Durability of concrete may be defined as "the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties." It normally refers to the duration or life span of trouble-free performance.
Let us consider building structures which are usually designed for a life span of 100 years. The concrete used as building material for these should have the ability to resist weathering action, chemical attack and abrasion for at least 100 years for the respective conditions of its exposure. Since the resistance capability for a type of concrete is different in different exposure, its properties need to be designed keeping in mind the exposure environment. So the importance of durable concrete, especially for building structures, cannot be emphasised enough.
But unfortunately, the real scenario of concrete use in most buildings in Bangladesh is not in conformity with the definition of durable concrete, and so, the life spans of such buildings are very short. The primary factor is the poor quality of the concrete used.
So what are the precautions one can take for ensuring that good quality durable concrete is used? This is actually not a very difficult task if one follows some rules for making concrete. If the ingredient of concrete is not good, concrete will not be good. If the mixing, transporting, placing, compacting and curing is not according to standard specifications, the concrete will not be a durable.
Considering exposure conditions in Bangladesh, minimum requirements for durable concrete:
* Concrete strength should not be less than 4000 psi and water-cement ratio should not be more than 0.45.
* For coastal areas and saline environments concrete strength should not be less than 5000 psi and water-cement ratio not more than 0.40.
* Use of brick chips should not be allowed. Stone chips and fine aggregates should be well-graded. Aggregates should be cleaned by washing. Aggregate should be inert.
* Use of suitable admixture is mandatory.
* Compaction needs to be ensured by using vibrator and water-tight forms.
* Concrete mix needs to be designed by trial mix.
* Specified clear cover should be maintained.
* Curing is equally important for ensuring hydration of cement.
Therefore first of all we need to select the ingredients as per specification. Every ingredient should maintain its engineering properties and specifications like strength, type and limits of mineral composition, gradation of aggregates etc. and be free from impurities. For example, for good concrete, aggregate should be well graded, strong, dense and inert. The ingredients in the mix should contain minimum impurities such as alkalis, chlorides, sulphates and silt.
Before mixing a batch of concrete, one has to go for a trial mix design for a particular batch of ingredient and final mixing of concrete should be in accordance with the mix design. For transportation and placement of concrete one needs to be careful about setting time of concrete and segregation. Concrete should not flow horizontally during placing. Good concrete needs good compaction but both under-vibration and over-vibration of concrete for compaction is harmful. Because of loss of water from exposed surface of concrete through evaporation, the concrete surface needs to kept moist for aiding hydration—this we call curing. Water for curing should be of the same quality (usually drinking quality) as that for the concrete mix.
Using specified ingredients and with proper mix design, permeability reduction, and attention to good jointing, placing and curing practices, one can create durable concrete structures that will resist the development of corrosive conditions and other deteriorating mechanisms. It is possible to measure the durability of concrete by conducting some tests like Air or Water Permeability test, Chloride Diffusion test, Electrical Resistivity tests etc. which reveal the porosity of the concrete.
Higher the porosity, higher the permeability. When porosity of concrete is high, it is easy for the elements to enter inside, create unwanted chemical reactions and deteriorating the concrete matrix. So this if an important factor which plays a more crucial role when it comes to durability than for strength. Strength too is affected due to high porosity. The required strength can be achieved by altering the mix design with a certain porosity level, but durability will get affected.
Durability of concrete depends upon the following factors:
Aggregates: Aggregates should be of specified quality and grading. It should not be soft or porous. Usually brick chips and mica containing sand is not good for durable concrete. Concrete made with brick chips shows very poor performance on durability.
Cement content: The required minimum quality of cement must be added to the mix as specified by the trial mix design.
Compaction: Inadequate compaction causes voids and porosity in the concrete which reduces durability.
Curing: It is very important to permit proper strength development, aid moisture retention and to ensure that the hydration process occurs completely.
Cover: Thickness of concrete cover must follow the limits set in codes.
Permeability: This is considered the most important factor for durability. Higher permeability is usually caused by higher porosity. Therefore, proper curing, sufficient cement, proper compaction, proper concrete cover, suitable aggregate and water-cement ratio could provide low permeability concrete.
Water: Water should be of at least drinking quality. Usually low water-cement ratio gives durable concrete.
Reinforcement: Corrosion of rebar (reinforcing bar) is one of the main causes for reducing the durability of concrete. Non-porous good quality concrete and suitable protective cover can protect from corrosion. For extreme exposure, such as in saline environments, foundation and/or the structure in contact with ground causes deterioration of concrete. Using Epoxy coated rebar, one can make the concrete durable in these cases.
Chemical admixtures: Addition of appropriate chemical admixtures makes concrete dense and less porous, making it durable.
Mohammad Abu Sadeque, PEng is Former Director, Housing and Building Research Institute (HBRI).