Micro-concrete is one of the most versatile materials for structural rehabilitation / retrofit activities. At times it is added with 5 mm down size aggregates for specified purpose/s. How does it perform?

Compressive strength

It is a principle which is taught in classrooms of Civil Engineering education that compressive strength is directly proportional to maximum size of aggregates. Maximum size of aggregates in micro-concrete (without any addition) is 2 mm. If this maximum size is increased to 5 mm by adding such aggregates, the compressive strength of the matrix is bound to increase. Although it is not mentioned in any of the text books, it is experienced from the long field experience on the subject that compressive strength of micro-concrete added with 5 mm down size aggregates in 1:0.5 proportion (1 – micro-concrete and 0.5 – 5 mm down size aggregates) is 20% higher than compressive strength of matrix with only micro-concrete.

Exotherm / Heat of hydration

It is a principle which is taught in classrooms of physics that thermal energy dissipates with increase in area of its domain. When 5 mm down size aggregates are added to the matrix of micro-concrete, the exotherm / heat of hydration generated is dissipated in larger specific surface and hence its effects of expansion causing thrust / pressure on shuttering get proportionately compensated. Although it is not mentioned in any of the text books, it is experienced from the long field experience on the subject that exotherm / heat of hydration of micro-concrete added with 5 mm down size aggregates in 1:0.5 proportion (1 – micro-concrete and 0.5 – 5 mm down size aggregates) is 07% lower than the exotherm / heat of hydration generated in matrix with only micro-concrete.

Limitation

Additional of 5 mm down size aggregates to matrix of micro-concrete has one significant limitation. The matrix with addition of 5 mm down size aggregates starts showing micro porosity. This is logical. Specific surface of micro-concrete then has to cover large size particles for which the ingredients of micro-concrete may fall short.

At the end:

Addition of 5 mm down size aggregates to micro-concrete has some distinct advantages but also has specific limitation. It is recommended to add 5 mm down size aggregates to micro-concrete in designed proportion (1:0.5 or 1:1 or otherwise) where incremental compressive strength and significant reduction in heat of hydration is recommended but it is to be critically reviewed when it is to be utilised for rehabilitation of the concrete members which need near to zero porosity. This is a basic requirement for a chemical industry where micro porosity can allow corrosive gases or corrosive liquid to enter the core concrete through micro porosity due to addition of 5 mm down size aggregates.