Optimal design of fresh sand fog seal mortar using response surface methodology (RSM): Towards to its workability and rheological properties

As a popular preventive maintenance material, sand fog seal (SFS) could fill the micro-cracks and improve the skid-resistance and wear-resistance of road surface, but little research has been conducted to explain how the components of fresh SFS mortar affect its workability and rheological behavior. In this paper, an improved workability evaluation system is established for fresh SFS mortar based on self-developed devices, including the basic indexes such as dynamic uniformity DUt,s, slump flow X, and seepage velocity Vs. There is a negative correlation between the fluidity index (slump flow and seepage velocity) and the uniformity index (dynamic uniformity), and both of them are important bases for the performance balance design of fresh SFS mortar. Then, the effects of water, filler and sand on the workability are evaluated using response surface methodology (RSM) with I-optimal design method. The results show that water mainly regulates the consistency of the SFS mortar, which in turn affects its flowability. The sodium bentonite filler could alter the suspension capacity of the fog seal paste and determine the state of dispersion of sand in the paste. Besides, the addition of sand plays a negative role in the fluidity of fresh SFS mortar. Meanwhile, three design intervals for fresh SFS mortar are proposed based on its workability, ① Easy-to-separate interval: DUt,s < 0.8; ② Hard-to-flow interval: X ≤ 54.9 mm and Vs = 0. ③ Recommended interval: DUt,s > 0.8, Vs > 0, and X > 54.9 mm, where the fresh SFS mortar can balance stability and fluidity well. Hence, to maximize the balance between uniformity and fluidity of fresh SFS mortar, a formulation design optimization guideline is proposed based on desirability optimization methodology (DOM). The formulation has the widest distribution of high desirability when the sand content is 25%. Finally, the results of rheological test show that fresh SFS mortar is a typical shear-thinning pseudoplastic fluid, which can fit the "Herschel-Bulkley" model well. Yield stress and plastic viscosity can be used as the important rheological indexes to predict the workability of fresh SFS mortar. In general, based on the approach proposed herein, an optimized fresh SFS mortar with both good uniformity and fluidity could be produced.