Effects of fines content on packing density of fine aggregate in concrete

• The effects of fines content in aggregate on concrete are still not clear.
• The allowable fines content has been a controversial issue for many years.
• The fines content would increase the surface area and thus the water demand.
• But the fines content would also increase the packing density as shown herein.
• Hence, the fines content has both undesirable and desirable effects.

The fines content (particles finer than 75 μm) in aggregate has substantial effects on the performance of concrete. Since the fines content has large surface area that would increase the water demand, maximum limits are often imposed. However, the fines content would also fill into the voids between larger particles to increase the packing density and thus reduce the volume of voids to be filled with cement paste. Hence, the fines content is not entirely undesirable and it has been suggested to raise the limits on the fines content. Somehow, due to measurement difficulties, the effects of fines content on the packing density are still not well understood. Herein, the packing density of fine aggregate with varying fines content was measured using both the dry and wet packing methods. It was found that under wet condition, the highest packing density generally occurs at a fines content of about 15%.