Con cer ning du ra bi li ty of the NPP ci vil en gi ne ering con crete struc tu res, ac cor ding to RCC -CW [16], the ma in objec ti ves are to:• pre vent in ter nal di sor ders re sul ting from Al ka li Si li ca Re ac tion (ASR) and De lay ed Et trin gi te For ma tion (DEF);• re du ce and/or pre vent crac king of con cre te and cor ro sion of ste el re in force ment and• con trol cre ep of con cre te. Al ka li -Ag gre ga te Re ac tions (AAR) may in vo lve si li ce ous ag gre ga tes (alka li -si li ca re ac ti vi ty, ASR) or very rare car bo na te ag gre ga tes (al ka li -car bo nate re ac ti vi ty, ACR), and fa ilu re to ta ke pre cau tions may re sult in pro gres si ve de te rio ra tion of con cre te struc tu res. AAR a re che mi cal re ac tions that oc cur betwe en cer ta in ty pes of mi ne ral in ag grega tes and the al ka li ne (Na+ and K+) and hy dro xyl (OH -) ions pre sent in the in ter sti tial so lu tion of ce ment pa ste in con cre te. The se dis so lu tion re ac tions oc cur due to the high so lu bi li ty in very alcaline solutions of cer ta in amor phous, di sor de red or po or ly cry stal li zed forms of si li ca. This re ac tion le ads to for ma tion of a hy gro sco pic al ka li ne gel that ab sorbs wa ter and expands, causing si gni fi cant expan sion and cha racte ri stic crac king of the con cre te. The ra te of expan sion cau sed by AAR ty pical ly has be en fo und to be 20 to 200 x 10 -6 mm/mm per year [1], de pen ding on the se ve ri ty of the re ac tion and the de gree of re stra int. The expan sion beco mes de tec ta ble in abo ut fi ve to ten years after con struc tion, and the most no ti ce able expan sion may be de tec ted in abo ut 15 to 25 years.The three pre re qu isi tes must be simul ta ne ously ful fil led for AAR expansion to oc cur: high con tent of al ka li in con cre te (usually ≥ 3 kg/m 3 ), pre sen ce of re ac ti ve mi ne rals in ag gre ga te and suf fi cient mo istu re sup ply (at le ast 80%). If any one of the se three con di tions is not met, expan sion due to AAR can not oc cur. Other con di tions, such as hi gher tem pe ra tu re, ac ce le ra te the spe ed of reac tion. ASR in vo lves a che mi cal re action, and for the re ac tion to oc cur, the follo wing com po nents must be pre sent: wa ter, re ac ti ve si li ca, and a high con centra tion of hy dro xyl ions -high pH [1]. Li ke wi se, the con cen tra tion and di stribu tion of the se com po nents and the am bient tem pe ra tu re ha ve a si gni fi cant in flu en ce on the ra te and de le te rio us ef fect of the re ac tion. A con cre te structu re with ASR com mon ly exhi bits wi dely dif fe ring si gns of de te rio ra tion in diffe rent pla ces. Con cre te expo sed to dry, in te rior envi ron ments wi tho ut wa ter nor mal ly do es not de ve lop crac king from ASR and the most vul ne ra ble parts of a con cre te struc tu re are tho se expo sed to a warm and hu mid envi ronment [1].In the absence of previous long-term experience EN 12620 [14] suggests to take one of the following precautions:■ limit the total alkali co...
