Prestressed Concrete mix design proportions should be chosen in such a way that strength, durability, workability etc suits the construction. The mix proportions for prestressed concrete are determined based on the recommendations provided by the standard codes of the region. Any mix design proportion derived for the specific construction has to go through quality tests and checks to finalize it for implementation.
Here the major requirements while choosing a mix design proportion for prestressed concrete are explained briefly.
Mix Design Proportions of Prestressed ConcreteDetermination of the proportions of cement, aggregates, and water to achieve the required strengths in concrete is made by designing the concrete mix. Such a concrete is called as a ‘Design Mix concrete’. Use of only the design mix concrete is recommended in the case of prestressed concrete construction.
1. Minimum Grade of ConcreteMinimum Grade of Concrete is generally M30 for post-tensioned and M40 for pre-tensioned members. Along with the grade of concrete following specifications are to be stated for prestressed concrete mix design.
- Grade designation
- Type of cement to be used
- Maximum nominal size of aggregates
- Cement content for the mix – Maximum and Minimum Value
- The maximum water-cement ratio of the mix
- Workability to be achieved
- Type of aggregates to be used
- Use of any admixture along with the conditions and requirements
2. Minimum Cement Content and Water-Cement RatioA maximum cement content is specified to restrict the shrinkage stains within the limits. In Indian standard code, a value of 530 kg/m3 is specified in the code so that shrinkage stains of concrete may be restricted within limits. As per BS 8110, the upper limit for cement content is 550kg/m3. The lower limit can go up to a value of 330kg/m3 which is decided based on the exposure conditions. In prestressed concrete, a lower water-cement ratio is provided to gain high strength concrete. This brings the issue to compaction. So, a water-cement ratio of 0.32 to 0.42 is used and compaction is performed under high frequency using a vibrator of 4000 to 9000 rpm.
3. Aggregates for Prestressed Concrete Mix DesignAggregates used for prestressed concrete must have consistent quality. For this standard codes recommends weigh batching instead of volume batching. Use of lightweight aggregates like pumice expanded blast furnace slag save the weight of prestressed structures. The density of concrete structures made from these aggregates ranges from 1300 to 2000 kg/m3. The use of lightweight aggregates helps increase fire and insulation properties.
High-Strength Prestressed Concrete Mix DesignThe major materials employed for the prestressed concrete mix design are high strength concrete and high strength steel. The ingredients used in the prestressed concrete mix design – cement, coarse and fine aggregate, water and admixture have to achieve the high strength performance criteria so that together they have minimum strength and performance based on the standard codes. With the application of vibration techniques, use of chemical admixtures, etc. we can produce concrete of a compressive strength in the range of 70-100 N/mm2. This concrete, called Ultra High-Strength concretes, are required to be used in prestressed concrete construction. Experimental investigations, in recent years, have shown that in high-strength concrete mixes, workability, type of aggregates (in terms of size, shape, their strength, porosity, permeability, water absorption, surface texture, etc.) and the strength requirements influence the selection of water-cement ratio. The use of high strength concrete helps:
- Reduce high stresses created in the anchorage sections
- Any cause of slippage between the pre-tensioned structures are avoided
- Initial compressive stress created is high that is reserved for future tension resistance.
- High strength prestressed concrete has reduced shrinkage and creep issues
- Indian standard code method.
- Erntroy and Shacklock’s empirical method.
- Mix design procedure of American Concrete Institute.
- British method, based on the work of Erntroy, which has replaced the Road Note No. 4 method of concrete mix design