Steel cooperation with concrete
One of the most important factors in the work of reinforced concrete construction is good adhesion of steel to concrete. It enables the correct transmission of tensile stresses from concrete to reinforcing bars, and in the case of higher stresses it ensures a specific slippage - scratches and cracks occur in the construction in the way assumed during the design process.
The standard for reinforcing steel grade B500SP as a criterion for the adhesion of steel to concrete provides the minimum relative surface area of the ribs fR. Under the standard conditions for this parameter, it is considered that the adhesion of the steel to the concrete after being built into the structure will be sufficient. Verification of the relative area of the ribs is currently performed in modern machines that measure the geometry of the entire bar by means of lasers. In the case of EPSTAL steel, the minimum surface area of the ribs fR is checked both in the place with EPSTAL and in the ribbing itself.
Regardless of the condition of the minimum relative rib area fR, the standard for reinforcing steel describes additional methods for verifying the adhesion of steel to concrete through laboratory testing of models. According to the standard PN-EN 10080 two main methods of this type can be distinguished:
- beam test,
- pull-out test.
For EPSTAL steel the Institute of Building Technology performed a pull-out test, which involves the removal of bars from concrete cubes. Concrete samples (class B15) were made in the form of cubes with a side length of 150 mm, with a EPSTAL steel bar of 18 mm anchored in each of them. The anchor length, which was assumed to be 5 bar diameters (5d = 80 mm), contained the EPSTAL inscription or self-ribbing. Three series of tests were carried out, each with 12 samples - 6 with EPSTAL inscription at the anchorage length and 6 without the inscription. The tearing force was increased incrementally by 5 kN so as to obtain about 10 steps for each sample.
Determining the pulling force of the rod and the displacement of its unloaded end allowed the determination of the traction stresses in the models tested. These stresses were several times greater than the minimum standard stresses (according to PN-EN 10080:2005) - both in ribbed zone and zone with EPSTAL inscription.
The applied adhesion stresses τbm / fcm for a given series of tests were:
- For samples without the inscription - from 0.56 to 0.71 (mean value is 0.63)
- for samples with 0.57 to 0.65 (average value is 0.61)
This means that the value of this stress on the samples with the inscription is less than 3.18% of the stress measured in the samples without the inscription. If we take into account the standard requirements for anchoring length for concrete B15 (according to PN-B-03264) - maximum 53 d minimum 22.5 d - we conclude that in the anchor bar made according to these requirements difference of adhesion stress for the zone with the inscription and without the inscription will be several times smaller than the calculated in the study, where the anchoring length was only 5d. Such small differences in stress do not affect the work of anchorage under computational loads.