Fundamental Study on the Method of Mixing Concrete Under Reduced Pressure

研究了减压搅拌混凝土新方法(MURP法)的基本问题以及由此产生的砂浆和混凝土的性能，期望如果新拌混凝土中的气泡行为符合波义耳定律，就可以生产出高强度和高冻融耐久性的引气混凝土。混合料在水蒸气压力下的失水率主要取决于抽真空系统的容量。然而，即使在真空到如此低的压力下，在所需的真空时间和搅拌时间内使用普通真空泵所损失的总水量与实际砂浆或混凝土混合料中的含水率相比微不足道，混合料的温度也没有变化。在保持AE剂用量不变的情况下，用MURP法搅拌水泥净浆、砂浆或混凝土时，夹带量与普通搅拌法几乎相同，且夹带量的变化规律与波义耳定律基本一致。然而，对于砂浆和混凝土，由于它们在从搅拌机和/或搅拌过程中排放时，由于空气从周围环境进入混合料，预期效果会受到影响，这种不良现象随着骨料含量的增加而变得更加突出，结果是，使用MURP方法对砂浆的预期效果降低了近一半，而对于混凝土，预期效果可以忽略不计。MURP法生产的混凝土具有气耗低、坍落度随时间推移的特点。与普通混凝土相比，其在振动作用下的空气损失也较低。

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.