Microstructural investigations on the skinning of ultra-high performance concrete

Elephant skin formation on UHPC surface: Effects of climatic condition and blast furnace slag content

“…Chen et al [16] reported that the most effective way to inhibit elephant skin formation is the one that can stop moisture escape from the surface, e.g., to cover the surface with a plastic sheet immediately. Similarly, a dependence of this skinning on lower humidity throughout the early curing was found by Wetzel et al [15]. In this study, 20°C-95% RH did not lead to vapor condensation, whereas 35°C-95% RH resulted in condensation on the surfaces.…”

“…3-6 at both the fresh and hardened state of UHPCs. This is an organic film that mostly consists of superplasticizer molecules as reported by Wetzel at all [15]. During SEM observations, it was realized that the organic film layer could be easily deteriorated during sawing and polishing operations due to mechanical abrasion and chemicals that are needed for polishing.…”

“…Thus, the effects of climatic conditions of the production area on elephant skin formation should be clarified. On the other hand, it was reported that GGBS can reduce required cement content while reducing the need for superplasticizer [23] which is the main reason behind the skinning problem [15]. In this study, the effects of elephant skin formation on the pore profile of UHPC were for the first time investigated under low and high RH conditions at two different early-age temperatures.…”

“…This increment of pore area just beneath the surface came into existence mostly due to the synergetic effect of an increase in both pore diameter and quantity. According to Wetzel et al [15], enrichment of PCE driven by capillary suction creates both an organic film that is only has a few lm thicknesses on the top surface, and a thicker layer with earlyhydration products beneath the film. Thus, the air void profile can be attributed to elephant skin formation observed within minutes after casting.…”

“…A phenomenon called ''elephant skin", therefore, can be observed on the casting surface of UHPCs as a side effect of an excessive amount of superplasticizer. Wetzel et al [15] found that capillary suction causes enrichment of polycarboxylate ether superplasticizer and early hydration products in the uppermost~50 mm. This enrichment resulted in the densification of the layer accompanied by an organic film of a few lm thicknesses on the top of it.…”