Gioi thieu & Phuong phap

Dinh luat Abrams

Duff Abrams da chung minh vao nam 1918 rang cuong do chiu nen cua be tong dam nen day du chu yeu phu thuoc vao ty le nuoc-xi mang. Dinh luat thuc nghiem cua ong co dang:

trong do:

• f_c la cuong do chiu nen (MPa)
• A la hang so dai dien cho cuong do toi da dat duoc cho mot to hop xi mang va cot lieu cu the — thuong tu 80–100 MPa cho OPC voi cot lieu tot
• B la hang so kiem soat toc do giam cuong do khi tang ty le N/X — thuong tu 8–25, gia tri cao hon tao ra duong cong doc hon
• w/c la ty le nuoc-xi mang theo khoi luong

Dinh luat Abrams hoat dong vi ty le N/X quyet dinh do rong mao dan cua ho xi mang da dong ran. Ty le N/X thap hon tao ra ho dac dac hon voi it lo rong hon va nho hon, dan den cuong do chiu nen cao hon. Moi quan he nay dung tot cho ty le N/X tu khoang 0,30 den 0,80.

Hieu chinh Bolomey

Bolomey (1935) mo rong phuong phap cua Abrams bang cach tinh den cach cac loai xi mang khac nhau anh huong den cac hang so A va B. Xi mang pha tron nhu PPC (Xi mang Portland Pozzolana) va PSC (Xi mang Portland Xi) thuy hoa khac voi OPC:

• OPC — xi mang Portland tieu chuan. Tang cuong do som nhanh, cuong do 28 ngay cao nhat cho mot ty le N/X nhat dinh. Su dung hang so A va B co so.
• PPC — pha tron OPC voi tro bay (15–35%). Phan ung pozzolan cham hon, nen cuong do som thap hon, nhung cuong do dai han co the bang hoac vuot OPC. A va B duoc dieu chinh giam de phan anh su phat trien cham hon o 28 ngay.
• PSC — pha tron OPC voi xi lo cao nghien min (25–70%). Giong PPC, cuong do som giam, nhung PSC co kha nang chong hoa chat tot. Cac hang so duoc dieu chinh them.

Trong MixRight, hieu chinh Bolomey dieu chinh cac hang so Abrams dua tren loai xi mang da chon, de du doan cuong do tinh den dong hoc thuy hoa khac nhau cua xi mang pha tron.

Phuong phap do chin Nurse-Saul

Nurse (1949) va Saul (1951) da xac lap rang su phat trien cuong do be tong phu thuoc vao ca nhiet do va thoi gian. Ham do chin ket hop chung thanh mot chi so duy nhat:

trong do:

• M la chi so do chin (°C·gio hoac °C·ngay)
• T la nhiet do bao duong trung binh trong khoang thoi gian Δt
• T₀ la nhiet do nguong ma duoi do qua trinh thuy hoa thuc te ngung — theo quy uoc la −10 °C
• Δt la khoang thoi gian

Diem mau chot la be tong bao duong o 40 °C trong 7 ngay co the dat cung do chin — va xap xi cung cuong do — nhu be tong bao duong o 20 °C trong 14 ngay. MixRight su dung dieu kien tham chieu 20 °C va 28 ngay lam co so, sau do tinh ty le cuong do du doan bang ty le do chin.

Dieu nay cho phep Cong cu Du doan Cuong do tinh den dieu kien bao duong thuc te: khi hau nong, thi cong thoi tiet lanh, hoac bao duong hap gia toc.

Mo phong Monte Carlo

Mo hinh tat dinh cho mot uoc luong diem duy nhat: "cap phoi nay se dat 32 MPa." Trong thuc te, moi dau vao — ty le N/X, cap cuong do xi mang, chat luong cot lieu, nhiet do bao duong — deu mang su khong chac chan. Mo phong Monte Carlo giai quyet dieu nay bang cach:

1. Mo hinh hoa moi dau vao khong chac chan nhu mot phan phoi xac suat (chuan, log-chuan hoac deu tuy truong hop)
2. Lay hang ngan mau ngau nhien tu cac phan phoi nay
3. Chay mo hinh ket hop Abrams–Bolomey–Nurse-Saul cho tung mau
4. Tap hop cac gia tri cuong do thu duoc thanh bieu do tan suat hien thi toan bo pham vi ket qua co the

Ket qua khong phai la mot con so duy nhat ma la mot phan phoi. Tu do, MixRight bao cao trung binh, trung vi, phan vi thu 5 va thu 95, va cuong do dac trung — cung cap cho ky su thong tin can thiet de thiet ke voi bien do an toan phu hop.

Tai lieu tham khao

1. Abrams, D.A. (1918). "Design of Concrete Mixtures." Bulletin 1, Structural Materials Research Laboratory, Lewis Institute, Chicago.
2. Bolomey, J. (1935). "Granulation et prévision de la résistance probable des bétons." Travaux, 19(30), 228–232.
3. Nurse, R.W. (1949). "Steam Curing of Concrete." Magazine of Concrete Research, 1(2), 79–88.
4. EN 206:2013+A2:2021. "Concrete — Specification, performance, production and conformity."
5. BRE (1997). "Design of Normal Concrete Mixes." 2nd Edition, Building Research Establishment.