4. Finishing of Concrete

Now, let us move on to the surface vibrators. So, let us see how to do the consolidation of the concrete using surface vibrators. So, these vibrators as the name indicates the vibration is applied at the surface of the concrete and from the surface the vibration will be transferred throughout the entire depth of the concrete.

Surface vibrators are a type of equipment used in concrete consolidation. They work by applying vibrations at the surface of the freshly poured concrete, which helps to settle and distribute the concrete evenly throughout its depth. The vibrations help in reducing air pockets and improve the material's density and strength. It is important to understand that although surface vibrators are effective, they do not perform as well as internal vibrators, which are inserted directly into the concrete.

Imagine a bowl of pudding that has air bubbles trapped under the surface. If you gently tap on the bowl's surface, the bubbles move up and burst, making the pudding smoother. Similarly, surface vibrators apply vibrations to the concrete's surface, helping to eliminate trapped air and create a denser material.

Generally speaking, the internal vibrators are the best one. They are more effective because we are directly applying the consolidation at the actual layer of concrete where the vibration is needed. But here the vibration is getting transferred from the surface to the entire depth of the concrete in the case of surface vibrators.

Internal vibrators are preferred over surface vibrators for consolidation because they are placed directly into the concrete. This direct contact allows them to carry out effective vibrations exactly where they are needed most – within the concrete mix itself. Surface vibrators, on the other hand, rely on the vibration moving through the concrete from the top down, which can be less effective, especially in thicker pours.

Think of internal vibrators as a singer's microphone directly amplifying their voice, while surface vibrators are like a loudspeaker playing music from afar. The microphone captures the sound at the source, while the loudspeaker can only project what it receives, often losing clarity.

But in some cases, we cannot use the internal vibrators. Say for example, if the slab thickness is very small and if your the casing or the head of the needle vibrator is not completely immersed in this slab because the thickness of the slab is very small.

There are situations where internal vibrators cannot be effectively used. For example, if you're working with very thin concrete slabs, the vibrator cannot be immersed completely without affecting the mix. In such cases, using a surface vibrator becomes necessary, allowing the vibratory action to take place at the surface edge.

Imagine trying to mix batter in a very thin pancake on a griddle with a whisk. If the whisk is too big, it will not penetrate the batter effectively, making it difficult to mix. Similarly, internal vibrators can struggle in thin sections where they can't be properly used.

On a similar note if the reinforcement is very much congested, it is densely reinforced slab. So, it is very difficult to find the spacing. It is very difficult to insert the vibrator in between the reinforcing bars. So, in that case also, it is not possible to use the internal vibrators in slabs where it is very densely reinforced.

When concrete slabs are heavily reinforced with rebar, it creates tight spaces that make the insertion of internal vibrators challenging. Here, surface vibrators are an alternative as they can consolidate the concrete from the top without the need to navigate around the dense reinforcement.

Consider trying to clean the food out of a blender full of stuck ingredients. If the blades are too crowded, a spoon (like an internal vibrator) can’t reach effectively. Using a spatula to smooth the top and scrape from above (like a surface vibrator) can still help achieve a better mixture.

Here, when you compare it with the frequency of the internal vibrator, you can see here the frequency range is only 3,000 to 6,000 vibrations per minute.

Surface vibrators typically operate at a frequency range of 3,000 to 6,000 vibrations per minute, which is significantly lower than internal vibrators that can exceed 12,000 vibrations per minute. This lower frequency may lead to less effective consolidation but is suitable for specific applications where internal vibrators are not feasible.

Think of frequency like the pace of a jog compared to a sprint. A jogger moves steadily at a controlled speed (like the surface vibrator), while a sprinter accelerates quickly, covering more ground faster (like the internal vibrator). Different situations will call for different paces of movement.

So, another type of surface vibrator is screed type vibrator vibrating screed. Screed in the sense it is a kind of finishing operation. Screeding refers to the finishing operation.

Screeding is a finishing operation that involves leveling off excess concrete. Screed vibrators combine consolidation and finishing by both smoothing and compacting the concrete surface. They are effective at ensuring an even, level surface while eliminating air pockets.

Imagine icing a cake—if you just pour frosting onto the top, it may not spread evenly. Instead, using a spatula helps to smooth the icing and make the surface even, similar to how a vibrating screed levels out and finishes concrete.

One thing to be noted here is this surface vibrators should not be used for a concrete with a very high workability above 75 mm.

Using surface vibrators on high-workability concrete over 75 mm can lead to segregation, where water and finer materials separate from the concrete mix and rise to the surface. This results in a weak surface layer, reducing the overall strength and wear resistance of the slab.

Think of trying to mix oil and vinegar—if you shake them too hard, they separate rather than blend. Similarly, excessive vibration on high-workability concrete can cause materials to separate instead of mixing uniformly.

The vibration transfer will be effective only for a depth of 150 mm. They are recommended for consolidating slabs up to 150 mm thick.

Surface vibrators are effective for consolidating concrete slabs that are up to 150 mm in thickness. Beyond this depth, the effectiveness of the vibration diminishes, leading to insufficient consolidation at lower levels of the concrete.

Imagine trying to shake a deep jar of marbles by only tapping the surface. The top layer will move effectively, but the marbles at the bottom won't budge much. In the same way, vibrations will only adequately affect the upper 150 mm of a concrete mix.

So, the next type of vibrator we are going to discuss is about the form vibrators. So, these are external type of vibrators. So, basically you are going to clamp your vibrator to the formwork.

Form vibrators are attached to the outside of the formwork and transfer vibrations to the concrete through the mold. While this method is less effective than internal vibration due to the indirect transfer, it is useful where internal vibrators cannot be used due to constraints.

Picture a speaker placed outside of a room—the sound travels through the walls to reach inside. Form vibrators work similarly, vibrating the outer formwork to indirectly consolidate the concrete inside.

So, one more thing to be noted here is the formwork should be very tight. So, if the formwork is not tight, when you vibrate the formwork itself concrete will start leaking from the formwork.

For form vibrators to be effective, the formwork must be securely tight. Loose formwork can lead to leakage of wet concrete during the vibration process, which not only wastes materials but also reduces the quality of the concrete structure being formed.

Think of a water balloon—if it’s not held tight, squeezing it will cause water to spray out instead of compressing it effectively. Similarly, if formwork isn’t tight when using form vibrators, the concrete won’t settle as desired.

Another important thing to be kept in mind is the consistency of the concrete and the vibrator need to be matched.

The effectiveness of vibration depends on matching the vibrator's specifications to the consistency of the concrete. Stiffer mix requires more vigorous vibrations, while more fluid concrete can be consolidated with less intense vibrations to avoid segregation.

Consider cooking different types of pasta—some need boiling water while others are best steamed. Similarly, the right approach to vibrate the concrete should be adjusted based on its consistency to achieve the best results.

So, the next one is the use of vibrating tables for consolidation.

Vibrating tables are commonly used in laboratories and precast concrete plants. They create vibrations that extend from the table to the mold and then to the concrete, ensuring thorough consolidation. Like form vibrators, they use indirect vibration but typically result in effective consolidation when set up correctly.

Think of a washing machine that shakes to agitate clothing for a clean. A vibrating table uses similar principles to shake the concrete effectively into place, ensuring a dense mix throughout.

So far we discussed about the consolidation. Now, we shall move on to the next step of the concrete making that is finishing of concrete. This is also a very important step.

Finishing of concrete is the final stage after consolidation. It involves various operations to enhance the surface quality of the concrete, including texture, smoothness, and strength. Proper finishing improves the overall appearance and performance of the concrete surface.

Like putting the final coat of paint on a wall, finishing concrete gives it a polished appearance and enhances its durability, making it not just functional but also visually appealing.

General practice is that excessive finishing of concrete surface may affect the surface strength.

Excessive finishing, especially when water is added to manipulate the surface, can lead to weaknesses in the concrete, particularly on floor slabs where strength and wear resistance are crucial. Maintaining a balance in finishing is vital for preserving the concrete’s integrity.

Imagine over-polishing furniture; too much polish can damage the wood instead of enhancing it. Similarly, over-finishing concrete can harm its strength rather than improving its surface quality.

This finishing is usually done in stages. So these are the different stages of finishing, screeding, floating and troweling.

Finishing concrete involves a sequence of stages: screeding, to level the surface; floating, to smooth it out; and troweling, for the final polish. Each step requires careful timing and technique to ensure the best results.

Think of baking a cake: first, you check if the cake is level (screeding), then you smooth the frosting (floating), and finally, you add decorative details (troweling). Each step is crucial for a delicious final product.

When should we start the finishing operation? The surface finishing should not be started before the initial set.

Surface finishing operations should begin only after the concrete has reached its initial setting point to ensure it has enough stiffness and strength to hold the intended texture. Starting too early can result in deformation and a poor finish.

Just like you wouldn’t frost a cake until it’s cooled and set, concrete must also be properly set before attempting to finish it.

So, the first thing we are going to discuss is the actual time of finishing. The first step is screeding. Screeding is nothing but strike off.

Screeding is the initial stage of finishing where excess concrete is removed to achieve a level surface. This step is crucial for ensuring that the subsequent finishing processes have an even material to work with, enhancing the quality of the final surface.

Consider leveling a canvas before painting. If the canvas has bumps or uneven spots, the paint job will suffer. Similarly, ensuring a flat concrete surface at the screeding stage lays a strong foundation for the final finish.

Following screeding, the next stage is floating. So, always give some time interval between every stage of finishing.

Floating aims to create a smooth surface by using flat tools to fill in voids and fix uneven areas. It helps push aggregates down slightly below the surface while sealing small imperfections, resulting in a well-prepared base for later stages.

Think of spreading a layer of icing on a cake. You want it smooth and even to create the best appearance for the final cake. Floating works similarly to create an ideal surface for subsequent finishing.

The next step of the finishing is troweling. So, troweling, what you do? As I told you the finishing we are doing it in stages.

Troweling is the final stage of concrete finishing that focuses on achieving a densely packed and smooth surface. It involves applying increased pressure to seal in any minor imperfections and finalize the surface quality. Proper technique is essential here to ensure a polished end product.

Much like polishing a piece of furniture to a high shine, troweling compresses and smooths the surface of the concrete to perfect its aesthetic and functional qualities.

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