What is Salt finish Concrete?
Salt finish concrete is a form of concrete surface finishing work where salt crystals are embedded into the surface of the freshly placed concrete using a roller or afloat. After the concrete sets, the salt crystals are washed out with a stream of water, leaving behind numerous shallow indents giving off a subtle texture to the concrete surface.
It is rather an easy, cheap, and traditional method of obtaining a decorative concrete surface finish with higher skid resistance than that of plain or broom finish concrete surfaces. It is cheap since it requires fewer additional equipment and materials. One of the most commonly used salt crystals in salt finish concrete is rock salt.
Rock salt, chemically termed as Halite, are the natural deposits of Sodium chloride usually resulting from the evaporation of seawater. Concrete finish with rock salt is known as rock salt finish concrete. Rock salt generally has no direct effect on the hardened concrete.
Salt finish concrete can implement in house courtyards, garden pathways, parking, areas around swimming pool etc
Manufacturing process of salt finish concrete
All steps before the salt finish concrete is similar to the normal concreting work which involves the preparation of the site, reinforcement mesh, formworks, concrete mixing, and placing of concrete.
Actual salt finish concreting work begins only after the freshly mixed concrete is placed into the formwork and its surface is made even and smooth using floats (hand and bull floats). Uniform, even and smooth surfaces results in superior finish quality. The steps afterward are briefly explained as follows
1) Spreading of Salt crystals
Coarse or Extra coarse rock salt is the most commonly used salt crystals for salt finish concrete. Before spreading the salt crystals, the concrete placed is left to dry off until it becomes stiff enough to prevent an indentation of at most a quarter of an inch when moderate pressure is applied by thumb and no concrete paste or grayish color gets stuck on the thumb when pressed against the surface.
The salt crystals are then evenly scattered over the concrete surface at a concentration ranging from 3 to 12 pounds (1.5 to 5.5 kg) per 100 square feet. This concentration varies upon the type of surface finish required by the client. The scattered salt crystals are then lodged into the concrete by pressing them with a trowel or salt roller or float.
The crystals must be lodged at most half of their diameter. The loose surface salt crystals are then cleaned off using a broom and the surface is then left to dry for at least 24 hours (initial hardening time of concrete). This process can be summarized as salting. It is the most crucial step in the salt finish concreting which requires many precautions and care.
2) Removal of Salt crystals
After a time-lapse of 24 hours, the concrete surface is rinsed with water. Water dissolves salt crystals entire or partially leaving behind dislodged salt crystals which are then washed off using a brush or powerful jet of water.
Pressurized water can directly be applied over the salt finish concrete surface for easy removal of salt crystals. Dislodged salt crystals leave behind shallow indentations on the concrete surface in a speckle pattern forming a subtle texture.
3) Application of sealants
Before applying the sealant let the surface and indentation dry off completely for at least 24 hrs and clean it using a broom or brush. Apply the first coat of sealant using a roller so that it is evenly spread over the surface. Let the first coat dry for at least 2 hours and then apply the second coat similar to before.
Effect of salt on Concrete
Specification for rock salt finish concrete
- Rock salt crystals of size ranging from 1/8 to 3/8 of an inch can be used for obtaining the desired results.
- Salt crystals larger than 3/8 of an inch should be avoided to prevent hazards resulting from larger indents
- Salt crystals should be pressed and lodged into the concrete surface up to a depth not exceeding half of their diameter at most.
- The concentration of salt crystal may vary according to the type of finish required ranging from 3-12pounds per 100 sq feet
- The surface before salting should be smooth and even obtain better finish, all the lines and marks left due to the edge of the floats and rollers should be smoothened
- The salt crystal should be spread evenly all over the surface to obtain better finish quality
- The stiffness of the concrete surface should be enough to prevent an indent larger than ¼ of an inch when the pressed with a thumb with moderate pressure
- Too wet or too dried surface results in poor finish quality
- The surface after salting should be washed properly with water removing all traces of salt content which can cause discoloration in the case of colored concrete.
- Sealant should be applied only after removing all traces of salt content
Pros of Salt finish concrete
- Salt finish concrete surfaces have good aesthetics when used with color concrete
- It is cheaper and quicker in comparison to other surface finishes
- Requires less material, equipment, and manpower
- Less time consuming
- Salt has no direct effect on hardened concrete
- It provides higher skid resistance
Cons of Salt finish concrete
Reminisces of the salt on the concrete surface and indentations attract moisture and lower the freezing point of the water. In the freezing and thawing environment due to high humidity and moisture content in the air and the presence of salt on the concrete surface, water gets trapped in the indentations on the concrete surface.
Prolonged exposure to water in absence of any sealant makes the concrete absorb moisture and saturate itself. A lower temperature in such climate causes water and moisture trapped in indentations on the surface and inside saturated concrete, to freeze and expand. It increases the water pressure inside such indentation resulting in a sharp increase of internal stresses and causing the concrete to crack.
This is the major cons of salt finish concrete which is mostly visible in cold environments where concrete is always exposed to high moisture which makes concrete saturated all the time.
In contrast to the above statement, the presence of micro air void (due to air-entraining agents) inside the concrete structure may help to release some stress due to such thawing and freezing but higher moisture content makes them over clogged thus resulting in cracks and faults.
Due to the above-mentioned reason a sealant is recommended to be applied to the salt finish concrete surface so that the surface can be made water repellant. This helps to prominently distinguish the inside and outside of the concrete and hence reduce the problem aroused due to freezing and thawing.
It is recommended to seal the salt finish concrete with an appropriate sealant. Such sealant can be either acrylic (film sealant) or siliceous (penetrating sealant).
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Zenish Shakya