in the composite structure.

Description

Method for producing cement-based composite structures

Field of the invention

The present invention relates to a method for producing cement-based composite structures suitable for use in safes, bank vaults, etc.

Background of the invention

In order to guard valuables such as cash against theft from safes or vaults, e.g. from automatic teller machines (ATMs) or bank vaults, such safes or vaults are constructed with a barrier that is difficult to penetrate with normal hand-held tools. These security barriers are therefore constructed from materials with a high resistance to attack using tools such as sledge hammers, hammer and chisel, impact drills, diamond core drills, cutting torches and electric hammers. Typically, such security barriers are composite constructions made with different types of steel plates and reinforcement and a hard matrix such as a high-strength concrete.

A typical construction comprises a front (outer) and back (inner) plate of a suitable quality steel, although some constructions have no front plate or no plates at all (being cast in a removable mould), a steel reinforcement and a matrix of e.g. a high-density, high-strength concrete. The concrete may be based on a mixture comprising fine Portland cement particles, a relatively large amount of ultrafine particles such as silica fume particles, a relatively large amount of a concrete superplasticizer and a relatively small amount of water, e.g. as described in US 5,234,754. It may further contain a strong aggregate, e.g. calcined bauxite, as described in US 4,588,443.

In addition to their barrier function, the front and back plates may function as shutter boards for the matrix, and the front plate may be provided with an aesthetically pleasing outer surface. A reinforcing mesh is often welded to the back plate in order to provide cooperation between the back plate and the matrix. A wall construction of this type results in a stiffness that is useful when the wall is subjected to impact. This type of wall construction is well known for use in e.g. vaults, safes and ATMs and is used by many manufacturers worldwide. A general requirement for such wall constructions is that they are as thin and as lightweight as possible, i.a. because they are often used in multi-storey buildings with limited capacity of the elevators, and with a requirement to generally keep floor loads to a minimum.

A typical wall construction of this type has a thickness of about 30-400 mm and contains a reinforcing mesh. After arranging the mould or the steel wall construction comprising the metal plates and the reinforcing mesh, the mould or the space between the plates is filled with the matrix material mass. For relatively thin constructions of this type, the mass is typically based on either a high-strength concrete as referred to above or a synthetic resin.

Safes and vaults comprising this type of wall construction are often attacked in such a manner that a burglar removes the front sheet using a cutting torch, thereby exposing the outer surface of the matrix. The attack is typically continued using one of the above mentioned tools. The back plate and the reinforcement can then be cut away using a torch, thus providing access to the interior of the safe or vault. The time required to penetrate the matrix depends on a combination of the stiffness of the overall construction and the strength and toughness of the matrix. The burglary resistance of this type of construction can therefore be significantly improved by increasing the construction's thickness. As noted above, however, it is generally desired to decrease the wall thickness and weight, so improving the burglary resistance by making the walls thicker is in most cases disadvantageous and unsuitable in practice. Therefore, increased burglary resistance is preferably obtained by increasing the penetration resistance of the matrix material itself, i.e. the high strength concrete, rather than by simply increasing the wall thickness. An additional consideration is that it is important that such wall structures can be produced relatively easily using production processes that are not prohibitively expensive.

US 4,780,141 (Cemcom) discloses cementitious composite materials for use in the manufacture of molds and tools for forming metal and plastics. In particular, the composite materials are prepared from a mixture of Portland cement, chemically reactive silica particles, inorganic oxide particles, e.g. ground crystalline silica or quartz, inorganic aggregate particles, e.g. steel particles, metal fibres and water. Specimens were after casting treated in a saturated lime solution at 60°C for 24 hours, after which the treated specimens were air dried for a minimum of 5 days and then cured at elevated temperatures (mainly 150-350°C, but in some cases up to 815°C) in an oven. In an article by Wise et al. ("The development of a high strength cementitous tooling/molding material", Materials Research Society Symposium Proceedings, Vol. 42, pp. 253-263, 1985), whose authors include the co-inventors of the above-mentioned US 4,780,141 , the same types of compositions as in this US patent are disclosed, as well as their preparation. It may be seen from this article that a curing cycle lasting approximately 9 days was used, including, in sequence, 1 day at ambient conditions, 1 day of steam cure at 60°C, 5 days of drying at ambient conditions, and finally a dry thermal cure for 1.5 days at 205°C followed by cooling.

US 5,522,926 (Bouygues) discloses a method for preparing high strength concrete elements from a mixture comprising Portland cement, fine sand, amorphous silica, ground quartz, metal fibres, steel wool, a dispersing agent and water. After setting, the concrete is cured at a temperature of not less than 250°C, and preferably not less than 400°C, in a high temperature curing sequence that typically lasts for a total of 10 days.

Compositions similar to those of US 5,522,926 are disclosed in an article by Richard & Cheyrezy, two of the co-inventors of this US patent ("Composition of reactive powder concretes", Cement and Concrete Research, Vol. 25, No. 7, pp. 1501-1511 , 1995).

US 4,636,345 describes a method for rolling of a cement-based material. A curing of the material is not mentioned although of course curing will take place. As no specific method of curing is mentioned, the curing taking place must be only curing methods available to the person skilled in the art. It is not possible from the document to deduct that any specially developed curing method should be employed.

EP 0 010 777 also describes methods and products for producing cement-based materials, but also this document does not address any specially developed or applied methods for curing the products. Thus, the same comments as the above apply, i.e. as no specific curing is mentioned, the curing taking place must be only curing methods available to the person skilled in the art.

EP 0 692 464 describes cement-based products and this document actually also describes a specific method for curing of these. Curing takes place by elevating the temperature of the material during a certain period of time. Prior art is mentioned, where the temperature is elevated by means of autoclave curing up to 200°C. Heating above said temperature is not mentioned and heating by other means than steam is not mentioned.

As it will be apparent to persons skilled in the art, production methods for high strength concrete materials based e.g. on Portland cement that involve very long curing procedures, for example a total of 9-10 days as in the references cited above, are not advantageous for industrial production. There is thus a need for new methods for the production of high strength, cement-based composite materials that enable such products to be produced in a simple manner and using short processing times. Of course, such methods should also avoid any decrease in the strength of the finished product compared to the known methods, and should ideally result in products with improved properties compared to those produced by currently available methods.

Surprisingly, it has now been found that high strength cement-based materials suitable for use e.g. in security barrier constructions, can be easily and quickly produced using a novel curing procedure.

Brief disclosure of the invention

In one aspect, the present invention relates to a method for producing a cement-based composite structure, comprising providing a mixture comprising fine particles of a cement binder having a size in the range of from about 0.5 to 100 μm, ultrafine particles having a size in the range of from about 5 nm to about 0.5 μm and water, and casting said mixture in a desired configuration followed by subjecting the cast mixture to a high temperature curing sequence comprising the following steps:

(a) heating the cast mixture to a temperature of not more than about 300°C over a time period sufficiently long to avoid steam explosions and heat-induced crack formation in the structure, (b) maintaining the cast mixture at an elevated temperature of not more than about 300°C for a period of at least about 2 hours, and

(c) allowing the cast mixture to cool to ambient temperature over a time period sufficiently long to avoid heat-induced crack formation to result in the composite structure. Further aspects of the invention relate to a composite structure produced by the above method, and a safe, vault, strong room, shelter or armoured vehicle comprising at least one such composite structure.

Detailed description of the invention

By means of the method of the invention, a series of important advantages are obtained over currently available methods for producing high strength cement-based materials suitable for use in security barriers. These advantages include:

• a significant increase in the compressive strength and penetration resistance of the structures,

• a lower curing temperature, typically not more than about 250°C, than that used according to the prior art, • a shorter process time, typically less than about 48 hours, than that used according to the prior art, and

• the possibility to use a simpler mixture composition, for example by avoiding the need for ground quartz, which is undesirable for environmental and health reasons.

Composite structures produced according to the invention are suitable for use in any type of construction in which increased resistance to impact is desired. Such uses include, but are not limited to, safes and vaults, e.g. in banks, businesses and homes, strong rooms, shelters and armoured vehicles such as armoured cars. Particular examples of applications for which the composite structures are especially suitable are bank vaults and automatic teller machines.

Although the composite structures of the invention will most often be used as a wall, or a part of a wall, for a safe, vault, strong room, etc., it will be understood that they are equally suitable for use as a floor, roof or door, in other words as a security barrier in general.

As noted above, the cement particles in the mixture used according to the invention are fine particles having a size in the range of from about 0.5 to about 100 μm. These are combined with ultrafine particles having a size in the range of from about 5 nm to about 0.5 μm. Typically, the average particle size of the ultrafine particles will be at least one order of magnitude smaller than the average size of the fine particles, thereby allowing the ultrafine particles to become substantially uniformly distributed in the voids between densely packed fine particles to result in an extremely hard and dense matrix that provides optimal resistance to penetration. In these embodiments, the fine particles will typically comprise at least one cement selected from the group consisting of Portland cement, low-alkali cement, sulphate-resistant cement, refractory cement, aluminate cement, slag cement and pozzolanic cement, and the ultrafine particles will typically comprise particles selected from the group consisting of silica fume and oxides such as iron oxide and titanium dioxide.

A matrix based on Portland cement and silica fume is especially preferred. In this case, and when using a combination of fine particles and ultrafine particles in general as discussed above, the matrix material will typically be prepared from a mixture comprising ultrafine particles in an amount of about 5-50% by volume based on the total volume of the fine particles and ultrafine particles in the mixture. More typically, the amount of ultrafine particles will be about 10-40% by volume, such as about 15-30% by volume, based on the total volume of the fine particles and ultrafine particles.

In such mixtures, the amount of water is preferably kept to the minimum required in order to wet the particles and provide a mixture with the required workability. Water is therefore normally added to the mixture in a volume ratio between water and fine+ultrafine particles of about 0.25-1.5, typically about 0.4-1.2, such as about 0.55-1.0.

When using a rather small amount of water as indicated above in a cement-based mixture, the mixture will typically be prepared using a suitable effective amount of a surface-active dispersing agent (also known as a water-reducing agent or plasticizer), preferably a dispersing agent of the type known in the art as "concrete superplasticizers". Examples of suitable concrete superplasticizers are naphthalene-based, melamine-based, vinyl-based, acrylic-based and carboxylic acid-based products, as well as mixtures thereof and derivatives such as vinyl copolymers.

The concrete superplasticizer or other dispersing agent is typically incorporated into mixture (either the dry mixture before water has been added or a wet mixture to which some or all of the water has already been added) in an amount of about 0.01-5% (dry weight based on the total weight of the fine and ultrafine particles), typically about 0.05- 4%, more typically about 0.1-3%, such as about 0.2-2%. It will be understood that the amount of superplasticizer to be used in each individual case will depend in part on the nature of the superplasticizer. For example, when using one of the new generation of highly effective vinyl-, acrylic- or carboxylic acid-based superplasticizers, the required dispersing effect can be obtained with a significantly smaller amount of superplasticizer than when using e.g. a naphthalene sulphonic acid/formaldehyde or melamine sulphonic acid/formaldehyde condensation product. Thus, the concrete superplasticizer should be used in an "effective" amount, i.e. effective for the given superplasticizer and in the given particle system to obtain the desired dispersing effect in the mixture using only the intended amount of water.

The matrix will typically further contain aggregate of a type known per se in the art, including metallic aggregate. In a preferred embodiment, the aggregate is a strong and hard aggregate with a strength exceeding that of ordinary sand or stone used as aggregate for ordinary concrete. Examples of such strong aggregate particles include topaz, lawsonite, diamond, corundum, phenacite, spinel, beryl, chrysoberyl, tourmaline, granite, andalusite, staurolite, zircon, boron carbide, tungsten carbide, silicon carbide, alumina and bauxite. A preferred strong aggregate material is calcined bauxite.

The matrix may in addition contain reinforcing bodies as is known in the art, in particular fibres or whiskers of any type suitable for use in concrete or mortar, for example steel fibres, organic fibres such as polypropylene fibres, glass or mineral fibres, inorganic non- metallic whiskers such as graphite whiskers, AI₂O₃ whiskers or SiC whiskers, metallic whiskers such as iron or steel whiskers, etc.

As will be understood from the above, the terms "mixture" or "cement-based mixture" and "matrix" are used interchangeably to refer to the combination of cement, ultrafine particles, water, fibres, aggregate, etc. which is mixed, cast and cured to form the composite structures of the invention.

In composite structures of the invention comprising inner and/or outer metal plates and/or reinforcing bars, these may be of any suitable material, e.g. iron or steel. In preferred embodiments of the invention, the metal plates and the reinforcing bars will all be of a suitable grade of steel. The reinforcing bars may optionally be wave-shaped bars as disclosed in European patent application No. 00610021.8. Composite structures of the invention prepared from a mixture comprising cement particles, ultrafine particles and water, and typically further comprising a dispersing agent, fibres or whiskers and/or a strong aggregate, may be prepared in a manner known perse in the art, e.g. by simply casting the mixture in a desired configuration, using vibration or 5 compaction if desired, followed by the curing procedure as described herein. Prior to the heat treatment, exposed surfaces are normally covered to avoid evaporation, e.g. by means of a steel plate that may be removed after heat treatment.

As described above, the curing process itself comprises three basic steps, namely: 10

(a) heating the cast mixture to a temperature of not more than about 300°C over a time period sufficiently long to avoid steam explosions and heat-induced crack formation in the structure (heating phase),

(b) maintaining the cast mixture at an elevated temperature of not more than about 15 300°C for a period of at least about 2 hours, typically about 2-72 hours (constant temperature phase), and

(c) allowing the cast mixture to cool to ambient temperature over a time period sufficiently long to avoid heat-induced crack formation to result in the composite structure (cooling phase).

20

Heating typically takes place using a hot air oven, i.e. an oven with forced air circulation, and at ambient pressure. In the heating phase (a), it is important that the heating is adjusted, taking into consideration both the nature and the dimensions of the cement- based mixture, so that steam explosions (explosive eruption) and heat-induced formation

25 of cracks in the material are avoided. In other words, the rate of temperature increase must in any given case be sufficiently slow so as to avoid these undesired effects. One way of ensuring that that the rate of temperature increase is sufficiently slow is by making sure that the difference between the average temperature of the structure (defined as the average temperature of a cross-section of the structure) and the surface temperature of

30 the cast mixture does not exceed about 20°C. Preferably, this temperature difference should not be more than about 15°C, such as not more than about 12°C or 10°C.

A suitable length for the heating phase will be able to be determined by persons skilled in the art based on the nature of the cement-based mixture, e.g. the nature and relative 35 amounts of the components of the mixture, including the water content, and the dimensions of the structure being produced, in particular the thickness of the structure. Phase (a) will typically last for a period of from about 2 to about 48 hours. Preferably, phase (a) is as short as possible, giving due consideration to the need to avoid possible damage to the cement-based matrix as a result of an excessively fast heating. The duration of phase (a) is therefore preferably less than about 25 hours, more preferably less than about 20 hours, most preferably less than about 16 hours, such as from about 4 to 14 hours, e.g. from about 6 to about 12 hours. This phase may also be divided into sub- phases, typically in which heating from room temperature to an intermediate elevated temperature of, e.g., about 80-100°C, takes place over a given number of hours, followed by maintaining the intermediate elevated temperature for, e.g., up to about 10 hours, such as about 0.5-5 hours, e.g. about 1-4 hours, and then by heating from the intermediate elevated temperature to a final elevated temperature. Maintaining the cement-based mixture at the intermediate elevated temperature promotes consumption of water due to the curing process, thus reducing the internal pressure and thereby the risk of explosive eruption.

The maximum temperature in phase (a), generally corresponding to the temperature in phase (b), is typically in the range of about 150-300°C, such as about 200-280°C, e.g. about 240-260°C.

During phase (b), the cast mixture is typically held at a substantially constant elevated temperature, i.e. the maximum temperature chosen for phase (a). It will be understood by persons skilled in the art, however, that a certain minor temperature fluctuation during phase (b) is acceptable. Although phase (b) may in certain cases last for up to about 72 hours, it is typically substantially shorter, i.e. up to about 48 hours, more typically up to about 24 hours, such as up to about 20 hours. Typically, phase (b) lasts for at least about 4 hours, such as at least about 8 hours. Phase (b) will thus often have a duration of about 2-16 hours, such as about 4-12 hours. As was the case for phase (a), a suitable duration for phase (b) in any given case will be able to be determined by persons skilled in the art based on factors such as the nature and dimensions of the material, including the thickness of the structure being produced.

In the cooling phase (c), the rate of cooling must be sufficiently slow so as to avoid the risk of crack formation due to excessively rapid cooling, but the cooling phase is otherwise preferably as short as possible. This phase normally has a duration of at least about 2 hours, and it may in certain cases last for up to about 48 hours or more. Typically, however, it will have a duration of about 6-24 hours, such as about 8-22 hours, e.g. about 10-20 hours. It will be understood that there is no upper limit for the duration of this phase, which may in practice be as long as desired, e.g. if the oven being used is not immediately required for other purposes.

The curing sequence below illustrates some typical curing times and temperatures for the different phases that have been found to be advantageous for producing composite structures with a thickness of about 40-150 mm suitable for use e.g. as security barriers. It should be understood that this curing sequence is only provided by way of example, and that both the times and the temperatures of the curing sequence may in any given case be adapted in accordance with the general guidelines given elsewhere in the present specification.

Treatment Typical time Approx. preferred

Heating from 20°C to 90°C 1-5 hours 3 hours Constant oven temperature of 90°C 0-3 hours 1 hour Heating from 90°C to 250°C 2-10 hours 8 hours Constant oven temperature of 250°C 5-12 hours 10 hours Cooling from 250°C to 20°C 10-20 hours 15 hours

Total time 18-50 hours 37 hours

If desired, the cast mixtures of the invention may be held at ambient conditions of temperature and pressure prior to the heat curing procedure. Preferably, the heat curing is initiated not more than about 10 days after casting, more preferably not more than about 7 days after casting, most preferably not more than about 5 days after casting. This is due to the fact that if too long a time at ambient temperature passes prior to the heat curing procedure, the compressive strength and penetration resistance may be adversely affected compared to the same material subject to heat curing shortly after casting. This is presumably due to structure formation having taken place in the cement-based matrix, thereby preventing the reactions which would otherwise occur at high temperature.

The total length of the curing sequence comprising steps (a), (b) and (c) is preferably not more than about 168 hours, more preferably not more than about 120 hours, more preferably not more than about 72 hours, still more preferably not more than about 60 hours, still more preferably not more than about 48 hours, such as not more than about 40 hours. After cooling, exposed surfaces of the composite structure may optionally be sealed with a layer impermeable to water vapour, e.g. a metal or plastic sheet or a vapour- impermeable polymer. Sealing the exposed surfaces in this manner, so as to prevent atmospheric moisture from entering the cured structure, has been found to contribute to long-term stabilisation of the strength of the structure, since such penetration of moisture into such structures has been found by the inventors to result in a slight long-term reduction in compressive strength.

The invention will be further illustrated in the following non-limiting example.

Example 1

A mixture with the following composition was prepared:

% by weight

Inducast TT-5™ is a cement-based product of the type described above containing Portland cement, ultrafine silica fume particles, calcined bauxite (0-1 mm) and a concrete superplasticizer (available from Densit A/S, Denmark).

After mixing of the components, the casting mass was cast into cylindrical steel moulds having a diameter of 100 mm and a height of 200 mm. The steel moulds were subsequently closed with a steel plate held in place with a clamp.

For heat treatment, the closed steel cylinders with the samples were placed in a temperature-controlled oven and with a sufficient distance between the samples to allow oven air free access to all surfaces of the samples. After arranging the samples, the oven door was closed and the samples were subjected to the following pre-programmed heat treatment: Heating from 20°C to 90°C 3 hours Constant oven temperature of 90°C 1 hour Heating from 90°C to 250°C 8 hours Constant oven temperature of 250°C 10 hours Cooling from 250°C to 20°C 15 hours

Total time 37 hours

Corresponding control samples were prepared as described above, but without being subjected to the heat treatment. The control samples were cured at room temperature for 28 days. Following the 37 hour heat treatment or the 28 days at room temperature, respectively, the compressive strength of the treated and control samples were determined.

The samples subjected to a heat treatment in accordance with the invention were found to have a compressive strength of 330 MPa, while the control samples had a compressive strength of 208 MPa.

Claims

1. A method for producing a cement-based composite structure, comprising providing a mixture comprising fine particles of a cement binder having a size in the range of from about 0.5 to 100 μm, ultrafine particles having a size in the range of from about 5 nm to about 0.5 μm and water, and casting said mixture in a desired configuration followed by subjecting the cast mixture to a high temperature curing sequence comprising the following steps:

(a) heating the cast mixture to a temperature of not more than about 300°C over a time period sufficiently long to avoid steam explosions and heat-induced crack formation in the structure,

(b) maintaining the cast mixture at an elevated temperature of not more than about 300°C for a period of at least about 2 hours, and

(c) allowing the cast mixture to cool to ambient temperature over a time period sufficiently long to avoid heat-induced crack formation to result in the composite structure.

2. The method of claim 1, wherein:

• the duration of step (a) is 2-48 hours, preferably less than about 25 hours, more preferably less than about 20 hours, most preferably less than about 16 hours, such as about 4-14 hours, e.g. about 6-12 hours,

• the duration of step (b) is about 2-72, such as 2-48 hours, typically up to about 24 hours, such as up to about 20 hours, e.g. about 2-16 hours, such as about 4-12 hours, and

• the duration of step (c) is up to about 2-48 hours, typically about 6-24 hours, such as about 8-22 hours, e.g. about 10-20 hours.

3. The method of claim 2, wherein the total length of the curing sequence comprising steps (a), (b) and (c) is not more than about 168 hours, preferably not more than about 120 hours, more preferably not more than about 72 hours, still more preferably not more than about 60 hours, still more preferably not more than about 48 hours, such as not more than about 40 hours.

4. The method of any of the preceding claims, further comprising, prior to heating in step (a), maintaining the cast mixture at ambient conditions for a period of not more than about 10 days, e.g. not more than about 7 days, such as not more than about 5 days.

5. The method of any of the preceding claims, wherein curing at an elevated temperature takes place in a hot air oven and at atmospheric pressure.

5 6. The method of any of the preceding claims, wherein curing is performed using a maximum temperature in the range of 150-300°C, e.g. about 200-280°C, such as about 240-260°C.

7. The method of any of the preceding claims, wherein the fine particles are selected 10 from the group consisting of Portland cement, low-alkali cement, sulphate-resistant cement, refractory cement, aluminate cement, slag cement and pozzolanic cement, and the ultrafine particles are selected from the group consisting of silica fume and oxides such as iron oxide and titanium dioxide.

15 8. The method of any of the preceding claims, wherein the mixture comprises ultrafine particles in an amount of about 5-50% by volume based on the total volume of the fine particles and ultrafine particles in the mixture, typically about 10-40% by volume, such as about 15-30% by volume, water in a volume ratio between water and fine+ultrafine particles of about 0.25-1.5, typically about 0.4-1.2, such as about 0.55-1.0,

20 and optionally a concrete superplasticizer.

9. The method of any of the preceding claims, wherein the mixture further comprises at least one component selected from fibres, whiskers and aggregate.

25 10. The method of any of the preceding claims, further comprising sealing exposed surfaces of the cooled composite structure with a layer impermeable to water vapour, e.g. a metal or plastic sheet or a vapour-impermeable polymer.

11. A composite structure produced by the method of any of claims 1-10. 30

12. A safe, vault, strong room, shelter or armoured vehicle comprising at least one composite structure according to claim 11.