Chúng ta đều biết chất lượng bê tông ảnh hưởng trực tiếp đến kết cấu của công trình xây dựng. Tuy nhiên kỹ thuật đổ bê tông là một yếu tố quan trọng quyết định đến độ bền vững của khối bê tông.
Kỹ Thuật Đổ Bê Tông
Để đảm bảo kỹ thuật các bạn cần làm theo các bước sau:
– Trước hết phải chuẩn bị chu đáo khung đổ bê tông: kiểm tra lại hình dáng, kích thước, khe hở của ván khuôn. Kiểm tra cốt thép, sàn giáo, sàn thao tác. Chuẩn bị các ván gỗ để làm sàn công tác.
– Chiều cao rơi tự do của bê tông (khoảng cách từ miệng ống đổ bê tông đến mặt đáy cần đổ bê tông) không quá 1,5m – 2m để tránh phân tầng bê tông.
– Trình tự đổ bê tông: đổ từ xa đến gần, từ trong ra ngoài, bắt đầu từ chỗ thấp trước, đổ theo từng lớp, xong lớp nào đầm lớp ấy. Dùng loại đầm thích hợp cho từng loại kết cấu bê tông: dùng đầm bàn cho sàn, đầm dùi cho cột, dầm, tường.
Đổ bê tông liên tục trong suốt quá trình, không tùy tiện dừng lại.
Tránh đổ bê tông khi thời tiết ẩm ướt, có mưa. Trường hợp trời mưa, phải che chắn, hạn chế để nước mưa rơi vào bê tông.
Đối với bê tông móng phải tạo nền đất cứng và làm sạch lớp đệm trước khi đổ bê tông. Bê tông cột có chiều cao nhỏ hơn 5m và tường có chiều cao nhỏ hơn 3m thì nên đổ bê tông liên tục.
Kỹ thuật đổ bê tông cột:
Đưa bê tông vào khối đổ qua cửa đổ thông qua máng đổ. Chiều cao rơi tự do của bê tông không quá 2m. Đầm được đưa vào trong để đầm theo phương thẳng đứng, dùng đầm dùi để đầm, chiều sâu mỗi lớp bê tông khi đầm dùi khoảng 30-50cm, thời gian đầm khoảng 20-40s.
Đổ bê tông cột bằng bơm cần.
Chú ý trong quá trình đầm tránh làm sai lệch cốt thép. Với kết cấu có cửa, khi đổ đến cửa đổ thì bịt cửa lại và tiếp tục đổ phần trên. Khi đổ bê tông cột lớp dưới cột thường bị rỗ do các cốt liệu to thường ứ đọng ở đáy, để khắc phục hiện tượng này trước khi đổ bê tông ta đổ 1 lớp vữa xi măng dày khoảng 10 – 20cm.
Kỹ Thuật Đổ Bê Tông sàn:
Đổ bê tông sàn.
Đổ bê tông liên tục kết hợp với đầm cho đến khi đạt độ dày cần thiết. dùng đầm bàn để đầm, khi đầm mặt phải kéo từ từ, các dải chồng lên nhau 5-10cm. Thời gian đầm ở 1 chỗ khoảng 30-50s.
Kỹ Thuật Đổ Bê Tông Móng:
– Bê tông được đổ trực tiếp vào khối đổ hoặc qua máng. Dùng đầm dùi.
– Quá trình đổ bê tông tươi và đầm bê tông hết sức quan trọng quyết định chất lượng của bê tông nên kỹ sư tư vấn đảm bảo chất lượng cần có mặt thường trực để chứng kiến công việc của bên nhà thầu.
– Chiều cao rơi tự do của bê tông không được quá 1.5 mét để tránh hiện tượng phân tầng. Nếu chiều cao rơi tự do quá 1.5 mét phải cho bê tông trượt qua máng nghiêng hay ống bạt, ống vòi voi. Khi đổ bê tông phải có người trực đề phòng bất trắc, rủi ro.
Các Kỹ Thuật Khác Cần Lưu Ý:
– Khi dùng các phương tiện gây ứng suất cục bộ lớn lên cốp-pha hay tạo xung lực mạnh, bên nhà thầu phải kiểm tra tính toán và kỹ sư tư vấn đảm bảo chất lượng cần kiểm tra rồi trình cho chủ dự án duyệt.
– Quá trình thi công phải đề phòng trời mưa và chuẩn bị phương tiện che chắn nếu có mưa. Đang thi công gặp mưa không được thi công tiếp mà phải đợi cho cường độ bê tông đạt đến 25 daN/cm2 (nếu thời tiết 25oC, khoảng 24 giờ) mới được thi công tiếp. Vì lẽ này mà khi đổ bê tông, giải phân cách các diện tích được đổ bê tông nên lựa chọn trùng với mạch ngừng thi công.
Đổ bê tông khi trời mưa
– Khi đủ cường độ để thi công tiếp, dọn sạch mặt tiếp giáp, nếu cần thiết phải đục xờm, lấy hồ xi măng và sikagrout (1:1) phết lên chỗ giáp mối khe ngừng với chiều dày khoảng 5mm làm vật liệu dán giữa lớp bê tông đã đổ và bê tông mới.
– Khi đầm cần chú ý không chọc dầm vào chỗ bê tông đã đổ và phải quan sát cho bê tông mới đổ đủ chảy làm mịn mạch nối. Chiều dày mỗi lớp đổ chỉ nên đạt 2/3 chiều sâu tác động của máy đầm.
– Không được tỳ đầm lên cốt thép và không dùng tác động của đầm làm cho bê tông dịch chuyển ngang. Không nên đầm một vị trí quá lâu mà chỉ cần vừa độ chặt, nghĩa là đầm đến khi trên mặt bê tông chớm xuất hiện nước xi măng. Đầm quá lâu một chỗ sẽ gây phân tầng bê tông.
– Khi đổ bê tông khối lớn (tạm quy ước đó là kết cấu có diện tích > 10m2, chiều cao kết cấu > 0.8 mét) mỗi lớp đổ nên là khoảng 30 cm và chờ cho bê tông sắp hết thời gian tươi mới nên đổ tiếp để tránh sự xuất hiện những vết nứt do ứng suất nhiệt gây ra. Mặt trên cùng của kết cấu bê tông vừa đổ cần được sửa sang bằng cách cán phẳng và xoa bằng bàn xoa.
Nếu cần xử lý đặc biệt bên thiết kế phải có chỉ dẫn riêng. cách chấn động lại. Biện pháp này phải được lập biện pháp riêng theo chỉ dẫn của chuyên gia.
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