The use of concrete admixtures greatly improves the flow properties of concrete and reduces the amount of cementitious materials in concrete. Therefore, concrete admixtures are widely used. In long-term production practice, it has been found that many mixing stations have misunderstandings about the use of admixtures, resulting in insufficient concrete strength, poor workability, or excessive concrete mix cost.
Mastering the correct use of admixtures can improve the strength of concrete while keeping the mix cost unchanged; or reduce the mix cost while maintaining the strength of concrete; keep the water-cement ratio unchanged and improve the working performance of concrete.
(1) Purchase admixtures at low prices
Due to fierce market competition, the mixing station has strict control over the procurement of raw materials. Mixing stations all hope to purchase raw materials at the lowest price, and the same is true for concrete admixtures. Mixing stations lower the purchase price of admixtures, which will inevitably lead to admixture manufacturers lowering their quality levels. Generally speaking, the acceptance criteria for admixtures are rarely specified in the mixing station procurement contract. Even if there is, it is only in accordance with the national standard requirements, and the national standard requirements are generally the minimum standards. This leads to the fact that when admixture manufacturers win bids at low prices, the admixtures they supply are of low quality and generally barely meet national standards, making it difficult to meet the functional requirements for mixing stations. Admixtures.
(2) Limit the amount of additives
The decision-making level of the mixing station strictly monitors the mix ratio cost, and even has clear requirements for the amount of cement and additives. This will inevitably lead to the technical department not being able to break through the decision-making layer's requirements for the maximum amount of additives when designing the mix ratio.
(3) Lack of quality monitoring and trial production verification of admixtures
At present, for the storage inspection of admixtures, most mixing stations inspect one or two of the technical indicators such as solid content, water reduction rate, density, and fluidity of the clean slurry. Very few batching plants carry out concrete testing.
In production practice, we found that even if the solid content, water reduction rate, density, fluidity and other technical indicators of the admixture meet the requirements, the concrete test may still not achieve the effect of the original trial mix, that is, the concrete water reduction rate is not enough. Or poor adaptability.
Due to the low quality level of admixtures purchased at low prices, in order to achieve sufficient water reduction effects, technical departments often increase the dosage of admixtures, resulting in low-quality, multi-purpose admixtures. On the contrary, some mixing stations with stable quality control and better mix ratio cost control use admixtures with better quality and higher prices. Due to high quality and low dosage, the unit cost of admixtures is reduced.
Some mixing stations limit the amount of admixtures. When the slump of concrete is insufficient, the technical department will either reduce the moisture content of the sand and gravel, or increase the water consumption per unit of concrete, which will directly lead to a decrease in concrete strength. Technical departments with a strong sense of quality will indirectly or directly increase the unilateral water consumption of concrete, and at the same time appropriately increase the amount of cementitious materials (keeping the water-cement ratio unchanged), resulting in an increase in the cost of concrete. Concrete mix proportions.
The mixing station lacks quality control and trial preparation verification of admixtures. When the additive quality fluctuates (decreases), the technical department still uses the original mix ratio. In order to meet the concrete slump requirements, the actual water consumption of concrete increases, the water-cement ratio increases, and the strength of concrete decreases.