4.2. Stages of Finishing

Now, let us move on to the surface vibrators. So, let us see how to do the consolidation of the concrete using surface vibrators. These vibrators, as the name indicates, apply vibration at the surface of the concrete, and from the surface, the vibration is transferred throughout the entire depth of the concrete. Generally speaking, internal vibrators are the best ones. 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. That is why internal vibrators are more effective when compared to surface vibrators.

Surface vibrators help consolidate concrete by creating vibrations at the surface that permeate the entire structure. Internal vibrators are usually preferred as they directly target the concrete layer needing consolidation; however, surface vibrators are essential when internal vibrators can't be used. They are particularly useful for thinner slabs or when the concrete is densely reinforced, making it difficult to insert an internal vibrator.

Think of it like trying to mix a drink. Using a straw (internal vibrator) lets you mix the drink directly, while shaking the cup (surface vibrator) can help, but depends on how well you can shake it so the mix moves throughout. If the cup is too full or has ice (like dense reinforcement), shaking will be less effective.

In some cases, we cannot use the internal vibrators. Say for example, if the slab thickness is very small and if 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. Similarly, if the reinforcement is very congested, it is very difficult to find the spacing to insert the vibrator among the reinforcing bars. Therefore, in these situations, we opt for surface vibrators, which are mostly used in slab construction.

Surface vibrators become necessary when internal vibrators cannot adequately access the concrete. This is often the case when dealing with thin slabs where the internal vibrator's head cannot fully immerse in the concrete, or when the reinforcing bars are too tightly packed together, preventing the insertion of an internal vibrator. By applying vibrations at the top, surface vibrators ensure some level of consolidation, keeping the connection between all particles in the concrete.

Imagine trying to reach for something in a crowded room. If the room is too packed (like a slab with too many bars), you can't just walk in (use internal vibrators). Instead, you might shake the walls (use surface vibrators) to get people to adjust and create space without directly going inside.

When you compare the frequency of the internal vibrator, you can see here the frequency range is only 3,000 to 6,000 vibrations per minute. However, internal vibrators may go as high as more than 12,000 vibrations per minute. This frequency range for surface vibrators is generally considered average.

Frequency indicates how often the vibrator operates within a given timeframe. While surface vibrators range from 3,000 to 6,000 vibrations per minute, internal vibrators operate at higher frequencies, allowing for more effective compaction of the concrete. The higher frequency of internal vibrators is more effective in densely populated concrete as it allows vibrations to penetrate deeper and encourage better material integration.

Think of it like sound frequencies. A low bass sound (surface vibrator) can make the walls vibrate, but a high-pitched squeak (internal vibrator) can penetrate through the structures more effectively, getting everyone dancing (ensuring the concrete particles settle properly).

One thing to be noted here is that surface vibrators should not be used for concrete with very high workability above 75 mm. If the workability exceeds this level, surface vibration can cause segregation, where lighter elements rise to the top, weakening the surface. This is particularly concerning for floor slabs, where surface strength is critical. Therefore, surface vibrators cannot be recommended for mixes with a slump in excess of 75 mm.

The workability of concrete impacts its behavior during consolidation. A very workable mix (slump above 75 mm) is prone to having water and fine materials separated during the surface vibration process. This segregation weakens the surface layer and reduces wear resistance, making it unsuitable for high-stress areas like floors. Hence, the use of surface vibrators should be carefully monitored in these situations to avoid weakening the final product.

Imagine stirring syrup into water. If you stir too aggressively when the syrup is too runny (high workability), it separates from the water, and you end up with a watery mix at the top rather than a uniform blend. This is similar to how concrete can become compromised if surface vibrators are used with too high of a workability.

Additionally, vibration transfer with surface vibrators is only effective for a depth of 150 mm. They are recommended for consolidating slabs up to this thickness. Beyond this thickness, effective transfer of vibration cannot be guaranteed.

The limitations of surface vibrators indicate that their effectiveness diminishes with increased concrete thickness. If the concrete element exceeds 150 mm, the vibration may not reach the lower layers sufficiently, leading to poor consolidation and potentially creating weak spots within the structure. For thicker slabs, it is essential to either consider internal vibrators or alternate methods of consolidation.

It's like trying to blow air into a balloon. If the balloon is too thick, your breath (surface vibration) won't inflate it properly. Thus, for very thick balloons (concrete slabs), you need to ensure you’re using the right technique to get the air all the way through.

Finishing is usually done in stages comprising screeding, floating, and troweling. The actual time period for finishing is between the initial setting time and final setting time. We should allow drying time between the stages to ensure a good finish, as trying to finish too soon can lead to complications.

Finishing concrete is a multi-stage process that ensures the final surface meets desired specifications. Each step—screeding, floating, and troweling—needs careful timing to align with the concrete setting times. This prevents damaging the poured concrete and helps achieve a smooth, durable finish by giving the concrete the required strength to withstand the finishing operations. By allowing adequate drying time, the concrete is in a suitable state for each type of finishing.

Imagine baking a cake. If you try to frost it too soon (like finishing concrete before it sets), the batter may still be soft and could ruin both the cake and the frosting. Waiting until it hardens will give you a clean and beautiful finish on the outside!

The first step is screeding, which is simply removing excess concrete from the surface using a wooden screed to level it to the correct elevation.

Screeding is a preliminary process in which excess concrete is struck off and the surface is leveled with the required elevation. By using a straightedge, workers ensure a flat surface free of excess material, setting the foundation for subsequent finishing steps. This technique is crucial as it establishes an even base on which the rest of the finishing can be conducted.

Think of screeding like leveling soil before planting—a gardener removes the bumps and excess to create a flat bed for seeds to ensure they root evenly. Just like in gardening, this initial step is essential for successful outcomes in concrete finishing.

Following screeding, the next stage is floating, aimed primarily at creating a smooth concrete surface. This can be done using a wooden or metal float to press down aggregates and fill any cavities.

Floating follows the initial screeding and serves to further smooth out the surface. Using a float helps embed aggregates lower into the concrete surface, ensuring any bumps or uneven areas are smoothed out. It’s an essential step before the final finish to ensure quality and aesthetics at the surface level.

Consider floating like smoothing out icing on a cake; even after the base is leveled, you want to make sure the icing is applied evenly and any imperfections filled in for a smooth finish that looks inviting.

Troweling is the final finish aimed at hardening and polishing the concrete. It’s done after the concrete has set and requires more pressure using a trowel blade angled for application.

Troweling is the final step in the finishing process, typically performed after the concrete has begun to harden, which requires applying more pressure to achieve a polished surface. The tool used during this stage is held at an angle to exert the necessary force to create a dense, smooth finish. This step improves surface quality, fills any small voids, and increases wear resistance, crucial for structural surfaces.

Think of troweling like buffing a car to a shine after washing it. Once the surface is clean and dry, you need to polish it to enhance its appearance and durability. The same effort applies to achieving a sleek finish on concrete.

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