Case Study Assignment Design - Concrete Mixes with Silica Fume

Assignment Design

I. Mix design

We initially propose two different concrete mixes for the structure.

Mix with silica fume

Silica fume gives very good improvement of the rheological as well as the mechanical and chemical properties of the concrete, improves also the durability of the concrete. Silica fume is a finer material than cement, so it fills up voids and empty spaces more effectively, 5-20% of cement can be replaced by silica fume. The strength of the concrete will be increased by replacing part of the cement by silica fume. Silica fume can react with calcium hydroxide to produce cementitious product which will be useful to form dense and impermeable concrete. However the use of silica fume will increase the amount of water required and that can be offset by using superplasticiser. Moreover, superplasticiser can enhance the workability of concrete. Hence, the use of silica fume with superplasticiser is very suitable for high strength concrete.

The proportion of fine aggregate depends on the workability of concrete, the maximum size of aggregate and the grading of the fine aggregate. However, generally self-compacting concrete requires the fine aggregate volume to be 40% of the mortar and pumpable concrete requires 35%-45% of the total aggregate. In this mix design, we assume the fine aggregate proportion to be 40% of the total aggregate in volume.

The process of mix design is shown in Table 1.1 and the final mix proportion of concrete is given in Table 1.2.

Table 1.1 Process of Mix Design

Stage Item Calculation Values

Characteristic strength Specified 100MPa at 28 days

Proportion defective See Note.i 2.5%

Standard deviation See Note.ii 8

Margin k s=1.96×8

16 MPa

Target mean strength =100+16

116 MPa

Aggregate type (coarse) Crushed

Aggregate type (fine) Crushed

Cement strength class Specified 52.5

Air content Specified 2%

KgRc See Note.iii 310 MPa

Quantity of silica fume See Note.iii s/c=12/88

Gravity of cement Specified

Gravity of silica fume Specified

Superplasticizer (Su) Specified 2L per 100kg of cement

Water to cement ratio De

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