CRC 528-T6: Design model and application technology for strengthened RC slabs under bending
Table of contents
Project data
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Titel | Title |
Report in the yearbook 2011
Flexural Strengthening of Elements
Crack pattern of a two layer strengthened steel reinforced slab
Within the transfer-project T6 of the Collaborative Research Centre 528 „Textile Reinforcements for Structural Strengthening and Repair” additional textile strengthened reinforced concrete elements under flexural loading were studied. Particularly, the already developed computation and design methods were verified to determine the amount of the required textile reinforcement. The model is based on the common design method for steel-reinforced structural members and was extended for an additional textile reinforced concrete layer. At the first time, the computation method was applied to RC-cross sections with different parameters. Properties like the thickness, structural depth, reinforcement ratio and steel diameter were varied. For the strengthening works, a recently developed carbon textile with a fineness of 800 tex and roving distances of 7.2 and 18.0 mm respectively was used. 15 small-sized specimens with a surface area of 1.80 × 0.60 m and thicknesses of 10 and 18 cm were loaded at the Otto-Mohr-Labor until they failed under bending.
Ten slabs were strengthened with one, two or three textile layer before testing, five remained unstrengthened as reference. Appropriate strengthening methods are meanwhile available for the users of textile reinforced concrete. The fine grained concrete can be sprayed and the textile fabric can be applied layer by layer. The necessary machinery, working steps, pre- and post-treatment measures have been developed. Especially, the ready-mixed concrete mixture of the Pagel Spezial-Beton GmbH is easy to use and allows a quick and economic execution of the shotcrete works.
The experimental studies and the following theoretical considerations led to the conclusion that the developed computational methods can be assessed as suitable and recommended for design calculations.
Another objective of the transfer project was the development of practical and easy-to-use design tables. Based on the known ones for reinforced concrete elements the necessary mathematical formula and design tables for strengthened cross-sections have been developed. They are currently still refined.
Report in the yearbook 2009
Flexural Strengthening of Slabs
RC slab strengthened with textile reinforced concrete under bending
In this project, there are two focuses:
1) the establishment of a design concept that is manageable for design engineers and 2) the development of an economical method of application.
Dimensioning model
The use of experimental research has demonstrated that the load capacity of reinforced concrete (RC) slabs under bending increases significantly with the addition of textile reinforced strengthening layers in the tensile zone. Load-bearing capacity could be increased by more than two fold depending on the properties of the textile reinforcement and the un-strengthened concrete member. In addition to load-bearing capacity augmentation, serviceability also benefits from strengthening since strengthened structural components remain in the uncracked state I for a longer period of time. A decrease in crack width (size) and crack spacing was observed during and after testing. Crack widths in the strengthened slabs were nearly 50 % smaller.
The design model developed is predicated on an expanded standard RC bending design model. The goal is to describe the interaction of steel and textile reinforcement by considering the differences in their respective bond properties. From this, a simplified design model must be derived at which time each particular bonding behavior is factored into the model as an appropriate bond coefficient. In order to accurately describe the internal balance of forces, textile bond properties are determined and incorporated into the design model.
Application technology
Defining a suitable technique for strengthening large RC members is crucial for practical use. An optimized sequenze of coordinated steps of alternating fine concrete spraying and textile lamination was established by means of large-scale tests on vertical, inclined and horizontal surfaces (from above and below). Thereby questions concerning fabric packaging, machine engineering, as well as pre- and post-treatments were, thereby, focused among others. Emphasis was placed on the following steps for developing the strengthening procedures:
- Wet spraying method;
- Surface pre-treatment by sandblasting (roughing of surface) and pre-wetting;
- Spraying with minimal rebound;
- Devices and mixing regime of concrete;
- Selection of suitable post-treatment.
An additional focus was on the development of a dry, ready mixed fine concrete admixture that meets the high standards of: fresh concrete consistency; machine workability; and properties of hardened concrete, as well.