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.

大部分建筑混凝土從攪拌機(jī)或卡車向模板內(nèi)輸送是采用底部卸料的吊斗、獨(dú)輪手推車、小斗車，或者由泵通過鋼管輸送完成的。在輸送過程中的主要危險(xiǎn)是離析。混凝土內(nèi)的不同成分因互異性而趨于分離。在容器或模板內(nèi)的過濕的混凝土中，較重的礫石成分趨于下沉，而較輕的材料，特別是水趨于上浮。當(dāng)橫向移動(dòng)，例如在模板內(nèi)流動(dòng)時(shí)，會(huì)導(dǎo)致粗礫石從拌合物的較細(xì)成分中分離出來。離析的危險(xiǎn)已經(jīng)使一些過去經(jīng)常使用的輸送方法，例如溜槽和輸送帶等被廢棄不用，而采用能使離析趨于最低的方法。

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.

澆筑是將拌好的混凝土通過輸送設(shè)備送到其在模板內(nèi)最終位置的過程。澆筑前，必須將鋼筋上的浮銹除去，必須將模板清理干凈，對(duì)模板上已有混凝土的硬固表層必須進(jìn)行清理和適當(dāng)?shù)靥幹谩仓蛽v實(shí)對(duì)混凝土最終質(zhì)量的影響是非常重要的。正確的澆筑必須避免混凝土的離析、模板或者其內(nèi)鋼筋的移位，以及相繼澆筑的混凝土層之間的不牢固黏結(jié)。澆筑完畢后，應(yīng)馬上使用手動(dòng)工具或振搗器將混凝土搗實(shí)。振搗可以防止出現(xiàn)蜂窩，確?；炷僚c模板和鋼筋的緊密接觸，并對(duì)此前可能出現(xiàn)的離析予以部分補(bǔ)救。振搗靠人工使用各種專用工具完成，但現(xiàn)在更常見和更成功的方法是采用電動(dòng)高頻振搗器。振搗器有沉入混凝土內(nèi)的內(nèi)振型，或者附著于模板上的外振型。前者比后者更為可取，但是當(dāng)遇到窄的模板或者因?yàn)?/span>其他障礙而使前者無(wú)法進(jìn)入時(shí)，必須采用后者作為補(bǔ)償。

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.

新澆筑的混凝土的強(qiáng)度在最初幾天和最初幾周內(nèi)的增長(zhǎng)最快。結(jié)構(gòu)設(shè)計(jì)通常28天的強(qiáng)度為依據(jù)，澆筑后的第一周的周末，可以達(dá)到這一強(qiáng)度的70%左右?；炷?/span>土的最終強(qiáng)度在很大程度上取決于其澆筑初期的濕度和溫度條件。在這期間適當(dāng)?shù)木S護(hù)條件稱為養(yǎng)護(hù)。混凝土過早的失水可使其強(qiáng)度損失30%或更多；如果在最初幾天內(nèi)混凝土的溫度下降到40°F以下，也會(huì)有同樣的強(qiáng)度損失，除非使混凝土在此后的一個(gè)長(zhǎng)時(shí)間內(nèi)一直保持潮濕。新澆筑的混凝土凍冰后，其強(qiáng)度降低可能會(huì)達(dá)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.

為了避免這類損害，至少在七天內(nèi)，對(duì)比較敏感的工程則延長(zhǎng)至14天內(nèi)，應(yīng)該防止混凝土失去水分。當(dāng)使用早強(qiáng)水泥時(shí)，養(yǎng)護(hù)期可縮減一半。進(jìn)行養(yǎng)護(hù)時(shí)要使外露表面一直保持潮濕，可采用灑水、浸水、塑料膜覆蓋等，或者正確地使用密封劑以形成蒸發(fā)阻滯膜。在適當(dāng)濕度情況下養(yǎng)護(hù)除了能改善強(qiáng)度外，還可以更好地控制收縮。為了保護(hù)混凝土在寒冷的季節(jié)不受低溫侵害，可以對(duì)拌合用水，有時(shí)也對(duì)骨料進(jìn)行加熱，如果有可能則采用保溫材料，還可以使用專門的外加劑，尤其是氯化鈣。當(dāng)氣溫非常低時(shí)，除了保溫之外還可能需要進(jìn)行外部加熱。

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