Question

Identify which contributes for strengthening of lime mortar structures and contributes for weaken- ing of RCC structures. (a) usage of water containing dissolved salts (b) addition of lime (c) absorption of \(\mathrm{CO}_{2}\) from atmospheric air (d) both \((2)\) and \((3)\)

Short answer

a) Water containing dissolved salts b) Addition of lime c) Absorption of CO2 from atmospheric air d) Both (2) and (3) Answer: d) Both (2) and (3)

Step-by-step solution

Analyze the effects of water containing dissolved salts

Water containing dissolved salts can lead to an increase of harmful ions such as chloride and sulfate ions. These ions can cause the corrosion of steel reinforcement in RCC structures, thereby weakening them. On the other hand, these dissolved salts might not affect lime mortar structures.

Analyze the effects of addition of lime

Adding more lime to lime mortar structures can improve their strength and durability due to the formation of calcium carbonate. However, adding lime to RCC can reduce the strength of the concrete by providing weaker bonds in the cement.

Analyze the effects of absorption of CO2 from atmospheric air

The absorption of CO2 from the atmospheric air can contribute to the carbonation process in lime mortar structures, which results in the formation of calcium carbonate, thus increasing their strength. This absorption of CO2 doesn't have the same strengthening effect on RCC structures. In fact, carbonation in RCC structures can lead to the corrosion of steel reinforcement, causing weakening.

Evaluate options and determine the correct answer

Based on the above analysis, we can conclude that: - Option (a) is incorrect because dissolved salts in water weakens RCC structures and doesn't strengthen lime mortar structures. - Option (b) is correct because the addition of lime strengthens lime mortar structures and weakens RCC structures. - Option (c) is correct because the absorption of CO2 from atmospheric air strengthens lime mortar structures and weakens RCC structures. - Option (d) combines options (b) and (c) which are both correct, so this is the correct answer. In conclusion, the correct answer is (d) both (2) and (3).

Key concepts

Dissolved Salts Impact

Dissolved salts in water can significantly impact construction materials, particularly reinforced cement concrete (RCC) structures. When water containing dissolved salts, such as chlorides and sulfates, infiltrates RCC, it can lead to serious problems. These salts are corrosive to the steel reinforcement within the concrete.
This corrosion process weakens the structural integrity of the RCC. It leads to the expansion of rusted steel, causing cracks and eventually, spalling of concrete surfaces.
In contrast, lime mortar structures do not incorporate steel reinforcement, and thus are less susceptible to the detrimental effects of dissolved salts. However, excessive salts could lead to efflorescence, which is the formation of white powdery deposits on surfaces, affecting aesthetics rather than structural stability.

Lime Addition Effects

Adding lime to lime mortar structures plays a positive role in enhancing their strength and durability. Lime, when combined with water, forms calcium hydroxide, which gradually interacts with carbon dioxide (CO2) to form calcium carbonate. This formation contributes to a denser and stronger mortar.
On the other hand, introducing additional lime into RCC structures can be counterproductive.
It can alter the mix's chemistry, weakening the bond strength of the concrete matrix, leading to a less durable structure. This is because the lime can partially replace Portland cement, which is critical for concrete hardness and cohesion.

CO2 Carbonation Process

The carbonation process refers to the chemical reaction between lime mortar and CO2 from the air. This reaction results in the formation of calcium carbonate, which enhances the strength and longevity of lime mortar structures. The process is slow and typically does not pose any negative effects.
However, when it comes to RCC structures, carbonation can be problematic.
The penetration of CO2 into the concrete can lower the pH levels, leading to the corrosion of steel reinforcement. Unlike in lime mortars, here, carbonation initiates the rusting process of steel, contributing to structural deterioration.

RCC Structure Weakening

The weakening of RCC structures can occur due to various chemical interactions and physical conditions. One significant factor is the inclusion of water with dissolved salts, which accelerates steel corrosion, compromising the structural frame.
Additionally, the absorption of CO2 through carbonation can further the corrosion process, as it reduces the alkalinity of the concrete, a critical aspect of protecting embedded steel.
Lime addition poses another risk, as excess lime lowers the adhesive bond within the concrete, focusing pressure on steel reinforcement, which cracks under strain. Careful management of these factors is crucial to ensure the long-term stability of RCC designs.