I often watch short videos on cellphone for entertainment after hard work. The following information comes from one of the programs that is  worth sharing with reader friends: 

砼 （tong,  the second tone.)  represents ‘concrete'.  Why？石 (shi，the second tone）means 'stone'； 人 （ren,  2 tone) looks like a walking body that is a ’man'； 工 (gong, the first tone) is  ‘work’or ‘job’.  So the whole word 砼 shows that it is a ‘man-made stone’.  -------- Yes，on the construction site，the worker put sand,  tiny stones, cement and water together.  The mixture is the  important material for building。

In Chinese we call ‘concrete’as ‘混凝土’: The character 混(hun，the fourth tone) means ‘mix’;  凝 (ning, the second tone) is ‘becoming solid’； 土 （tu, the third tone） is ‘earth’.   Now you see that 砼 is a simple symbol for ‘混凝土’.   It is said a clever professor invented the special character for teaching job in 1953.  

This case is quite rare in the language history.