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What is the reason for the rapid loss of slump?

After the concrete was delivered to the construction site, it was found that the slump dropped quickly. Many people on site will ask whether too little water-reducing agent was added.

This judgment cannot be said to be completely unreasonable. Still, if we only focus on “adding or not adding water-reducing agents” every time, the problem will easily become more and more chaotic as we deal with it.

Sometimes adding a little water-reducing agent can restore the current state. In a few days, as the material changes, the temperature rises, and the transportation time lengthens, it will start to fluctuate again. In the end, the higher the dosage, the higher the cost, and the on-site feedback is still unstable.

So when encountering rapid slump loss, the first reaction is not necessarily to continue adding water reducing agents, but to sort out the following steps.

Check if the sand and gravel have changed

Problems with the condition of concrete often do not occur suddenly. Changes in sand moisture content, increases in mud content, changes in mechanical sand grading, and increases in stone powder content will all affect the performance of additives. Especially for stations with large fluctuations in sand and gravel materials, the same mix ratio and water reducing agent dosage may result in different states in the morning and afternoon.

Sometimes on site, one may feel that ‘this batch of water reducing agents is unstable’, but if we look ahead, it may be that the sand and gravel have changed first.

You can first ask a few simple questions:

Is the moisture content of sand lower than usual?
Has the mud content increased?
Has there been any significant change in the grading of machine-made sand?
Is the stone powder content too high?
Have there been any batch changes in cement, fly ash, and mineral powder recently?

These issues may seem basic, but many on-site fluctuations are often hidden within these fundamental links.

Has the cementitious material changed?

Water-reducing agents do not work alone; they must work together with cementitious materials such as cement, fly ash, and mineral powder.

Some cement adsorbs quickly. In the early stages, if the admixture is used too much, it looks okay when it comes out of the machine, but the loss is obvious later on. Some powders require more water, and the system will also consume more additives. In addition, the fineness, mineral composition, and activity of different batches of rubber materials vary, so it is not surprising that additive performance changes accordingly.

That’s also why the same water-reducing agent performs well in one material system, but may not perform the same when transferred to another system.

In this situation, simply increasing the dosage may not necessarily be the optimal solution. What should be done more is a small-scale comparison to see if the problem lies in the initial water reduction rate, collapse retention ability, or the compatibility of the cementitious material.

Observe long-term state

Many stations judge the condition of concrete, but it is easy to only see the slump of the machine.

But for the construction site, what really matters is whether it can be used after arrival, whether it is stable during the pumping process, and whether it still has construction performance after waiting for a period of time.

In summer, the temperature is high, and the hydration reaction is fast; Long transportation distance and long travel time; The construction site has been waiting for a long time, and the status continues to deteriorate. Combined, these factors mean that even if the status is normal when exiting the station, it may significantly deteriorate upon arrival.

So when looking at slump loss, we cannot just look at one point in time.

You can pay more attention to a few data points:

How will it be in 30 minutes?
How much slump is left after 60 minutes?
Can it still be pumped after waiting on site?
Does the concrete have any stickiness, bottoming, segregation, or bleeding?

These are more valuable references than simply observing the machine status.

Adjust the proportion, not just the dosage

If all factors such as sand and gravel, rubber materials, water, mixing, and transportation have been investigated and the problem still exists, then we need to return to the additive solution itself.

But the adjustment mentioned here is not just a simple sentence of ‘add more points’.

Some projects require an initial water reduction rate; some prioritize collapse protection after 1 hour; some projects need to balance wrapping and pumping; and some stations also need to control overall costs. The requirements for external additive solutions vary depending on the material, season, and transportation radius.

The truly effective solution is not only to determine the slump of the concrete, but also to ensure its stability upon arrival at the site.

4.Quality Control

Regular quality control tests should be conducted to ensure that the chemical admixtures are of the required quality and that their addition to the concrete results in the desired properties.

This includes testing the workability, strength, and durability of the concrete.

Our suggestion for the rapid loss of slump

The slump loss is fast; don’t rush to assign responsibility to the water reducer first.

We suggest looking at it in this order:

First, check if the raw materials have changed, especially the moisture content, mud content, grading, and powder batch of the sand and gravel.
Check if the production control is stable, including metering, water usage, mixing time, and transportation waiting time.
Then check the compatibility of the adhesive material and additives, and if necessary, compare them through small-scale trials.
Finally, adjust the admixture scheme to balance water reduction, slump retention, workability, and cost.

Many on-site issues are best avoided by relying on experience to add materials as soon as they arise. It may be resolved in the short term, but there may be even greater fluctuations in the future.

Identifying the reasons and adjusting the plan is usually more stable and cost-effective.

If you also encounter situations such as rapid slump loss, large fluctuations in dosage, and unstable on-site conditions, you can conduct a systematic investigation based on the raw materials and construction conditions. Often, the problem is not that it cannot be solved, but that we need to find the right direction first.