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Combination of Unbonded Post-Tensioning with Void Slab

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As a structural engineer we often come across a situation where the architectural requirements are not complementing structural aspects and hence either of them has to compromise based on the intensity of the aspect. Specifically when clear soffit of the floor is a prime requirement, eventually it turns out to be a heavy design which impacts mostly on all associated structural elements such as beams, column, footing which is required to design for more loadings than the conventional slab-beam framing system.  On the other hand, architectural preference is to provide column-free ambience in the most important spaces. Consolidation of such requirements / preferences escalates the consumption of construction material especially concrete and steel. It impacts mostly on the self-weight of the structure and hence overall structural design becomes heavy which directly affects the sustainability criteria globally.

To make the system work without hurting the architectural preference and without impounding more self-weight, void slab system could be an appropriate approach. There are many ways available to achieve void slab construction. Some options are related to shape of the void created within the slab such as sphere, ellipsoidal, cylindrical, cuboid, triangular prism etc… which affects the practical aspects of placing them actually on site. Analysis and design part remains merely the same, which requires appropriate consideration of section properties, based on the shape and profile.

After having this brief overview, it is worth to share our experience of working with I-Sects system which has a most appropriate shape as far as simplification of analysis and design is concerned  and also for the material handling, storing, transporting and on-site installation which improves its of overall performance. It has a stackable shape hence, it requires minimum space to store, transport and its handling. The most eye-catching feature is it facilitates simplified concrete pouring sequence. It does not require a prepared base portion of the slab which is inevitable requirement in most of the void shapes listed above. Normally, the slab bottom solid portion is cast first with the bottom reinforcement and shear links are installed in. Need to wait till it gets enough strength for next stage. Then, the void former units are installed with top reinforcement. After this, concrete pouring is carried out for remaining depth up to top of the slab. This is how activities shall be planned out. Where as in case of I-Sects system, there are legs provided so that it can be installed altogether with reinforcements. After installation of the I-Sects and reinforcement in totality, the casting shall be carried out in two phases. First pouring is to be carried-out up to solid part at bottom of slab with more workability having slump more than 120 mm. Now-a-days, RMC concrete is available with pump-ability criteria which would provide enough workability to reach below the I-Sects basin evenly. The depth clearance of bottom solid portion is worked out based on the maximum aggregate size which is 20 mm normally. Once the first pouring is completed which generally requires 1-2 hours as minimum casting time, second casting shall be carried out from the location from where first casting was carried out. Second casting shall be done with normal workability criteria. This makes the on-site process much simplified.

I-Sects system has been explored in many projects across India and received expected performance in the “Indian way” of construction also. Apart from weight saving, there are several other advantages such as thermal insulation, noise insulation, stiffness to volume ratio is better than solid section, hence better stiffness aspects of the floor, etc…

Furthermore onto providing RC void slab, the system has also been blended with mono-strand unbondedpost tensioning system and experienced extraordinary performance of the blended system. The Beauty of this blend is – mono-strand unbonded provides flexibility to keep one or many based on the minimum requirement of PT strands in a single rib created between adjacent I-sects units such that it requires minimum width of the rib to resist pre-compression force induced by the strands. This makes the force distribution more even than multi-strand PT system. There is an impressive optimization in terms of weight of the structure as well as apparent overall depth of the floor. This blending is effective in case of large spans more than 10 m where flat soffit is the prime concern.

One such application undertaken by Post Tension Services India Pvt. Ltd. (PTSI) was the Shaktidham Temple located in Aurangabad, Maharashtra. A post-tensioned two-way void slab was laid in a column free area measuring 50 feet by 60 feet with hidden beams which formed a two-way spanning flat plate of apparently 480mm thickness. The slab was made to rest on peripheral columns; 4 at corners and remaining 10 on the side edges. It significantly reduced the concrete material by 21 cubic meters than the equivalent solid flat plate option.

For Further Details
Post Tension Services India Pvt. Ltd.
PTSI, Off Genda Circle, Opp. Baroda People’s Co-Op. Society, Alkapuri, Vadodara – 390 007, Gujarat.
Ph: +91 265 2341298, +91 7567 878 798
Email: info@ptsindia.net
Web: www.ptsindia.net

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