GB1603405A - Curing of concrete - Google Patents

Description

(54) CURING OF CONCRETE (71) 1, ISTVAN DOV KAFRY, an Israeli citizen of 40 Chatsworth Road, Stamford, Lincolnshire, formerly of 34 Western Avenue, Easton Hill, Stamford, Lincolnshire, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed to be particularly described in and by the following statement :- This invention relates to an accelerated curing method of concrete, either reinforced or not. Such concrete must set in the mould into which the mix has been poured, until it becomes of sufficient strength for demoulding without detriment to its shape and further strength development. It is applicable to slabs, beams, columns, railway sleepers, concrete pipes, or any other configuration, precast or on site concreted.

Accelerated curing of concrete by the application of heat, hitherto has been using predominantly heat brought from. an external heat source to the concrete surface ; either by means of some heat carrying agent, such as steam, hot water, heated oil and hot gases, or by the generation of heat in the vicinity of the concrete, such as infra red radiators or electric heating pads.

Although these methods shorten the curing time considerably as against natural curing, they suffer from the disadvantages of high capital cost (especially steam curing) and low thermal efficiency ; less than 25 En for steam (overall), 814 n for hot water and hot gas insta!!ations,25-35'forinfra red radiators, etc.

The object of the present invention is :- a) To shorten substantially curing times b) To reduce curing costs by a more efficient energy exploitation. c) To obtain higher strength within a predetermined time inverval.

According to the invention, an electric current is passed through the mass of the concrete, and/or currents are induced within the concrete and its reinforcement.

In this way, heat is generated within the concrete, which is used to raise its temperature at a thermal efficiency of 75 90%. (Distinct from the chemically developed heat, due to the process of hydration.) Although there are methods known, by which an electric current passing through the reinforcement (connected to an external power source) is used to produce heat within the concrete, the method to be described is basically different on two accounts :- 1) When reinforcement is used as the "heating element", the comparatively low resistance of steel necessitates the application of very low voltages (usually less than 6V), even for elements of considerable length, such as electric poles. This in turn, means a comparatively large current for a certain power, requiring heavy and costly equipment.

With the method to be described, current is carried by the concrete, which has a specific resistance roughly 4.5x IOB times that of steel. Thus, even for short paths of a few cm length, such as across the thickness of a slab or beam, appreciably higher tensions are used, as illustrated in the examples. In extreme cases and specific applications, tensions as high as 250V may be used.

2) With reinforcement as the heat source, the heat transfer to the concrete is effected over the circumference of the steel bars, presenting a rather restricted area.

Accordingly, the power levels that can be safely handled by this method are low ; otherwise, high temperature gradients would be created around the reinforcing bars, detrimental both to concrete and eventually to the reinforcement itself. This fact restricts the use of the method of thin, long configurations.

With the method of curing to be described, the heat is generated over the whole sectional area of the configuration, perpendicular to the direction of the current fiow in case of a slab, that equals the actual surface area, i. e. length x width, presenting no such limitations. The heat source is distributed, instead of being concentrated as in the former case. Powers of 2-20 Kw/m' concrete can be used, that is on an average 50 times higher values than with the previous system.

The present invention provides a method of curing concrete which comprises the steps of introducing a mass of uncured concrete mix into a mould, applying alternating current whereby to cause current to flow in the mass of the concrete itself, and controlling the alternating current to cure the concrete.

Tension, current and frequency to be used vary according to size and shape, as well as strength requirement of the elements to be cured and length of curing cycle allowed in the process. The examples given in the following, will show the wide variations both of voltage and current according to product requirements. The frequencies used in all these demonstrations were 50 Hr, although this parameter can also be varied advantageously. In general, the application of higher powers and temperatures result in reduced curing times for equivalent demoulding strength.

The electrical resistance of the concrete during the first curing phases diminishes with rising temperature; subsequently, with the gain in maturity it rises sharply.

Optimum curing temperatures were observed in the range of 50-90 C.

In order that the present invention be more readily understood, embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which :- Figure 1 is the perspective view of a concrete slab being cured by a method according to the invention; Figure 2 illustrates a possible solution for the continuous curing of a rectangular beam, by way of currents induced in the concrete mass; and Figure 3 shows a block diagram of a control circuit used in another application of the present invention.

Referring to Figure I there is shown a wooden base I with two mild steel plates 2,3 mounted thereupon substantially mutually parallel. Wooden spacers 4,5 are provided to complete the mould. Concrete mix 6 is then poured into the mould, and is cured by application of a voltage, u from a power source 7 to the plates 2,3 which act as electrodes. The wooden spacers 4,5 ensure that no current can flow between the plates except via the concrete mix. Any reinforcement present in the mix may combine with the plates 2,3 to form the electrodes which was the method used in the examples below.

Referring to Figure 2 there is shown a mould 10 for a beam comprised of three rectangular timber portions forming a duct.

Concrete mix 11 is poured into the mould, and the mould is passed through a coil of wire 12 which, when connected to a power source 13, induces heating currents directly in the concrete mix 11. These currents are represented by the circular current paths 14.

Rollers 15 are shown as providing one means of motion of the mould in a direction indicated by arrow A through the coil 12.

Examples No. 1. Precast purlin, reinforced.

Product data: L (length) =4.5 m ; W (weight) =) 50 kg ; V (volume) =0.0625 m' ; Water/Cement (WiC) ratio=0.4 Cement content: 440 kg Ordinary Portland Cement (O. P. C.)/m' concrete Curing data: U (applied tension) at 50 Ho 15 V 1, (initial curing current) 105 A Pi (initial curing power) 1575 W Th (heating time) 2 h te (ambient temperature) 7.5 C tmaX (max. curing temperature 66 C E (energy consumption). 4.5 Kwh (specific energy consumption 72 Kwh/m3 After T, (cooling time)-2h, the mould was turned over and the purlin removed intact. It was self supporting, of stony appearance, and transported immediately to the storage site, unharmed.

No. 2 Centrifugated precast pipe, reinforced Product data: I. D. =0.375 m; L=1. 883 m (incl. spigot & socket); W=305 kg ; V=0. 125m' ; W/C ratio=0.3; Cement content=483 kg ; O. P. C./m3.

Curing data: U at 50 Hz 24 V 1, 130 A P, 3120 W Th Ih 30' ta 24 C tmax 52 C E 5. 58 Kwh E, 44.7 Kwh/m' After TC=lh 25'the pipe has been stripped. It was self supporting and has been rolled to the storage site without injury No. 3 Eight prestressed concrete sleepers, cured simultaneously Product data: L (unit) =2.51 m; W=8x290 kg; V=8x0 113 m' ; W/C ratio=0.36 Cement content=435 kg ; Rapid Hardening Portland Cement (R. H. P.) per m'.

Curing data: U at 50 Hz 33 V 1 ; 270 A P, 8900 W Th 4 h ta 15 C tmax 68 C E 36.1 Kwh

Es 39.9 Kwh/m3 After Tc=io h the prestressing has been released, the sleepers demoulded and successfully tested at 28 Ton (minimum up to first hair crack), applied at the load bearing surfaces.

In the above examples, some form of current/voltage regulation was provided, to cater for the changing electrical resistance during the process: although this is not mandatory in every case.

Equipment One of the additional advantages achieved by the method according to the present invention is, that the equipment needed for carrying it out is comparatively simple, requires very little maintenance, no skilled attendance, and the capital investment is but a fraction of the one needed for a steam-plant of equivalent capacity.

In genera), it consists of an appropriate (usually"taylor made") transformer with or without a power regulator, associated switch gear, energy meter, indicating instruments (optional) and thermostatic control, plus the necessary cables, clamps, etc. The transformer in certain applications may be advantageously replace by rotating machinery.

A further application of direct electric curing is in the curing of test samples of concrete in order to establish the strength of a concrete structure.

The usual procedure for establishing the strength of a concrete structure or product at a certain time-usually at 3, 7,24 or 28 days-is to crush a standard size sample in the shape of a cube or cylinder, taken from the same mix and cured under identical conditions. Its ultimate strength is supposed to be the indicator of the product strength.

However, the hardening process is a near quadratic function of the temperature and under the same ambient conditions this will differ within the concrete product and the sample. The reason na that during hydration (i. e the setting and hardemng process) heat is being developed within the concrete structure, which is better retained and consequently reaches a higher value within the sizable mass of the structure than within the small sample. Accordingly, at a certain time the actual product strength could be considerably higher, than the one indicated by the sample. Thus demoulding time, i. e. the removal time of the shuttering, could be substantially shortened.

For some years now this fact has been realised and put to practical use. In one instance, the sample specimens are submerged in a water filled tank. The temperature of the product is continuously compared-through an electronic device -with that of the curing bath; the latter being heated to an equalising temperature, whenever deviation is indicated by the apparatus. The control is fully automatic.

Another method uses air as the heating medium, instead of water.

The present invention obviates the need of either curing tank or air heating equipment. The temperature comparison is done directly between the product and the concrete within the sample-in this case a cube mould. Thereby the result can be made not only more accurate, but more consistent, the parameters being measured directly. The equipment is much simpler, lighter and consequently cheaper: in fact it is just a modification of the cube moutd.

The sample mould 22 is constructed of a moisture and heat resistant insulating material, with two steel plates built in as heating electrodes and arranged to directly contact opposed faces of the sample to be cured in a somewhat similar manner to that shown in Figure 1. The actual temperatures within the product 21 and the sample are monitored by the temperature sensors 25,26 in both the test sample and the structure to an electronic comparator 23. Whenever the sample temperature is lower than that of the product-whether naturally cured or otherwise-current is switched on to the sample mould, flowing through the concrete and heating it up to the product temperature (or slightly higher) ; then it is automatically switched off. The electric power source is a small transformer 24 fed from the mains. Figure 3 shows the general set-up.

The whole equipment is mounted in a portable cabinet, fitted with a flexible cord and plug to standard mains supply socket outlet (either 220-240 V or i IS V).

Claims (10)

1. The mould can be built for a single or several cubes Alternately, several single moulds can be cured in parallel, monitored by a single sensor WHAT WE CLAIM ! S :- i A method of curing concrete which composes the steps of introducing a mass of uncured concrete mix into a mould, applying alternating current whereby to cause current to flow in the mass of the concrete itself, and controlling the alternating current to cure the concrete.
2. 2. A method according to claim 1, wherein the voltage current and frequency of the alternating current are adjusted to create a maximum curing temperature in the mass of concrete in the range of 50 to 90 C.
3. 3. A method according to claim 2, wherein the energy applied to the mass of concrete is in the range of 2 to 20 KW/m3 of concrete.
4. 4. A method according to claim 2 or 3, wherein the voltage of the power source is in the range of 15 to 250 volts.
5. 5. A method according to claim 1, wherein electrodes are provided on opposite internal faces of the mould, the alternating current being applied to said electrodes.
6. 6. A method according to claim 1, wherein the mass of concrete is provided with reinforcing members, the alternating current being applied to respective reinforcing members.
7. 7. A method according to claim 1, wherein a coil surrounds the mould, and the alternating current is applied to said coil whereby to induce said current flow in said mass of concrete.
8. 8. A method of monitoring the curing of a concrete product comprising preparing a concrete mix, introducing a mass of uncured concrete mix into a first mould for forming a concrete product, introducing a sample mass of uncured concrete mix into a further mould, monitoring the temperature of the concrete mix in the first mould, applying alternating current whereby to cause current flow in the sample mass of concrete itself, and controlling the alternating current whereby to maintain the temperature of the sample mass of concrete mix to be equal to that of the mass of concrete mix in the first mould.
9. 9. A method according to claim 8, and applying alternating current whereby to cause current ílow in the mass of concrete in the first mould, the alternating current being controlled whereby to cure the concrete in said first mould.
10. 10. A method of curing concrete substantially as hereinbefore described with reference to Example 1, 2, or 3.

    I I. A method of monitoring the curing of a concrete product substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.