Conveying, Placing, Compacting, and Curing of Concrete

Conveying of most building concrete from the mixer or truck to the form is done in bottom-dump buckets or in wheelbarrows or buggies or by pumping through steel pipelines. The chief danger during conveying is that of segregation. The individual components of concrete tend to segregate because of their dissimilarity. In overly wet concrete standing in containers or forms, the heavier gravel components tend to settle, and the lighter materials, particularly water, tend to rise. Lateral movement, such as flow within the forms, tends to separate the coarse gravel from the finer components of the mix. The danger of segregation has caused the discarding of some previously common means of conveying, such as chutes and conveyor belts, in favor of methods which minimize this tendency.

大部分建筑混凝土从搅拌机或卡车向模板内输送是采用底部卸料的吊斗、独轮手推车、小斗车，或者由泵通过钢管输送完成的。在输送过程中的主要危险是离析。混凝土内的不同成分因互异性而趋于分离。在容器或模板内的过湿的混凝土中，较重的砾石成分趋于下沉，而较轻的材料，特别是水趋于上浮。当横向移动，例如在模板内流动时，会导致粗砾石从拌合物的较细成分中分离出来。离析的危险已经使一些过去经常使用的输送方法，例如溜槽和输送带等被废弃不用，而采用能使离析趋于最低的方法。

Placing is the process of transferring the fresh concrete from the conveying device to its final place in the forms. Prior to placing, loose rust must be removed from reinforcement, forms must be cleaned, and hardened surfaces of previous concrete lifts must be cleaned and treated appropriately. Placing and compacting are critical in their effect on the final quality of the concrete. Proper placement must avoid segregation, displacement of forms or of reinforcement in the forms, and poor bond between successive layers of concrete. Immediately upon placing, the concrete should be compacted by means of hand tools or vibrators. Such compacting prevents honeycombing, assures close contact with forms and reinforcement, and serves as a partial remedy to possible prior segregation. Compacting is achieved by hand tamping with a variety of special tools, but now more commonly and successfully with high frequency, power-driven vibrators. These are of the internal type, immersed in the concrete, or of the external type, attached to the forms. The former are preferable but must be supplemented by the latter where narrow forms or other obstacles make immersion impossible.

浇筑是将拌好的混凝土通过输送设备送到其在模板内最终位置的过程。浇筑前，必须将钢筋上的浮锈除去，必须将模板清理干净，对模板上已有混凝土的硬固表层必须进行清理和适当地处置。浇筑和捣实对混凝土最终质量的影响是非常重要的。正确的浇筑必须避免混凝土的离析、模板或者其内钢筋的移位，以及相继浇筑的混凝土层之间的不牢固黏结。浇筑完毕后，应马上使用手动工具或振捣器将混凝土捣实。振捣可以防止出现蜂窝，确保混凝土与模板和钢筋的紧密接触，并对此前可能出现的离析予以部分补救。振捣靠人工使用各种专用工具完成，但现在更常见和更成功的方法是采用电动高频振捣器。振捣器有沉入混凝土内的内振型，或者附着于模板上的外振型。前者比后者更为可取，但是当遇到窄的模板或者因为其他障碍而使前者无法进入时，必须采用后者作为补偿。

Fresh concrete gains strength most rapidly during the first few days and weeks. Structural design is generally based on the 28 day strength, about 70 percent of which is reached at the end of the first week after placing. The final concrete strength depends greatly on the conditions of moisture and temperature during this initial period. The maintenance of proper conditions during this time is known as curing. Thirty percent of the strength or more can be lost by premature drying out of the concrete; similar amounts may be lost by permitting the concrete temperature to drop to 40 °F or lower during the first few days unless the concrete is maintained continuously moist for a long time thereafter. Freezing of fresh concrete may reduce its strength by as much as 50 percent.

新浇筑的混凝土的强度在最初几天和最初几周内的增长最快。结构设计通常28天的强度为依据，浇筑后的第一周的周末，可以达到这一强度的70%左右。混凝土的最终强度在很大程度上取决于其浇筑初期的湿度和温度条件。在这期间适当的维护条件称为养护。混凝土过早的失水可使其强度损失30%或更多；如果在最初几天内混凝土的温度下降到40°F以下，也会有同样的强度损失，除非使混凝土在此后的一个长时间内一直保持潮湿。新浇筑的混凝土冻冰后，其强度降低可能会达到 50%。

To prevent such damage, concrete should be protected from loss of moisture for at least 7 days and, in more sensitive work, up to 14 days. When high early strength cements are used, curing periods can be cut in half. Curing can be achieved by keeping exposed surfaces continually wet through sprinkling, ponding, covering with plastic film, or by the use of sealing compounds, which, when properly used, form evaporation-retarding membranes. In addition to improved strength, proper moist-curing provides better shrinkage control. To protect the concrete against low temperatures during cold weather, the mixing water and, occasionally, the aggregates are heated, temperature insulation is used where possible, and special admixtures, particularly calcium chloride, are employed. When air temperatures are very low, external heat may have to be supplied in addition to insulation.

为了避免这类损害，至少在七天内，对比较敏感的工程则延长至14天内，应该防止混凝土失去水分。当使用早强水泥时，养护期可缩减一半。进行养护时要使外露表面一直保持潮湿，可采用洒水、浸水、塑料膜覆盖等，或者正确地使用密封剂以形成蒸发阻滞膜。在适当湿度情况下养护除了能改善强度外，还可以更好地控制收缩。为了保护混凝土在寒冷的季节不受低温侵害，可以对拌合用水，有时也对骨料进行加热，如果有可能则采用保温材料，还可以使用专门的外加剂，尤其是氯化钙。当气温非常低时，除了保温之外还可能需要进行外部加热。