Concrete composition

Abstract

The concrete composition includes, in a mixture with water, a hydraulic binder, sand and aggregates, wherein the hydraulic binder includes a Portland cement of high reactivity, and the hydraulic binder is present in an amount of 280-340 kg per cubic meter of concrete, a shrinkage reducing admixture is present in an amount of 0-4 L per cubic meter of concrete, and water is present in an amount of 140-160 L per cubic meter of concrete.

Claims

1. A concrete composition comprising, in a mixture with water, a hydraulic binder, sand and aggregates, wherein the hydraulic binder comprises a Portland cement of high reactivity, wherein the Portland cement has a Blaine fineness of 4500-6500 cm.sup.2/g, characterized in that the hydraulic binder is present in an amount of 280-310 kg per cubic meter of concrete, that a shrinkage reducing admixture is present in an amount of 3-4 L per cubic meter of concrete, wherein the shrinkage reducing admixture is an organic admixture, and that water is present in an amount of 140-160 l per cubic meter of concrete.

2. A concrete composition according to claim 1, wherein the reactivity of the hydraulic binder is selected so as to obtain a 28d compressive strength of the concrete of >52.5 MPa.

3. A concrete composition according to claim 1, wherein the reactivity of the hydraulic binder is selected so as to obtain a 1d compressive strength of the concrete of >20 MPa.

4. A concrete composition according to claim 1, wherein the hydraulic binder comprises a supplementary cementitious material.

5. A concrete composition according to claim 4, wherein the supplementary cementitious material is present in an amount of 1-20 wt.-% of the hydraulic binder.

6. A concrete composition according to claim 4, wherein the supplementary cementitious material has a Blaine fineness of 4500-6500 cm.sup.2/g.

7. A concrete composition according to claim 1, further comprising an expanding agent in an amount of 0.5-3 wt. % of the hydraulic binder.

8. A concrete composition according to claim 1, further comprising a water reducing agent.

9. A concrete composition according to claim 8, wherein the water reducing agent comprises poly-carboxylate ether or polynaphthalene sulfonate.

10. A concrete composition according to claim 1, wherein the concrete composition is freshly mixed and the amount of water and hydraulic binder is selected so as to reach a flow of the freshly mixed concrete at 10 min of at least 100 mm+/10 mm.

11. A concrete composition according to claim 1, wherein within less than 50 days after placing the concrete has a relative humidity of 80% according to ASTM F2170.

12. A concrete composition according to claim 1, wherein the shrinkage reducing admixture is based on polypropylene glycol, polyethylene glycol and/or a glycol ether derivative, and is, optionally, in a liquid mixture with surfactants.

13. A concrete composition according to claim 4, wherein the hydraulic binder comprises, as the supplementary cementitious material, ground granulated blast furnace slag, fly ash, pozzolans, ground limestone or mixtures thereof.

14. A concrete composition according to claim 7, wherein the expanding agent comprises CaO.

15. A concrete composition according to claim 8, wherein the water reducing agent, is present in an amount between 0.4 and 1 wt. % of the hydraulic binder.

16. A concrete composition according to claim 1, wherein within less than 40 days after placing the concrete has a relative humidity of 80% according to ASTM F2170.

17. A concrete composition according to claim 1, wherein within less than 30 days after placing the concrete has a relative humidity of 80% according to ASTM F2170.

18. A construction element comprising concrete obtained from a concrete mix composition according to claim 1.

Description

(1) According to a preferred embodiment of the invention the hydraulic binder comprises ordinary Portland cement and a supplementary cementitious material, such as granulated blast furnace slag, fly ash, pozzolans or mixtures thereof. The supplementary cementitious material is preferably present in an amount of 1-20 wt.-% of the hydraulic binder. In order to achieve sufficient reactivity, the supplementary cementitious material may preferably have a Blaine fineness of 4500-6500 cm.sup.2/g.

(2) According to the invention the concrete composition may contain a shrinkage reducing admixture in an amount of 0-4 L per cubic meter of concrete. In this connection, it was surprisingly found that the use of a shrinkage reducing admixture at lower binder amounts per cubic meter of concrete significantly lowers the drying time of concrete, whereas at higher binder contents, it tends to slow the drying of concrete. When used in specific conditions, the shrinkage reducing admixture accelerates the drying of hardened concrete.

(3) In light of the above, a preferred embodiment of the invention provides for the shrinkage reducing admixture being present in an amount of 3-4 L per cubic meter of concrete and the hydraulic binder being present in an amount of 280-310 kg per cubic meter of concrete. Thus, a relatively high amount of shrinkage reducing admixture is used to reduce the drying time of concrete at lower binder amounts per cubic meter of concrete. In contrast, in another preferred embodiment of the invention, no or a little amount of shrinkage reducing admixture is used at higher binder amounts per cubic meter of concrete. In particular, the shrinkage reducing admixture may be present in an amount of 0-1 L per cubic meter of concrete and the hydraulic binder may be present in an amount of 310-340 kg per cubic meter of concrete.

(4) In the context of the invention, conventional liquid shrinkage reducing admixtures may be used. Chemical shrinkage reducing admixtures are commonly used to mitigate drying shrinkage of concrete. They are in most cases a liquid mixture of surfactants and glycols. Their mechanism of action relies on the reduction of the surface tension and capillary forces upon free water evaporation in the pores of less than 50 nm diameter. These chemical admixtures do not cause the concrete to expand, nor prevent it from shrinking: their main purpose is to reduce the extent of shrinkage.

(5) Preferably, the shrinkage reducing admixture is an organic admixture, in particular based on polypropylene glycol, polyethylene glycol and/or a glycol ether derivative, preferably in a liquid mixture with surfactants.

(6) In order to improve the workability of the concrete mass, the mixture may preferably comprise a water reducing agent, in particular a polycarboxylate ether based or a polynaphthalene sulfonate based plasticizing admixture, the water reducing agent preferably being present in an amount of between 0.4 and 1.5 wt.-% of the hydraulic binder.

(7) Plasticizing admixtures are sometimes also referred to as dispersing agents or flow agents and are deployed in order to improve workability with regard to flowability. Such admixtures are long chained organic molecules, that group around the cement particles and thus either bring about electrostatic repulsion between the particles or steric stabilisation of the particles, thereby increasing flowability of the building material.

(8) According to a preferred embodiment of the invention, the concrete composition further comprises an expanding agent, such as CaO, preferably in an amount of 0.5-3 wt.-% of the hydraulic binder. Calcium oxide is a known expanding agent that is able to compensate the natural drying shrinkage that concrete undergoes during its service life. This reduces the formation of cracks resulting from plastic shrinkage of the concrete mass, which in turn increases the water tightness of concrete structures. It was found that the addition of calcium oxide at a dosage of up to 3.0 wt.-% does not have a significant impact on concrete drying times.

(9) The concrete composition of the invention, in particular by adjusting the amount of water and hydraulic binder, allows to achieve a flow of the freshly mixed concrete at 10 min of at least 100 mm+/10 mm.

(10) In a particularly preferred embodiment, the concrete, within less than 30-50 days, in particular less than 30-40 days, after placing has a relative humidity of 80% according to ASTM F2170.

(11) The invention will now be described with reference to the following exemplary embodiments.

(12) Concrete was mixed according to several designs as described in Table 1. The aggregates used in all concrete compositions were identical and followed a 0-16 Fuller size gradation. All concretes contain 150 L of water per cubic meter. The dosage of the admixture was adjusted to reach a concrete slump at 10 minutes of 100 mm+/10 mm. Mortar of concrete is sieved and concrete slabs of 30 by 90 cm size and 15 cm thickness were casted and cured in room at 20 C. and 65% relative humidity. The mortar is placed in a bowl and let to sit 45 minutes. The bowl is then gently tilted and the water that appeared is collected and weighed. Bleeding is then expressed as a percentage of total water in the concrete composition. The relative humidity is measured as function of time following the procedure described in ASTM F2170, and Table 2 reports the time in days required to reach 80% relative humidity.

(13) TABLE-US-00001 TABLE 1 Total Slag Water Shrinkage Mix binder content reducer reducing CaO # (kg/m3) (wt-%) (wt-%) agent (L/m.sup.3) (wt-%) 1 280 15 0.5 0 0 2 280 15 0.7 0 3 3 340 15 0.5 4 0 4 340 15 0.6 4 3 5 280 15 0.5 4 0 6 280 15 0.7 4 3 7 340 15 0.5 0 0 8 340 15 0.7 0 3

(14) The results displayed in Table 2 show that under the described laboratory conditions, the reference concrete (mix #1) requires 100 days to dry to an internal relative humidity of 80%. Adding 3% calcium oxide does not have a significant impact on drying times. Comparing lines 1 and 2 to lines 7 and 8 shows that increasing the binder content decreases significantly the drying times. Interestingly, line 6 shows that the same shorter drying times can be achieved with leaner concrete that contains 4 L/m3 of shrinkage reducing agent. On the contrary, comparing lines 3 and 4 to lines 7 and 8 shows that adding the shrinkage reducing agent to a concrete containing more binder per cubic meter increases the drying times, that means the contrary effect to that observed at 280 kg of binder per cubic meter.

(15) In all systems, bleeding water is collected, and remains highest in the reference concrete (mix #1). Even the concrete compositions that give the lowest drying times have acceptable bleeding water amounts.

(16) TABLE-US-00002 TABLE 2 Time to reach 80% Bleeding at Mix relative humidity 45 minutes # (days) (wt-%) 1 100 4.6 2 105 3.2 3 69 3.2 4 75 2.2 5 39 4.5 6 47 3.8 7 40 3.1 8 39 2.4

(17) To sum up, the invention describes concrete compositions that combine several opposing physical properties: reduced drying times, but good fresh concrete workability and capacity to generate bleeding water at the concrete surface once placed. Further, the concrete compositions of this invention exhibit no stickiness. It was found that the use of shrinkage reducing admixtures, in specific conditions of water and binder amounts per cubic meter of concrete, act as an accelerator for concrete drying.