How do fines in aggregate affect the performance of concrete?

Fines in aggregate have a significant impact on the performance of concrete, primarily through their effect on water demand and workability. When fine particles are present in the aggregate mix, they tend to fill the voids between larger coarse aggregates. This can lead to an increase in the overall surface area that requires moisture to achieve proper hydration.

As a result, an increased proportion of fines can lead to higher water demand since the mix requires more water to achieve the desired consistency and workability. If the water-to-cement ratio becomes too high to accommodate the increased demand, it can weaken the concrete and reduce its performance, countering the benefits of a well-graded aggregate mix. Additionally, high fines content may decrease the ease of handling and placement of the concrete, impacting its workability. Therefore, while fines do serve a role in filling spaces within the aggregate matrix, their presence can compromise the balance needed for efficient mixing and optimal concrete performance.