Shrinkage-compensating concrete

Abstract

A shrinkage compensating concrete does not require restraint. The expansive forces developed during hydration compensate for concrete shrinkage, obviating the need for any added internal or external restraint element. Using this new shrinkage compensating concrete, substantially crack-free slabs may be built without using restraining steel bars, fibers, or other separate restraining element. The shrinkage compensating concrete includes a cement that develops internal expansive forces that never exceed the tensile strength of the concrete, such that the internal expansion compensates for the concrete shrinkage. The expansive cement may be an ASTMS, M or S cement, or other expansive cements may also be used.

Claims

1. A method of using a shrinkage-compensating mortar, comprising: making a shrinkage-compensating mortar by combining a hydraulic cement, an expansive additive, sand and water; forming the shrinkage-compensating mortar into a mortar structure, with substantially no internal restraint in the mortar structure and with no external restraint acting on the mortar structure; and the structure having an expansion exceeding the limits of ASTM C 845.

2. The method of claim 1 wherein expansive forces developed during hydration compensate for shrinkage.

3. The method of claim 2 wherein internal expansive forces are at all times lower than the tensile strength of the mortar such that internal expansion compensates for shrinkage, resulting in shrinkage-compensation without the need for reinforcement.

4. The method of claim 1 wherein the expansive additive comprises an ASTM Type K cement.

5. The method of claim 1 wherein the expansive additive comprises an expansive cement.

6. The method of claim 5 wherein the expansive cement comprises an oxide or sulfate that expands upon hydration.

7. The method of claim 6 wherein the expansive cement comprises calcium sulfoaluminate.

8. The method of claim 7 wherein the hydraulic cement comprises Portland cement.

9. A method of using a shrinkage-compensating mortar consisting essentially of a calcium sulfoaluminate cement, an expansive additive, sand and water, comprising: mixing the cement, the expansive additive, the sand and the water to provide a shrinkage-compensating mortar; forming the shrinkage-compensating mortar into a structure and allowing the structure to hydrate with substantially no internal restraint in the mortar structure and no external restraint acting on the structure.

10. The method of claim 9 wherein expansive forces developed during hydration compensate for shrinkage.

11. The method of claim 10 wherein internal expansive forces are at all times lower than its tensile strength, such that internal expansion compensates for shrinkage, resulting in shrinkage-compensation without the need for reinforcement.

12. The method of claim 9 having an expansion exceeding the limits of ASTM C 845.

13. The method of claim 12 wherein the mortar consists of the calcium sulfoaluminate cement, the expansive additive, the sand and the water.

14. A method of using a shrinkage-compensating mortar consisting essentially of a hydraulic cement, an expansive additive, sand and water, comprising: mixing the hydraulic cement, the expansive additive, the sand and the water to provide a shrinkage-compensating mortar having an expansion exceeding the limits of ASTM C 845; forming the shrinkage-compensating mortar into a structure and allowing the structure to hydrate with substantially no internal restraint in the mortar structure and no external restraint acting on the structure.

15. The method of claim 14 wherein the hydraulic cement is calcium sulfoaluminate cement, and the expansive additive is hydrated lime and/or calcium sulfate.

16. The method of claim 1 wherein the hydraulic cement is calcium sulfoaluminate cement, and the expansive additive is hydrated lime or calcium sulfate.

17. The method of claim 1 wherein the mortar has no internal restraint.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a graph of typical shrinkage as a function of time for a self-leveling hydraulic cement composition, with and without an expansive additive.

(2) FIG. 2 is a graph of expansion of a cement mortar composition with various amounts of expansive additives.

DETAILED DESCRIPTION

(3) Definitions

(4) Expansive cement: a cement that when mixed with water forms a paste that, after setting, tends to increase in volume to significantly greater degree than Portland cement paste. ACI 223-98.

(5) Shrinkage compensating cement: A cement that, when mixed with water, produces a paste that, after setting, increases in volume to a significantly greater degree than does Portland cement paste. ACI 223-98.

(6) Shrinkage compensating concrete: The ASTM standard specification for expansive hydraulic cement (ASTM C 845) defines shrinkage compensating concrete as a concrete that is internally restrained with resilient reinforcing and made with expansive cement with induces both compressive stress in the concrete and positive steel strain that approximately offsets tensile stresses induced by drying shrinkage.

(7) Several methods may be used to measure the expansive properties of concrete (See Rice, U.S. Pat. No. 3,779,085). The restrained expansion of mortar is generally measured using ASTM C 806: Test Method for Restrained Expansion of Expansive Cement Mortar. The compressive strength of the expansive cement is measured using ASTM Test Method C 109/C109M, except that a water-cement ratio of 0.50 shall be used, the specimens shall be covered with a polyethylene sheet or other suitable material for preventing loss or gain of moisture at the surface of the specimens during the moist storage period in the molds, and the specimens shall remain in the molds for 3 days.

(8) The scope of ASTM C 806 covers the determination of length changes of expansive mortar, while under restraint, due to the development of internal forces resulting from hydration of the cement. The apparatus used is as follows:

(9) Molds: The molds for casting test specimens, when used in conjunction with the restraining cage described below, shall provide for forming either 2 by 2 by 10 in. prisms having a 10-in. gage length, or 50 by 50 by 250 mm prisms having a 250 mm gage length. The molds shall otherwise conform to the requirements of Practice C 490, except that the cage stud holder, gage stud and spacer screws described in that specification shall not be used.

(10) Restraining cage: The cage consists of a threaded steel rod with steel end plates held in place by nuts. The rod shall be provided with cap nuts for the prevention of corrosion. The rod shall conform to specification A 307 grade A steel.

(11) The method for measuring the restrained expansion in concrete is ASTM C 878 Restrained Expansion of Shrinkage Compensating Concrete. The scope of ASTM C 878 covers the determination of the expansion of concrete made with shrinkage compensating cement. Its significance and use are as follows:

(12) Since the potential for expansion, under conditions of controlled restraint, of concrete made with shrinkage compensating cement cannot always be satisfactorily predicted from tests of mortars made in accordance with Test Method C 806, a need has been recognized for a test method in which concrete specimens are tested. This test method can also be adapted readily to studies of expansion involving degrees of restraint, comparison of cements, effects of cement contents, aggregates, mixture proportions, schedules or environmental treatments that differ from the standards procedures prescribed by this test method. The test restrains expansion using internal steel.

(13) External restraint: A restraint element external of the concrete conventionally used to restrain shrinkage compensating concrete. External restraint typically may be a surrounding structure, such as a previously existing concrete slab or a wall, that prevents a newly poured shrinkage compensating concrete from expanding. Pouring forms, such as plates, pipes, plywood, etc. used to simply confine the concrete are not external restraint. These types of forms confine, but do not restrain, the concrete.

(14) Internal restraint: A restraint element within the concrete used to restrain the expansion of the concrete. Typically these are steel bars or rods, or steel or polymer fibers.

(15) Substantial restraint in a slab. Substantial restraint for the purposes of this invention is defined in ACI 223R-10, section 5.2.2 as a ratio of steel reinforcement area to gross concrete area of 0.15%. Consequently, a slab built with a ratio of 0.015% or less shall be considered as lacking substantial restraint.

(16) Discussion

(17) The inventors have discovered that shrinkage compensating concrete may be used without any restraint element, while still largely avoiding cracking. Using the concrete of the invention, tensile stresses created by shrinkage remain smaller than the tensile strength at any given time, in the absence of restraint, thereby preventing cracking. Two examples are provided below: a slab on grade (Example 1) and a self-leveling composition (Example 2).

Example 1Restraint-Free Shrinkage Compensating Concrete

(18) A new shrinkage compensating concrete may be used with no internal or external restraining elements, and will still resist shrinkage cracking as well as expansive concrete structures using restraining elements, such as steel bars or steel or non-metal fibers. This unrestrained shrinkage compensating concrete may be made in various ways. One example is made with the mix proportions in Table 1.

(19) TABLE-US-00001 TABLE 1 ABS. BATCH VOL. SPEC. MATERIAL AMOUNT SOURCE WT (LB) FT.sup.3 GRAV Cement ASTM C150 382 1.94 3.15 Type II/V Cement CTS 123 0..63 3.15 Type K Komponent Water 33.3 277.4 4.45 1.00 gallons No. 3 35% Vuln. Bg 1141 6.9 2.65 Aggregate Rk Crk SV No. 4 26% Vuln. Bg 843 5.12 2.65 Aggregate Rk Crk SV Concrete 39% Vuln. Bg 1272 7.69 2.65 Sand Rk Crk SV C494 4.0 oz/cwt Eucon NW 20.2 oz/ Type A C + P cubic WR yard Material 4043.2 27.00 Totals Air 1% 0.27 Content Plastic 149.7 Unit pcf Weight
Komponent is an expansive material per ASTM Type K available from CTS Cement Mfg. Co. Cypress, Calif.

(20) Table 2 shows the aggregate gradation used for this concrete.

(21) TABLE-US-00002 TABLE 2 Size (mm) 0.15 0.075 4.75 2.36 1.18 0.6 0.3 No. No. 37.5 25 19 12.5 9.5 No. 4 No. 8 No. 16 No. 30 No. 50 100 200 FM No. 3 100 95 69 42 16 4 0 0 0 0 0 0 7.11 Agg. No. 4 100 100 91 16 3 0 0 0 0 0 5.90 Agg. Concrete 100 98 83 67 45 21 8 2 2.78 Sand Comb. 100 98 89 80 68 44 33 26 18 8 3 1 5.11 Grad.

(22) The concrete in Example 1 was cast as a 6 inch slab on ground. Two layers of thick plastic were placed between the sub-base and the concrete slab. The expansion in the concrete as measured by C 878 bars was 0.04% in seven days. Concrete strength by ASTM C 39 was 2,314 psi at 7 days and 2,632 psi at 28 days. The mortar expansion as measured by ASTM C 806 was 0.1830% at 7 days for 24 wt % Komponent. This expansion also places the cement of the outside the bounds defined by ASTM C 845. An inspection of the slab after five months found the slab to be crack free.

Example 2Self-Leveling Floors

(23) The concept described above can be extended to construction materials used as underlayment, self-leveling floors, and/or toppings. A self-leveling hydraulic cement-based topping mix usually exhibits high flow characteristics. It is typically used to create a flat and smooth surface with a compressive strength similar to or higher than that of traditional concrete prior to installing interior floor coverings. When it is poured, it has a viscosity similar to pancake batter. The low viscosity is obtained through the addition of polymers and/or large amounts of water. Since all of this water is not needed in the hydration of the cement, its evaporation can lead to drying shrinkage and cracking.

(24) The addition of controlled amounts of expansive additives will adjust the expansion of the floor topping to minimize drying shrinkage and result in a near-zero dimensional change during drying. As a result, the self-leveling floor is essentially crack-free.

(25) FIG. 1 shows the typical shrinkage as a function of time for a self-leveling hydraulic cement composition (TRU, available from CTS Cement Manufacturing Co, Cypress Calif.) with and without an expansive additive (Komponent ASTM Type K expansive cement, available from CTS Cement Manufacturing Co, Cypress Calif.). The measurement was made using a shrinkage cone apparatus fitted with a laser beam measurement device (Schleibinger Testing Systems). The advantage of the shrinkage cone technique, compared to the traditional ASTM C 878 bar is that the shrinkage is tested in the absence of metallic restraint. After 3 days, the shrinkage of the self-leveling composition was 0.005 in. without expansive additive, and 0.0027 in. (or about half that of the unmodified composition) with 10% of the expansive additive Komponent. With 12% of the additive, the material showed an expansion of 0.002 in. and 0.004 in. with a 14% addition of Komponent ASTM Type K expansive cement.

Example 3Restraint-Free, Shrinkage-Compensated Mortar

(26) Mortar can also be modified to exhibit crack-free expansion compensated to zero-shrinkage. Length change in such materials is usually tested using ASTM C157 and ASTM C596 standards. Is this example, the specimens were removed from the mold 30 minutes after final set and a first reading was taken. They were then stored in lime-saturated water for 7 days. Length change measurements were taken every 30 minutes for 3 hours after the initial reading, then daily for the seven days. After 7 days, specimens were taken out of lime water, and stored in air (733 F., 504% humidity).

(27) FIG. 2 shows that the early expansion caused by additives is able to compensate the subsequent shrinkage overtime-in the absence of restraint, which is a departure from the prior art. FIG. 2 shows shrinkage for the base material (CSA cement) reduced to near zero with the use of expansive additives (such as, but not limited to Calcium Sulfate and hydrated lime).

(28) Mix Proportions

(29) TABLE-US-00003 Mix 1 2 3 CSA Cement 33.33% 31.83% 31.50% ASTM C778 Silica 66.67% 66.67% 66.67% Sand Hydrated Lime 0.00% 0.20% 0.20% Calcium Sulfate 0.00% 1.30% 1.63% Water to Cement 0.47 0.47 0.47 Ratio

(30) As described, a concrete may include an expansive cement such that the expansive forces developed during hydration compensate concrete shrinkage, obviating the need for internal or external metallic restraint.

(31) A concrete may include an expansive cement such that the expansive forces developed during its hydration compensate concrete shrinkage, obviating the need for restraint and resulting in a substantially crack-free slab.

(32) A concrete may include a cement developing internal expansive forces at all times smaller than its tensile strength, such that the internal expansion compensates concrete shrinkage, resulting in a shrinkage-compensating slab without the need for internal reinforcement. The expansive cement may be a Type K cement. The expansive cement may include calcium sulfoaluminate or any other oxide or sulfate expanding upon hydration.

(33) A shrinkage-compensating concrete or mortar may be provided substantially without internal restraint in which the tensile strength of the concrete or mortar exceeds the expansive forces in the concrete

(34) A self-leveling hydraulic cement-based topping composition may be provided such that the internal expansion compensates shrinkage, resulting in a shrinkage-compensating topping without the need for internal reinforcement.

(35) A self-leveling hydraulic cement based topping composition may include an expansive agent causing internal expansion to compensate for shrinkage while remaining below the tensile strength, resulting in an essentially crack-free self-leveling floor. The expansive agent may be a calcium sulfoaluminate such as Komponent ASTM Type K cement.

(36) A method is provided for placing essentially crack-free slabs of concrete in which an expansive cement is mixed with hydraulic cement. The expansion compensates for shrinkage, so that the need for internal or external reinforcement is obviated.

(37) A method is provided for placing essentially crack-free self-leveling floors wherein an expansive compound is mixed with a self-leveling topping composition. The expansion compensates for shrinkage so that the need for internal or external reinforcement is obviated.

(38) Thus, a novel concrete and method has been shown and described. Various changes may of course be made without departing from the spirit and scope of the invention. The invention, therefore, should not be limited except by the following claims and their equivalents.