AT393501B - REPROCESSING OF CONCRETE MIXTURES - Google Patents

Description

AT 393 501B For larger construction contracts, the concrete mix is normally not produced on site, but delivered in trucks. The central concrete mill is located close to the site, so that the mix can be poured within 90 minutes. It often happens, especially at the end of a working day, that a truck comes back from the construction site with a lot of unused mixture. Until now, this mixture has not been used and had to be disposed of as waste before hardening, which is undesirable for economic and environmental reasons

It has been found that excess concrete mix can be made reusable by adding enough retarders to prevent hardening overnight or over the weekend, mixing the delayed concrete mix with fresh concrete mix the next working day and canceling the delay effect by adding an accelerator by choosing one suitable retarder it is also possible to omit the dilution with fresh concrete mix, i.e. H. the whole amount of the delayed concrete mixture is accelerated again and can be used as a fresh concrete mixture.

The invention thus relates to a process for the reprocessing of unused concrete mixtures according to claim 1. If desired, the procedure can be carried out according to subclaims 2 and 13.

The retarders used in the process according to the invention must be effective for a long time in the amounts used (ie be able to delay hardening more than 6 hours) and be reversible in their action, so that the addition of an accelerator basically returns the concrete mixture to its original state. Preferred retarders are phosphonic acid derivatives, preferably those with hydroxy and amino groups that can bind calcium ions as chelates. Compounds that are sold as Dequest (R.T.M.) Products by Monsanto Co. are particularly preferred, namely:

Dequest 2000: Aminotri (methylenephosphonic acid)

Dequest 2006: Aminotri (methylenephosphonic acid & pentasodium salt Dequest 2010: l-hydroxy-l, l-diphosphonic acid Dequest 2016: l-hydroxy-l.l-diphosphonic acid tetrasodium salt Dequest 2041: ethylenediaminetetra (methylenephosphonic acid)

Dequest 2047: ethylenediaminetetra (methylenephosphonic acid) calcium / sodium salt Dequest 2051: hexamethylenediaminetetra (methylenephosphonic acid)

Dequest 2054: hexamethylenediaminetetra (methylenephosphonic acid) potassium salt Dequest 2060: diethylenetriaminepenta (methylenephosphonic acid)

Dequest 2066: Diethylenetriaminepenta (methylenephosphonic acid) sodium salt

Other suitable retarders are hydroxycarboxylic acids and their salts such as lemon, gluconic, wine, fumaric, itacon, malonic and glucoheptanic acid; Polycarboxylic acids and their salts, such as polymaleic, polyfumaric, polyacrylic and polymethacrylic acids, preferably of low molecular weight; Antioxidants such as ascorbic and isoascorbic acid; Polymers such as sulfonic acid-acrylic acid copolymers, polyhydroxysilanes and polyacrylamides, preferably with a low molecular weight; Carbohydrates such as sugar and corn syrup; and lignosulfonates such as calcium lignosulfonate, hydroxycarboxylic acids, polycarboxylic acids, isoascorbic acid and polyhydroxysilanes are preferred among them

Particularly suitable retarders are mixtures of at least one phosphonic acid derivative and at least one other retarder. Since many of the retarders, which do not belong to the group of phosphonic acid derivatives, also have water-reducing properties, these can also rarely increase the compressive strength of the hardened concrete.

Particularly preferred retarders are mixtures of a Dequest product with citric acid, especially Dequest 2000 and citric acid. Preferred ratios of Dequest product to citric acid range from 1: 1 to 2: 1.

Suitable accelerators for the addition in the last step of the process according to the invention are those which are described in ASTM C494 as " Type C admixtures " be designated. Preferred accelerators do not contain chloride ions and and z. B. calcium salts such as calcium nitrate and calcium formate, thiocyanates, triethanolamine and glycolurils such as trimethylolglycoluril. A particularly preferred accelerator is a product sold by Master Builders Inc. under the name Pozzolith 555A.

Preferred water reducing amplifiers, which if desired are added together with the accelerator in the last step of the method according to the invention, are those which are referred to in ASTM C494 as "Type A admixtures". These water reducing agents themselves have no significant deceleration or acceleration properties. A particularly preferred is the product sold by Master Builders Inc. under the name Pozzolith Polyheed.

It is also possible to use " Type C admixture " as accelerator and " Type A admixture " as a water reducing agent, a " Type E admixture " use what is a water reducing accelerator. However, a combination of accelerator and water-reducing agent is preferably used. -2-

AT 393 501B

The amounts of retarder and accelerator which are used as a percentage by weight of the concrete mixture in the process according to the invention depend on various factors which the person skilled in the art can readily recognize. The following should be mentioned as such: 1) The formulation of the retarder and accelerator used. 2) The duration of the desired delay. Usually this is 12-18 hours (overnight) or 36-90 hours, preferably 60-72 hours (over the weekend). 3) The type of cement. ASTM types I-IV can be used, but type I and Π are preferred. The level of fly ash used to substitute cement affects the amount of accelerator in the last step of the process: the more fly ash contained, the more accelerator is needed. 4) The time between making the concrete mix and adding the retarder. This can be added to the unused part of the concrete mixture as soon as the used part has been poured or when the truck with the mixture returns to the central concrete mill, as long as the concrete mixture has the required flow properties, air content and density. The retarder is preferably added within 1 to 4 hours after the mixture has been prepared. The longer this period, the more retarders must be added. 5) The curing time of the original mixture. For a mixture with slow curing, fewer retarders and more accelerators are needed than for one with faster curing. 6) The temperature of the concrete mix. The higher the temperature, the faster the mixture hardens and the more retarder is used. Because of the faster hardening, the retarder should preferably be added at temperatures above 20 ° C within 3 hours after the concrete mix has been made. 7) The volume of the concrete mix to be treated. A larger quantity of concrete mix will harden faster at a certain temperature than a smaller quantity at the same temperature, because the larger quantity cannot give off the heat of hydration as quickly, which leads to a greater increase in temperature and faster hardening.For example, a volume of 2 cubic meters becomes Concrete mix needs more than twice as much retarder than a volume of 1 cubic meter. 8) The ratio of prepared concrete mix to fresh concrete mix in the final mix. The proportion of the mixture prepared can be from 5 to 100% of the final mixture, but is preferably 10 to 50%. The higher the proportion of processed mixture, the more accelerator must of course be added. 9) The type and amounts of additives in the original and freshly added cement mix.

In a preferred embodiment of the method according to the invention, when a truck with an unused concrete mixture returns to the central concrete mill, the temperature of this mixture is first measured with a probe, the volume of the load is estimated and how much is calculated on the basis of this data and the known properties of the concrete mixture Delayers must be added to keep the load processable until the next working day. This calculation is advantageously carried out using a microcomputer programmed in advance and the calculated amount of retarder (preferably as an aqueous solution) is added using an automated metering system. After a short mixing process, the load can easily stand still until the next working day. Then the desired amount of fresh concrete mix is mixed with the prepared mix, the temperature is measured again and the required amount of accelerator (and - if necessary · water reducing agent) is calculated. The addition can again be done with an automated dosing system under computer control.

The following examples illustrate the invention. Unless otherwise stated, parts and percentages are by weight. The additives to the concrete mix are calculated (calculated as dry »active substance) in percent by weight based on the dry weight of the cement material (i.e. cement and fly ash).

Standard formulation of the concrete mix *

Part of the quantity in kg / m of concrete

Kaiser cement type I / Π 250

Fly ash (Jim Bridger, CF517 at 70 ° F 46.5

Sand 864

Gravel 1008

Water 175 -3-!

AT 393 501B

Delay formulation R aqueous solution containing 12.8% aminotri (methylenephosphonic acid) = Dequest 2000 5 8% citric acid

Speed up formulation C

Pozzolith SSSA (Master Builders) based on calcium nitrate (chloride-free) as an aqueous solution containing 10 48.7% active substance.

Water reducing agent: Formulation A

Pozzolith Polyheed (Master Builders), a slightly retarding water reducing agent based on 15 lignosulfonate as an aqueous solution containing 39% active ingredient.

example 1

Four samples of the standard formulation of the concrete mix are mixed and left in 20 liter containers for 20 hours. Three of these vessels are 5% aqueous solutions from Dequest 2000, with or without

Citric acid, as indicated in Table 1, was added to the fourth vessel with only 5% water. The concrete mixes were remixed and left overnight. After 24 hours, the three delayed mixes are mixed with fresh concrete mix in a ratio of 1: 5 and reactivated with 3.1% formulation C and 0.3% formulation A. Table 1 shows the compressive strength measured after 14 and 28 days

Table 1 No.% Dequest 2000% citric acid compressive strength in kg / cm ^ after 14 28 days 1 0.318 ... 290 390 2 0.227 - 277 370 3 0.318 0.200 321 432 4 - - 265 325 40

Example 2

Example 1 is repeated with the addition of 0318% Dequest product and 03% citric acid as a retarder in 2.5% aqueous solution. Reactivation is carried out as in Example 1. The results are summarized in Table 2 and the air content, the curing time (after reactivation) to strength and the compressive strength after 1 and 7 days are given.

Table 2 50

No.Dequest product% air curing time compressive strength in hours in kg / cm ^ after 1 day after 7 days 1 2000 2.0 4.0 153 443 2 2006 1.8 4.25 147 418 3 2010 1.9 3.875 140 402 4 2016 1.8 4.0 144 419 5 2041 1.8 5.0 121 423 -4- 55

AT 393 501 B

Table 2 (continued)

No.Dequest product 6 2051 7 2054 8 2060 9 2066 10% air hardening time in hours 2.0 3.5 1.9 3.25 1.8 3.875 2.2 3.75 0.7 5.5

Compressive strength in kg / cm after 1 day after 7 days 138 416 144 416 135 414 156 - 86 256

Example 3

As a function of the temperature of the concrete mix and the volume of this mix, it was experimentally determined which dosage of the retarder formulation R (in liters of the solution per 100 kg of the cement material) are necessary for deceleration overnight and over the weekend. The results are shown in Table 3.

Iabgüg-3.

Concrete temp. Overnight dosing Weekend dosing in ° C up to 1.5 na * over 1.5 m ^ 30-35 3.4 4.4 9.0 25-30 3.1 4.0 8.4 20-25 2 .5 3.4 7.8 15-20 2.2 2.8 7.1 10-15 1.5 1.5 6.5 5-10 1.2 1.2 5.9 0-5 0.9 0.9 5.3 with the

The delayed concrete mix can be reactivated after mixing one part with 5 parts fresh concrete mix in the dosage of accelerator formulation C given in Table 4 (in liters of the solution per 100 kg of cement material).

Act 4

Concrete temp. Overnight dosing Weekend dosing 25-30 1.9 5.3 20-25 2.5 6.5 15-20 3.1 7.1 10-15 3.7 7.5 5-10 4.4 7. 8 0-5 5.0 8.1 -5

Claims (16)

AT 393 501B PATENT CLAIMS 1. Process for the reprocessing of unused concrete mixtures, characterized in that a) a retarder is added before the concrete has hardened, in an amount which delays the hydration up to 90 hours; b) at the end of the desired delay period, an accelerator is added to bring the concrete mixture back into a hardenable state. 2. The method according to claim 1, characterized in that the delayed concrete mixture is diluted with fresh concrete mixture before the addition of the accelerator. 3. Process according to claims 1 or 2, characterized in that a phosphonic acid derivative is used as a retarder. 4. The method according to claim 3, characterized in that as a retarder at least one compound from the group of aminotri (methylenephosphonic acid), aminotri (methylenephosphonic acid) pentasodium salt, 1-hydroxyethylidene-1,1-diphosphonic acid, l-hydroxyethylidene-l, l- diphosphonic acid tetrasodium salt, ethylenediaminetetra (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid) -calcium / sodium salt, hexamethylene diamine tetra (methylenephosphonic acid), hexamethylenediaminetetra (methylenephosphonic acid) potassium salt, diethylenetriaminophenonic acid (methylenephosphonate) (methylenephosphonate) (methylenephosphonic acid) (methylenephosphonic acid), methylenediaphthalenopenta (methylenephosphonic acid) 5. Process according to Claims 1 to 4, characterized in that at least one compound from the group of hydroxycarboxylic acids or polycarboxylic acids and their salts, ascorbic acid, isoascorbic acid, sulfonic acid-acrylic acid copolymers, polyhydroxysilanes, polyacrylamides, carbohydrates and lignosulfonates is used as a retarder. 6. The method according to claim 5, characterized in that at least one compound from the group of hydroxycarboxylic acids, polycarboxylic acids, isoascorbic acid and polyhydroxysilanes is used as retarder. 7. The method according to claims 1 to 6, characterized in that the retarder from - at least one compound from the group of phosphonic acid derivatives and - at least one compound from the group of hydroxycarboxylic acids or polycarboxylic acids and their salts, ascorbic acid, isoascorbic acid, sulfonic acid-acrylic acid copolymers , Polyhydroxysilanes, polyacrylamides, carbohydrates and lignosulfonates 8. The method according to claim 7, characterized in that the retarder from - at least one compound from the group of aminotri (methylenephosphonic acid), aminotri (methylene-phosphonic acid) pentasodium salt, l-hydroxyethylidene-l, l-diphosphonic acid, l-hydroxyethylidene ll-diphosphonic acid tetrasodium salt, ethylenediaminetetra (methylenephosphonic acid), ethylenediaminetetra (methylenephosphonic acid), hexamethylenediamine tetra (methylenephosphonic acid) potassium salt, diethylene triaminophenonic acid and - citric acid exists 9. The method according to claim 8, characterized in that the retarder consists of aminotri (methylene-phosphonic acid) and citric acid 10. The method according to claim 9, characterized in that aminotri (methylenephosphonic acid) and citric acid in a ratio of 1: 1 to 2: 1 are used. 11. The method according to claims 1 to 10, characterized in that the accelerator consists of at least one compound from the group of calcium salts, thiocyanates, triethanolamine and glycolurils. 12. The method according to claim 11, characterized in that the accelerator consists of a mixture of calcium nitrate, sodium thiocyanate, triethanolamine and trimethylolglycoluril -6- AT 393 501 B. 13. The method according to claims 1 to 12, characterized in that a water-reducing amplifier is added together with the accelerator. 14. The method according to claim 13, characterized in that the water reducing amplifier is a 5 lignosulfonate derivative. 15. The method according to claim 2, characterized in that the reused concrete mixture makes up 10 to 50 percent by weight of the total mixture. 16. The method according to claims 1 to 15, characterized in that the retarder is added about 1 to 4 hours after the preparation of the original mixture.