COATED STEEL FIBER FOR REINFORCEMENT OF A CEMENTITIOUS MATRIX

Abstract

A steel fiber adapted for the reinforcement of a cementitious matrix is provided with a zinc aluminium alloy coating. The amount of aluminium ranges from 0.05 wt % to 0.5 wt %. The remainder of the coating being zinc and unavoidable impurities. The small amount of aluminium delays the growth of the alloy layer and delays the appearance of brown rust stains on the surface.

Claims

1. A steel fiber adapted for the reinforcement of a cementitious matrix, said steel fiber being provided with a zinc aluminium alloy coating, the amount of aluminium in the zinc aluminium alloy coating ranging from 0.05 wt % to 0.5 wt %, the remainder of the coating being zinc and unavoidable impurities.

2. The steel fiber of claim 1, wherein said steel fiber with the coating is end drawn.

3. The steel fiber of claim 1, said steel fiber having a steel core, a zinc aluminium alloy coating and an intermetallic layer with Fe.sub.2Al.sub.5 between said steel core and said zinc aluminium coating, said intermetallic layer having a thickness being smaller than 1 micrometer.

4. The steel fiber according to claim 1, wherein said zinc aluminium alloy coating is present in amounts above 25 g/m.sup.2.

5. The steel fiber according to claim 1, wherein said steel fiber has a length over diameter ratio L/D ranging from 40 to 100.

6. The steel fiber according to claim 1, wherein said steel fiber is provided with anchorages formed by cold deformations such as undulations, bendings, nail heads.

7. The steel fiber according to claim 6, said steel fiber having a middle portion between said anchorages, said middle portion having an elongation at maximum load A.sub.g+e being greater than 4%.

8. A bundle of steel fibers according to claim 1, said bundle of steel fibers being glued together by means of a water solvable glue, imidazoles, triazoles or tetrazoles being present in said glue.

Description

MODE(S) FOR CARRYING OUT THE INVENTION

[0030] A steel fiber according to the present invention may be manufactured along following lines.

[0031] Starting product is a steel wire rod with a diameter of 5.5 mm or 6.5 mm and with following steel composition: A minimum carbon content of 0.35 wt %, a manganese content ranging from 0.40 wt % to 0.70 wt %, a silicon content ranging from 0.15 wt % to 0.30 wt %, a maximum sulfur content of 0.05 wt %, a maximum phosphorus content of 0.05 wt %. Possible minor amounts of chromium, nickel or copper may be present up to 0.20 wt %.

[0032] The steel wire rod is dry drawn until a half product steel wire with an intermediate diameter ranging between 1.5 mm and 3.5 mm.

[0033] The thus drawn steel wire is provided with a metallic coating of a zinc aluminium alloy with 0.2 wt % aluminium in the coating. This coating operation is preferably done by means of a hot dip process where the steel wire goes through a bath of the molten zinc aluminium alloy. The coating weight on the half product steel wire ranges from 70 g/m.sup.2 to 200 g/m.sup.2.

[0034] The thus coated half product steel wire is then dry drawn until a steel wire with a diameter below 1.0 mm. The coating weight on the final steel wire is higher than 20 g/m.sup.2, preferably higher than 30 g/m.sup.2. The tensile strength of the drawn steel wire may range from 1200 MPa to 2700 MPa, mainly depending upon the final reduction and the carbon content in the steel composition. The surface of the steel wire does not show any rust or colouring departing from the normal appearance of a zinc alloy coating.

[0035] Analysis by means of an optical microscope shows the presence of a very thin intermetallic Fe.sub.2Al.sub.5 layer with a thickness ranging from 50 nm (nanometer) to 150 nm.

[0036] A classical wrapping test carried out on the steel wire shows no flaking of the zinc alloy coating, which means that the adhesion is good, despite the very thin intermetallic layer.

[0037] Measurement of coating centricity has resulted in values ranging from 5% to 30%, which are rather low values but still within an acceptable range. These lower values were expected since it is generally known that absence of intermetallic layers or very thin intermetallic layers have a negative influence on centricity particularly with low coating weights around 30 kg/m.sup.2.

[0038] A stress relieving treatment at a temperature in the range of 420° C. to 450° C. is applied to the steel wire during a couple of seconds, in order to increase the elongation at maximum load A.sub.g+e to more than 4%, e.g. more than 5%. This stress-relieving temperature is higher than in case of a pure zinc coating (only 400° C.-410° C.). Despite this higher temperature, no brown rust spots are noticed on the surface of the steel wire with the Zn0.2Al coating when exposed to a corrosive environment.

[0039] This is in contrast to a steel wire with a pure zinc coating. In the case of a pure zinc coating, the iron zinc alloy layer quickly grows during stress relieving, even up to the surface of the coating. So iron is present in the top layers of the zinc coating. After short exposure in a corrosive environment, brown rust stains are appearing, despite the abundant presence of zinc and despite the galvanic protection.

[0040] The coated and stress-relieved steel wire is then cut and bent to result in several steel fibers provided with bendings at the end. Examples of anchorages in the forms of bendings are disclosed in U.S. Pat. No. 3,900,667, EP-B1-0 851 957, EP-B1-2 652 221, and EP-B1-2 652 222.

[0041] Before being cut and bent, the steel wires may be bound together by means of a water solvable glue in order to form a strip according to U.S. Pat. No. 4,284,667. According to the teaching of WO-A1-2006/067095, Benzimidazole is added to the glue in order to prevent evolution of hydrogen gas.

Three Point Bending Test

[0042] In order to check the compatibility of the newly coated steel fibers according to the invention with concrete, standard three point bending tests have been carried out according to norm EN14651. Beams with dimensions 150 mm×150 mm×600 mm and made of C35/45 concrete have been reinforced with steel fibers having a zinc 0.2 wt % aluminium coating and provided with bendings according to EP-B1-2 652 222. Tests have been done for three different dosages of steel fibers.

[0043] Following parameters have been calculated:

limit of proportionality f.sub.L=3/2×F.sub.L×/(b×h.sup.2) [0044] with F.sub.L=maximum load between 0 and 0.05 mm deflection [0045] residual flexural strength f.sub.R,1

CMOD=0.5 mm

deflection=0.47 mm [0046] residual flexural strength f.sub.R,2

CMOD=1.5 mm

deflection=1.32 mm [0047] residual flexural strength f.sub.R,3

CMOD=2.5 mm

deflection=2.17 mm [0048] residual flexural strength f.sub.R,4

CMOD=3.5 mm

deflection=3.02 mm

f.sub.R,i=3/2×F.sub.R,i×/(b×h.sup.2) [0049] with F.sub.R,i=load at the corresponding CMOD or deflection

[0050] The table hereunder summarizes the results. The mentioned values are average values of five to six tests.

TABLE-US-00001 TABLE Dosage f.sub.L f.sub.R, 1 f.sub.R, 2 f.sub.R, 3 f.sub.R, 4 (kg/m.sup.3) (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2) (N/mm.sup.2) 20 5.54 2.80 3.54 3.88 3.79 30 5.69 5.07 5.99 5.81 5.56 40 5.58 4.69 5.87 6.03 6.12

[0051] The above results have been compared with results obtained from steel fibers with same geometry and composition but with a standard 100% zinc coating. Some differences are noticed but these differences are within the acceptable standard range.

Passivation Test

[0052] Thin test plates with following standard concrete composition have been made: [0053] 400 kg/m.sup.3 of CEM I 45.5 R HES; [0054] 1200 kg/m.sup.3 of river sand 0/4; [0055] 200 kg/m.sup.3 of crushed limestone 4/7.

[0056] Three test plates have been made with varying coating thicknesses: 28 g/m.sup.2, 33 g/m.sup.2 and 40 g/m.sup.2.

[0057] A number of invention steel fibers are placed at the bottom of each mold before filling it with concrete. The samples are vibrated for 30 seconds and cured in a humid environment before demolding. After a curing time, the bottom of each sample is inspected.

[0058] None of the test plates showed formation of hydrogen.