Karim Hariri

Effects of a clay additive on the properties of no-slump concrete

The strength of no-slump concrete is mainly achieved by a high particle packing density (ppd), which causes a strong interlocking of the coarse aggregates. Also inert fillers like fly ash are enhancing the strength by help of interlocking of micro-sized particles (Bornemann 2005). Both the compressive strength in the green and hardened state is enhanced. However, additional water or liquifier is needed to keep a sufficient workability. Accordingly there’s often a lack of water for the cement hydration process, that is either retained by the filler or is replaced by high superplasticizer content. Anyway there is a discrepancy to be noted between high green strength (little water needed) and high compression strength of hardened concrete (more water needed). The stickiness of a filler-optimized mixture causes hindrances in manufacturing, yet another problem. Another theory, wherein green strength is explained by capillary forces between micro-sized particles (Wierig 1972) seems to fail. Capillary forces have a maximum effect on particles with diameters from 0.1-1 mm, while friction forces are dominant when particle diameters exceed 1 mm (Bornemann 2005).

Assessment of the State of Condition of Prestressed Concrete Structures with Innovative Measurement Techniques

The reliability of tensile elements is a requirement for the safety of prestressed structures. Besides many other data, the assessment of the load-carrying capacity of a prestressed concrete structure requires the knowledge of the actual force of the tendons and their corrosion state as well as the localization of fractures. Neither for grouted interior nor for ungrouted exterior tendons non destructive and reliable methods to measure the prestress and to assess the corrosion activity or to locate tendon fractures are available today. This contribution reports on actual research projects, dealing with fiber optic sensors to measure deformations and dealing with the variation of high frequency damping of steel tendons in concrete members due to corrosion. Furthermore a high-frequency-method is used to locate fracture in tendons. First applications of some of these innovative techniques on a real structure are presented: the Herrenbrücke-bridge in Lübeck. Since three winters, measurements on this prestressed concrete bridge assessing its state of condition are carried out.