Cho cấu kiện bê tông cốt thép chịu nén có f’c(Mpa), khối lượng riêng wc(T/m3 ). Cấu kiện được thi
công trong môi trường không khí có độ ẩm H(%). Bảo dưỡng bằng hơi nước, khi cường độ bê tông đạt 70% theo thiết
kế, kết cấu chịu tải trọng thường xuyên gây ra ứng suất nén là 18Mpa.
a) Sau bao nhiêu ngày kết cấu bắt đầu chịu tải? (tính tròn ngày).
b) Biến dạng từ biến sau 600 ngày là bao nhiêu? Biết: Hệ số ks=1.025

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?

Different types of steels contain different elements that alter the characteristics of the steel. for each of the following elements, explain what the element does when alloyed with steel.

Apipe with an outside diameter of 15 cm is exposed to an ambient air and surrounding temperature of -20°c. the pipe has an outer surface temperature of 65°c and an emissivity of 0.85. if the rate of heat loss from the pipe surface is 0.95 kw per meter of length, the external convective heat transfer coefficient (h) is: (a) 12.5 w/m"k (b) 18.6 w/mk (c) 23.7 w/mk (d) 27.9 w/mk (e) 33.5 w/mk