Environmental Factors affecting the strength of the concrete structureArticles > Environmental Factors affecting the strength of the concrete structure
Environmental conditions affecting the strength of Concrete:
The focus of attention of a civil engineer remains largely on the strength and durability of the structure he constructs. The finishing look and decoration of the building hold a secondary status. The primary objective of the construction is to achieve durable and long-lasting results. The durability and the strength of the concrete structure are dependent on the compressive and the tensile strengths of the concrete. As far as the tensile stress of the structure is concerned, it is largely dependent on the reinforced bars and partially depends on the binding strength of the concrete itself, while the compressive strength of the concrete is totally dependent on the aggregate and cementitious material (binder). Here in this article, we are discussing the strength of the concrete affected by the environmental conditions. The environmental conditions are directly affecting the compressive strength by means of weakening and abrading the structure, and indirectly affecting the tensile strength because the tensile strength is partially dependent on the concrete material. There are tens of factors that directly or indirectly affect the strength of the concrete. Among which the most important factors are as under.
(1). Permeability of the Structure:
The permeability of the structure is of primordial importance. In the case of a porous structure, water and other chemicals like acid rains, and sulfate are allowed to enter the structure and weaken the bonding of the concrete material. In most of the successful cased, the concrete is designed in a way to minimize its permeability, so that external liquid may not penetrate into the structure.
(2). Alkali-Silica Reaction:
Alkali-Silica Reaction (ASR) is the expansive chemical reactions among the chemicals present in the concrete mixture. A remarkable quantity of Silica is present in the aggregate of concrete, while potassium and sodium alkalis are present in cement paste. The silica of aggregate reacts with potassium and sodium of cement. This reaction produces expansion in the concrete, thus developing a network of cracks, spalling of joints, and movement of certain parts of the structure. All these reactions weaken the strength of the concrete structure.
(3). Sulfate in Water:
There is a significant amount of Sulfate mixed in water and soil. The sulfate of soil and water reacts with the sodium and calcium of binding material (cement), thus forming Calcium Sulfate and Sodium Sulfate. The Sulfate attack in seashore regions and other regions enriched with Sulfate leads to the disintegration of the concrete structure. The addition of Sulfate in the structure puts pressure on the concrete structure and caused disintegration in it.
(4).Impact of Freeze-Thaw Cycle on Concrete:
The Freez-thaw cycle is another challenge for the strength of the concrete structure. On freezing, the water present inside the pores and cracks of the structure expands and exerts pressure on the walls of the pores and cracks. The expansion on freezing further increases the cracking of the structure. Thaw is the melting of ice and snow due to the higher temperature. One thawing conditions the ice present inside the pores and cracks of the concrete structure is melt, and reduces its volume, thus allowing further water to accommodate within the spaces of the pores and cracks. In the night time, the temperature again falls down to a remarkable level. On falling of the temperature, the water present inside the pores again freezes and expands its volume. The expanded volume of the water exerts more pressure on the walls/sides of the cracks, thus further damaging the structure. The cycle of freeze and thaw is very harmful to the concrete structure, even for the natural rocks. In geological studies, the cycle of freeze and thaw is considered to be a pivotal factor causing the weathering of the rocks.
(5). Abrasion Resistance of the Concrete:
The resisting ability of a surface being worn away by friction and scratching is known as abrasion. This ability is dependent on the strength of the concrete, exposure conditions, properties of aggregate, binder (cementing material), surface finishing, and the curing method. If the above precautionary steps are not taken, the abrasion will completely wear away the structure in a short time period. Abrasion also badly influences the reinforced material by wearing away the concrete cover. The reinforcing bars can retain the strength of the structure only when these are tightly covered in the concrete. Both the compressive and the tensile stress are interdependent on each other. Failure of each can fail the other.
(6). Impact of the Rain and Humidity:
The pH level of the concrete is directly affected by the moisture and environmental conditions. Affecting the pH level of the concrete is known as carbonation. Concrete is not directly damaged by the process of carbonation, but, the alkaline concrete environment of the rebars (steel) is lost. The loss of an alkaline concrete environment causes corrosion of the steel rebars.
(7). Chloride and Steel:
Chlorides in de-icing chemicals can make their way down to the reinforcing steel. Chemical reactions between the chlorides and the steel initiate the corrosion process which can create significant damage to structural concrete. Deck overlays, surface treatments, and especially coating reinforcing steel can help protect bridge concrete from corrosion due to chlorides.
(8). Effect of the Chemical Attacks:
Concrete is resistant to most natural environments and many chemicals. Some chemicals, however, can attack concrete and cause deterioration. Concrete with low permeability is more resistant to chemicals.
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