Durability and Spalling

Durability
The ability to endure. In this context, the durability of concrete structures however is synonymous with the ability of the concrete structures to last long without significant deterioration. Since durability under one set of conditions does not necessarily represent durability under another, thus a general reference to environmental factors is usually included when defining durability of concrete structures. According to ACI (American Concrete Institute) committee, durability of Portland cement concrete is defined as its ability to resist weathering action, abrasion, chemical attack or any other form of deterioration process. Durable concrete structures will retain its original form, quality and serviceability when exposed to environment. However, properties of materials change with time due to environmental interactions thus affecting the durability and causes failure of concrete structure. Concrete structure is assumed to reach the end of service life when its properties have deteriorated to the extent that continuing use of that structure is ruled either unsafe or uneconomical.

There are two main causes of concrete structures durability problems. First and foremost, the material related failures. Concrete deterioration can be caused by inappropriate use of materials. Concrete structures are mainly composed of cement (mainly Portland cement) as well as other cementitious materials such as fly ash, slag cement, chemical admixtures and et cetera. Each of these materials is added to serve different purpose. For example, chemical admixtures such as calcium chloride are added to accelerate the setting and early strength development of concrete. It is used particularly in cold weather applications and early removal of framework. Therefore, appropriate materials should be used depending on application and improper use of materials may result in failure. Additionally, poor construction practice is also one of the contributing factors to concrete deterioration. Missing frame anchors, misalignment of truss cleats, minimal fixings for window, incorrect truss spacing and poor fixing installation are typical example of poor construction practices that may lead to structures failure.

The second cause of concrete structures durability problems is environmental related failure. There are various damages caused by environment which result in concrete failure. The following are environmental condition that can affect the concrete durability:
1. Temperature
2. Moisture
3. Physical factors
4. Chemical Factors
5. Biological
These factors are may be due to weathering conditions, abrasion, attack by natural or industrial liquid and gases, and biological agent. Consequently, these are contributing factors to major problems faced by concrete structures which are corrosion and spalling.

Spalling
Spalling of concrete structures can be described as deterioration of concrete causing breaking of layers or pieces of concrete from the surface of the concrete cover. In corrosion, spalling occurs when a substance of concrete sheds tiny particles of corrosion products as the corrosion reaction progresses. These corrosion products are not soluble or permeable. Water damage is one of the most common causes of concrete spalling. Water can penetrate into cracks in concrete and, when it freezes it can break apart the concrete. Also, water can rust the reinforcing steel bars inside a concrete structure; the expansion of rust will then caused spalling.