Search results
Results from the WOW.Com Content Network
Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Autogenous shrinkage also occurs when the concrete is quite young and results from the volume reduction resulting from the chemical reaction of the Portland cement.
For small specimen thickness, the creep during drying greatly exceeds the sum of the drying shrinkage at no load and the creep of a loaded sealed specimen (Fig. 1 bottom). The difference, called the drying creep or Pickett effect (or stress-induced shrinkage), represents a hygro-mechanical coupling between strain and pore humidity changes.
Fibers are usually used in concrete to control cracking due to plastic shrinkage and to drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion, and shatter resistance in concrete. [7]
It is not a problem as long as the very fresh concrete is still in a liquid, or a sufficiently plastic, state and can easily accommodate volume changes (contraction). Plastic shrinkage. Later in the setting phase, when the fresh concrete becomes more viscous and starts to harden, water loss due to unwanted evaporation can cause "plastic shrinkage".
The shrinkage limit (SL) is the water content where further loss of moisture will not result in more volume reduction. [2] The test to determine the shrinkage limit is ASTM International D4943. The shrinkage limit is much less commonly used than the liquid and plastic limits.
The presence of fibers in the mixture significantly reduces drying shrinkage, with larger fibers exhibiting a more pronounced effect than finer ones. This reduction is attributed to the increased water content required for workability when adding more and finer fibers.
ASA is mildly hygroscopic; drying may be necessary before processing. [4] ASA exhibits low moulding shrinkage. [5] ASA can be used as an additive to other polymers, when their heat distortion (resulting in deformed parts made of the material) has to be lowered. [6]
Figure 3: Simplified chemical reactions associated with curing of a drying oil. In the first step, the diene undergoes autoxidation to give a hydroperoxide. In the second step, the hydroperoxide combines with another unsaturated side chain to generate a crosslink. [4] Epoxy resins are typically cured by the use of additives, often called hardeners.