【正文】 的應(yīng)力，將導(dǎo)致磚塊沿最大應(yīng)力方向斷裂，這種斷裂將形成一條從墻頂?shù)綁δ_，貫穿墻體的鋸齒狀裂縫。 10 對由各種不同強(qiáng)度的砂漿建成的磚墻已經(jīng)做了大量的實(shí)驗(yàn)。 砂漿與其他膠凝材料，如混凝土、熟石膏和灰泥有著密切的關(guān)系，但是若把砂漿和這些材料混同起來，或是想用這些材料來替代砂漿是錯誤的，因?yàn)槊恳环N材料均具有各自的特點(diǎn)，互不相同。作為骨料組成部分的砂子所含表面水分也應(yīng)包括在 6加侖加水量之內(nèi)。這便是為什么在混凝土內(nèi)總存在一些孔隙的原因?？墒前凑者@種水灰比加工出來的拌合物太干不能操作。其結(jié)果 是經(jīng)常調(diào)配出強(qiáng)度非常低的混凝土。 混凝土的水灰比是它最終強(qiáng)度的決定因素。萬一這種現(xiàn)象出現(xiàn)，混凝土 養(yǎng)護(hù)完后，濕氣會很容易地進(jìn)入其內(nèi)。在施工中，如果這些脹縮運(yùn)動超出允許限度，混凝土就會開裂。 因此，常常將鋼筋埋人混凝土中。它能承受很高的壓力荷載。骨料通常占一塊特定混凝土體積的百分之七十五。 二、 混凝土的優(yōu)缺點(diǎn)及其水灰比 混凝土是波特蘭水泥、水、砂子與卵石或碎石的拌合物。亞布拉姆斯公布了他的研究及觀測結(jié)果。這個時期大多 數(shù)混凝土含水量很低，要用大量手工作業(yè)才能使之夯實(shí)就位。 ?其后，水泥回轉(zhuǎn)窯的開發(fā)致使大規(guī)模的水泥生產(chǎn)遍及全世界。然而，大約在 1850年，這門工業(yè)不僅在英國，而且在德國和比利時都已牢固確定。之所以如此命名是因?yàn)橛眠@種水泥制成的混凝土被認(rèn)為很象是從英國波特蘭附近采集來的石灰石。通過燒制石灰與粘土的天然混合物來制造水泥的方法便是其中的 一項(xiàng)發(fā)現(xiàn)。該工程的設(shè)計(jì)師及工程師約翰很多古羅馬建筑都是用某種混凝土建造的，使用的就是這些材料，石材建筑物也是用含有類似 成分的砂漿粘合的。希臘人用的材料是一種取自圣多林島的火山灰，至今仍在世界的那個地區(qū)使用。這種建筑結(jié)構(gòu)在世界上一些干燥的沙漠地區(qū)曾經(jīng)很常見。在最初時期，建筑物是由土夯實(shí)而成，或者在沒有任何連接或粘接物的情況下，用石頭一塊一塊地壘砌而成。s final strength. At one time the instructions for preparing a batch of concrete would have contained proportions such as 1:2:4, indicating that 1 part of Portland cement to 2 parts of sand to 4 parts of gravel by volume were to be mixed together, after which sufficient water was to be added to obtain a 4 workable mixture. This procedure ignored entirely the importance of the watercement ratio. It also resulted very often in the preparation or a very weak concrete, since the natural tendency is to add enough water to make placement of the concrete as easy as possiblethe sloppier the better, as far as the workmen are concerned. This manner of specifying the proportions of concrete is obsolete and should never be followed. In theory, it takes only 3 gal of water to hydrate pletely 1 cu ft of cement. (A sack of cement contains 1 cu ft exactly, and the sack weighs 94 lb). But this watercement ratio produces a mixture that is too stiff to be worked. In practice, therefore, additional water, between 4 and 8 gal per sack of cement, is used to obtain a workable mixture. But the greater the proportion of water in a watercement ratio, the weaker the final concrete will be. The additional water that is necessary to achieve a workable batch will only evaporate from the concrete as the concrete sets, and it will leave behind in the concrete innumerable voids. This is the reason there will always be some porosity in concrete. When an excessive amount of water has been used, there will be an excessive number of voids, which may cause the concrete to leak badly. If these voids should be filled with moisture when cold weather es, they will cause the frost damage alluded to above. As a general rule, therefore, 6 gal of water per sack of cement should be the maximum amount used for making concrete。 1 CONCRETE AND MORTAR 1. Early History of Cement and Concrete Shelter from the very beginning of man/ existence, has demanded the application Of the best , available technology of the contemporary era. In the earliest ages, structures consisted of rammed earth, or stone blocks laid one on another without benefit of any bonding or cementing medium. Stability of the stone structures depended on the regular setting of the heavy stones , The earliest masonry probably consisted of sundried clay bricks, set in regular courses in thin layers of moist mud. When the moist mud dried, a solid clay wall resulted. Construction: of this kind was mon in the dry desert areas of the world. Burnt gypsum as a cementing material was developed early in the Egyptian period and was apparently used in construction of some of the pyramids. Later the Greeks and Romans discovered methods of burning limestone to produce quicklime which was subsequently slaked for use in making mortar. Both the Greeks and the Roman