Question

5. Figure 5 shows the theoretical change in the statistical distribution of the resistance capacity, R, and operational stresses, S, of a structure over time. What are the implications of such changes for those responsible for the selection and specification of construction materials? R,S Time Figure 5 Resistance capacity, R, and operational stresses, S, as a function of time.

Answer 1

IMPLICATIONS FOR CHANGE IN RESISTANCE CAPACITY AND OPERATIONAL STRESSES FOR SELECTION AND SPECIFICATION OF CONSTRUCTION MATERIALS:

• Based on strength, durability and economy, a variety of building materials are used throughout the world. But their impact on resisting stresses induced varies adversely. Nowadays, heavily loaded structures are built using modern technologies. This definitely would lead to go for materials that can withstand more loads and satisfy the durability & economical considerations.
• The selected materials should possess more strength so that they can resist the operational stresses efficiently
• On selecting building materials, we must select appropriate materials for appropriate structures. For example, 1.wood can be used as building material for small houses but it can not resist heavy loads. 2. For heavily loaded structures such as dams, we can not use ordinary portland cement instead we should use low heat portland cement. Because if we use OPC, it evolves more heat of hydration.

AQUEOUS CORROSION :

Aqueous corrosion is the process of destruction or loss of any metallic material due to electrochemical reaction taking place between the material and moisture present in the atmosphere.

We use steel for reinforcement, window grills, etc in our constructions. First let us take reinforcement rods for better understanding of corrosion. Concreting under sea water or concreting in adverse conditions leads to reinforcement corrosion. Sometimes, the reinforcement in columns or beams may be exposed to the environment. This may happen due to improper compaction during concreting, usage of inferior materials in concrete, poor workmanship etc. We know that moisture in present in the surrounding air. So when the reinforcement is exposed to the environment, it tends to react with the moisture. Steel is an alloy of iron and carbon. The iron reacts with the moisture in the presence of oxygen and forms oxides of iron (rust).

IMPLICATIONS FOR CHANGE IN RESISTANCE CAPACITY AND OPERATIONAL STRESSES FOR SELECTION AND SPECIFICATION OF CONSTRUCTION MATERIALS: Based on strength, durability and economy, a variety of building materials are used throughout the world. But their impact on resisting stresses induced varies adversely. Nowadays, heavily loaded structures are built using modern technologies. This definitely would lead to go for materials that can withstand more loads and satisfy the durability & economical considerations. The selected materials should possess more strength so that they can resist the operational stresses efficiently On selecting building materials, we must select appropriate materials for appropriate structures. For example, 1.wood can be used as building material for small houses but it cannot resist heavy loads. 2. For heavily loaded structures such as dams, we cannot use ordinary Portland cement instead we should use low heat Portland cement. Because if we use OPC, it evolves more heat of hydration. AQUEOUS CORROSION:

EFFECTS OF AQUEOUS CORROSION ON R AND S:

• The steel imparts tensile strength to the concrete. If it gets rusted due to some reasons, then its ability to resist the tensile stresses gets reduced which may also leads to failure of the structure.
• Due to corrosion, the bond between the steel and concrete gets affected adversely. Hence it will reduce the integrity of the structure as a whole and may also collapse it.

STEPS TAKEN TO PROTECT METAL STRUCTURES FROM AQUEOUS CORROSION:

• Providing sufficient cover in concrete
• Using galvanized or other protective coated steels in concrete
• Using corrosion inhibitors

1. PROVIDING SUFFICIENT CONCRETE COVER

Sufficient cover in concrete prevents the exposure of reinforcement to the environment. Hence it prevents the corrosion of the reinforcement. Some of the values of clear cover are given as follows.

STRUCTURAL ELEMENTS	CLEAR COVER (mm)
SLAB	15
BEAM	25
COLUMN	40
FOOTING	50

2. USING GALVANIZED OR PROTECTIVE COATINGS

Applying protective coating on reinforcement prevents the corrosion of the reinforcement. Galvanizing is the process of coating zinc over the steel rods to prevent them from rusting. Tinning is another process to prevent corrosion. It is the process of coating tin on steel or iron materials.

3. USING CORROSION INHIBITORS

Corrosion inhibitor is the substance addded to the concrete as admixtures to prevent the steel from corrosion. It is of 2 types namely anodic inhibitors and cathodic inhibitors.