Volume 1, Issue 6 (June 2004)
Beneficial Reuse of Foundry Sands in Controlled Low Strength Material
A study was conducted to determine how the unconfined compressive strength and flow of flowable fills prepared with foundry sand depends on the bentonite content of the sand. The study showed that there are several advantages of using foundry sands with bentonite content > 6% as the fine aggregate in flowable fill. These advantages include: (i) lower long-term strength gain (making the design of excavatable mixtures simpler and less risky), (ii) less flow loss, (iii) fewer components and fewer interactions between components that are difficult to characterize, and (iv) a larger fraction of inexpensive foundry sand being used in the mixture. The unconfined compressive strength (UCS) of flowable fills prepared with foundry sands is sensitive to the water-cement ratio (W/C), at least when the W/C spans a broad range (4–11). Mixtures with W/C < 6.5 generally will have excessive UCS, whereas a suitable UCS is generally associated with W/C > 6.5. Bentonite content does not affect the UCS systematically, but it does have an indirect effect in that foundry sands with more bentonite require more water to flow, which affects strength. The amount of water required to achieve adequate flow primarily is a function of the bentonite content of the foundry sand. In general, as the bentonite content of the foundry sand increases, the water content of the mixture should increase correspondingly. The amount of fly ash has only a modest effect on the amount of water required. The most important factor affecting flow loss is the presence of cementitious fly ash in the mixture. Flow loss can be reduced appreciably by using a foundry sand with at least 6% bentonite so that fly ash fines need not be added to the mixture.