Four Types of Asphalt Mixing Plant Failures

In asphalt pavement construction, asphalt mixing plant failures occasionally occur. The efficiency of the asphalt mixing plant and the quality of the concrete produced are related to whether the asphalt concrete mixing plant failures occur, as well as the type and probability of failures. In this paper, we analyze the causes of asphalt concrete mixing plant failures by combining years of experience in asphalt concrete production and construction. This paper provides some experiences to promote the development of asphalt concrete and ensure the construction quality of high-grade asphalt pavement.

Here are four types of asphalt mixing plant failures and their cause analysis.

Unstable output and low equipment production efficiency

In construction production, we often encounter this kind of phenomenon. Equipment capacity is seriously insufficient. The actual production capacity is far below the equipment specification capacity. This causes equipment waste and low efficiency. The main reasons for such failures are as follows.

Improper asphalt concrete mix ratio.

The asphalt concrete mix ratio is divided into the target mix ratio and production mix ratio. The target mix ratio controls the proportion of cold material delivery of sand and gravel. Production ratio is the design of the asphalt concrete mix ratio of various types of sand and gravel materials in the finished material. The production mix ratio is determined by the laboratory and directly determines the grade standard of the finished asphalt concrete.

The target mix ratio is set to further ensure the production mix ratio. It can be adjusted during production according to the actual situation. When the target mix ratio or the production mix ratio is not suitable, the stone stored in each measurement of the mixing plant will be out of proportion. Some of them overflow and some of them wait for material. It is not possible to weigh in time, and the mixing cylinder is idle so that the output is reduced.

Sand and stone material grade is not qualified.

Each kind of specification of sand and stone material has a grade range. If the incoming material is not strictly controlled and the gradation is seriously over the range, a large amount of waste will be produced and the measuring bin will not be able to measure in time. It not only causes low output but also wastes a lot of raw materials.

Sand and stone material moisture content is too high.

The production capacity of the drying cylinder of the asphalt mixing plant is matched with the equipment model. When the moisture content of sand and stone is too high, the drying capacity decreases. The quantity of sand and gravel materials supplied to the metering bin to reach the set temperature per unit time is less, thus reducing the output.

The fuel oil burning value is low.

Asphalt mixing plant burning oil has certain requirements. It generally burns diesel, heavy diesel or heavy oil. In construction, for the sake of cheap, sometimes mix oil is burned. This oil combustion value is low, less heat, seriously affect the heating capacity of the drying cylinder.

Equipment operating parameters are not set properly.

This is mainly reflected in the dry mixing, wet mixing time set improperly, as well as the bucket door open, and close time adjustment improper. In general, each mixing production cycle is 45s, which is just the rated capacity of the equipment.

For example, if the mixing cycle is 45s, the output per hour is Q = 2×3600/ 45= 160t / h. 

If the mixing cycle time is 50s, the hourly output Q = 2×3600/ 50= 144t / h (Note: The rated capacity of 2000 type mixing equipment is 160t / h). This requires the mixing cycle time to be shortened as much as possible under the premise of quality assurance.

Unstable Asphalt Concrete Discharge Temperature

The production process of asphalt concrete has strict requirements for temperature. If the temperature is too high, the asphalt is easily “burnt”, commonly known as “paste material”, which is worthless and must be thrown away. If the temperature is too low, the asphalt and gravel materials do not adhere well, commonly known as “white material”. The loss of “paste” and “white material” is very alarming. The cost of each ton of material is generally about $40. The more waste material an asphalt concrete production site discards, the lower the level of management and operation. There are two reasons for this kind of failure.

Asphalt heating temperature control is not accurate.

The temperature is too high, resulting in “paste material”, and the temperature is too low, resulting in “flower white material”.

Inaccurate control of heating temperature of gravel material.

Improper flame size adjustment of the burner, failure of the emergency air door, change of moisture content in the sand and gravel material, lack of material in the cold bin, etc., all tend to cause waste. This requires careful observation, diligent measurement and a high sense of quality responsibility in the production process.

The unstable oil-to-stone ratio of Asphalt mixing plant failures

The oil-to-stone ratio is the ratio of the mass of asphalt to the mass of sand and other fillers in asphalt concrete. It is the most important indicator to control the quality of asphalt concrete. If the oil-to-stone ratio is too large, the pavement will become “oil cake” after paving and rolling. The oil-to-stone ratio is too small, the concrete material is scattered, not rolled into shape, are serious quality accidents. The main reasons are the following two aspects.

In sand and stone, the content of mud and dust seriously exceeded the standard.

Although dust removal, but the filler contains too much mud, most of the asphalt and filler combined, which is commonly known as “oil absorption”. Stone surface adhesion asphalt less, rolling is not easy to form.

Metering system failure.

Mainly asphalt metering scale and mineral powder metering scale metering system zero point drift, resulting in measurement errors. Especially the asphalt metering scale, an error of 1kg will seriously affect the oil to stone ratio. In production, the metering system must often be calibrated. In actual production, because of the impurities in the mineral powder, often cause the mineral powder metering bin door is not closed, leakage occurs, which seriously affects the quality of asphalt concrete.

Dusty & polluted construction environment

In the construction, some mixing plant dust flying, serious polluting the environment, affecting the health of the staff. The main reasons are the following two aspects.

Sand, stone in the amount of mud, dust is too large, seriously exceed the standard.

Secondary dust removal system failure.

Asphalt mixing plant is generally used dry secondary bag dust removal. It is made of small pores, good permeability, and high temperature resistant special materials. Expensive, but the effect is good, can meet the requirements of environmental protection. The main reason for pollution is the bag pulse air pressure is too low. Or some units in order to save money, the bag is damaged and not replaced in a timely manner. Bag broken or blocked, incomplete combustion of fuel, impurities adsorbed on the surface of the bag, resulting in blockage, resulting in dust at the entrance of the drying cylinder cold material. The bag is broken, or not installed, the smoke is “yellow smoke”, which is actually dust.

Read more:

7 Key Points for Controlling the Asphalt Mixture Quality

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