选择混凝土混合物比例

Concrete mix proportions is a process of optimization of several desirable characteristics based on the project requirements.

The characteristics to be optimized are经济，，，，力量，，，，耐用性， 和可安置性。

1。经济

The primary reason for systematically determining mixture proportions is economy. The maximum economy can be achieved by minimizing the amount of cement used and where appropriate, by replacing portland cement with usually less expensive pozzolana or GGBF slag. Economy is also improved by using the largest nominal maximum size aggregate consistent with the dimensional requirements of the structures on the project, and available to the project.

2。Strength

Strength is an important characteristic of concrete but other characteristics such as durability, permeability, and wear resistance may be equally or more important. These may be related to strength in a general way but are also dependent on other factors.

For a given set of materials, strength is inversely proportional to thew/c。

Since the materials which make up concrete are complex and variable, an accurate prediction of strength cannot be based solely on the selected w/c but must be confirmed by tests of cylinders made from trial batches with the materials to be used on the project. Strength at the age of 28 days is frequently used as a parameter for structural design, concrete proportioning, and evaluation of concrete. When mass concrete is used, the design strength is generally required at an age greater than 28 days, generally 90 days, because mixtures are proportioned with relatively large quantities of pozzolana or GGBF slag to reduce internal heat generation. The early strength of mass concrete will be low compared to that of structural concrete; therefore, mass concrete should be proportioned for an adequate early strength as may be necessary for form removal and form anchorage. A compressive strength of 500 psi at 3 days age is typical of that necessary to meet form-removal and form anchorage requirements.

3。Durability

混凝土必须抵抗暴露的环境的恶化，包括冷冻和融化，润湿和干燥，化学攻击和磨损。混凝土必须满足三个要求，然后才能被视为霜冻作用。它必须由非易感的骨料和适当的空气系统制成，并且必须在重复冷冻和解冻时，在混凝土重复融化之前，必须达到适当的成熟度。除非在特定情况下证明它是不适当的，否则兵团放置的所有暴露混凝土都应被夹住。当混凝土的抗压强度约为3500 psi时，存在适当的成熟度。通常，使用低W/C，由于这种降低了渗透性和侵袭性液体的渗透，还可以提高耐用性。

4。Placeability

安置性，包括令人满意的完成特性，包括由“可加工性”和“一致性”术语描述的特征。可工作性是新鲜混合混凝土的特性，它决定了可以将其混合，放置，合并和完成的易度性和同质性。一致性是新鲜混合混凝土流动的相对迁移率或能力。

可行性体现了诸如可可的性，凝聚力和紧凑性等概念，并受到骨料的分级，粒子形和比例的影响；所使用的水泥材料的数量和品质；新利客户存在或不存在夹带的空气和化学混合物；以及混合物的一致性。低迷测试，ASTM C 143是唯一可用的测试，用于测量“可容纳性”一词中包含的几个特征的任何方面。

可塑性、粘结性、压实性和鳍ishability are mostly evaluated by visual observation, and, therefore, the evaluations are somewhat subjective. Typically, the Contractor will evaluate these characteristics from a different perspective than the government personnel involved, and within the Contractor’s organization, the placing foreman may evaluate the placeability differently than the finishing foreman. In general, the Contractor would like a high-slump mixture, while the Government desires a closely controlled slump. The key consideration must be a carefully proportioned concrete mixture which is placeable by the conveying and placing equipment to be used on the project without the addition of water at the placement site. Simply adjusting the water content of a mixture that was proportioned for placement by crane and bucket will not assure that it is pumpable or that such an adjustment will result in concrete that meets strength and durability requirements. Mass concrete mixtures are particularly susceptible to placing problems if not correctly proportioned. Care must be exercised to assure that the mortar content of lean, mass concrete mixtures is sufficient to provide suitable placing and workability. Water-reducing admixtures should not be used to reduce the paste content and the resulting mortar content of these mixtures to a level which causes the mixture to be harsh and unworkable.