Even if the concrete is inspected at the plant and found to be workable, dry aggregates increase the likelihood of water being added at the time of installation. Because aggregates that are drier than the SSD condition absorb water from the paste over time, they can cause slump loss in transport. Dry aggregates also require more cement per unit volume, which increases the cost of construction.
How do you control the moisture content of the aggregate? It depends on how out-of-date this information is at your plant. If the material is still within its recommended maximum moisture content, then there is no need to control the moisture level. However, if the material is past its expiration date, it should be kept as dry as possible. This can be done by storing it in a covered area away from any source of humidity such as piles of wet sand or gravel, or by heating it before use.
If you choose to use damp aggregates, be sure to add sufficient water to bring the mix to the required slump weight. Also remember that aggregates with higher levels of moisture will require more cement per unit volume, which increases the cost of construction.
Concrete that is too moist may not set up properly, which could lead to problems such as bleeding (where water migrates into the mortar) or spalling (where large pieces of concrete break off).
Too much water in the mix can also cause problems.
Stronger than coarse aggregate of a bigger size Cook discovered that when the water-to-cement ratio decreases and test age increases, the differential in compressive strengths owing to aggregate size is bigger. The lower the coarse aggregate size, the greater the flexural strength of the concrete. This phenomenon has been reported by many other researchers since then. Tufnell reported that at low volume fractions (0.5) coarse aggregates as small as 2mm can increase the strength of concrete.
Concrete containing coarse aggregates smaller than 4mm will always be stronger than ordinary concrete with fine aggregate of similar shape and type. This is because the maximum stress in the concrete occurs at the interface between the coarse and the fine aggregate. If this area is reduced, then the maximum stress available for transfer into the cement matrix is also reduced. Therefore, coarse aggregate becomes more important when designing strong concrete.
The reason why coarse aggregate affects the strength of concrete is not clear but several hypotheses have been made: (1) Coarse aggregate creates microscopic cracks that allow water to enter the concrete causing it to shrink and become less dense; this reduces its strength. (2) Coarse aggregate causes concrete to be more heterogeneous which makes it more likely to contain areas where water can be trapped causing it to expand and break down more easily. (3) The surface area of coarse aggregate is much larger than that of fine aggregate so more per unit mass of concrete is involved in transferring load.
Aggregate materials aid in the compacting of concrete mixtures. They also minimize cement and water consumption and add to the mechanical strength of concrete, making them an essential component in the building and maintenance of stiff structures.
The three main types of aggregate are: natural, manufactured, and recycled.
Natural aggregates such as sand and gravel provide a consistent texture and quality throughout the concrete mix and allow for maximum use of space during storage and placement. These features make them ideal for most applications. However, they can be expensive and location dependent. Also, they may contain harmful substances such as metal chips and glass shards that could potentially damage your health if you were to ingest them.
Manufactured aggregates are products produced by quarrying rock and then processing it into granules or powders. This type of aggregate is the least expensive option and comes in a wide variety of sizes and shapes. It can also be engineered to meet specific performance requirements. For example, crushed brick is used in place of stone to reduce cost while still providing good compaction. After placement, the molded shape of the brick allows for tight packing which reduces slumping.
Recycled aggregates are materials reclaimed from old structures or waste materials that have value for reuse in new projects.
Aggregates account for 60–80 percent of the volume and 70–85 percent of the mass of concrete. Aggregate is also critical for concrete strength, thermal and elastic characteristics, dimensional stability, and volume stability. Shrinkage is more likely to harm cement. Concrete's ability to resist compression is reduced as the percentage of coarse aggregate in the mix is increased.
The quality of the aggregate has a major impact on the performance of the concrete. A properly graded mixture of coarse and fine aggregates produces a concrete with good workability, closes the pores, and increases the strength of the material. Poorly graded mixtures may not dry completely, resulting in ineffective waterproofing and high permeability. Coarse-grained concrete tends to be stronger but also less flexible than fine-grained concrete.
Concrete that is low in air content (less than 10 percent) will not dry out and remain workable for long periods. This type of concrete must either be used immediately after mixing or baked in an oven at temperatures greater than 100 degrees Fahrenheit for several hours to days before it is ready for use.
Concrete that is high in air content (more than 20 percent) will dry very slowly because there is not enough surface area to absorb much water vapor. These mixtures should never be left outside over night because they will eventually become hard due to chemical reactions between the water vapor and the cement.
Fine aggregates are defined as any particle with a diameter smaller than 4.75 mm that can pass through a #4 sieve yet be retained on a #200 screen. Concrete that has excessive amounts of coarse aggregate will not cure properly because there are not enough small particles to make up the bulk of the concrete.
Coarse aggregates are defined as any particle with a diameter greater than 4.75 mm. These include stones, gravel, and sand. The amount of coarse aggregate used in a concrete mix determines its density. If too much coarse aggregate is used, the concrete will be heavy and dense. If not enough coarse aggregate is used, the concrete will be light and voids may appear inside the mixture. Coarse aggregate is important for three reasons: first, it makes the concrete stronger; second, it provides support for the concrete as it cures; third, it affects the rate at which water drains from the mixture during dry periods.
Water reduces the friction between grains of coarse aggregate, allowing water to seep between the grains and drain away from the concrete before it sets. This is called "water-retaining capacity". Coarse aggregate with large holes or vugs allows water to drain faster because there are fewer solid surfaces to prevent water from escaping.
Flaky, elongated, or a mixture of both types of aggregates are deemed undesirable for concrete mixing. Flaky aggregates have a tendency to be aligned in one plane, presenting problems with concrete durability. When the smallest dimension of an aggregate is less than 60% of its mean dimension, the aggregate is deemed flaky. Elongated aggregates have a length-to-width ratio of more than 4:1. They tend to come from wood products and may contain knots and other defects that can cause concrete to fail under stress.
The presence of dirt or moisture inside the box or bag where the aggregate is stored will affect the quality of the aggregate you use. If the aggregate is dirty, remove any large rocks or debris before using it in concrete projects.
Also, avoid using rubberized asphalt roofing material as concrete aggregate because the small bits of rock and gravel inside the plastic sheeting will decrease the strength of the finished product.
Finally, use caution not to include too much coarse aggregate in your mix. Too much coarse aggregate can cause your concrete to be harder than necessary for easy finishing. This can lead to etching (or scraping) the surface to make it look nice and smooth.
Coarse aggregate includes anything larger than 1/4" in diameter. Examples include sand, gravel, crushed stone, and recycled materials such as glass and metal.