Fundamental Study on the Method of Mixing Concrete Under Reduced Pressure

本文研究了减压搅拌混凝土新方法（MURP法）的有关基本问题，以及用该方法生产的砂浆和混凝土的性能，以期在新拌混凝土中气泡的行为符合波义耳定律的情况下，生产出高强、高抗冻融耐久性的引气混凝土.混合物料在水蒸气压作用下的失水速率主要取决于抽气系统的抽气能力。然而，即使抽真空到如此低的压力，在所需的抽真空时间和搅拌时间内，通过使用普通真空泵损失的水的总量与实际砂浆或混凝土混合物中的水含量相比可以忽略不计，并且混合物的温度不会改变。2.当采用MURP法搅拌水泥净浆、砂浆或混凝土时，保持AE剂的剂量不变，夹带的空气量与普通搅拌法几乎相同，并且夹带空气的行为与波义耳定律所预期的相似。然而，在砂浆和混凝土的情况下，在它们暴露于大气条件之后，由于在它们从混合器排出和/或搅拌期间空气从它们的周围侵入混合物中，预期效果受损。随着骨料含量的增加，这种不希望的现象变得突出。结果，通过使用MURP方法的预期效果在砂浆的情况下减少到接近一半，在混凝土的情况下减少到可以忽略的程度。MURP法生产的混凝土具有空气损失小、坍落度随时间变化小的特点。它在振动下的空气损失也比普通混凝土低。

Basic problems related to the new method of mixing concrete under a reduced pressure (MURP method) and properties of mortar and concrete produced by the method were studied, expecting that air-entrained concrete of high strength and high freeze-thaw durability may be produced if air bubbles in fresh concrete behave In accordance with the Boyle's law.The followings are main findings or conclusions.1. The rate of water loss from mixing materials under water vapor pressure is dependent mainly on the capacity of evacuating system. However, even when evacuated to such a low pressure, the total amount of water lost through the use of ordinary vacuum pumps during the required evacuating time and mixing time is negligible compared with the water content in actual mortar or concrete mixes, and the temperature of the mixes is not altered.2. When cement paste, mortar or concrete is mixed by MURP method keeping the dosage of AE agent constant, nearly the same amount of air as that by ordinary mixing method is entrained, and the entrained air behaves in a similar way as can be expected by Boyle's law. However, in the cases of mortar and concrete, the expected effects are impaired after their exposure to the atmospheric conditions, due to the intrusion of air into the mixtures from their surroundings during their discharging from mixer and/or agitation.This undesirable phenomenon becomes prominent as the content of aggregate is increased.As the result, the expected effects through the use of MURP method are reduced to nearly a half in the case of mortar, and to negligible extent in the case of concrete.3. Concrete produced by MURP method has characteristics of low losses of air and slump with time. Its air loss under vibration is also lower compared with ordinary concrete.