A high-strength concrete mix is simply a type of high-performance concrete mixture with a specified compressive strength of 40 MPa and above. This type of concrete mix is intended to accomplish specific needs of certain special concrete characteristics such as flexural strength, modulus of elasticity, and durability for concrete applications such as marine foundations, dam structures, retaining walls, grandstand roofs, heavy-duty industrial floors, and parking garages.
This article is aimed at discussing how to make high-strength concrete mix and the various methods of making the high-strength concrete mix.
Suitable Design Mix For High Strength Concrete
High strength concrete mix of M50 to M70 grades of concrete is manually designed. A suitable mix for a typical high-strength concrete in practice can be obtained from a concrete mix design ratio where the water-cement ratio is between 0.33-0.35 or lower and the binder material (cement) of 400-450 kg or higher per cubic meter of concrete with cement, sand and aggregate ratios applied as per the design mix.
Mix Design of High Strength Concrete Procedure
- First, the mean design strength of the high-strength concrete mix is determined by applying suitable control factors to the specified minimum strength of the concrete.
- Depending on the cement type and aggregates used, the design strength reference number in correspondence to the design strength at a given age of the concrete is interpolated from a graph of compressive strength against the reference number.
- The corresponding water-cement ratio to the design strength reference number is obtained from the graph for aggregates with maximum sizes of 10mm and 20mm to achieve the required workability.
- The aggregate-cement ratio to obtain the desired workability with the given water-cement ratio is then determined by the absolute volume method.
- Finally, the batch quantities of the concrete are worked out after adjustments of moisture content in the aggregates.
Materials Used for High Strength Concrete
The materials used for making high-strength concrete must be of high quality to achieve desired concrete compressive strength. High-quality materials will definitely meet the performance requirement for strength, durability, workability, and other properties. The followings are common materials used for the high-strength concrete mix:
- Low C3A cement
- Coarse aggregates with maximum size ranging from 10-20mm
- Supplementary cementitious materials such as blast-furnace slag, fly ash, and natural pozzolans.
- Silica fume
- Well-graded fine aggregates of high fineness modulus of about 3.0
- Clean water
Methods of Making High Strength Concrete
There are many different methods of making the high-strength concrete mix. The commonly known techniques and methods of producing high-strength concrete are high-speed slurry mixing, Sulphur impregnation or Sulphur filling, re-vibration, the use of admixtures, seeding, and the use of cementitious aggregates.
High-Speed Slurry Mixing
High-speed slurry mixing is a method of producing high-strength concrete which involves the preparation of cement and water mixture in advance, before adding the aggregates to the mix to produce a high-strength concrete. The high compressive strength achieved in using this method of concrete mix is associated with the efficiency of cement particles, and the quantity of water saved in the accurate mixing of the cement paste.
Sulphur Impregnation or Sulphur Filling
This method of making high-strength concrete involves the impregnation of Sulphur into low-strength porous concrete. The impregnation process of the concrete is carried out in the laboratory using moist cured fresh concrete for 24hours, a dry specimen at 120C for 24hours, then the immersion of the fresh and dry specimens in a molten Sulphur under vacuum for a period of 2 hours, and then the vacuum is then released after 2 hours, after which the samples are then kept in molten Sulphur for another half an hour, then the specimens can finally be tested to confirm the compressive strength of the concrete. Using the Sulphur impregnation technique a concrete compressive strength of 58MPa can be achieved.
Using this method of producing high-strength concrete mix, a controlled re-vibration of concrete usually would decrease the detrimental effect of concrete segregation, water bleeds, and air voids that causes shrinkage which will adversely affect the strength of concrete. For high-strength concrete, a controlled re-vibration is highly recommended.
Use of Admixtures
The use of admixture is one of the broad techniques in the production of high-strength concrete which helps in reducing the water content, this method of high-strength concrete production majorly decreases pores in concrete. Most admixtures such as fly ash and silica fume which are pozzolanic materials are primarily used to reduce the water content in concrete and therefore increase the strength of the concrete and as well accelerate the strength achievement compared to plasticizers and superplasticizers.
Seeding in high-strength concrete production is a less practical method of producing high-strength concrete when compared to other methods. This involves the addition of a percentage of fully hydrated Portland cement, finely ground, to a fresh concrete mix. This increases the cement ratio of the concrete mix, thus developing high-strength concrete in the process.
Use of Cementitious Aggregates
Cementitious aggregates, such as glass fiber and fondu, which is also a glassy slag clinker cementitious material can be used to produce high-strength concrete. The use of cementitious aggregates can produce a compressive strength as high as 125 MPa with a water/cement ratio of 0.32.
Advantages of High Strength Concrete
In building construction there are many advantages of high-strength concrete application which can not be achieved using conventional concrete mix. The following are the major advantages that are achievable in using a high-strength concrete mix.
- Easy to place and consolidate without affecting the strength of the concrete.
- The reduction in the size of structural members creates more usable area. Consequently, reducing the volume and the cost of concrete.
- The sizes and numbers of structural members such as beams and columns are drastically reduced since lesser sections are capable of bearing high structural loads.
- Reduction of floor slab thickness and supporting beam sections which form the major component of the self-weight and cost of structures.
- High seismic resistance when compared to conventional concrete.
- Highly resistant to abrasion
- Reduces formwork area and its cost of construction.
- Because of high early strength gain by high strength concrete, shoring and stripping time are decreased.
- The life span of the structure in severe environments is increased with the use of high-strength concrete.
- Superior long-term service performance under static, dynamic, and fatigue loading.
- Low shrinkage and creep.
- High stiffness resulting from high modulus.
- High resistance to chemical attack, freezing, and thawing significantly improves the durability of the concrete, and reduces crack propagation.
- Reduced cost of maintenance and repairs.