Analysis of the causes of large fluctuations in concrete performance during the use of water-reducing agents

Due to its own molecular structure, the requirements for concrete raw materials are more stringent than those of traditional high-efficiency water-reducing agents such as naphthalene series and aliphatic ones. The current shortage of sand and gravel aggregates and the irregular production of commercial concrete make it easier to bring mud into concrete than before. The use of water-reducing agents is an important addition to the concrete industry. The reasons for the large fluctuations in concrete performance during use are: Which?

　1. Frequently Asked Questions

　(1) Slight changes in raw materials lead to too small water-reducing agent dosage, resulting in poor fluidity and rapid loss of concrete.

(2) Slight changes in raw materials lead to excessive amounts of water-reducing agent, resulting in concrete delamination, segregation, and serious bleeding.

　2. Cause Analysis

When the raw materials change little, the water-reducing agent will cause fluctuations in the workability of concrete due to its high water-reducing rate. Let's compare the active ingredient content (that is, solid content) of naphthalene-based water-reducing agent and water-reducing agent. It is not difficult to understand why the dosage of water-reducing agent easily fluctuates. compare results.

Use the above two kinds of water-reducing agents and use the same raw materials to achieve the same water-reducing rate and make a C30 mix ratio.

It can be seen that under normal circumstances, the solid content of naphthalene water-reducing agent is about 4 to 5 times. It can be calculated that the dosage of naphthalene water-reducing agent (liquid) is 2.4% of the total cementitious material. The effective ingredient (solid content) of the water-reducing agent per cubic meter of concrete is 2.4kg, and the active ingredient content is 0.65% of the total cementitious material; the water-reducing agent (liquid) content is 2.2%, and the effective ingredient of the water-reducing agent per cubic meter of concrete is 2.4kg. (Solid content) is 0.46kg, and the active ingredient content is 0.12% of the total cementitious material. It is because the water-reducing rate of the water-reducing agent is high and the amount of active ingredients is small, measurement errors and production errors change the actual dosage of the water-reducing agent during production, resulting in unstable workability of concrete. And currently, in order to show that their products have a high water-reducing rate and low dosage, most manufacturers deliberately increase the solid content and control the dosage of liquid water-reducing agent between 1.5% and 2.0%. (C30) mix ratio is an example. The amount of water-reducing agent per cubic meter is between 5.55 and 7.40kg. The lower the amount of water-reducing agent, the greater the variability of production will be.

Analysis of the influence of the moisture content of manufactured sand on water-reducing agents. Since the machine-made sand (with a large content of machine-made sand and gravel powder) has strong water absorption, it also has a certain adsorption effect on the water-reducing agent. If sand with too low moisture content comes into direct contact with the water-reducing agent when mixing concrete, the actual amount of water-reducing agent will be reduced. Nowadays, most commercial concrete companies privately adjust the mixer feeding sequence and mixing time in order to improve production efficiency. Thinking that this can save electricity bills, in fact, it increases the amount of water-reducing agent, increases material costs, affects the performance of concrete, and the gain outweighs the loss.

Test 1 is based on different feeding sequences to study the effect of sand on the workability of concrete due to the adsorption of water-reducing agent by sand. Test mix ratio.

Note: (1) The cement is medium-crystalline P·O42.5;

(2) The mineral powder is Danyang S95 grade;

(3) Powder The coal ash is Wulong Grade II;

(4) The sand is manufactured sand, Zone II, with a fineness modulus of 2.8 and a stone powder content of 8.3%.

For the first time, stir the water-reducing agent directly with the aggregate for 30 seconds, then add the cementitious material and water together and stir for 60 seconds. The initial slump is 160mm and 120mm after half an hour.

For the second time, stir the cementitious materials and aggregates together for 30 seconds, then dissolve all the water-reducing agent into the water and add them together for 60 seconds. The initial slump is 180mm and 165mm after half an hour.

For the third time, stir one-half of the water and aggregate together for 30 seconds, then dissolve all the water-reducing agent into the remaining water and add the cementitious material together and stir for 60 seconds. The initial slump is 195mm and 185mm after half an hour.

Test 2 uses three different feeding methods and different amounts of water-reducing agent to achieve the same workability.

Using the first feeding method, 9.62kg of water-reducing agent was used, the slump was 210mm, and the workability was good.

Using the second feeding method, 8.88kg of water-reducing agent was used, the slump was 205mm, and the workability was good.

Using the third feeding method, 8.14kg of water-reducing agent was used, the slump was 205mm, and the workability was good.

It can be seen from the two tests that the mix ratio is the same. The first feeding method has a slump 35mm smaller than the third feeding method, and the second feeding method is 30mm smaller than the third feeding method. If you want to achieve the same slump, the first feeding method requires 20% more water reducing agent than the third method, and the second method requires 8% more water reducing agent than the third method. Sand and gravel adsorption water reducing agent will have a great impact on the workability of concrete. However, the moisture content of the sand, the feeding sequence and the mixing time during actual production will aggravate or reduce this effect. Similarly, because the naphthalene series has more active ingredients (solid content), while the water-reducing agent has less active ingredients (solid content), it further aggravates the fluctuation of the water-reducing effect of the water-reducing agent during the production process.

　3. Solutions

　(1) The solid content of the water-reducing agent should not be too high, and the amount of liquid water-reducing agent should be controlled at 2.5% to 3.0% of the total cementitious material. %. In this way, if the total amount of cementitious material is 370kg, then the amount of water reducing agent per cubic meter will increase from the original 5.55 to 7.40kg (the original dosage is 1.5% to 2.0%) to 9.25 to 11.1kg. In this way, each cubic meter of concrete water-reducing agent increases, but the solid content decreases, and the cost of each cubic meter of concrete water-reducing agent remains unchanged. Due to the increased unit dosage, measurement errors and production errors are reduced, ensuring the stability of concrete production.

(2) The moisture content of the sand must be strictly controlled during actual production. Excessive moisture content will cause concrete to use water.�The fluctuation is large, and the moisture content is too small, which increases the adsorption of the water-reducing agent by the sand. The moisture content of the sand should be controlled between 4% and 6%. When feeding, the water-reducing agent should be avoided from direct contact with the dry sand, and the mixing time should be correctly controlled. </p