The principal components in every construction of a building are concrete and reinforced steel. However, concrete is the pivotal component the construction begins with. Concrete is a composite of multiple items namely cement, water, sand and aggregates. The aggregates include limestone, gravel, crushed stone, slug etc. About 60 to 80% of the concrete mix is made of aggregates. They provide the required compressive strength and bulk to concrete.
Importance of aggregates
In a concrete mix, the aggregates composition is very much important. It must be clean, strong and free of chemicals or coatings that can deteriorate the concrete. Typically the aggregates are of two types which are coarse and fine. In coarse aggregates, the particles have diameters greater than 4.75mm. The usual range employed is between 9.5mm and 37.5mm in diameter whereas, in the fine aggregates, they are usually sand or crushed stone and have diameters less than 9.55mm.
What is No fines Concrete?
As the name suggests, it is a type of lightweight concrete which has no fine aggregates and only contains cement, water and coarse aggregates. It is known by many names such as porous concrete, permeable concrete etc. Since it contains coarse aggregates, they are typically surrounded and held by a thin layer of cement paste which provides the strength to the concrete.
So what makes it special from the regular type of concrete?
The main highlight is that it is much lighter than regular concrete. It is mainly used for the construction of external load-bearing walls, small retaining walls etc. In No fines concrete, the strength primarily depends on the water/cement ratio, aggregate cement ratio and unit weight of concrete. It is generally made between 6:1 to 10:1 aggregate-cement ratio. Another important factor in no fines concrete is its water-cement ratio. It must be carefully chosen because if the ratio is too small, the cement paste will get dried that results in less adhesion between aggregates and paste. The compressive strength of no-fines concrete is the range between 1.4 MPa to 14 MPa.
But another possibility presented by no fines concrete is in pavement of walkways and roads. One major challenges presented in the construction of walkways, is the problem of runoff water preventing groundwater recharge. No fines concrete exhibiting porous characteristic because of containing only coarse aggregates is a permeable concrete that allows runoff water to settle down to ground allowing groundwater recharge. Such practices is a must for safeguarding and to fight against the severe and now common drought scenario.
Pros and Cons
The biggest advantage is that it is lightweight which makes it suitable for lightweight concrete construction work. Also, when compared to regular concrete, the thermal insulating characteristic of no fines concrete is far more better. Another advantage is the saving of materials as no-fines concrete is composed of water, cement, coarse aggregates and does not include fine aggregates, only less amount of cement is required to produce 1 m3 of no-fines concrete. The density of this concrete is the range of 25 to 30% which is less than the regular concrete. And last but not the least is the possibility of groundwater recharge possible through paving of walkways using no fines concrete. Even with this much advantages, there are definitely some cons.
Since it does not have fine aggregates, the cohesive property of no-fines concrete is weak. Also, the compressive strength of No-fines concrete is lower than conventional concrete, therefore, it is not suitable for reinforcement. If it is to be used for reinforcement, then a thin layer of cement paste must be applied on the concrete before using it. This improves the bonding of reinforced steel with the concrete.