A brief overview of post tensioning has been presented here, followed by its advantages and economics.
BRIEF
Post tensioning is the technique of pre-loading the concrete in a manner which eliminates, or reduces, the tensile stresses that are induced by the dead and live loads; High strength steel wires called strands are arranged to pass through the concrete floor. When the concrete has hardened, each set of strands is gripped by means of a hydraulic jack and stretched to a pre-determined force. At this stage the strands are locked in a purpose made device called an anchorage, which has been cast in the concrete; this induces compressive stress in the concrete. The strand is there after held permanently by the anchorage.
The non jacking end of the strand may be bonded in concrete or may be fitted with similar device on the other end as the case may be. In order to allow the strand to stretch in the hardened concrete under the load applied by the jack, bond between the concrete and strand is prevented by a tube through which the strands pass. This tube is termed as duct or sheathing and is made of galvanised iron. After stressing the duct is filled with grout material. Grout filling of ducts transfers load to the entire concrete element and ensures corrosion protection.
Advantages
Post Tensioning offers various advantages, some of which have been listed below:-
- Larger Clear Spans
- Thinner Slabs (Spans of 7mm to 8mm = 210mm thick slab)
- Lighter Structures
- Reduced Cracking
- Reduced Deflection
- Rapid Construction
- Better Water Tightness
- Less Manpower
- Reduced Passive Reinforcement
Economics
Conventional reinforced concrete slabs are best suited for spans up to 7m to 8m and also where the spans are uniform and form work is simple, so that form workers can work quickly. This is common in simple residential blocks up to three storeys. Further, in the driveways often wider spans are required to provide parking access. More columns and thicker slabs are required for conventional reinforced slabs, and this is where the labour costs come in.
As in post tensioned slabs the slabs will be thinner, with larger spans and thereby reduced columns and formworks. The works can be carried out faster, and also the slab can be de shuttered quicker than conventional slabs.
Growth Trend/Demand for Post Tensioning
With the above mentioned advantages and economics becoming prominent, post tensioning in buildings is becoming a trend in the region and world over. The economics are being harnessed in each and every possible way by the client and the contractors, by selecting post tensioning for their projects. Thus in all there is a good scope of growth for post tensioning ahead.
Service
Feasibility and cost estimate for your project
Design for Post-Tension Slab Conform to ACI or AIT Standard
Sales and Installation Service
- Post-Tensioned Slab
- Precast Concrete Wall Panel
- Precast Concrete Element
Product
The Post-tensioning product are designed and manufactured under quality program to meet international standards. The products comprise of complete range of post tensioning components including.
– Unbonded grease and P.E. coated P.C. strand
– Bonded P.C. strand
– Post-tensioned anchorage components for bonded and unbounded tendons
– Tendon support chairs
– Corrugated metal ducts for flat and round tendon.
Slab Post-tensioning System
There are 2 systems for Post-tension slab. One is unbounded system that consists of cast iron (FCD45) anchorage and mono-strand with nominal diameter of 0.5” (12.7mm.) inserted in PE. Duct with grease
The other is bonded system that consists of 3 Type of anchorage systems (3BS13, 4BS13, 5BS13) with multi-strand flat duct.
The C-Post Co. uses “ANCHORA post-tensioning system” that Developed by Dr. Bijan O. Aalami and our company.
The strands are individually gripped in one flat anchor head unit that consist of Block and Guide. The strands are stressed individually by means of mono-strand jack and their pre-stressing forces shall be transmitted by means of flat type anchorage system. The strands are contained in one flexible flat duct which is made of corrugated galvanized metal. To ensure corrosion protection and to give adequate bond strength, the tendons are filled with suitable cement grout mix after complete stressing of the strands.
The same anchorage for stressing anchorage can be used as dead end anchorage in which case the wedges are pre-seated in the anchor head unit by applying nominal force to the strand using the stressing jack.
Alternatively an onion dead end anchorage can be used with simpler solution and more cost effective. In this case each strand is formed into an onion shape by means of special forming jack.
Slab Anchorage
The other civil structure such as bridge, silo etc. are normally applied by multi strand system. The system is normally adopted for bonded tendons that consist of a bundle of strand with a nominal diameter of 0.5” (12.7 mm) or 0.6” (15.2 mm). The number of strands per tendon can be from 7 strands up to 19 strands. The tendons are contained in one round duct which is made of corrugated galvanized metal.
