Civil Engineering - Concrete Technology - Discussion

Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading. All portland cement concrete undergoes drying shrinkage or hydral volume change as the concrete ages. The hydral volume change in concrete is very important to the engineer in the design of a structure. Drying shrinkage can occur in slabs, beams, columns, bearing walls, prestressed members, tanks, and foundations.

Drying shrinkage is dependent upon several factors. These factors include the properties of the components, proportions of the components, mixing manner, amount of moisture while curing, dry environment, and member size. Concrete cured under normal conditions will undergo some volumetric change. Drying shrinkage happens mostly because of the reduction of capillary water by evaporation and the water in the cement paste. The higher amount of water in the fresh concrete, the greater the drying shrinkage affects. The shrinkage potential of a particular concrete is influenced by the amount of mixing, the elapsed time after the addition of water, temperature fluctuation, slumping, placement, and curing. The makeup of concrete is also very important. Each aggregate and cement type has distinctive characteristics, each contributing to concrete shrinkage. The amounts of water and admixtures used during mixing also have direct and indirect effects on drying shrinkage of concrete. Concrete shrinkage occurs mostly due to the evaporation of the mixing capillary water. The severity of this shrinkage depends on the physical properties of the concrete including size of the structure, location of the structure, and the surrounding temperature.

Properties and Proportions of Components.

The compositional makeup of concrete contributes directly to the drying shrinkage of concrete. Loss of moisture in the hydrated cement paste results in shrinkage. Different compositions and fineness of cement have variable effects on the shrinkage of cement paste. The difference in shrinkage is reduced significantly due to the adjustment of the amount of gypsum added to the different cement compositions. The size of aggregate is not as important but has an indirect influence on the water content of concrete. Shrinkage decreases with the volumetric increase of aggregate concentration causing a linear relationship between free shrinkage and cracks width. High-density aggregates and high modulus of elasticity of aggregates will decrease the compressibility and increase the shrinkage of concrete. The use of admixtures may alter the hydration reaction, which results directly in a high increase of drying shrinkage.

Moisture:

The concrete properties influence on drying shrinkage depends on the ratio of water to cementitious materials content, aggregate content, and total water content. The total water content is the most important of these. The relationship between the amount of water content of fresh concrete and the drying shrinkage is linear. The increase of the water content by one percent will approximately increase the drying shrinkage by three percent. Constant water to cementitious materials ratio coincides with changes in the amount of aggregate used.

Dry Environment:

The amount of drying shrinkage depends on the environmental conditions; relative humidity, temperature, and air circulation. Concrete subjected to a dry atmosphere will, in most cases, have a greater drying shrinkage than if subjected to an alternate wetting and drying. Lower temperatures generally produce a decrease in drying shrinkage because of higher humidity and slower evaporation.

The correct answer is:*The shrinkage of concrete** is primarily *proportional to water content in the mix*.

Here’s an explanation of each option:.

1. *Proportional to Water Content*l*: Higher water content leads to greater evaporation after the concrete sets, resulting in more shrinkage as water leaves the cement paste. This is the primary factor influencing shrinkage.

2. *Proportional to Cement Content*: While not directly proportional to cement content alone, a higher cement-to-aggregate ratio in the mix (common in richer mixes) can contribute to increased shrinkage because these mixes typically retain more water.

3. *Increases with Age*: Shrinkage occurs most during the early stages as concrete dries and continues at a decreasing rate over time. It does not indefinitely increase with age but stabilizes after initial curing and drying shrinkage.

So, the correct answer here would be "is proportional to water content in the mix"as the dominant factor in shrinkage behavior.

Shrinkage increases with age of concrete is true but its rate of shrinkage slows down starting from fresh concrete to older one. And its overall effect is the found to be increased with time (here we are not talking about rate but the changes in concrete due to shrinkage will be more as earlier because shrinkage, hydration, strengthening of concrete are continuous process that takes place throughout itself).

Shrinkage occurs with age due to carbonation shrinkage (reaction of CO2 from the atmosphere and Ca(OH) 2 from that of concrete.

When this occurs a crystal of Ca (OH) 2 converts into other compounds having lower volume causing volume to decrease.

No, it's wrong.

Because shrinkage decreases with the age of concrete!

Please explain the Effect of shrinkage on the age of concrete.

Shrinkage decreases with time.