New Test Methods for the Textile Reinforcement of Concrete

Fiber-reinforced plastics for the textile reinforcement of concrete are gaining popularity. For tensile tests required in quality assurance, existing clamping solutions are either not suitable or too time-consuming due to the sensitive high-performance fibers. A new development shows how glass and carbon reinforcements with hydraulic jaws can make testing quick and economical. Manufacturers as well as research institutions and testing centers want to achieve reproducible and economical tensile testing of reinforcements made of fiber-reinforced plastics (FRP reinforcements). The special requirements for this test are especially evident in that high-performance fibers are extremely brittle and carbon fibers are especially sensitive to lateral pressure. Therefore, a conventional clamping solution with steel jaws is not suitable. This also applies to the versions that were specially developed in the past few years for tensile testing on stiff reinforcements impregnated in epoxy resin.

For example, this includes a test arrangement as per ISO 3341:2000-05 in which yarn is held in place via deflection rollers and tested. The arrangement has been tried and tested for impregnated, uniform fiber strands (rovings) and fiber strands with soft, impregnated masses, however, not for stiff fiber strands with thermoset impregnation masses. Clamping fiber strands using steel plates screwed across from each other or standard specimen grips with rubber-like compensating layers does not produce the required results either since, depending on the properties of the fiber strands, they can negatively influence the strength in the transitional area to clamping. The only method so far that enabled fiber-compliant load application is the adhering of the fiber strands in sleeves. However, this method is very time consuming and as a result not ideal for larger test programs.

Polymer compensation layer reduces clamping pressure For this reason, solidian GmbH (Albstadt, Germany) developed a clamping solution using special material-compliant jaws together with the testing machine manufacturer, ZwickRoell (Ulm, Germany). This complies with the requirements of the RFP reinforcements and, at the same time, enables quick, real-world testing to be performed. The new testing fixture's basis is a parallel closing hydraulic wedge grip that is designed to handle maximum tensile forces of 250 kN. Only the grip inserts were replaced, which usually feature level steel surfaces, with a milled pyramid grid for example.

When directly clamping fiber strands with the steel jaws, the serrations of the jaw surface will press into the comparatively soft epoxy resin and damage filaments. Independent of that, there is an abrupt switching of the fiber's strain at the transition to the clamping area due to the evenly distributed clamping pressure. To perform the load entry more carefully in the transition area especially, the clamping pressure was continually reduced in the direction of the middle of the fiber using to two measures. For one, the jaws protrude several centimeters from the specimen grip so that only indirect clamping pressure transferred via the stiffness of the jaws is applied to the fiber strand in the transitional area. For the second measure, the developers use a special compensating layer made from thermoplastic polymers with corundum filling. It is placed so that the 5mm thickness of the insert increases to 8 mm in the upper area. Due to the lower overall depth, the clamping pressure is also reduced in the transitional area.  

Influence of the cross strand Six fiber strands of a series with a length of 440 mm from are removed for testing. The cross strands are cut short so that, in total, they reach a total a length between 10 mm and 15 mm. For the reinforcement grid nodes that are especially thick or for extremely high tensile strength, it may be beneficial to remove the cross strand to avoid strength minimizing of concentrated tension in node areas. The influence on the respective evaluation type can be evaluated in advance by comparing the tensile strength of fiber strands with and without cross strands. During installation, the specimen is aligned to the screwed-in stops and held in place until a closing pressure of 30 bar has been reached above and below. Before starting the test, the clamping pressure is increased. Ideally, the clamping pressure is determined as low as possible during pretesting. For extremely high-strength fiber strands that have a force at break of over 20 kN, clamping pressures of approximately 100 bar are required. The control is displacement-controlled via the crosshead. For the free length of 160 mm tested here, the crosshead speed is set to 3 mm/min. For other lengths, the speed must be adjusted so that there is a similar strain rate. The test is ended either when there is a tensile failure of the fiber strand or when the fiber is removed from the clamping area. Conclusion Previous experience has shown that also sensitive fiber-reinforced plastics with hydraulic jaws can be tested if the clamping technology is adapted to the lateral pressure sensitivity of the material. In the first comparison series with similar fiber strands which were adhered inside sleeves, no significant differences such as a lower tensile strength due to clamping were observed.

Image source: solidian GmbH

Article in the trade magazine Bauingenieur