Problems and solutions of concrete mixed with superplasticizer

The concrete mixture mixed with concrete superplasticizer will have some problems, such as sticking can, false setting, non-setting, too fast slump loss or low strength after hardening, and so on. In order to avoid the above phenomena, let's understand the causes of the problems and solutions.

Settlement joint

Phenomenon: after pouring, there will be a number of short, straight, wide and shallow cracks before and after initial setting.

Reason: the concrete mixed with superplasticizer is more sticky, does not secrete water and is not easy to settle flat, and mostly appears above the steel bar.

Solution: apply pressure to the cracks before and after the initial setting of the concrete until the cracks disappear.

Sticking can

Phenomenon: cement mortar is partially glued to the wall of the mixer tube, resulting in uneven concrete and less ash.

Reason: sticky concrete often occurs in drum mixers with similar shaft-diameter ratio after adding retarder and water reducer.

Solution: 

1. Pay attention to the removal of residual concrete in time.

2. First add aggregate and part of water to stir, then add cement, residual water and superplasticizer to mix.

3. Use a large shaft diameter ratio or a forced mixer.

False condensation

Phenomenon: after leaving the machine, the concrete quickly loses its fluidity and even cannot be poured.

Reason: 1. Insufficient content of calcium sulfate and gypsum in cement leads to excessive hydration of calcium aluminate; 2. The adaptability of water reducer to this kind of cement is poor; 3. When the content of triethanolamine is more than 0.05-0.1%, it will initially set but not final.

Solution:

1. Change the type or batch number of cement.

2. Change the variety of superplasticizer if necessary, it is not necessary in general.

3. Reduce the water reduction rate by half.

4. Reduce the mixing temperature.

5. The retarding content of Na2SO4 is 0.5-2%.

Non-condensation

Phenomenon:

1. After adding superplasticizer, the concrete has not solidified for a long time, even for a day or night;

2. The surface secretes plasma and is yellowish brown.

Reason:

1. The amount of superplasticizer is too large, which is likely to be more than 3-4 times of the recommended dosage.

Overdose of retarder

Solution:

1. If the recommended dosage is not more than 2-3 times, the strength decreases slightly, but the 28d strength decreases less and the long-term strength decreases even less.

2. Properly raise the curing temperature after the final setting and strengthen the watering maintenance.

3. Remove the formed part and re-pour it.

Low strength

Phenomenon:

 1. The strength is much lower than the trial results of the same age; 2.

Although the concrete has solidified, its strength is extremely low.

Reason: 1. The amount of air-entraining superplasticizer is too large, which makes the air content in concrete too large;

2. After mixing with air-entraining superplasticizer, the vibration is not enough;

3. Without reducing the water or increasing the water-cement ratio;

4. The dosage of triethanolamine is too large;

5. The quality of superplasticizer is not up to the requirements, such as the content of active ingredients is too low.

Solution:

1. Adopt other reinforcement measures or re-pouring.

2.Strengthen the vibration after pouring.

3. Take measures against the aforementioned reasons.

4. Identify this batch of superplasticizer.

Slump loss is too fast

Phenomenon: concrete loses and workability very quickly, slump decreases by 1-3min for every 2-2cm extension, and it has obvious bottom sinking phenomenon, which is more likely to occur in large slump concrete.

Reason: 1. The adaptability of water reducer to the cement used is poor; 2. The bubbles in concrete continue to overflow and water evaporates, especially when using air-entraining superplasticizer; 3. High mixing temperature or ambient temperature of concrete; 4. The concrete has a large slump.

Solution:

1. Take measures against the cause.

2. The post-blending method was used.

The superplasticizer is mixed after mixing concrete for 1-3min, even before pouring, and then re-mixed.

3. Be careful not to add water.

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