Test: Concrete Technology - 1 - Civil Engineering (CE) MCQ

The initial setting time of concrete is affected by temperature. At different temperatures, the behaviour of concrete changes significantly.

• At temperatures below 0°C, concrete can freeze, which slows down the setting process considerably.
• Specifically, at -10°C, the setting time is extremely prolonged, making it difficult for the concrete to gain strength.
• At -4°C, concrete still experiences a significant delay in setting.
• At 0°C, concrete can begin to set, but this occurs much slower than at warmer temperatures.
• At 4°C, the setting time improves, but it is still slower than at higher temperatures.

In summary, the initial setting time of concrete is notably affected by cold temperatures, with 0°C being the threshold where it can start to set effectively, although still at a slower rate.

The function of fine aggregate in concrete mixtures is essential for achieving desired properties. It plays several key roles, including:

• Workability: Fine aggregate helps to improve the workability and uniformity of the mixture, making it easier to handle and apply.

• Cement Suspension: It assists the cement paste in holding the coarse aggregate particles in suspension, ensuring an even distribution throughout the mix.

• Plasticity: Fine aggregate promotes plasticity, which prevents the segregation of the paste from the coarse aggregate, maintaining a consistent texture.

In summary, fine aggregate is crucial in enhancing the performance and stability of concrete mixtures.

The increase in the volume of a given mass of fine aggregate caused by the presence of water is known as bulking. The bulking of fine aggregate is caused by the films of water which push the particles apart. The extent of bulking depends upon the percentage of moisture present in the sand and its fineness.

gap-grading is defined as a grading in which one or more intermediate-size fractions are absent. The term continuously graded is used to distinguish the contentional grading from gap-grading. On a grading curve, gap-grading is represented by a horizontal line over the range of the size omitted.

Lead nitrate is a destructive impurity in mixing water. Its effects can include:

• Toxicity: Lead is a harmful substance that can poison humans and animals.
• Corrosion: It can cause corrosion of metals and deterioration of concrete.
• Adverse reactions: Lead nitrate may react negatively with other materials, affecting structural integrity.

In contrast, other impurities like calcium chloride, alkalies, and algae may have lesser or different impacts, but they are not as destructive as lead nitrate.

Admixtures are the chemical compounds in concrete other than hydraulic cement (OPC), water, aggregates and mineral additives that are added to the concrete mix immediately before or during mixing, to modify one or more of the specific properties of concrete in the fresh or hardened state. In normal used, admixture dosages are less than five percent by mass of cement. Some of the important admixtures and purposes for which they could be used are: Accelerators: They-speed up the initial set of concrete, i.e., rate of development of strength at early ages. Typical materials used are CaCl₂ and Retarders: The slow down the initial set of concrete, i.e., keep the concrete workable for a longer time for placement. These are generally used in large or difficult placement conditions where partial setting before the placement is complete is undesirable.

Adding an accelerator to concrete influences various properties. Here are the main effects:

• Rate of hydration of cement increases, leading to faster setting times.
• Shrinkage may also increase, which can affect cracking.
• The rate of heat evolution is enhanced, aiding in the curing process.

However, the addition of an accelerator does not improve resistance to alkali-aggregate reaction. This reaction can cause expansion and cracking in concrete, and an accelerator does not mitigate this issue.

CaSO₄ is known as gypsum and is commonly used as retarder. Rest other compounds like Calcium Chloride, Sodium Chloride and Sodium Sulphate are used as accelerators.

Mineral additives also called supplementary cementing material or pozzolana are fine grained siliceous inorganic material which, as such, do not process cementing properties in themselves, but react chemically with calcium hydroxide Ca(OH)₂ released from the hydration of Portland cement at normal temperature to form compounds of low solubility having cementing properties. The action is termed pozzolanic action.
Some of the advantages obtained through use mineral additives are:
1. Improved workability with lesser amount of water.
2. Lower heat of hydration and thermal shrinkage.
3. Improved resistance to attack from salts and sulphates from soils and sea water.
4. Reduced susceptibility to dissolution and leaching of calcium hydroxide.
5. Reduced permeability.
6. Lower costs.

They increase the workability of green or fresh concrete to have higher flowability without increasing the water content which results in faster rate of concrete placement, easy of placement in relatively poorly accessible locations without vibration, good shutter finish for highly reinforcement concrete members, and reduction in cement content. Superplasticisers, high-range water-reducing admixtures, are a class of plasticisers which have fewer deleterious effects. Advantages of water reduction in hardened state of concrete are increased strength, density, durability, volume stability, abrasion resistance, reduced permeability and cracking.