What is concrete aggregate? What is its function? Don’t say you don’t know yet!

Thread Content

Why does some concrete remain in excellent condition after 10 years, while other concrete deteriorates after just 2 years? In concrete, you can think of cement as the glue and aggregates as the bones; if the bones are not good, no amount of good glue will be able to hold everything together. I. What is concrete aggregate? Aggregates refer to the sand (fine aggregate) and gravel (coarse aggregate) in concrete; they make up 70% to 80% of the volume of concrete. To use an analogy: cement paste is the \"flesh,\" while aggregates are the \"bones.\" Without bones, the flesh is nothing but a pile of mud. Fine aggregate: with a particle size of less than 4.75 mm, mainly sand, which fills the gaps between the larger stones. Coarse aggregate: with a particle size greater than 4.75 mm, mainly consisting of crushed stone or gravel, and bearing the main load. II. The key role of aggregates in concrete: Function 1 – Structural support, bearing pressure. Concrete’s resistance to compression relies not on the cement paste, but on the strength of the stones. Make a test block using pure cement slurry, apply pressure to it – it will break at 30 MPa ； With gravel, it can easily reach 50 MPa. The aggregate acts like thousands of \"small columns,\" distributing and transmitting the load. The particle size, strength, and gradation of coarse aggregates directly determine the upper limit of concrete strength. Using weathered rock as aggregate is pointless no matter how high the cement grade is. Effect 2: Volume stability to prevent cracking. As cement paste hardens, it contracts (due to drying shrinkage, autogenous shrinkage, or temperature changes), and cracking occurs as a result of this contraction. What about the aggregate? It does not shrink; the aggregates act like a \"skeleton\" to support the volume and prevent shrinkage. Well-graded aggregates, with small stones filling the gaps between larger stones and sand filling those gaps between the stones, result in the lowest porosity and thus the least shrinkage. Conversely, if the grading is poor, there are many voids and excess paste, leading to significant shrinkage and thus cracks. Effect 3: Cost savings – Cement is much more expensive than aggregates; by using aggregates to replace part of the cement paste, several dozen dollars can be saved per cubic meter of concrete. One building less, and millions saved. III. How to select aggregate quality? 4 key points: 1. Gradation: It should be continuous without any gaps. Good aggregates: There should be particles of various sizes – large stones, medium-sized stones, small stones, and sand – which fill each layer together to create a tight, seamless structure. Poor-quality aggregates: either all coarse gravel or all fine sand, resulting in a high porosity; the cement paste cannot fill the gaps, leading to poor strength. 2. Clay content: The lower, the better. Clay covering the surface of the stones prevents the cement paste from adhering to them. The mud content in coarse aggregate is ≤1.0%, and that in fine aggregate is ≤3.0%. High-strength concrete has stricter requirements. If the mud level is too high, wash it before using again. 3. Flaky shape: The fewer flaky stones there are, the better. Flaky stones (which are thin and flat) are prone to breaking under stress. The content of flaky particles shall be ≤15%, and for high-strength concrete, it shall be ≤10%. 4. Durability: Frost-thaw cycles must not damage structures in northern regions; the aggregates must be able to withstand such cycles. When tested with a sodium sulfate solution, the mass loss rate is ≤5%–12% (depending on the environment). IV. A Truth That Challenges Assumptions: Many people think that aggregates are simply \"rock debris\" that do not participate in any reactions. Wrong! Alkali-aggregate reaction: Active components in certain aggregates react with the alkalis in cement, causing volume expansion and leading to cracking in concrete. This kind of damage takes 5-10 years to become apparent, and once it occurs, it cannot be completely cured. Therefore, suppression tests must be conducted on aggregates suspected of having alkaline activity. END concrete is not a \"cement glue\"; it is a \"aggregate framework + cement glue\". You spend a lot of money on high-grade cement but cut costs on aggregates, which is like using good bricks in building a structure but poor-quality rebar. Next time someone says \"You can use any stones,\" show them this article. Only with the right aggregate can concrete be strong ； If the aggregates are of poor quality, it’s pointless to use expensive cement.