Influence of formwork wall effect on fiber orientation of UHPC with two casting methods

This paper investigated influence of formwork wall effect on fiber orientation and flexural properties (flexural strength and toughness)of ultra-high performance concrete (UHPC)prepared by a direct cast and flow control method. Fiber orientations in samples with different distance from formwork were quantitatively evaluated using image analysis technique. Based on experimental results, the better fiber orientation and higher flexural performance can be obtained near formwork boundaries due to restriction effect on fiber rotation. Considerable variations of flexural strength and toughness with position were induced by the existence of mold sidewalls for directly cast UHPC specimens, reaching up to 54.9% and 49.6%, respectively. This is related to the significant fiber orientation coefficient variation of 21.1%–30.3%. When adopting the flow control method, the variability of fiber orientation coefficient was decreased to the range of 6.0%–12.3%, inducing the reduction of flexural strength and toughness differential to lower than 19.5% and 18.6%. Therefore, the flow control casting method can significantly inhibit the wall effect of formwork and is favorable for improving the homogeneity of UHPC in real structures.