The mix design for high strength concrete is determined by various factors such as the properties of cement, sand, aggregates, and water-cement ratio. The primary goal is to achieve a compressive strength above 40 MPa. To attain such strength, it is necessary to use the lowest possible water-cement ratio. However, this approach has a downside as it reduces the workability of the mix. As a result, special vibration techniques are required for proper compaction.
Currently, high strength concrete with a desired 28-day compressive strength of up to 70 MPa can be achieved by proportioning the ingredients appropriately. This can be accomplished using normal vibration techniques for compacting the concrete mix. Such a technique eliminates the need for special vibration methods that are required for mixes with lower workability.
Overall, mix design plays a critical role in the production of high strength concrete. By considering the properties of cement, sand, aggregates, and water-cement ratio, it is possible to achieve the desired strength without compromising on the workability of the mix.
Erntroy and Shacklock have proposed a set of empirical graphs that show the relationship between the compressive strength of concrete and a reference number for mixes made with crushed granite, coarse aggregates, and irregular gravel. These graphs are presented in figures 1-4, with figures 1 and 2 representing mixes made with ordinary Portland cement, and figures 3 and 4 representing mixes made with rapid hardening Portland cement.
Figure 5 shows the relationship between the water-cement ratio and the reference number for 20mm and 10mm maximum size aggregates, with four different degrees of workability considered. The range of degrees of workability varies from extremely low to high, corresponding to compacting factor values of 0.65 and 0.95, respectively.
Tables 1 and 2 provide the aggregate-cement and water-cement ratios required to achieve the desired degree of workability with a given type and maximum size of aggregate, for two different types of cement. However, these design tables were obtained using aggregates containing 30 percent of the material passing the 4.75 mm IS sieve, so adjustments may be necessary if other ingredients are used.
It is generally recommended to first conduct trial mixes and make suitable adjustments in grading and mix proportions to achieve the desired results, due to the considerable variations in the properties of aggregates. Aggregates available on-site can be combined using the graphical method to satisfy the requirement of 30 percent of the material passing the 4.75 mm IS sieve.
The table shows the amount of cement needed to achieve different levels of workability using rapid hardening cement at varying water-cement ratios. The workability is measured by the slump test, and four different levels of workability are considered: very low, low, medium, and high. The water-cement ratios range from 0.2 to 0.6, and the corresponding aggregate cement ratios required for each level of workability are listed in the table. These ratios are determined based on the standard mix design procedure and take into account factors such as the desired strength, durability, and economy of the concrete mix.
Mix Design of High Strength Concrete -Procedure
To determine the mean design strength of a construction material, appropriate control factors are applied to the minimum strength that is specified. This calculation is done based on the type of cement and aggregates used. Using Figures 1 to 4, the reference number for the design strength at a specific age can be interpolated. In order to achieve the required workability corresponding to this reference number, the appropriate water-cement ratio can be obtained from Figure 5, taking into account the maximum size of the aggregates used (either 20mm or 10mm).
To determine the correct aggregate-cement ratio for achieving the desired workability with the known water-cement ratio, the absolute volume method is used. After calculating these ratios, batch quantities can be determined by adjusting for any moisture content present in the aggregates.
Fig.1: Relation between compressive strength and reference number (Erntroy and Shacklock)
Fig-2: between compressive strength and reference number (Erntroy and Shacklock)
Fig-3: Relation between compressive strength and reference number (Erntroy and Shacklock)
Fig-4: Relation between compressive strength and reference number (Erntroy and Shacklock)
Fig-5: Relation between water-cement ratio and Reference Number
Fig-6: Combining of Fine aggregates and Coarse aggregates
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High Strength Concrete Mix Design Example
In this scenario, the goal is to design a high strength concrete mix suitable for producing precast prestressed concrete. The specified 28-day works cube strength is 50 MPa, and a very good degree of control is required with a control factor of 0.80. The degree of workability required is very low.
The type of cement used is ordinary Portland cement, and the coarse aggregate used is crushed granite with an angular shape and a maximum size of 10mm. The fine aggregate used is natural sand, and its specific gravity is 2.60. The specific gravity of cement is 3.15, and the specific gravity of coarse aggregates is 2.50.
Both fine and coarse aggregates contain moisture, with fine aggregates having 5% moisture and coarse aggregates having 1% moisture. The grading characteristics of the fine and coarse aggregates are also provided in detail.
The aim is to formulate a high strength concrete mix that meets these requirements, taking into account the properties of the cement, coarse and fine aggregates, and moisture content.
| IS sieve size | Percentage Passing | |
| Coarse aggregate | Fine aggregate | |
| 20mm | 100 | – |
| 10mm | 96 | 100 |
| 4.75mm | 8 | 98 |
| 2.36mm | – | 80 |
| 1.18mm | – | 65 |
| 600 microns | – | 50 |
| 300 microns | – | 10 |
| 150 microns | – | 0 |
Design of Concrete Mix
The given information provides a set of calculations and ratios needed to determine the required proportions by weight of dry materials to make concrete. The mean strength required for the concrete is 63 MPa, based on a reference number in Figure 1. The water cement ratio is 0.35, as indicated in Figure 5.
To achieve the desired workability with a 10mm maximum size aggregate and very low workability, the aggregate-cement ratio must be 3.2, as per Table-1. The aggregates must be combined using the graphical method illustrated in Figure 6 so that 30% of the material passes through the 4.75 mm IS sieve. The ratio of fine to total aggregate is 25%.
Using these ratios and calculations, the required proportions by weight of dry materials can be determined. The weight of cement required per cubic meter of concrete is denoted as C. The weight of cement required is 1, the weight of fine aggregates required is [(25/100)x3.2] = 0.8, and the weight of coarse aggregates required is [(75/100)x3.2] = 2.4. The required water content is 0.35.
In summary, the given information provides a set of calculations and ratios needed to determine the required proportions by weight of dry materials to make concrete, based on the mean strength, water cement ratio, aggregate size, and desired workability.
