Q. My finishers are really happy with the concrete mix we have been using lately. The mix has enough paste to allow the workers to easily produce a smooth, flat surface. However, even though the ready mix supplier says that he is sending us the same mix for each project, the finishability of the concrete isn't the same each time. I think the fine aggregate has changed. The supplier says that the sand is from the same pit, but from a new location in the pit. He believes that since he is still using the same mix design and sand from the same pit, the finishability of the concrete should be the same. I know that's not true. Any suggestions as to what is causing the difference in finishing and how to correct it?
A. From what you've described, you're right. A change in fine aggregate is probably causing differences in finishability. Using the same ratio of coarse to fine aggregate and the same total weights in the batch mix doesn't guarantee consistent finishing characteristics. The fineness of the sand plays an important role in the finishability of the concrete mix, and even small changes in fineness can affect finishing. The fineness modulus is a single parameter that describes the grading curve and can be useful in checking the uniformity of grading. It is defined as the summation of the cumulative percent retained on standard sieves divided by 100. Typically the fineness modulus varies between 3.2 and 2.8. A small number indicates a fine grading, and a large number indicates a coarser material. The fineness modulus of the fine aggregate is required for mix proportioning, since sand gradation has the largest effect on concrete workability. Concrete made with a fine sand (low fineness modulus) has much higher paste requirements for good workability. ASTM C 33, "Standard Specification for Concrete Aggregates," requires that the fineness modulus not vary by more than 0.2 from the value used for mix design purposes. If it does vary by more than 0.2, changes in the mix proportions are needed to provide the same workability. Have your ready mix supplier check the fineness modulus of the fine aggregate currently used in the concrete mix, and then compare it to the fineness modulus for the mix design. A difference of 0.2 could be the reason that finishability has changed. Changes in the mix design, typically additional cement, might be required to provide the same finishing characteristics.