Eksplosiv afskalning af beton: Nyudviklet prøvningsmetode

This project BK2 is a part of the larger project “Centre Contract – Resource Saving Concrete Structures”, also called “Green Concrete”. The large project was split into 3 phases: Ground Package, Big Package and Enhanced Package. This part of the project BK2 has been occupied with developing a new test method for explosive spalling of concrete. The method should be relatively simple and should give an indication beforehand if different types of concrete were critical in terms of explosive spalling. The advantage of this method is that the test specimen is relatively small, Ø 150 mm x 300 mm standard cylinder. The method is faster and easier to use than tests in full scale. The project BK2 was split into several parts. First the experimental arrangement was construct-ed, tested, and modified by means of some concrete cylinders from the stock on BYG•DTU and some cylinders made at Danish Technological Institute from a recipe of high strength concrete and some drill samples and a flag of concrete from a test in an earlier test facility. After this, several types of concrete from the “Green Concrete” project and a single concrete from BYG•DTU were tested in the Big Package and in the Enhanced Package. The experimental arrangement includes a steel mantle, 50 mm thick in two parts, surrounding the concrete cylinder by means of 12 pieces of Ø 36 mm steel bolts. Between the concrete cylinder and the steel mantle a pressure distributing material is placed, which is volume constant, to fill out the space between the cylinder and the mantle because they do not fit exactly into each other. The steel mantle is constructed in such a way that it can resist the same pressure from the ther-mal expansion that will develop at the surface of a concrete wall in a fire. The pressure from the thermal expansion is one parameter that is assumed necessary to give an indication of explosive spalling. Besides, the arrangement contains a mobile oven. After heating the oven to 1000°C, the end surface of the concrete cylinder is exposed. The cylinder end is centrally exposed in an area of Ø 100 mm and sounds from spalling etc. are noted with time etc. After one hour the test is terminated, the oven is removed, and the end of the cylinder is examined. In next phase, Big Package, 6 types of concrete in class aggressive environment and 6 types in class passive environment were tested for spalling. Furthermore, one concrete type was tested that should not explode. 6 types in class aggressive environment and 4 in class passive environ-ment, all with micro silica showed clear sign of spalling while 3 types without micro silica can be considered to be without risk of explosive spalling. After the tests, 4 types of cracks on the concrete were obtained. Two types of cracks; spalling and stretch cracks, are cracks that are expected at fire while the other two types, “tophat” and mid crack, are not wanted and may be considered as a result of the arrangement. The 2 last crack types are sought to be eliminated partly by mounting an “end stop” on the steel mantle, so that the pressure distributing material will not be forced out at the end of the steel mantle, and partly by means of demounting the upper part of the steel mantle just after the end of the test, which should give free expansion of the concrete cylinder by means of temperature movement. In Enhanced Package only 4 concrete types in class passive environment were chosen for test of explosive spalling. It was because of time and budget circumstances. The management of the centre contract decided that 4 concrete walls should be tested at Danish Institute of Fire Technology (DBI) of the same concrete type as the cylinders as standard of comparison for the new test arrangement. By use of a relatively thick wall there will, in accordance with the calculations, be built up a pressure in the surface of the wall in a fire situation because the middle of the wall is cold and therefore makes a force against the expansion in the surface. 3 walls were cast at Unicon in Jutland and one was cast at Sydsten in Sweden. Thereafter they were transported to BYG•DTU. The 4 walls 1.2 m long x 1.2 m high and 0.3 m thick were dried up in the test building at BYG•DTU in about 5.5 months to a moisture level of about 5 weight %. At DBI the walls were tested in a 6 m large oven, at a standard fire in one hour. None of the walls showed signs of explosive spalling, but there were a lot of small cracks in the surface and by an analysis it was concluded that there were several perforating cracks. These cracks are developed in two steps: At first some cracks are developed in the middle of the wall because there is tensile stress because of the expansion of the surface due to the fire. In the cooling phase the surface will contract, but is hindered as the middle is warm, and cracks at the surface will develop. All in all there will be cracks all way through the wall, but no spalling because of the minimal compression stress. It was decided in Enhanced Package to use a heat curve of the oven for explosive spalling that was of the same shape and level as the standard fire curve. At these tests there was no sign of spalling. The temperature was lower at the surface of the concrete cylinder than in the oven shown in a further analysis which certainly was the reason why no spalling was observed. Furthermore the distance from the oven to the surface of the concrete cylinder was also 10 mm longer than in Big Package. Crack type “tophat” seemed to be eliminated after mounting the “end stop”. On the other hand mid cracks still develop, even if the concrete cylinder is relieved from the pressure after one hour. But this crack type is harmless to the result. The results show that this test method can be used to give a rough estimation if a concrete with given moisture content has a risk of explosive spalling. To be certain that the new developed arrangement “catch” all concretes with the risk of explosive spalling, a temperature load is prescribed as in Big Package i.e. a momentary influence on the concrete cylinder surface from a 1000°C oven that gives about 800°C at the concrete surface.