聚集体的属性

Approximately 75% of the volume of concrete is occupied by aggregates, so quality of aggregates plays an important role in determining properties of concrete. Aggregates are chemically inert, solid bodies held together by the cement. Aggregates come in various shapes, sizes, and materials ranging from fine particles of sand to large, coarse rocks. Natural aggregates are formed by process of weathering and abrasion or by artificially crushing large parent mass.

Because cement is the most expensive ingredient in making concrete, it is desirable to minimize the amount of cement used. 70 to 80% of the volume of concrete is aggregate in order to keep the cost of the concrete low. The selection of an aggregate is determined, in part, by the desired characteristics of the concrete. For example, the density of concrete is determined by the density of the aggregate. Soft, porous aggregates can result in weak concrete with low wear resistance, while using hard aggregates can make strong concrete with a high resistance to abrasion. Aggregates are of two basic types:

• Coarse: crushed rock, gravel or screenings.
• 精细：细的沙子和破碎机罚款

Aggregates should be clean, hard, and strong. The aggregate is usually washed to remove any dust, silt, clay, organic matter, or other impurities that would interfere with the bonding reaction with the cement paste. It is then separated into various sizes by passing the material through a series of screens with different size openings. The final properties of the concrete will depend on the cement characteristics, the type and amount of aggregate, the water-cement ratio, and the completeness of the reaction subject to time, humidity, and temperature.

Examples of classes of concrete aggregate are shown below.

聚合的类型

班级	用途	Examples
超轻量级	Lightweight concrete which can be sawed or nailed, also for its insulating properties	ver石陶瓷球
轻的	Used primarily for making lightweight concrete for structures, also used for its insulating properties.	expanded clay shale or slate crushed brick
normal weight	用于普通混凝土项目	压碎的石灰石沙河碎石碎的再生混凝土
重量级	用于使高密度混凝土屏蔽核辐射	steel or iron shot steel or iron pellets

骨料的选择取决于建议的混凝土使用。通常，沙子，砾石和碎石被用作骨料制成混凝土。应当良好地分析骨料，以提高包装效率并最大程度地减少所需的水泥糊。另外，这使混凝土更加可行。下面讨论了聚集体的一些重要特性。

聚集体的属性

The property of aggregates are mentioned below.

1.健全

由于润湿和干燥而导致体积变化的岩石很少见。但是，聚集体易于在冷冻和解冻周期中变化。冻结会导致内部压力在聚集体内冻结和扩展的水中积聚。可以计算出临界大小，在该临界尺寸下，冻结应力不是问题。但是，对于大多数岩石而言，它比正常大小要大。

2.Wear Resistance

A good aggregate will be hard, dense, strong, and free of porous material. The abrasion resistance of aggregate can be tested by the Los Angeles abrasion test; however, this test does not match well with concrete wear in the field.

3.Alkali-Aggregate Reaction

An expansive reaction between some reactive forms of silica with the aggregate and alkalis in the cement paste. The result is overall cracking in the structure, manifesting itself in map or pattern cracking at the surface. This reaction can be controlled most easily by using low-alkali cements. However, due to changes in manufacturing, low-alkali cements may not be feasible. A better approach is to avoid aggregate with the potential or proven record of reactivity. A low厕所比率非常不可渗透，会减慢反应，但不会阻止反应。没有外水就不会发生不良反应。

4.其他碱性硅反应

在堪萨斯州和内布拉斯加州的河流系统中发现的砂粒具有高度反应性，并导致地图破裂。用压碎的石灰石替换30％的骨料可有效减少损坏。基本上，它导致扁平粘土矿物的分离，从而导致非常缓慢的膨胀。

5。碱碳酸盐反应

涉及碳酸盐岩石的广泛反应。反应可以通过使用低叠碱水泥或将骨料与其他反应性较低的材料混合来控制。ASTM已为骨料中的有害物质设定了标准，取决于应用。这可以分为两类：

6。Impurities

Solid materials – particles passing a 200-mesh sieve. These fine particles may increase water requirements and interfere with surface bonding between cement and coarse aggregates.

可溶性物质 - 有机物可能会化学干扰影响设定时间的碱性水泥糊。从海洋中获得的聚集体应进行彻底清洁，以避免盐污染中的问题。

7。Unsound particles

软颗粒（例如粘土团块，木材和煤炭）会在表面引起斑点和缩放。可以释放有机化合物，从而干扰设置和硬化。还应避免低密度的低密度材料