During the concrete mixing water is an essential requirement which is used to make the product compact and strong. However, for the concrete to achieve its maximum strength, water must forcefully be removed from the concrete before hardening. This process creates what is referred to as the vacuum concrete and uses vacuum mats connected to a vacuum pump. The process requires the following:
First, the concrete is mixed well until it achieves a good workability and then placed on the form works. The fresh concrete enters the water filled channels. The vacuum pump is the used and a large amount of vacuum is created on the surface of the concrete. A large amount of water is extracted from the hardening concrete. This reduces the water to cement ratio and therefore making the concrete more strong. The vacuum concrete is known to be of low density, low permeability, stronger, durable, and has an increased abrasion rate.
It is advised not to prolong dewatering process in the sense of having an extra strong concrete because the dewatering process has a critical value where beyond it there would be no significance impact. The critical value depends on the mix proportion and how thick the concrete layer is.
The vacuum is applied on porous mats which are connected to a vacuum pump. The mats are then placed on filter pads to prevent removal of cement during the dewatering process. The most economical time and of a great value has been found to be between 15 to 25 minutes. Any time beyond this has no or little reduction of water.
The process increases the concrete strength up to 25%.
It allows for clean and smooth finish surfaces.
The formwork is removed early.
It offers a good bond with the existing concrete. This allows it to be used for road repairs and resurfacing slabs.
The intrinsic permeability of the concrete permits oil, grease, and water to enter through, which can with time weaken the concrete.
This is a type of concrete which is highly flowable and spreads easily to the form without the use of mechanical vibration. It is a form of non-segregation concrete that is placed using its own weight. one of the most important aspects of Scc is that it meets the performance prospects and at the same time maintaining all of its characteristics. In some occasions admixtures such as superplasticizers are used to reduce segregation and bleeding. Concrete segregation leads to low strength and forms honeycombed area next to the form work. Well-designed Scc has excellent stability and does not segregate.
The properties of SCC
A good Scc must be able to resist segregation
It must flow easily and placed easily
Should fill the special forms on its weight
It must have an improved strength compared to the vibrated concrete
Must be placed on higher velocity than the regular concrete
Can be used in areas having congested reinforcement
The uses of Scc
The Scc has been for a long time been used to build bridges and on the pre cast sections. The Akashi kaikyo is an example of a bridge built using the Scc. There are other more uses of Scc which are:
Areas with high numbers of pipes and rebars
The scc has several advantages over using the regular concrete
It reduces labor
It leads to improved construction
Improves the integrity of a structure
Reduces the construction time compared to the time it would have taken if regular concrete was used to construct the same structure
Wearing of equipment is highly reduced
Minimizes errors on reinforced areas
The finishes produced are superior
It does not require skilled labor during working
Creates a good bond with reinforced steel
Pumping procedures are easier
Lowers the noise level which is produced by the vibrated concrete
The surfaces produced are uniform
Allows for designs and other architectural features
Highly recommended for long span and deep sections applications
Some of the factors affecting the Scc
Long transportation can reduce its flowability
Additional of water on site may not always yield to the flowability and may even affect its stability. Interested to learn more about this subject, read self-consolidating concrete guide from Wikipedia, one of the authentic sources of information on the web.