Cau 3: Mot loai be tong nang co cap phoi theo thiet ke la 1:x:y(z)= 1:2:4(0,65). Hon hop be tong co ρv= 2400(kg/m3). Khi cho them phu gia tang deo ma van giu nguyen do deo yeu cau, nguoi ta co the giam duoc 30 lit nuoc nhao tron cho 1m3 be tong. Hoi khi do mac be tong thay doi nhu the nao? Biet xi mang su dung co mac PC40, vat lieu chat luong trung binh.

Bai giai:

Ap dung cong thuc ve cuong do cua Bolomey-Skramtaev:

Rb= A.Rx. (X/N-0,5) (vi N/X=0,65);

→Rx=Rb/[A.(X/N-0,5)]

Khi ko co phu gia: R1=Rb/[A.(X/N-0,5)]

Khi co phu gia: R2=Rb/{A.[X/(N-30)  -0,5]}

So phan tram tang cuong do be tong la:

∆R= (R2-R1)/R1 .100%

     = (R2/R1-1).100%

     = {[X/(N-30)  -0,5]/ (X/N-0,5) -1}.100%

     = {[X/(0,65X-30)  -0,5]/ 1,04 -1} .100%

Ma ta co X= 2400/(1+2+4+0,65)= 314(kg)

→ ∆R={[314/(0,65.314-30)-0,5]/1,04  -1}.100%= 25%