21 Write the chemical reaction that occurs to silica fume wh

21. Write the chemical reaction that occurs to silica fume when it is used as an SCM in concrete. 22. C2s and C3S are two of the compounds that are Portland cement. What are the other two? (1/2 pt. ea.) 23. List 3 of the main causes of contraction strain in structural concrete. (1/2 pt. each) POSSIBLE EXTRA CREDIT: (1 pt. poss.) If the concrete plans show the cover space between edge of rebar and forms to be 1.25 inches, and min. clear spacing between rebars is 1.5 inches, then what is the maximum allowable size of coarse aggregate?

Solution

21 ) Silicate fume react with the lime to produce calcium silicate hydrate(C-S-H gel)

3Ca(OH)₂ +SiO₂= 3CaO.SiO₂.3H₂0

22 ) C₃A and C₄AF

C-CaO

A-Al₂O₃

F-Fe₂O₃

S-SiO₂

23 ) Temperature changes:- Due to decrease in temperature conrete contracts.

Shrinkage:- Due to the evaporation of mosture from the voids of concrete at high temperatures conrete shrinks.

Due to the applicaton of load

The allowable size of the coarse aggregate should be less than the minimum of two so that concrete can move easily at the time of placing. There for maximum allowable size of aggregate is 1.25 inches.