This study proposed a method to estimate the compressive strength of early-age concrete before demolding using a pin penetration device. Small holes were prepared on molds to apply for this method. A strong correlation was obtained between pin penetration depth and compressive strength despite the materials and mix proportions difference. This method was applicable between 5 and 15 MPa compressive strength, which is the demolding criterion. Within the experimental results, the penetration depths of 12.5, 9.5, and 8.5 mm (Flat), and 13.0, 10.0, and 9.0 mm (Sharp) were sufficient to demolding criteria of 5, 10, and 15 MPa, respectively.

：Average temperatures over most of Mongolia are below freezing from November through March. The lowest temperatures in January are as low as −36℃ to −40℃. However, in the extremely cold climate of Mongolia, it is often executed the concrete works even outside temperature is below −20℃. Therefore, in this experimental work, we investigated the required time span of pre-curing in order to achieve the potential strength at 28 days before being exposed to extremely low temperature (−20℃) , simulating Mongolian weather conditions. Moreover, internal temperature changes and microstructure analyses have been executed. We have found that concrete freezing temperature decreases remarkably from 0℃ to −15.6℃ with increasing pre-curing time, however, depending on the watercement (W/C) ratio. Our experiments have shown that there was no freezable water after 14 days when the W/C ratio is below 0.39. Our research concluded that concrete should at least be cured at 20℃ for 14 days before being exposed to sub-zero temperatures, in order to reach in fair to middling potential strength at 28 days.