FI78639C - Process for making mixed concrete - Google Patents

Description

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Method for preparing ready-mixed concrete

The invention relates to a method for producing ready-mixed concrete, the method comprising a first mixing step for producing mortar by mixing cement, fine aggregate and primary water, and a second mixing step for producing ready-mixed concrete by mixing mortar, coarse aggregate and secondary water. and in the second mixing step, a water reducing agent.

The concrete shrinks as the water evaporates and the concrete hardens. the more water the concrete contains, the more strongly it shrinks and the more likely it is to crack and the lower its strength.

U.S. Patent No. 4,403,863 discloses a method of making concrete by means of a fixed mixing device comprising an overhead mixer for making mortar by mixing cement, fine aggregate and water; and a bottom mixer that receives the mortar from the top mixer and mixes it with the coarse aggregate. With this method, a slightly lower amount of water can be obtained to produce concrete, but it cannot be expected to form concrete with a satisfactory settling value and compressive strength as the cement begins to harden in the mixer below.

DE-A-1 1 255 045 discloses an apparatus for removing water from sand in a storage silo. The purpose of the dewatering is that the sand could be transported by truck without difficulty. If the sand is too wet when it is loaded onto the truck platform, you will still have to wait for the sand to dry by parking the car. Otherwise, wet sand will leak out when transported to the road, endangering traffic safety. The invention according to the present invention seeks to eliminate these problems, the dewatering being carried out as quickly as possible by means of a hollow, perforated body oscillating from the low silo described therein. The publication mentions that this equipment achieves a moisture content of about 5% by weight on the sand.

5 GB application publication 0 020 722 further discloses a method for mixing and spraying concrete, and on pages 6 to 37 of this publication, lines 37-39 show the addition of one or more admixtures to fresh, watery concrete.

The object of the present invention is to increase the compressive strength of concrete and to minimize the variation in the quality of concrete by improving the hydration of the cement particles while reducing the amount of water required.

This object is achieved by the method according to the invention, characterized in that the water content of the surface of the fine aggregate is adjusted by wetting the fine aggregate with water and keeping it in storage for at least 48 hours at a height of at least 15 m before feeding it to the first mixing step, the variation of the surface water content can be kept within + 1% by weight.

The present invention makes it possible to reduce by about 10

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kg / mJ or 5-9% of the amount of water required in the conventional method for preparing concrete by single-stage mixing in a roller mixer. According to the invention: the compressive strength of the concrete produced by the method is 8-12 S higher than that of the conventional product produced by one mixing step, if their settling height and water / cement ratio are the same. The water / cement ratio of the product according to the invention is 3-8% higher than that of the conventional product if they have the same compressive strength. In other words, the invention allows the amount of cement to be reduced from 35 to 60 kg / m 2, which is required in the production of concrete. These reductions in water and cement volumes mean 35 major cost reductions in concrete production. In addition, the method according to the invention allows easy and economical production of concrete, with minimal variation in quality.

The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 is a schematic representation of a concrete and manufacturing method applying the present invention, Fig. 2 is a graph showing the variation of fines surface water content obtained by conventional sorting stock by way of example. , which shows by way of example the variation of the water content of the surface of the fine aggregate fed from a high storage silo for adjusting the water content of this surface according to the present invention.

Fig. 4 is a cross-sectional view showing, by way of example, a high storage silo used in accordance with the present invention to control the water content of the surface of finely divided aggregate.

The term "finely divided aggregate" as used herein means a aggregate having a relatively small particle size of 20 to 20 ni and usually means sand. The term "coarse aggregate" refers to a aggregate that is relatively large in size, and typically refers to gravel.

In the method according to the invention, the water content of the surface of the fine aggregate or sand is first adjusted.

It is preferred to use sand with a particle size of up to 5 mm. Although the water content of the surface of the fine aggregate depends on which material is used, it is important to ensure that the surface water content of a given amount of the fine aggregate used to make a given amount of concrete is kept within + 1% by weight.

Conventionally, the surface water content of the fine aggregate fed from the sorting depot was measured at regular intervals. The results varied over a wide range of about 3.8-7.6%, as shown in Figure 2, for example.

35 Sand whose surface water content varies so much is not able to adhere evenly to the cement, and when concreting concrete, a product with uneven compressive strength and other properties is obtained.

According to the invention, a high silo is used to control the water content of the surface of the fine aggregate. A system 5 which can be used in applying the method according to the invention is shown by way of example in Figure 1. The finely divided aggregate is conveyed by means of a suitable medium, such as a belt conveyor 1, and water is sprayed onto this aggregate by means of a sprinkler 2. The moist aggregate is stored in a high silo 3 with a height of, for example, 25 m. The height of the finely divided aggregate in silo 3 is at least 15 m and its volume is preferably at least 250 m3. The aggregate acquires a substantially uniform surface water content, varying in the range of 15 ± 1% by weight, if it is kept in silo 3 for at least 48 hours. Although the mechanism by which a uniform surface water content is obtained is not clear, it is very likely that the potential energy of water on the surfaces of aggregate particles and the weight of aggregate particles can cause excessive water dripping along the particle surfaces, replacing water on the particles. a stable water film covering the particles.

The rate at which water exits in droplets until the amount of water on the surface of the aggregate has stabilized is inversely proportional to the specific surface area of the aggregate, and directly proportional to its storage height. Thus, it is advantageous to use a high storage silo and store the aggregate in it so that its height becomes high. Fully moistened sand, reaching a height of 20 m and 30 with a volume of 800 m3, was stored in a silo at a height of 25 m for 48 hours. The sand was removed from the silo through an outlet at its bottom at regular intervals and the water content of its surface was measured. The results are shown in Figure 3. As can be seen from Figure 3, the water content of the sand pin-35 nan remained at 5 + 1% by weight.

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Figure 4 shows an example of a high storage silo. It comprises an outer jacket 31 having a substantially square cross-section and divided by two central partitions 32 into two compartments. The outer jacket 31 has a floor 34 provided with an outlet 33 for aggregate and a roof 36 provided with an inlet 35 for aggregate. The roof 36 has a cover 28 which contains a aggregate supply device 37. It is possible to use several silos having such a structure, which are arranged 10 in parallel with each other.

The aggregate, the surface water content of which has been adjusted, is transferred to the silo well 5 by means of a belt conveyor 4, and the water content of its surface is measured by means of a water content meter 6, as shown in Fig. 1. The aggregate is then transferred 15 to the aggregate tank 8. Cement and concrete aggregate are fed from the cement tank 7 and the aggregate tank 8 to the overhead mixer 14 for the first mixing, respectively. Simultaneously or after mixing them with each other to some extent, a first amount of water 20 is fed from the primary water tank 19 to the overhead mixer 14 to knead the mixture of cement and aggregate. An air-mixing agent is added from the tank 12 to the primary water. It is possible to use any commercially available air-blending agent, including a product sold by Hercules 25 Co of USA and known under the trade name "Vinsol". It is sufficient to add the amount normally used. It is preferable to mix the semen and aggregate and add primary water containing an air to concrete mixer to knead the mixture. The advantage of this procedure is to achieve a satisfactory bonding strength between the aggregate whose surface water content is stabilized and that of cement and improves air uptake.

The mortar prepared by the first mixing step, as described above, is transferred from the upper mixer 14 to the lower mixer 15. Simultaneously, coarse aggregate is fed from the coarse aggregate tank 6 to the lower mixer 15. Simultaneously or after mixing the mortar and coarse aggregate, some from the secondary water tank 11 to the mixer 15 below. The secondary water contains a suitable amount of water reducing agent fed from the water reducing agent tank 13. It is possible to use any commercially available water reducing agent including Super Plasticizer NP-10 or Water Reducing Agent Pozzolith No. 70 or 5L (Mater Builders Co., USA) or Super 10 Plasticier Mighty FD (Kao Soap CO., Japan). It is preferable to mix the mortar and coarse aggregate to some extent and to add secondary water containing a water-reducing agent to knead the mixture. This procedure allows the water-reducing effect of the water-reducing agent to be fully utilized and produces concrete with substantially no settling losses.

As will be apparent from the foregoing, the method of the present invention comprises two steps, i.e., a first mixing step in which mortar 20 is prepared and a second mixing step in which concrete is prepared. It is characterized by the use of a fine aggregate with a controlled surface water content and an air-mixing agent and a water-reducing agent, which are added during the various stages. These features ensure the production of a mortar which has good water retention properties and improved adhesion of water to the fine aggregate and thus achieves improved adhesion of coarse aggregate to the mortar. The method according to the invention thus makes it possible to produce concrete at all times which has excellent compressive strength and durability. It is a very economical method that can produce concrete from a smaller amount of cement than is required in any conventional method to produce concrete with the same compressive strength.

According to the most preferred embodiment of the invention, the cement and the fine aggregate are mixed with each other for 5-25 seconds in an overhead mixer and the first water containing the air-absorbing agent is then added to the mixture to knead it for at least 25 seconds to make a mortar. The mortar is mixed with the coarse aggregate in a bottom mixer for 5-25 seconds and secondary water containing a water reducing agent is added to the mixture to knead it for at least 25 seconds to form the mixed concrete.

Although the ratio of primary water (including water on the surface of fines) to secondary water is not particularly limited, it is preferably in the range of 4: 6 to 9: 1.

Table I shows by way of example the amounts of materials used to prepare concrete by the most preferred method according to the invention, as described above. Table II is similar to Table I, but deals with the conventional method of mixing cement, fine and coarse aggregate, and additives in a roller mixer.

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Table I

Nominal V / s Las- H / a Material quantities (kg / m3) strength ratio '' boiling ratio (2 '(kp / cm2) (weight%) (cm) (vol.%) 5ementt ^ 3) Water Sand Gravel Ap (4) C 8 42.6 191 135 855 1.184 0.478 180 70.5 18 45.5 224 158 873 1.075 0.560 21 48.2 241 170 904 998 0.602 8 41.7 209 135 831 1.192 0.523 210 64.5 18 44.5 245 158 847 1.083 0.613 21 47.3 264 170 878 1.003 0.660 8 40.9 219 136 811 1.202 0.543 255 62f0 18 43.5 258 160 821 1.093 0.645 10 21 46.2 277 172 850 1.016 0.693 8 40, 4,256,137,787 1,192 0.640 270 53.5 18 42.9 303 162 790 1,083 0.758 21 45.5 325 174 816 1.005 0.813 8 39.8 258 138 774 1,200 0.645 300 50.0 15 40.9 284 152 772 1,144 0.710 18 42.0 307 164 772 1.093 0.763 15

Table II

Nominal V / S Las- H / A Material quantities (kc / m3) strength ratio boiling ratio'2 '(kp / cm2) (% by weight) (cm) (tjiav.%) Cement3 * Water Sand Gravel AD ^ 8 42 , 6,232,145,829 1,149 0.530 180 62.5 18 45.5 269 168,844 1,039 0.672 20 21 48.2 288 180 873 963 0.720 8 41.7 250 145 805 1,158 0.625 210 58 18 44.5 290 168 818 1,048 0.725 21 47.3 310 180 847 972 0.775 8 40.9 278 146 780 1.158 0.695 255 52.5 18 43.5 324 170 785 1.048 0.810 21 46.2 347 182 811 972 0.868 25 8 40r4 294 147 764 i'158 0.735 270 '50 18 42.9 342 172 767 1.048 0.855 21 45.5 368 184 790 972 0.920 8 39.8 315 148 743 1.158 0.788 300 47 15 40.9 345 162 741 1.098 0.862 18 42.0 370 174 738 1.048 0.925 30 Note. (1) V / S ratio = water / cement ratio; (2) H / A ratio = sand / aggregate ratio; (3) contains 10% by weight of fly ash; (4) AD = water reducing agent Pozzolith No. 70; (5) density; cement = 3.16, sand = 2.59 and gravel - 2.66.

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As can be seen from these tables, the concrete produced by the process of this invention contains about 10 kg / m less water than the product produced by the conventional process. The compressive strength of the concrete according to the present invention is 8-12% higher than that of the concrete prepared according to the conventional method, which has the same settling value and the same water / cement ratio. Thus, the water / cement ratio of the concrete produced by the method of the present invention is 3-8% higher than that of the product produced by the conventional method having the same compressive strength. Taking into account the reduction in the amount of water required, a 3-8% increase in the water / cement ratio corresponds to a reduction in the amount of cement required of 35-60 kg / m 2.

The invention is further illustrated by the following Examples 15.

Examples 1-4

A series of experiments were performed by changing the timing of the addition of the water-reducing agent during the preparation of the concrete in a two-stage fixed mixer comprising 20 upper and lower mixers. 245 kg of cement and 847 kg of sand were charged to the overhead mixer, and after mixing for 5 seconds, 126 kg of primary water containing 30 cm of air mixing agent known as Vinsol was added to knead the mixture for 25 seconds to make a mortar. The mortar and 1083 kg of gravel were placed in the mixer below and mixed together. Thirty-two kilograms (32 kg) of secondary water containing 2450 cm 2 of a water-reducing agent known as Pozzolith No. 70, were added (1) simultaneously with gravel (Example 1), five seconds after gravel (Example 2), (3) 15 seconds after gravel (Example 3) and (4) 25 seconds after gravel (Example 4), respectively. . The compressive strength of the concrete thus obtained was measured 3, 7 and 28 days after its preparation. The results are shown in Table III. Each compressive strength value is the average of the test results, which were repeated several times.

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Table III

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Example Deposition Air Compressive strength (kp / cm) no_cm _% _ 3 d_7 d_28 d 5 1 18.5 4.5 81 163 248 2 18.5 4.6 79 160 245 3 18.5 4.6 80 159 243 4 18.5 4.5 82 165 253

Claims (2)

A method for producing ready-mixed concrete, comprising a first mixing step for producing mortar by mixing cement, fine aggregate and primary water, and a second mixing step for producing ready-mixed concrete by mixing mortar, coarse aggregate and secondary water, the first mixing step adding water the water content of the surface of the fine aggregate is adjusted by wetting the fine aggregate with water and keeping it in a storage silo (3) for at least 48 hours as a mass at least 15 m high before it is fed to the first mixing stage, the variation in water content can be kept within + 1% by weight. Method according to Claim 1, characterized in that the volume of the fine aggregate mass 3 in the storage silo (3) is at least 250 m.