Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28C—PREPARING CLAY; PRODUCING MIXTURES CONTAINING CLAY OR CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28C5/00—Apparatus or methods for producing mixtures of cement with other substances, e.g. slurries, mortars, porous or fibrous compositions
  • B28C5/003—Methods for mixing

Definitions

• the invention presented relates to a process for the batch wise production of cement-concrete by intimate mixing of aggregates comprising a coarse and a fine fraction, cement, optional filler, and water in a concrete mixer with proportioning means for the components of the concrete mass, mixing means, and discharge means wherein the fine fraction of the aggregates is added to and absorbed in a premixture of the remaining components, wherein the coarse fraction of the aggregates is coated with a paste, consisting of cement, optional filler, and water.
• the fine fraction particles will increase the viscosity of the paste, which is high as it is, due to the filler content, but the paste will remain liquid. It was assumed that the compressive strength and the workability of the concrete would be increased, with an increasing quantity of fine fraction incorporated in the cement paste during the mixing.
• French Patent FR-A-1,575,874 deals with a problem occurring during continuous concrete mixing in a slightly inclined horizontal rotary mixing drum provided with means for continuously introducing water, cement, and fine and coarse aggregates. Traditionally, streams of these components were introduced at the upper part of the mixing drum. However, it was observed that improper moisturizing of the coarse aggregates often occurred during the mixing period, i.e. during the retention time of the material in the rotating mixing drum.
• FR-A-1,575,874 provides an improved moisturizing of the coarse aggregates by introducing water, cement and coarse aggregates at the upper part of the drum (providing an initial premixing of these components in a premixing zone in the upper part of the drum), and separately introducing the fine aggregates at an intermediate position in the drum (providing a final mixing zone in the remaining (lower) part of the drum in which the fine aggregates are mixed with the "pre-mix" formed in the premixing zone).
• the mixing principle according to this patent may be considered as a special (continuous) version of the following general mixing principle: Premix water, cement and coarse aggregates to a premixture during a period of time corresponding to the retention time in the premixing zone; introduce fine aggregates; and mix the fine aggregates with the premixture during a period of time corresponding to the retention time in the final mixing zone.
• EP-A-22738 (corresponding to US-A-4,369,066) and U.S. 3,812,076 do not actually concern concrete mixing.
• EP-A-22738 concerns production of mortar (i.e., a mixture of cement, sand and water) wherein cement and water are mixed by vigorous, intense stirring followed by addition of moist sand centrally in the funnel-shaped profile formed by the stirring.
• US-A-3,812,076 deals with effectively dispensing finely divided ethylene oxide polymers and other organic additives in a hydraulic cement-aggregate composition by a method wherein these fine organic materials are introduced into a mixing vessel as a suspension in a stream of gas. In order to prevent sticking together of the fine organic particles, they may be initially mixed with an inert, high surface area (almost colloidal) solid particulate material, such as fumed silica (with an average particle diameter in the micron range).
• an inert, high surface area (almost colloidal) solid particulate material such as fumed silica (with
• the present invention rests on the surprising realization that to achieve the results aspired in the above-mentioned SE patent specification it is necessary to overcome not only one but two prejudices, and further the adding of filler is not obligatory for the achievement of good strength qualities.
• the concrete produced in accordance with the process of the invention, shows in addition to the improved homogeneity a reduced tendency to separation, and the concrete shows no sign of accumulations of free washed stones, the so-called “stone-nests", as all stones are neatly surrounded by cement paste.
• the concrete liberates only a reduced quantity of water before and after vibration in which way bleeding on the concrete surface and thus strength reduction are avoided.
• the compressive strength measures show a reduced standard strength deviation compared to concrete produced according to the normal process. Due to the improved homogeneity of the concrete and the demonstrated reduced standard deviation of strength it will also by the process according to the invention be possible to reduce the cement content in a given concrete mix, while maintaining the strength values.
• the cement paste has the predetermined water content, and all the cement particles are preground and possibly activated by the mixing with the coarse fraction.
• the successive adding of the fine fraction water is bound by the formation of water meniscus between the fine grains.
• the viscosity of the paste and the volume of fines in the mix are increased.
• the concrete passes from a wet to a less wet stage.
• the qualities of the concrete mass such as homogeneity and workability increase with increasing fine fraction volume which contributes to the above improved qualities of the ready concrete.
• a particularly appropriate adding of the fine fraction is achieved by controlled vibration technique, more precisely by the application of a so-called vibratory feeder.
• Such vibratory feeds have a wide field of application within the most diversified lines of material handling, from the transportation of granulated sugar in sugar factories to the charging of pellets in blast furnaces.
• the application of a vibratory feeder in the process according to the invention permits an especially effective control with the material flow of the fine fraction which can be adjusted according to requirements.
• the successive adding of the fine fraction is advantageously adjusted so in relation to the rotation of the mixing means that the fine fraction added does not meet previously added fine fraction material, not yet absorbed in the paste, which would then form a double layer of fine fraction which it would be difficult homogeneously to work into the mass.
• filler may be desirable to add filler, among other things to be able to adjust the viscosity of the cement paste. Such an adjustment could be carried out by measuring means for the determination of the viscosity of the paste, connected to the proportioning means for the filler.
• filler is here used in the widest sense of the word, and not only with the meaning fine fraction material with particle sizes smaller than 1/4 mm, but also covering mineral admixtures such e.g. as silica, fly ash, puzzolanes etc., and in addition more special admixtures such as colours and inactive fillers such as fineground quartz etc.
• mineral admixtures such e.g. as silica, fly ash, puzzolanes etc.
• colours and inactive fillers such as fineground quartz etc.
• the filler materials are normally added together with the cement and/or used for later adjustments.
• the coarse fraction of the aggregates can with advantage be moistened with part of the water before cement, optional filler, and the remaining water quantity is added. It is also important to establish a suitable premixing time before successively adding the fine fraction. Hereby the qualities of the cement paste and thus the workability of the mass is improved.
• the invention further relates to a plant for the realization of the process according to the invention comprising a mixing container with proportioning means for the components of the concrete mass, mixing means and discharge means, and the plant is characterized in that the proportioning means for the fine fraction of the aggregates is equipped with devices for the control and regulation of the flow of the fine fraction so as to gradually increase the volume of the fine fraction in the premixture.
• the proportioning means for the fine fraction is a vibratory feeder, providing a very accurate control with the proportioning and which can be adjusted to provide different proportioning periods, in accordance with the composition and qualities of the concrete mass in question.
• the proportioning means can be adapted to add the fine fraction on more than one zone of the mixer. E.g. this can be achieved by the application of two or more proportioning means or by a turnable proportioning device.
• the plant may also include means for the determination of the viscosity of the cement paste, connected to the proportioning means for the filler and control the filler flow.
• the material flow in the mixer is under the influence of an external pressure from the mixing means, thus establishing high and low pressure zones in the concrete mass, and that the fine fraction is added in a high pressure zone.
• the mixing means of the plant are primarily designed so as to establish alternating high and low pressure zones which can further be supported by adapting the mixing means to move in a curved path, e.g. a planetary movement.
• the invention can be applied in the production of any type of concrete for varying purposes and with varying strength qualities.
• a person skilled in the art will be able to determine the optimum mixing process for a given type of concrete by varying the components of the mass, the grain sizes of the aggregates, the types of cement, different admixtures, mixing time etc.
• Stone and half of the water were mixed for 5 s. to moisten the stone material.
• the sand quantity was added at a time, and the mixing was continued for 65 s. Total mixing time 90 s.
• Stone, water, containing a plasticiser, and cement were mixed as under I. Hereafter sand was added over a period of 20 s., and the mixing was continued for 45 s. Total mixing time 90 s.
• Mix I was homogeneous and had a workability, expressed by a Vebe measure of 4 s.
• Mix II was also homogeneous and demonstrated an improved workability, expressed by a Vebe measure of 2,5 s.
• test II shows improved early strength values, not least if the time differences are taken into account. It is assumed that the diferences would be even more significant if a mixture providing high and low pressure zones had been used.
• the mixer is a pan mixer, operating on the basis of the counter-current mixing principle, by which the mixing is effected in a fixed pan by means of 4 shovels, driven in pairs by a planetary gear box.
• the mixer is equipped with 2 off side scrapers, incorporated with the purpose to lead the concrete to the most active mixing zone.
• the speed of the side scrapers is approx. half the speed of the shovels.
• This specific counter-current mixing principle makes the concrete mass move approx. one round per each 10-12 s. and brings about the effect that high and low pressure zones are continuously developed in the concrete mass.
• the stone fractions 4-8 and 8-16 mm and the sand were added at a time. After a dry-mixing period of 5 s. water was added over a period of 20 s., and after 15 s. mixing time the cement was added. The total adding lasted approx. 30 s., and hereafter the batch was mixed for approx. 30 s. The total mixing time was thus 60 s.
• the stone fractions 4-8 and 8-16 mm were added all at a time together with the water. After a 5 s. moisturing the cement porportioning was started and lasted for approx. 15 s., whereafter the batch was mixed for another 10 s. (paste mixing time). Then sand was added successively over a period of 15 s. by means of a SKAKO vibratory feeder. To reach the same total mixing time the batch was ready-mixed for another 15 s., but the concrete was homogeneous already after 5 s. ready mixing.
• the cylinder cores were stored in a water bath for 14 days at a constant temperature of 21°C. The results achieved are listed in the below tables.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Structural Engineering (AREA)
• Preparation Of Clay, And Manufacture Of Mixtures Containing Clay Or Cement (AREA)
• Artificial Fish Reefs (AREA)

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Publications (2)

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• 1982
  • 1982-11-15 SE SE8206489A patent/SE8206489D0/xx unknown
• 1983
  • 1983-08-15 IT IT49338/83A patent/IT1174795B/it active
  • 1983-11-11 GR GR72950A patent/GR79036B/el unknown
  • 1983-11-14 EP EP83903538A patent/EP0126105B1/en not_active Expired
  • 1983-11-14 ES ES527247A patent/ES527247A0/es active Granted
  • 1983-11-14 JP JP58503667A patent/JPS59502017A/ja active Pending
  • 1983-11-14 WO PCT/DK1983/000107 patent/WO1984001918A1/en active IP Right Grant
  • 1983-11-14 AU AU22602/83A patent/AU574655B2/en not_active Ceased
  • 1983-11-14 DE DE8383903538T patent/DE3366672D1/de not_active Expired
  • 1983-11-14 PT PT77660A patent/PT77660B/pt not_active IP Right Cessation
  • 1983-11-15 CA CA000441226A patent/CA1238038A/en not_active Expired
• 1984
  • 1984-07-09 FI FI842744A patent/FI842744A/fi not_active Application Discontinuation
  • 1984-08-31 ES ES535577A patent/ES535577A0/es active Granted

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