WO2007080157A1

WO2007080157A1 - Apparatus and method for in situ measuring of evaporation from a surface

Info

Publication number: WO2007080157A1
Application number: PCT/EP2007/050153
Authority: WO
Inventors: Ole Mejlhede Jensen
Original assignee: Curing Meter A/S
Priority date: 2006-01-09
Filing date: 2007-01-08
Publication date: 2007-07-19

Abstract

The present invention concerns a method for in situ measuring of evaporation from a surface and an apparatus for measuring the evaporation from a surface. The apparatus comprises an evaporation surface made of a porous, hydrophilic material, which surface is connected to a reservoir. The reservoir is in shape of a spiral capillary tube with a calibrated scale. Also included is a method for manufacturing the apparatus.

Description

APPARATUS AND METHOD FOR IN SITU MEASURING OF EVAPORATION FROM A SURFACE

BACKGROUND

The present invention concerns a method for in situ measuring of evaporation from a surface and an apparatus for measuring the evaporation from a surface. Curing technology concerns, among other things, adjustment and control of temperature and humidity conditions in hardening constructions and elements of concrete. The curing technology comprises further e.g. measurement/adjustment/control of moisture conditions in the early hardening phase of the concrete in order to achieve an optimal development of properties in the hardening concrete ("moisture curing").

During the latest decades, the development within concrete technology has formed the basis of a new concept: High-Performance Concrete. Typically, a water/cement ratio

(w/c ratio) in the range of 0.40-0.60 is used for conventional concrete, but today's super plasticizers have made it possible to manufacture relatively fluid concrete with a w/c ratio of 0.20-0.30 when up to 20% silica fume is added. With these extremely dense concretes, concrete strengths of 200-400 MPa can be achieved industrially, whereas in comparison, conventional concrete typically has concrete strengths of 30-

50 MPa.

Danish concrete research has had a central role in the theoretical and experimental development of this new concept and several Danish companies are today involved in the industrial implementation of High-Performance Concrete in targeted special productions.

In curing technology terms the concept High-Performance Concrete means that the requirements for optimal and controlled moisture curing during hardening are significantly increased. At low w/c ratios, even modest losses of water in the early hardening phase may be detrimental to the subsequent hardening and property development of concrete. In the field of High-Performance Concrete, it can be expected that the coming years will witness a growing need for simple, operational methods for the measurement/adjustment/control of the moisture curing conditions of concrete in the manufacturing process.

PRESENTATION OF PROBLEM

It is well known to measure the rate of evaporation of a liquid for many purposes. In particular in the agricultural field is it important to determine the amount of moisture present in the soil, the precipitation, and how long the water remains in the soil, in order to determine correct watering, drainage etc. The importance of these questions has increased, as water resources has become increasingly scarce. This in turn has led to a multitude of devices suitable for determining various conditions relating to evaporation of water. An example is disclosed in US 4,324,132.

The device according to US 4,324,132 comprises an evaporation surface, covered by a sheet of fibrous material, and an open ended tube extending from the surface, such that in preparing a measurement the fibrous material and the open ended tube is saturated/filled by a liquid, for example water. By using a rather large evaporation surface, connected to a small diameter tube, the evaporation from the surface will cause rapid movement of the liquid front in the tube, whereby the actual evaporation may be determined.

The present invention is directed at solving very specific problems arising in the field of casting concrete, and in particular monitoring the fresh concrete during the initial stages. This particular application presents problems which the prior art devices are not able to address satisfactory, reliably and just as important cheaply enough.

During the first hours after mixing and casting, concrete is plastic and workable. The setting of the concrete - the time when it stiffens and becomes rigid - will normally occur 4-8 hours after adding water and mixing. During the setting period and immediately thereafter, the strength of the concrete is very low, and in this condition the cast concrete is very susceptible to any form of mechanical influence. If concrete is exposed to heavy desiccation in the period before and during setting, detrimental cracking in the concrete surface may occur. These cracks - plastic shrinkage cracks - occur because the surface tension of the pore water in menisci builds up critical capillary tensile stresses in the hardening binder phase.

For conventional concrete, crack damage due to plastic shrinkage in the early hardening phase traditionally presents a problem when casting at high temperatures, low relative humidity and high wind velocity. The damage is likely to be particularly severe when concrete/mortar is cast in thin layers, e.g. when shotcreting in connection with repair work. As result of the thinner layers, even a limited loss of water by evaporation may in these cases cause crack damage due to plastic shrinkage.

Furthermore the chemical aggressiveness of the concrete environment as well as the chemical composition of the liquid on the surface of a concrete structure provides for special problems.

The requirement that a method must be cheap in use and at the same time also reliable must be seen with due respect to the application. When casting concrete structures, such as for example roads, bridges, car parks and other large surface structures it is desirable to be able to monitor and measure in a plurality of positions on such a structure in order to obtain as true a picture of the actual conditions as possible. If the equipment is too expensive, or requires frequent replenishment, adjustment or other action/maintenance, in practice a representative measuring position will be selected in order to save on costs relating to the control. This however is not always an ideal situation, in mat the hardening process of concrete is an extremely complex process, where many different factors may have an influence. It is therefore desirable to be able to measure as many places as possible.

These and other associated problems were extensively discussed in the prior pending application of the inventor published as WO 03/006986. This application is hereby in its totality incorporated in this application. The device and method according to inventor's own earlier published invention (published as WO 03/006986) did solve the problems, but nevertheless had some drawbacks.

The original device was created with a straight capillary tube. In practice, however, this created at least three problems.

Firstly, the amount of liquid in the capillary tube was not sufficient to provide monitoring of the evaporation over longer periods of time.

Secondly, although the capillary phenomena exhibits large forces as a combination between the diameter of the tube and the surface tension of the liquid, the liquid in the tube had a tendency to "hang" in the tube. This caused the liquid string in the tube to separate whereby uncertainty about the actual measurement was caused.

Thirdly, the orientation of the tube also proved to be of importance to the measurements. When the surface on which the measurements were to be made was horizontal, orientation of the tube had no influence on the result, but on inclined surfaces the tube's orientation in relation to the evaporation surface proved to have influence, such that gravitational influence on the liquid in the capillary tube affected the results.

In order to address these and other minor problems, a new device was developed which overcomes these problems.

Accordingly the invention provides an apparatus for in situ measuring of evaporation of a substance from a surface, where the apparatus has an evaporation surface of a well-defined area and consisting of a hydrophilic, porous material; and in that the evaporation surface is in open connection with a reservoir containing the same substance as the substance evaporating from the surface, wherein the reservoir is in the shape of a curved capillary tube.

Tests with the prior art device indicated that the maximum lifting height corresponded to the theory, which means that for a capillary tube having an interior diameter of approximately 0,8 mm made in glass, the lifting height for water is approximately 37 mm, corresponding to the longest usable length of a straight capillary tube. For other materials, for example polycarbonate plastic, the lifting height is less, i.e. approximately 11 mm. This is due to the contact angle between the material and the liquid. Glass/water has a contact angle of 0° whereas polycarbonate/water has a contact angle of approximately 65°. Therefore, by arranging the capillary tube in a curved shape, and maintaining the distance between the evaporation surface and the point furthest away in the capillary tube to approximately 37 mm, a longer capillary tube is provided. This in turn provides for more liquid in the tube, and hence the possibility of monitoring/registering the evaporation during a longer time period, as the evaporation from the evaporation surface is not dependent on the length of the capillary tube, but only on the lifting height of the liquid and the contact angle between material and liquid.

To be able to maintain the lifting height within 37 mm and at the same time provide for a rather long capillary tube together provides a very usable apparatus. It is however very important to maintain a coherent liquid string in the capillary tube, as disintegration of the string will cause the apparatus to give incorrect and unusable measurements.

The curved shape is preferably in one plane, co-planar with the apparatus, and may have any shape. Preferably, the capillary tube is arranged to surround the evaporation surface, such that the orientation of the device has no influence on the measurements, in that when the capillary tube is arranged substantially evenly distributed around the evaporation surface whereby the influences due to orientation will be compensated due to the symmetrical arrangement.

The provision of the capillary tube in one plane and the apparatus having a substantially plate shape is also important for the proper workings of the apparatus. During the hardening process of concrete, the cements reaction with water develops heat. It is therefore important in order to carry out a reliable measurement that the conditions close to the apparatus are identical to the conditions on/in the surface of the concrete. By having a planar shape, it is possible to create an intimate thermal contact between the concrete and the apparatus, simply by slightly pressing the apparatus into the surface. In this context it must be assured that air does not get trapped between the apparatus and the concrete, as air will isolate and thereby not convey the same amount of heat to the apparatus, and thereby alter the evaporation conditions on the apparatus compared to the actual conditions without an insulating air layer.

The intimate thermal contact in combination with a relatively thin planar shape of the apparatus also provides substantially identical micro climatic conditions around the apparatus as the conditions elsewhere on the concrete surface, which in turn provides more representative measurements.

In a further advantageous embodiment, the capillary tube is in the shape of a spiral.

This configuration provides a substantially even distribution of the capillary tube surrounding the evaporation surface. Also, in this connection it is made possible to reduce the overall size of the apparatus, such that the apparatus itself does not in any measurable way have influence on the evaporating surface.

The spiral shape, furthermore, provides for a relatively long capillary tube. In this context, it should also be noted that any cross-section such as round, semi-round, triangular or rectangular may be used, without departing from the inventive principle, as the rules of physics apply to these cross-sections as well as to any other conceivable cross-sections. The spiral shape provides in practice for a capillary tube approximately 75 mm long. A balance has to be reached between the length of the spiral, i.e. how many windings about the evaporation surface, and the need for a readable scale which must be applied to the apparatus for reading the evaporation. The linear distance between the evaporation surface and the end of the spiral has to remain within the limits mentioned above, which in practice when a readable/useable scale is provided, provides a capillary tube of approximately 75 mm length, where the cross-section of the capillary tube is approximately 0,8 x 3 mm.

In order to further improve the user-friendliness of the apparatus in that colour is added to the substance in the tube and the apparatus is provided with a calibrated scale on which the amount of evaporation from the surface can be read directly in a still further advantageous embodiment. These aspects may be further improved in a further embodiment where the scale on the apparatus has been calibrated to show an integrated evaporation loss of the substance from the evaporation surface of the apparatus.

In order to provide that the substance in the reservoir evaporates at substantially the same rate as the water from the surface onto which the apparatus is intended to measure evaporation, the hydrophilic, porous material arranged in the evaporation surface of the apparatus is selected among woollen felt, foam or sponge rubber, sintered glass, metals or ceramics or materials exhibiting similar characteristics, and that the capillary tube is made in a material selected from glass, polycarbonate plastics or other transparent materials. The determining factors for selecting an appropriate material is mainly two factors, namely that the substance in the reservoir does not destroy or disintegrate the material, and further that it may be assured that the material must be able to maintain a steady transport of substance, where the substance, in most cases, will be water, through the selected material.

In a further advantageous embodiment of the invention, in particular in embodiments where the capillary tube is not made from glass, the inside surfaces of the capillary tube may be coated/metallized with a material which reduces the contact angle between the substance in the capillary tube and the material in which the capillary tube is formed. This coating may advantageously be formed in order to avoid that influences between the material from which the apparatus is made, and the substance arranged in the reservoir/capillary tube effect the overall evaporation material.

The lower the contact angle between the sides of the capillary tube and the substance, e.g. water, the less interaction there will be between these two, and the less effect this interaction will have on the measured evaporation result. It has been found that when designing the apparatus, an advantageous length of the capillary tube is between 50 mm and 100 mm long, more preferred 60 mm and 90 mm, and most preferred between 70 mm and 75 mm long, and that the interior surface of the capillary tube optionally is coated/metallized with a material which reduces the contact angle between the substance in the capillary tube and the interior surface of the capillary tube. This will for the normal applications of the apparatus, i.e. where it is desirable to measure the evaporation from a newly cast fresh concrete surface, be sufficient in order for the concrete to reach the desired maturity state, i.e. sufficient time for curing of the concrete, when the capillary tube has a length in these intervals.

One factor in determining the length of the capillary tube in relation to the design of the capillary tube around the evaporation surface is the fact that the distance between the furthermost point of the capillary tube and the opposite side of the evaporation surface in the apparatus shall be slightly less than the capillary tube's ability to elevate water.

In the apparatus, an evaporation surface is provided at one end of the reservoir, i.e. the capillary tube, and at the opposite end of the tube, a vent opening is provided such that a vacuum will not arise inside the capillary tube, which also could hamper the evaporation from the surface in that it would be very difficult for the evaporation process to overcome the low pressure created by the substance's/water's movement inside the capillary tube. When the apparatus is delivered to the user, the evaporation surface and the vent opening in the opposite end of the capillary tube are sealed by a removable seal, and the apparatus is thereafter packaged in a sealed, substantially hermetically sealed, package. In this manner, an apparatus is provided ready for use in that both seals across the evaporation surface and the vent opening must be removed, and the apparatus placed on the surface to be measured on, whereafter the apparatus will register and measure the evaporation from that surface. The seals as well as the sealed package provide the apparatus with a shelf life such that it may be produced, transported, stored and unpacked at substantially different points in time without detrimental effects on the apparatus or the results which are obtained by such an apparatus.

It has also, within the scope of the invention, been found that the method for manufacturing an apparatus according to the description above, which comprises the following steps: - an apparatus body is injection moulded from a transparent polycarbonate material, such that the capillary tube is formed as an open groove in one side of the body;

- a scale is printed along the capillary tube;

- the hydrophilic, porous material in the shape of a pre-cut element is placed in an aperture, which aperture is open to both sides of the apparatus; - to the lower side of the apparatus in which the groove is formed is applied a sealing, heat conducting folio;

- on the other side of the apparatus a removable sealing member is applied to cover the evaporation surface; - the apparatus is inserted into a sealable package, provides a very rational, industrial and economically feasible manufacturing process, which assures that the apparatus in action will have the accuracy and user friendliness as described above.

Finally, the invention it also directed towards the use of such an apparatus where it is important that at least part of the apparatus opposite the evaporation surface is in firm and substantially complete contact with the surface on which it is desirable to measure the evaporation.

Different designs and layouts of the apparatus are foreseen within the scope of the invention, but the main points and important aspects are as listed above.

Below, the invention will be described in more detail with reference to the accompanying drawing. In this connection, it should be mentioned that the drawings filed with the inventor's earlier pending patent application published as WO 03/006986 relating to the theoretically based aspects of this principle are incorporated by reference.

Fig. 1 illustrates a schematic perspective of one embodiment of the invention, Fig. 2 illustrates different configurations of the capillary tube, and

Fig. 3 illustrates a cross-section through the apparatus depicted in fig. 1.

In fig. 1, an apparatus 1 manufactured from a transparent material is illustrated. In the apparatus, an evaporation surface 2 is provided which is in fluid connection with a reservoir in the shape of a capillary tube 3. The capillary tube is depicted in dashed lines as the capillary tube is formed as grooves in the underside of the apparatus, as may be seen in fig. 3. In fig. 1, apertures 4 are also depicted which go straight through the material thickness of the apparatus such that the zone in which the evaporation is measured surrounding the evaporation surface 2 is as undisturbed by the device as possible. For practical purposes, it may be desirable to provide an extension 5 of the apparatus 1 such that it becomes easier to handle and place the apparatus 1 in its appropriate position. Furthermore, the extension 5 may also be used for commercial purposes or for information relating to the proper use of the apparatus 1.

In fig. 2, various configurations of the capillary tube 3 in relation to the evaporation surface 2 are illustrated. The important aspect in this context is the fact that the capillary tube is arranged substantially evenly distributed around the evaporation surface 2 such that influences from orientation, inclination and the like are minimized to the largest degree possible. Although, only three different layouts of the capillary tube 3 in relation to the evaporation surface 2 have been depicted, any other layout/design of the capillary tube which provides a substantially even distribution of the capillary tube in relation to the evaporation surface 2 should be construed as being within the scope of the present invention.

Turning to fig. 3, the cross-section along the line A-A is depicted. The evaporation surface material, in this case for example a woollen felt 2 is placed in an aperture going from the overside 6 to the underside 7 of the apparatus 1. On the underside of the transparent plate member 5 a spirally shaped groove 3 is formed, which at one end terminates, in open fluid connection, with the evaporation surface material 2 and at the opposite end terminates in a venting hole 8 where the venting hole is in free communication with the ambient atmosphere. In this embodiment, both the evaporating surface 2 and the venting hole 8 are sealed with removable seals, for example in the shape of tape strips 9,10. In order to activate the device, the tape strips

9,10 are removed, and the bottom surface 11 of the device will conduct the heat from the material/surface on which it is desired to measure the evaporation. The underside 11 of the device is covered by a heat conducting foil 12 which at the same time delimits the grooves 3 such that a capillary tube is formed.

The underside may be sealed in any suitable manner, for example by applying a sealing foil having an adhesive, for example a hot melt adhesive, which will adhere to the material in which the capillary tube is formed, and not be dissolvable due to the influence of the liquid in the capillary tube. Alternatively, the grooves formed in the underside of the device constituting the capillary tube may be sealed by applying a foil, such as a plastic foil, for example polycarbonate, where the apparatus is made from polycarbonate, and thereafter weld, either by heat, ultrasound, microwave or infrared, the foil to the body of the apparatus.

Claims

1. Apparatus for in situ measuring of evaporation of a substance from a surface, where the apparatus has an evaporation surface of a well-defined area and consisting of a hydrophilic, porous material; and in that the evaporation surface is in open connection with a well defined reservoir containing the same substance as the substance evaporating from the surface, characterised in that the apparatus has a generally plate shape, having a top surface and a bottom surface separated by a material thickness, wherein the reservoir is in the shape of a curved capillary tube and that said reservoir is formed in said material between the top and bottom surfaces substantially symmetrically around the evaporation surface, and where the bottom surface of said apparatus is adapted to be brought into intimate contact with the surface from which evaporation is to be measured .

2. Apparatus according to claim 1, characterised in that the capillary tube is in the shape of a spiral.

3. Apparatus according to claim 1 or claim 2, characterised in that colour is added to the substance in the tube and that the apparatus is provided with a calibrated scale on which the amount of evaporation from the surface can be directly read.

4. Apparatus according to claim 3, characterised in that the scale on the apparatus has been calibrated to show an integrated evaporation loss of the substance from the evaporation surface of the apparatus.

5. Apparatus according to claim 1, wherein the hydrophilic, porous material is selected among woollen felt, foam or sponge rubber, sintered glass, metals or ceramics or materials exhibiting similar characteristics, and that the capillary tube is made in a material selected from glass, polycarbonate plastics or other transparent materials.

6. Apparatus according to claim 1 or 5, wherein when the material in which the capillary tube is made is not glass, the inside surfaces of the capillary tube may be coated/metallized with a material which reduces the contact angle between the substance in the capillary tube and the material in which the capillary tube is formed.

7. Apparatus according to claim 1 wherein the capillary tube is between 50 mm and 100 mm long, more preferred 60 mm and 90 mm, and most preferred between 70 mm and 75 mm long, and that the interior surface of the capillary tube optionally is coated/metallized with a material which reduces the contact angle between the substance in the capillary tube and the interior surface of the capillary tube.

8. Apparatus according to claim 1 or 7 wherein the radial distance from the centre of the evaporation surface, to the part of the capillary tube furthest away from the center is between 20 mm and 50 mm, more preferred between 30 mm and 40 mm and most preferred between 35 mm and 38 mm.

9. Apparatus according to claim 1, wherein the evaporation surface and a vent opening in the opposite end of the capillary tube are sealed by a removable seal, and the apparatus thereafter is packaged in a sealed, substantially hermetically sealed, package.

10. Method for manufacturing an apparatus according to any of claims 1 to 9, comprising the following steps:

- an apparatus body is injection moulded from a transparent polycarbonate material, such that the capillary tube is formed as an open groove in one side of the body;

- a scale is printed along the capillary tube; - the hydrophilic, porous material in the shape of a pre-cut element is placed in an aperture, which aperture is open to both sides of the apparatus;

- to the lower side of the apparatus in which the groove is formed is applied a sealing, heat conducting folio;

- on the other side of the apparatus a removable sealing member is applied to cover the evaporation surface;

- the apparatus is inserted into a sealable package.

11. Use of an apparatus according to any of claims 1 to 9 in order to determine the evaporation from a surface, wherein the apparatus is placed in said surface, such that substantially complete contact is established between the surface and the apparatus, at least the portion of the apparatus surface being opposite to the evaporation surface of the apparatus.