JPH0215519B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for recycling concrete. Concrete required for major construction operations is not mixed on site and concrete is approximately 90% removed from mixing.
It is now common practice for concrete to be supplied in mixed form from a central mixing plant located close enough to the site that it can be transported within minutes. It often happens, especially at the end of the day's work, that mixed trucks return from the job site loaded with unused portions of concrete. Concrete returned from the site unused in this way is called recycled concrete. At present, this concrete cannot be used and is simply dumped in a dump before it hardens. Although this current situation is undesirable for both economic and environmental reasons. Now, such excess concrete can be prepared by adding enough retarder to prevent this concrete from hardening even for one night or one weekend, and then converting the delayed concrete into fresh concrete on the next working day. It has been found that this can be made usable by adding an accelerator which has the effect of counteracting the effect of the retarder. In fact, by appropriately selecting a retarder, the step of mixing with fresh concrete can be omitted, and the entire concrete that has been delayed in setting can be accelerated to set again, allowing it to be used as if it were fresh concrete. It is possible to do so. Accordingly, the present invention provides a method for reclaiming mixed virgin concrete, which method comprises: 1) delaying the concrete by an amount sufficient to delay hydration for up to an additional 90 hours before it hardens; 2) optionally diluting the retarded concrete with fresh concrete at the end of the desired setting retardation period; and 3) restoring the retarded concrete to a hardenable condition. adding an accelerator to the retarded concrete. If desired, a water-reducing strength agent may be added to the diluted concrete along with an accelerator. The retardant used in the method of the invention is one that has a sustained action (i.e., is capable of delaying hydration for more than 6 hours) when used in appropriate amounts.
It must have a reversible action so that the addition of the accelerator can substantially restore the concrete to its original condition. Preferred retarders are phosphonic acid derivatives, more preferably capable of acting as calcium chelators. It contains hydroxy and amino groups. Particularly preferred retarders of this type are the Dequest(TM) series of compounds sold by Monsanto Co. (St. Louis), in particular: Dequest2000: Aminotri(methylenephosphonic acid) Dequest2006: Aminotri(methylenephosphonic acid) Pentasodium salt Dequest2010: 1-hydroxyethylidene-1,
1-diphosphonic acid Dequest2016: 1-hydroxyethylidene-1,
1-Diphosphonic acid tetrasodium salt Dequest2041 Ethylenediaminetetra (methylenephosphonic acid) Dequest2047: Ethylenediaminetetra (methylenephosphonic acid) calcium sodium salt Dequest2051: Hexamethylenediaminetetra (methylenephosphonic acid) Dequest2054: Hexamethylenediaminetetra (methylenephosphonic acid) potassium Salt Dequest2060: Diethylenetriaminepenta(methylenephosphonic acid) Dequest2066: Diethylenetriaminepenta(methylenephosphonic acid) sodium salt. Other suitable retarders include polycarboxylic acids and their salts, including hydroxycarboxylic acids and their salts, citric acid, gluconic acid, tartaric acid, fumaric acid, itaconic acid, malonic acid and glucoheptanoic acid, e.g. Polymaleic acid, polyfumaric acid, polyacrylic acid and polymethacrylic acid, preferably of low molecular weight, antioxidants, e.g.
including ascorbic acid and isoascorbic acid, polymers such as sulfonic acid-acrylic acid copolymers, polyhydroxysilanes and polyacrylamides, preferably of low molecular weight, carbohydrates such as sucrose and corn syrup and lignosulfonates such as calcium lignosulfonate. . Among these, hydroxycarboxylic acids, polycarboxylic acids, isoascorbic acids and polyhydroxysilanes are preferred. Further preferred retarders are mixtures of at least one phosphonic acid type retarder and at least one different type of retarder. Since many of the non-phosphonic acid type retarders also have water-reducing properties, they also have the effect of increasing the compressive strength of the final cured concrete. Particularly preferred retarders are mixtures of the Dequest series and citric acid, especially Dequest 2000 and citric acid. The preferred ratio of Dequest to citric is 1:1 to 2:1. Preferred accelerators to be added in step 3) of the process of the invention are those classified as Type C mixtures in ASTM C494. Preferred accelerator compositions are chlorine-free and include, for example, calcium salts such as calcium nitrate and calcium formate, thiocyanates, triethanolamine and glycolurils such as trimethylol glycoluril. A particularly preferred accelerator of this type is from Master Builders Inc.
(Cleveland) under the trademark Pozzolith 555A. In step 3), it can be added together with an accelerator if desired. Preferred water-reducing strength agents are those classified as Type A mixtures in ASTM C94, ie, water-reducing agents that do not themselves have a substantial retarding or accelerating effect. A particularly preferred material is Master Builders Inc.
It is sold under the trademark Pozzolith Polyheed by . Further, instead of using a combination of a type C mixture as an accelerator and a type A mixture as a water reducing agent, it is also possible to use a type E mixture as a water reducing agent alone. However, it is preferred to use a combination of type C and type A. In the process of the present invention, the amount of retarder and accelerator that must be added as a percentage of the concrete weight will vary according to a number of different factors that will be readily understood by those skilled in the art. Dew. These include: 1) Composition of retarders and accelerators used. 2) The length of time the delay is desired. This is usually 1
During the night (12-18 hours) or weekends (36-90 hours, preferably 60-72 hours). 3) ASTM Type of Cement. Type 1~
Types 1 and 2 are preferred, although Type 4 can be used in the process of the invention. The degree of substitution of the flyash in the cement will influence the amount of accelerator to be added in step 3) of the process of the invention. Generally, as the degree of fly ash substitution increases, more accelerator is required. 4) Time of addition of retarder to concrete, defined as the length of time between mixing of the concrete and addition of the retarder. The retarder may be added to the unused portion of the concrete immediately after use;
Alternatively, it may be added when the mixed truck returns to the ready-mixed concrete production industry, or as long as the concrete maintains the desired slump, air content, and unit weight. Preferably, the retarder is added to the cement 1 to 4 hours after initial mixing. If the addition time is longer, more retarder will be required. 5) Hardening time of the original concrete. If the concrete has a very slow setting rate, less retarder will be needed and more accelerator will be needed than one with a longer setting time. 6) Concrete temperature. The higher the temperature, the faster the cure rate and therefore the greater the amount of retarder that will be required. For faster curing, for concrete temperatures above 20°C (70°C), the retarder is preferably added within 3 hours of initial mixing. 7) Volume of concrete to be processed. More voluminous concrete at a given initial temperature will cure faster than less voluminous concrete at the same initial temperature.
This is because larger volumes do not lose the heat of hydration as quickly, thus giving a larger temperature rise and higher curing rate. Therefore,
For example, 2 cubic meters of concrete is 1
It would require more than twice the amount of retarder than a cubic meter of concrete would require. 8) Ratio of recycled concrete to new concrete in the final mix. Old concrete contains 5-100% by weight of the total mixture, preferably 10
It may be in the range of ~50% by weight. Obviously, the higher the proportion of recycled concrete, the greater the amount of accelerator. 9) Type and amount of mixture in the original cement and newly added cement. In a preferred method of carrying out the process of the invention, as the mixed truck returns to the ready-mixed concrete road mill with a portion of fresh concrete, the temperature of the load is measured with a probe;
The volume of the load is estimated and, based on these data and the known properties of the concrete batch, the amount of retarder required to hold the load usable until the next day of operation is calculated. Advantageously, this is carried out by a suitably programmed microcomputer and the desired amount of retarder (preferably as an aqueous solution) is automatically metered into the load. The load is briefly mixed and then left without further mixing until the next day of operation. The desired amount of new concrete is then mixed into the reclaimed concrete, the temperature is measured again, and the desired amount of accelerator (and amount of water reducer if necessary) is calculated. Again, the addition is preferably carried out under automatic computer control. The following examples are intended to illustrate the invention. Parts and percentages are by weight unless otherwise specified. example

[Table] Retarder composition R 12.8% aminotri(methylenephosphonic acid)
(Dequest2000) and an aqueous solution containing 8% citric acid. Accelerator Composition C Pozzolith 555A (Master Builders), a calcium nitrate-based chlorine-free accelerator supplied as an aqueous solution containing 48.7% active material. Water reducer composition A Pozzolith Polyheed (Master Builders), 39%
A water reducing agent based on low-retardation lignosulfone, supplied as an aqueous solution containing active substances. In the examples below, all additive additions to concrete are given as weight percentages based on the dry weight of cementitious material (i.e. cement + fly ash) in the concrete. Percentages are based on dry active material unless otherwise specified. Example 1 Four samples of a standard concrete composition are mixed and left in a 5 gallon container for 2 hours. 5% by weight as shown in the table in three of the containers.
Add an aqueous solution of Dequest 2000 alone or with citric acid. For the fourth sample, only 5% by weight of water is added as a reference. The concrete sample is mixed and left overnight. twenty four
After an hour, the three delayed samples were divided into 1 part recycled concrete and 5 parts fresh concrete.
diluted with fresh concrete in the ratio of
3.1% accelerator composition C and 0.3% water reducer composition A
to reactivate it. The table shows the compressive strength measured after 14 and 28 days.

[Table] Example 2 For each sample, 0.318% Dequest material plus 0.200% citric acid was used as a retarder, and 2.5%
Example 1 was repeated by adding the solution in an aqueous solution. Reactivation was carried out as in Example 1. The table shows the results for % air content, cure rate to initial setting after reactivation and compressive strength after 1 and 7 days.

[Table] (Reference)
Example 3 Appropriate amount of retarder R for overnight delay and weekend (2 day) delay (cementitious material 100
The number of liters of solution per kg) is expressed as a function of the concrete temperature and the amount of concrete to be treated (delayed), the experimental results of which are shown in the table.

[Table] Retarded concrete is prepared by mixing 1 part of old retarded concrete with 5 parts of fresh concrete, then adding a predetermined amount of accelerator composition C (solution per 100 kg of cementitious material) as shown in the table below. l number).

【table】

Claims (1)

Claims: 1. A method for reclaiming mixed virgin concrete, comprising: 1) adding a retarder to the concrete before it hardens in an amount sufficient to delay hydration for up to an additional 90 hours; 2) adding an accelerator to the retarded concrete to restore the retarded concrete to a hardenable state at the end of the desired set retardation period. 2. The retarded setting concrete is diluted with fresh concrete before addition of the accelerator;
A method according to claim 1. 3. The method of claim 1 or 2, wherein the retarder comprises at least one phosphonic acid derivative. 4 The retarder is aminotri(methylenephosphonic acid), aminotri(methylenephosphonic acid) pentasodium salt, 1-hydroxyethylidene-1,1
-Diphosphonic acid, 1-hydroxyethylidene-
1,1-diphosphonic acid tetrasodium salt, ethylenediaminetetra (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid) calcium sodium salt, hexamethylenediaminetetra (methylenephosphonic acid), hexamethylenediaminetetra (methylenephosphonic acid) potassium salt ,
The method according to claim 3, comprising at least one selected from diethylenetriaminepenta(methylenephosphonic acid) and diethylenetriaminepenta(methylenephosphonic acid) sodium salt. 5. The retarder comprises at least one compound selected from hydroxycarboxylic acids or polycarboxylic acids and salts thereof, ascorbic acid and isoascorbic acid, sulfonic acid-acrylic acid copolymers, polyhydroxysilanes, polyacrylamides, carbohydrates, and lignosulfonates. 5. The method according to any one of claims 1 to 4, comprising: 6. The method according to claim 5, wherein the retarder comprises at least one compound selected from hydroxycarboxylic acids, polycarboxylic acids, isoascorbic acids, and polyhydroxysilanes. 7 The retardant is at least one compound selected from phosphonic acid derivatives, and hydroxycarboxylic acids or polycarboxylic acids and salts thereof, ascorbic acid and isoascorbic acid, sulfonic acid-acrylic acid copolymers, polyhydroxysilanes, and polycarboxylic acids. 7. The method according to any one of claims 1 to 6, comprising at least one compound selected from acrylamide, carbohydrate, and lignosulfonate. 8 The retarder is aminotri(methylenephosphonic acid), aminotri(methylenephosphonic acid) pentasodium salt, 1-
hydroxyethylidene-1,1-diphosphonic acid,
1-hydroxyethylidene-1,1-diphosphonic acid tetrasodium salt, ethylenediaminetetra (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid) calcium sodium salt,
At least one compound selected from hexamethylenediaminetetra (methylenephosphonic acid), hexamethylenediaminetetra (methylenephosphonic acid) potassium salt, diethylenetriaminepenta (methylenephosphonic acid), diethylenetriaminepenta (methylenephosphonic acid) sodium salt, and citric acid. 8. The method of claim 7, comprising an acid. 9. The method of claim 8, wherein the retarder comprises aminotri(methylenephosphonic acid) and citric acid. 10. The ratio of aminotri(methylenephosphonic acid) to citric acid is from 1:1 to 2:1. The method of claim 9. 11. The promoter comprises at least one compound selected from calcium salts, thiocyanates, triethanolamines and glycolurils.
A method according to any one of claims 1 to 10. 12. said accelerator consists of a mixture of calcium nitrate, sodium thiocyanate, triethanolamine and trimethylol glycoluril;
A method according to claim 11. 13. The method according to any of claims 1 to 12, wherein the water-reducing strength enhancer is added together with the accelerator. 14. The method of claim 13, wherein the water-reducing strength enhancer is a lignosulfonate derivative. 15 Recycled concrete accounts for 10 of the total mixture
3. A method according to claim 2, in the range .about.50% by weight. 16. The method of any of claims 1-15, wherein the retarder is added about 1 to 4 hours after initial mixing of the concrete.