Fresh concrete that is anticipated to produce the greatest outcomes must have the attribute of workability. This is the most crucial characteristic of new concrete. Fresh concrete should be able to distribute uniformly without causing aggregate segregation. Aggregate segregation occurs when the different types of aggregate are placed in an unbalanced manner which can cause poor performance and failure of the concrete later on.
The second most important factor for fresh concrete is its ability to resist freeze-thaw cycles. The greater this attribute, the longer the concrete will last before requiring some type of reinforcement. Concrete that has been exposed to extreme temperatures, such as those found in areas where ice storms occur, requires additional time to cure before it can be used.
The third most important factor for fresh concrete is its durability. Concrete that has been exposed to air pollution or organic materials such as wood chips that may accelerate its degradation need to be cured under controlled conditions to prevent contamination.
The final important factor for fresh concrete is its appearance. Concrete that has not had enough time to harden properly will appear dull and gray instead of white or brown like old concrete. This can affect how people feel about your home if it looks bad even though it does a good job keeping out heat and cold.
Fresh concrete stiffens with time and loses workability, but it is not settling or developing strength. Some water is absorbed by aggregate after mixing concrete, some is lost by evaporation, and some is needed for first chemical reactions. The loss of workability over time is caused by a variety of circumstances, including: temperature changes, which cause the concrete to contract or expand, resulting in cracking; rain, which washes away some of the additive packages used to improve plasticity; and voids created by coarse aggregate falling into small holes or cracks. Coarse aggregates such as gravel fall through the concrete's surface because they are too large to be retained by the cement paste.
The workability of fresh concrete can be improved by adding more water under certain conditions. If the concrete has been stored before use, it should be watered until no dry spots remain. This will prevent mold growth and ensure good flow during placement.
As concrete cures, it becomes less fluid and more rigid, which improves its durability. Concrete that has not been exposed to air continues to cure even under humid conditions. As it cures, it forms a solid mass that cannot be remixed without destroying its integrity. However, concrete that has been exposed to air continues to cure even under moist conditions. As it cures, it forms a hard mass that can be worked upon later with additional tools.
How Do You Make High-Quality Concrete?
The higher the water content, the greater the fluidity of the concrete, which is one of the key elements influencing workability. More paste is available in rich concrete with a lower aggregate/cement ratio to make the mix cohesive and greasy for greater workability. Less paste means less cohesion and thus less workability.
Concrete that is too wet may not dry uniformly causing "slump" (the tendency of fresh concrete to slump when placed in the form). This can cause problems when casting large or multiple forms because the operator will need to keep an eye on the height of the concrete as it sets to prevent it from overflowing the mold.
If concrete is too dry, it will be hard to work with and may crack when squeezed. Drying time depends on the cement type but generally needs to be done within 24 hours if using ordinary portland cement, or up to 90 days if using high-performance cements such as HPCC or CPC.
Concrete that is either too wet or too dry affects the workability of the mixture and therefore its ability to be molded into place. If it's too wet, this will increase the amount of hand mixing required; if it's too dry, the concrete will be difficult to compress while still maintaining its shape, resulting in cracking.
Although aggregates are often thought of as inert fillers in concrete, their various qualities have a significant influence on the strength, durability, workability, and economy of concrete. These various aggregate qualities provide designers and contractors with the greatest amount of flexibility in meeting their design and building needs. Aggregates can be classified by their physical type, such as rock or sand, and by their relationship to the cement paste, which determines their effect on the concrete.
The three main types of aggregates are natural, manufactured, and recycled. Natural aggregates include: Rock (gravel, stone, brick), Sand (gravel, shell, glass), and Gravelite (a crushed waste product from steel mills). Manufactured aggregates include: Crushed Brick (CBB) for use in high-strength concrete, Granulated Concrete Aggregate (GCAA) for use in low-strength concretes, and Slag for use in shotcrete.
Recycled aggregates are those that are processed into new materials or products instead of being disposed of after use. Recycling reduces the impact of using natural resources and helps reduce landfill waste. Recycled aggregates include: Crushed Glass (CAG) for use in colored concrete, Rubber Tire Dust (RDD) for use in lightweight concrete, and Wood Chips (WC) for use in structural concrete.