1. Core composition rule The basic composition of concrete
is "cementitious material + aggregate + water + admixture". Cementitious material (cement + admixture) determines the strength basis, the proportion of aggregate exceeding 70% affects the volume stability, and the admixture precisely regulates the performance.
2. The core principle
of strength is that the water-binder ratio is the key to determine the strength of concrete. The smaller the water-binder ratio, the higher the strength (the premise is to ensure the workability). The water-binder ratio of ordinary C25-C40 concrete should be controlled at 0.40-0.55.
3. The principle
of cement selection is to match the engineering requirements according to the strength grade. P · O42.5 cement is suitable for C25-C55 concrete, and P · O52.5 cement is suitable for high grade above C60. Cement with unqualified soundness is strictly prohibited.
4. Aggregate control rule
: select sand (fineness modulus 2.3-3.0), silt content ≤ 3%, machine-made sand powder content ≤ 10%; select 5-25 mm continuous gradation of crushed stone, crushing value ≤ 16%, reasonable gradation can reduce the amount of cementitious materials.
5. Admixture matching rule
: fly ash is preferentially used in medium and low grade concrete, and the water demand ratio is ≤ 105%; slag powder can improve the strength more significantly, and the dosage of S95 grade is not suitable for 40% of supercementitious materials.
6. The core principle
of admixtures is that polycarboxylate superplasticizer is the mainstream, and the dosage is usually 0.8% -1.2% of cementitious materials. The key is to ensure the compatibility with cement and avoid segregation or slump loss.
7. The mix proportion design rule
follows the principle of "meeting the strength, ensuring the workability, taking into account the economy, low carbon and environmental protection". The sand ratio is very important for pumping concrete, which should be controlled at 38% -45%. If it is too small, it will be easy to segregate, and if it is too large, it will increase the consumption of materials.
8. Workability judgment rule
The slump directly reflects the fluidity, and the pumping concrete should be 180-220mm; the expansion degree evaluates the cohesiveness, and it is qualified if it is ≥ 500mm and there is no obvious bleeding.
9. Avoidance rule
of bleeding segregation When bleeding occurs, water-binder ratio (too large), sand ratio (too small) or admixture dosage (too high) should be checked first, which can be solved by reducing water-binder ratio, increasing sand ratio or compounding thickener.
10. Setting time control rule
: If the initial setting is too short, it is easy to crack, and if the final setting is too long, it will affect the progress; add retarder (such as sodium gluconate) at high temperature, add early strength agent (such as sodium sulfate) at low temperature, and control the dosage accurately.
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