Chemical prestressing of carbon reinforced concrete
Table of contents
Project data
Titel | Title |
Report from the annual report 2022
Expansion tests on concrete samples
Prestressing of concrete structures is the process of introducing compressive stresses into the concrete members before using them. With the help of prestressing, concrete elements can be considerably stiffened and their stress state can be favorably influenced. Structures can thus achieve better crack resistance and less deformation. Prestressing is well established for steel reinforced concrete, but for structures with carbon and especially textile reinforcement, the prestressing process is extremely complex, mainly because of the transverse pressure sensitivity of the fiber materials. Furthermore, it is not feasible to provide a large number of hydraulic jacks to prestress the large number of carbon fiber rovings.
One of the most promising alternatives for hydraulic prestressing is chemical prestressing. Hence, the aim of this project is to use the chemical prestressing technology to prestress carbon reinforced concrete. The technology is based on the addition of swelling admixtures in the concrete. When the volume of the concrete increases, it exerts tensile stresses on the carbon fiber reinforcement, which in return, apply compressive stresses in the concrete. Thus, prestressing takes place without mechanical devices or additional operations.
One of the most important phases in this project is to develop a suitable concrete mixture that gives reasonable amount of expansion with maintaining good concrete characteristics. Therefore, a number of concrete mixtures were manufactured. All mixtures had the same components except for the swelling agent, which substituted between 10 and 20 % of the cement by weight. The results have shown that expansive agent additions of more than 17 % lead to very high expansion; however, the compressive strength is decreased significantly. On the other hand, expansive agent additions of below 11 % leads to small expansion. The most appropriate additions were found to be between 12–15 %, which produced good expansion, while maintaining a reasonable compressive strength. This percentage will be used in the next phase of the project, which includes reinforcing the concrete with textile carbon reinforcement to study the prestressing force.
Report from the annual report 2021
Chemical prestressing of CRC
With the help of pre-stressing, concrete components can be considerably stiffened and their stress state can be favourably influenced. Structures can thus achieve better crack resistance and deform very little despite minimised weight. Prestressing is well established for steel reinforced concrete, but for structures with carbon and especially textile reinforcement, the prestressing process is extremely complex, mainly because of the transverse pressure sensitivity of the fibre materials.
In our own preliminary work, the technology of chemical prestressing (or self prestressing) of concrete elements was therefore further developed and perceived as an efficient method for introducing prestress into carbon reinforced concrete (CRC). The technology is based on the application of swelling admixtures in the concrete, which, in interaction with the strain restraint, induce tensile forces in the reinforcement and consequently compressive forces in the concrete. The prestressing thus succeeds without mechanical devices or additional operations.
The aim of the subproject is to develop methods for the chemical prestressing of shell-shaped and folded CRC structures. Compared to the prestressing of plane components, additional stiffening effects are activated in spatially prestressed elements, which lead to the expectation of very favourable component behaviour. First, suitable material combinations are designed and their potential for chemical prestressing is investigated in detail. Subsequently, folded or curved chemically prestressed element cells are analysed. Individual test specimens are examined in a long-term test with regard to the tension force losses due to creep and relaxation.
The main gain in knowledge of the project consists in answering the question of whether the chemical swelling process of the concrete matrix can lead to a dimensionally stable and time-stable prestressing of spatially curved or folded thin-walled carbon reinforced concrete elements and whether this prestressing can lead to a significant increase in the efficiency of the components.