Several characteristics of the good adaptability of concrete admixtures

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Several characteristics of the good adaptability of concrete admixtures

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Concrete admixtures are the "technical soul" of modern concrete. Their compatibility with cementitious materials, aggregates, and mixing water not only determines the workability of concrete but also directly affects its strength and durability. In production and construction, the compatibility of admixtures can be accurately identified through a combination of visual observation and tactile assessment in the following aspects.

1. The concrete and aggregate are fused together seamlessly , forming a natural whole.

The highly adaptable admixtures can completely break down the flocculated structure of cement particles, forming a uniform and stable colloidal suspension system, so that the paste and aggregate can achieve a perfect state of integration.

(1) Performance during the mixing process.

The concrete rotates smoothly and smoothly in the mixer without any jamming or dead angles, and there will be no "shaft binding", "clumping" or "wall sticking". The resistance is uniform when the mixing blades pass over the concrete, without any sudden increase or decrease.

(2) Discharge status

During discharge, the concrete flows out continuously in a "waterfall" shape, and the coarse and fine aggregates are evenly mixed in the slurry. There is no separation phenomenon of "thin slurry flowing first and then stones falling". After discharge, the inner wall of the mixer is clean and tidy, with only a thin layer of slurry film remaining, and no large pieces of concrete clumping.

(3) Overall texture

When a shovel is inserted into the concrete, it can be felt as a uniform "paste" texture, without any hardened aggregate lumps or loose slurry. When the shovel is pulled out, it will not bring out a large number of loose stones.

(4) Encapsulation characteristics

Each grain of sand and gravel, regardless of size, is tightly encapsulated by a layer of uniform, continuous and complete slurry. Even when the concrete is spread out, no exposed stone surface can be seen, and there is no clear boundary between the aggregate and the slurry.

2. Good workability and stability

three interrelated and mutually restrictive aspects: fluidity, cohesiveness, and water retention. Admixtures with good adaptability can achieve a dynamic balance among these three properties and maintain stability over a relatively long period.

(1) Excellent liquidity

The concrete discharged from the mixer can flow naturally under its own weight, filling the corners of the formwork and areas with dense reinforcement without the need for external vibration. After the slump cone is poured, the concrete can spread out quickly and evenly in all directions, with a large expansion and smooth transition at the edges, without sudden breaks or bulges. The pumping pressure is stable, the pump pipe vibration is small, and there will be no pump blockage or pipe burst.

(2) Moderate cohesiveness

Concrete has good resistance to segregation and will not separate or stratify even after prolonged bumpy transport. When dropped freely from a height of 2 to 3 meters, the concrete can still maintain its integrity and will not break into a pile of stones and slurry. There will be no "grout leakage" or "grout run-out" during pouring, and the concrete can maintain a good shape.

(3) Good water retention

The concrete surface will not release a large amount of free water, nor will it exhibit phenomena such as "bleeding," "grout leakage," or "bottom peeling." The surface will remain moist for 1 to 2 hours after the concrete is poured.

(4) The slump remains stable

At room temperature (20-25℃), the slump loss should not exceed 20mm after 1 hour and 40mm after 2 hours.
Even at high temperatures (above 30℃), the slump can still meet construction requirements within 1 hour by adjusting the formula. Throughout the transportation and construction process, the workability of the concrete changed gradually, without any sudden "caking" or "thinning".

3. The slurry has a glossy appearance and uniform color.

The appearance of the paste is the most direct and quickest way to judge the adaptability of admixtures. A well-adapted concrete paste is typically characterized by its oily sheen, uniform color, and smooth texture.

(1) The pulp is rich and full

The concrete surface has a distinct, continuous layer of slurry with uniform thickness. When gently turned over with a shovel, you can see that the aggregate is well-coated and flowing, without the dry state of "more aggregate and less slurry".

(2) The slurry is oily and shiny

The surface of the slurry exhibits a natural, soft sheen, like silk. This is because the admixtures fully disperse the cement particles, resulting in a uniform distribution of hydration products and the formation of a smooth surface film. Higher gloss generally indicates better dispersion of the admixtures.

(3) The color is pure and uniform

Normal silicate cement concrete slurry is bluish-gray in color, with a uniform color and no uneven spots or streaks.
Concrete with fly ash added is light gray, and concrete with mineral powder added is dark gray; both colors should be uniform and pure. The color of concrete from the same batch and truck should be completely consistent, with no color difference.

(4) No impurities float

The concrete surface should be clean and free of floating oil, black flocculent matter, colored streaks, or other impurities. The presence of floating oil indicates an excessive amount of air-entraining agent or defoamer in the admixture. The presence of floating black matter usually indicates an adverse reaction between the admixture and certain components of the cement.

4. The flow pattern is uniform, forming a round cake, and it moves forward while being coated with batter.

Pouring concrete onto a flat, clean steel plate or ground and observing its flow process and its shape after it stops is one of the standards for judging the suitability of admixtures.

(1) Uniform flow process

After the concrete flows out of the slump cone, it spreads outwards at a uniform speed. The flow process is smooth and there is no phenomenon of sudden speed change.

(2) Synchronous flow of aggregates

Throughout the flow process, coarse and fine aggregates are always enveloped by the slurry and move forward synchronously with the slurry, without the phenomenon of "the slurry flowing away first and the stones being left behind".

(3) After stopping , it forms a standard round cake shape.

After the flow stops, the concrete forms a standard round cake with smooth edges, uniform thickness, and flat surface. The height difference between the center and the edge of the cake is very small, and there is no obvious "straw hat" (high in the middle and low at the edge) or "pot bottom" (low in the middle and high at the edge) deformation.

(4) The aggregate is evenly distributed.

The aggregate on the surface of the disc is evenly distributed, and there is no phenomenon of stones being concentrated at the edges or in the middle. Each stone is evenly coated with the paste, and there is no exposed stone.

5. The bubbles are of appropriate quantity, stable, small, and uniform.

Appropriate, stable, and uniform microbubbles can significantly improve the workability and freeze-thaw resistance of concrete, but too many or too large bubbles can severely reduce the strength of concrete. Admixtures with good adaptability can precisely control the number, size, and distribution of bubbles.

(1) Appropriate amount of air intake

There are only a small number of evenly distributed tiny air bubbles on the concrete surface, with a diameter generally between 0.1 and 1 mm. When the concrete is gently turned over with a shovel, a large number of air bubbles will not overflow.

(2) Stable bubbles

Bubbles will not easily break or merge into large bubbles during concrete mixing, transportation, pouring and vibration.

(3) No large air bubbles

There are no large air bubbles with a diameter greater than 3 mm on the concrete surface, and no dense honeycomb-like air bubble clusters. After vibration, the surface air bubbles can be smoothly discharged, and no large number of air holes will be left on the formwork surface.

(4) Uniform distribution of air bubbles

Air bubbles are evenly distributed inside the concrete, and there is no phenomenon of too many or too few air bubbles in some areas.

6. Lightweight and non-sticky to operate, smooth application.

The adaptability of admixtures ultimately manifests in the ease of construction. Adaptable concrete can significantly reduce the labor intensity of workers and improve construction efficiency.

(1) Easy to shovel and transport

When shoveling concrete, it feels light and effortless, without any heaviness or stickiness. After pouring out the concrete, the shovel surface is clean, with almost no concrete residue, and it can be easily shaken off completely.

(2) Non-stick container

Concrete does not stick to the tank wall during transportation in the tanker truck, and unloading is clean and thorough, leaving very little residue inside the tank. It will not stick to the formwork during pouring, and the concrete surface is smooth and flat after demolding, without any sticking marks.

(3) Easy to vibrate

The vibrator can be easily inserted into the concrete with minimal resistance. During vibration, air bubbles are quickly and evenly expelled, preventing segregation caused by over-vibration or honeycomb-like pitting caused by under-vibration. After vibration, the concrete surface is smooth with no obvious vibration marks.

(4) Smooth surface

When applying a finishing treatment before the concrete has initially set, the surface should feel smooth to the touch, not stick to the trowel, and the surface should be smooth and flat after finishing.

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