DE3626506A1 - Process for reducing surface cracking in rods and wires of concrete-reinforcing steels - Google Patents

Abstract

The invention relates to a process for reducing surface cracking in rods and wires of steels for the steel-reinforcement of concrete, subject to dynamic stress. The steel reinforcement of concrete is here carried out with non-prestressed steels. According to the invention, there is a restriction to rods and wires of pronouncedly soft steels with less than 0.23% of carbon, a yield strength of less than about 650 N/mm<2> and a tensile strength of less than about 750 N/mm<2>. These steels are preferably processed by the user in the hot-rolled state with edge decarburisation caused by the manufacture. According to the invention, they undergo a shot-peening treatment which is carried out to produce roughness. As a result, no cracking is detectable even in the case of more than 2,000,000 load changes with stresses which correspond to the static strength properties.

Description

The invention relates to a method in the preamble of claim 1 designated Art.

The steels intended for reinforced concrete reinforcement are in not pre-stressed. They are subject to built-in State of a dynamic load, however, so that their fatigue strength is of considerable importance. The static strength values can therefore only be one Approach a fraction, which depends on the one hand on the amount of Strength and on the other hand on the surface condition of the steel depends. These are the relationships familiar to the expert Understanding that vibrations or fatigue cracks starting from superficial cracks.

To improve the dynamic property values of the steels therefore find thermal and mechanical processes for surface hardening or influence application, which usually the steelmaking process, either by Connection to the exit of the steels from the last scaffolding a rolling mill can be carried out or by in cold state undergo a limited deformation, that for work hardening leads. From the processor, for example Manufacture reinforcement meshes from hot-rolled, ribbed wires Such methods are therefore not applicable.

For steels of higher strength, such as tempering steels, that hardened can be spread by shot peening Avoid vibration cracks below 0.65 mm in length. When crossing the fatigue strength is formed below the Surface new cracks that cause a break (VDI report No. 506, 1984, pages 32/33).

With steels intended for the reinforced concrete reinforcement one knows also galvanic and plastic coating processes, through which in particular corrosive influences that can also lead to cracks, should be prevented.  

In this case, however, the anchoring behavior of the natural Steel surface affected in concrete.

The invention is based on the task, in the introduction designated steels to reduce the surface Cracking to such an extent that the dynamic load values correspond to the static ones Property values of the steels can choose. The process is supposed to not necessarily tied to the manufacturing process be, but also carried out by the consumer can. The adhesion between the steel surface and the Concrete should be preserved or even an improvement Experienced.

The invention solves this problem according to the proposal of Labeling part of claim 1, for which the subclaims Provide 2 to 6 advantageous developments.

The invention therefore limits those used Steels both analytically and in terms of their mechanical Property values. This does not exclude that part of carbon is replaced by known carbon equivalents can be as intended by British Standard. The The hardness of the steels mentioned is below about 400 HB or HV. Due to the manufacturing decarburization and surface topography is the fatigue strength usually relatively small. Surprisingly, it now works here with a roughness-forming shot peening treatment Surface cracks appear under dynamic load conditions largely avoided. A roughness like that for the anchoring of the steels in the concrete is desirable by given the proposed type of shot peening.  

The shot peening can be conveniently carried out under one Pressurized fluid jet grains or by means of centrifugal wheels run accelerated abrasive grains. While at that the latter case, several centrifugal wheels made of different Directions to the steel surface for exposure bring are, in the former case, the ball currents Nozzles brought up to the steel surface.

When applied by means of nozzles, these are expediently in lying radially aligned with the steel to be treated, with their exit directions about each other form the same angle and the inside diameter of the nozzles much smaller than the diameter of the to be treated Steel are. In this way it is ensured that the radiation energy optimally absorbed by the steel to be treated the distance between the nozzle and the workpiece is such to be chosen is that the beam unfolding as little as possible over the Thickness of the steel to be treated goes beyond.

There can certainly be a mutual impairment of the Avoid nozzles if their exit directions are longitudinal of the steel to be treated are slightly inclined. For example this angle of inclination can be approximately 10 °.

The invention is further with reference to the following Illustrated embodiment illustrated.

The steel to be treated was a class III S reinforcing steel, Diameter 16 mm, the first try and a reinforcing steel Class IV S, diameter 28 mm, used as the basis for the second experiment.

The shot peening treatment was as follows in both cases specified stated:  

Blasting media S 230S (shot blasting shot),
Blasting media diameter: 0.5 to 1.0 mm 54 HRC abrasive hardness
Abrasive amount per surfacem / a = 2 g / mm^2nd Distance nozzle / workpieceH _D  = 100 mm Nozzle diameterd _D  = 10 mm Abrasive amount per time  = 5 kg / min Pressure in the blasting systemp = 5 bar Pressurized air.

As a result of this treatment, the following results were achieved in the two samples designated A and B in a dynamic test:

Dynamic tests on straight, rebars not concreted in concrete

• A) Reinforcing steel III S, ○ 16 mm
  Upper load: (0.7 × actual yield point × actual cross section)
  Vibration range of the tension: 2 s _a = 215 N / mm ²
  untreated shot-peened 597 000 ≦ λτ 2 013 000 761 000 ≦ λτ 2 251 100 1 530 600 ≦ λτ 2 017 800
• B) Reinforcing steel IV S, ○ 28 mm
  Upper load: (0.7 × actual yield point × actual cross section)
  Stress range: 2 _a = 200 N / mm ²
  untreated shot peened 623 0002 460 000 589 6002 165 000

While the untreated samples under dynamic load Load changes between 350,000 and 1,530,000 broke, were able to use the treated samples without exception at load change numbers of more than 2,000,000 no cracks were found will. The invention thus solves the underlying problem.

Claims (6)

1. A method for reducing the surface cracking of bars and wires made of steels for reinforced concrete reinforcement, which are subject to dynamic loading, characterized in that those made of soft steels with less than 0.23% carbon, a yield strength of less than about 650 N / mm ² and a tensile strength of less than about 750 N / mm ² existing rods and wires in the hot-rolled state with conventional edge decarburization are subjected to a roughness-forming shot peening treatment. 2. The method according to claim 1, characterized, that blasting with blasting grains under pressure fluid is performed. 3. The method according to claim 1, characterized, that the blasting was accelerated by means of blast wheels Grit is executed. 4. The method according to claims 1 and 2, characterized, that used for blasting nozzles lying in one plane that are radial to the steel to be treated are aligned and their exit directions in relation form approximately equal angles to each other, the inner diameter the nozzles are substantially smaller than the diameter of the steel to be treated. 5. The method according to claims 1, 4, characterized, that nozzles are used for the blasting, their exit directions to the longitudinal direction of the to be treated Steel are inclined. 6. The method according to claim 5, characterized, that the angle of inclination of the exit directions of the nozzles about 10 ° to the longitudinal direction of the steel to be treated in the direction of the steel pass.