4.2.2. Floating

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.

In this section, we are being introduced to surface vibrators, which are tools used during the process of consolidating concrete. The term ‘surface vibrators’ means that these devices work by applying vibration directly to the surface of the concrete. The vibrations do not remain purely at the surface; instead, they are transmitted down through the concrete, helping to remove air pockets and ensuring a more uniform mixture throughout.

Think of surface vibrators like shaking a bowl of jelly from the top. When you shake the surface, the ripples travel through the entire jelly, changing its consistency. Similarly, when surface vibrators shake the surface of the concrete, they help to evenly distribute the mixture below.

So, 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 often considered more effective than surface vibrators because they can apply vibrations directly where they are needed, deep within the concrete. This direct application means that they work more efficiently to eliminate air pockets and ensure proper blending of materials. In contrast, surface vibrators work from the top down, which may not be as effective in achieving the same level of consolidation throughout the concrete's depth.

Imagine using a blender for a smoothie. When you use a hand blender (internal vibrator), you can blend ingredients right where they are, resulting in a well-mixed smoothie. But if you only stir from the top with a spoon (surface vibrator), you may miss some ingredients that are sitting at the bottom, making for an uneven mixture.

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. So, in that case we cannot use a needle vibrator for such thin slabs.

There are circumstances where internal vibrators cannot be used effectively, particularly in thin slabs where the internal vibrator may not be long enough to reach the entire depth of the concrete. In such cases, the use of surface vibrators becomes necessary as they can work effectively without needing to penetrate deeply into the material.

Consider trying to use a long straw to drink from a very shallow dish. The straw can't reach the liquid if the dish is not deep enough. Similarly, internal vibrators can’t reach their full potential on very thin slabs, so surface vibrators are instead used to consolidate the concrete.

So, these are mostly used in the slab construction. So, you can see that the surface vibrators are applied at the top surface of the concrete. And it consolidates the concrete from top down. And 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. But your needle vibrators are having very high frequency as high as more than 12,000 vibrations per minute.

While surface vibrators are effective in slab construction, they come with limitations. The frequency at which they operate (3,000 to 6,000 vibrations per minute) is significantly lower than the frequency of needle vibrators (which can exceed 12,000 vibrations per minute). This lower frequency means that surface vibrators may not achieve the same consolidation effectiveness, particularly in thicker slabs or where a higher density is required.

If you think of the vibrations as music beats, surface vibrators play a slower, softer tune, while needle vibrators blast out a fast-paced rhythm that can more profoundly shake things up. Just like a slow song may not energize a crowd as much as a fast one, surface vibrators may not achieve the same effect in consolidating dense concrete mixtures.

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. In this vibrator, what we do is we do the finishing as well as the consolidation together.

The screed type vibrator serves a dual purpose: it combines both finishing (screeding) and consolidation of the concrete. Essentially, this tool smooths the top layer of concrete while also ensuring it is adequately consolidated. This is particularly beneficial in achieving a level surface before it sets completely.

Think of a chef smoothing frosting on a cake while also ensuring there are no air bubbles underneath — that’s what a vibrating screed does. Just as a chef aims for a smooth finish while ensuring the cake is solidly frosted, the screed vibrator ensures the top layer of concrete is both flat and thoroughly mixed.

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. If the workability is above 75 mm for such high workable concrete mixes, we are not supposed to use surface vibration.

Surface vibrators are not suitable for use with concrete that has a high workability (more than 75 mm). Using surface vibration on such mixes can lead to segregation, where materials separate, causing the surface layer of concrete to become weaker. This is especially critical for applications where surface strength is paramount, such as floor slabs that need to endure wear and tear.

Imagine pouring syrup on a stack of pancakes. If the syrup is too runny (high workability), it can easily run off the sides and create a mess rather than staying evenly spread. Similarly, high workability concrete can lose its consistency when vibrated on the surface, thereby affecting its final strength.

And one more thing to note that is the vibration transfer will be effective only for a depth of 150 mm. So, they are recommended for consolidating slabs up to 150 mm thick. Beyond that the transfer will not happen effectively.

The efficacy of surface vibrators diminishes beyond a certain depth; they are effective only up to 150 mm thick slabs. For concrete slabs thicker than this, the vibrations will not reach the lower layers properly, hindering proper consolidation, which can result in defects or weaknesses in the concrete structure.

Imagine trying to shake the bottom of a deep box filled with sand by only pressing down on the top — no matter how much you shake the top, the bottom remains unmoved if the distance is too great. This illustrates the limitation of surface vibrators, as their influence cannot reach too deep.

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