The cement temperature is too high, resulting in the loss of concrete slump too fast.
In the process of pouring C30 beam and slab in June, it was found that the slump loss of concrete was very fast, resulting in the concreting in the roller.
Cause analysis: the temperature of cement is 80 ℃ and the cement is generally fine, which increases the water demand. When the water consumption is insufficient, the slump loss is too fast.
Preventive measures: from May to October in summer and autumn, for cement directly transported from cement plant or grinding station for short distance, the temperature of cement must be less than 65 ℃.
The additive crystallized in winter and blocked the pipeline.
From November to February of the next year, due to the rapid drop of temperature, more crystals will appear when the additive pump pumps the material to the scale bucket, blocking the pipeline and butterfly valve, resulting in slow metering.
Cause analysis: Na2SO4 in naphthalene superplasticizer used by the company reaches supersaturation and precipitates crystal at low temperature.
Preventive measures: negotiate with the water reducing agent supplier to adjust the formula of the water reducing agent, and reduce the solid content from 32% to 20%, which has basically solved the crystallization problem.
Rainwater or flushing water into the admixture tank, resulting in the concentration reduction, affecting the concrete slump.
The slump of the concrete supplied on a certain day was found to be too small when it was checked by the test bench before leaving the factory.
Cause analysis: check that the feed inlet above the superplasticizer tank is not covered properly after being opened, rainstorm and water that may flush the tank top into the tank cause the concentration of superplasticizer to drop.
Preventive measures: after the feed supplier pumps the superplasticizer into the tank, the upper cover shall be closed in time, and the material man shall strengthen the inspection.
The wood calcium superplasticizer was misused as naphthalene superplasticizer to cause retarding.
In the supply of the second floor beam and slab of a project, the construction unit reflected that the setting time was too long, and it did not set 24 hours after pouring.
Cause analysis: according to the record of mix proportion cutting, ordinary superplasticizer is input into the computer as high range superplasticizer. High range superplasticizer is mainly composed of naphthalene, with the dosage of 0.5% ~ 0.8% according to the powder, while ordinary superplasticizer is mainly composed of calcium lignosulfonate, with the dosage of 0.25% ~ 0.3%. When the amount of wood calcium exceeds 0.4% of cementitious material, serious retarding will occur.
Corrective measures: the dosage of wood calcium superplasticizer should be strictly controlled, and the input of mix proportion must be checked by another operator.
Different brands of waterproof agent mixed use.
During the c40p8 construction of the basement exterior wall of a project, it was found that the slump of a truck of concrete was too large.
Cause analysis: after checking the purchase records, two kinds of waterproof agents purchased in the same period were used in two projects, one had obvious water reducing effect, the other had less water reducing rate, and the workers used mixed waterproof agents when carrying bags.
Corrective measures: each batch of waterproof agent (including expansive agent, etc.) must be rechecked and qualified before use. Materials in the warehouse shall be isolated and marked, and shall not be mixed.
[example 6] the sand contains large pieces of mud and pebbles.
During the pumping construction in a construction site, it is found that there are many boulders and mud blocks in the hopper of the pump truck.
Cause analysis: the screen mesh of river sand supplied by sand yard was damaged during sand pumping, and the pumping hole was inserted too deep, which pumped large stones and mud masses at the bottom of riverbed into sand.
Corrective measures: the sand yard should regularly check whether the screen of the sand extractor is damaged, and the company's sand and gravel inlet and the main building's waiting trough should be equipped with barriers to prevent debris from entering the concrete.
[Example 7] the mud content of crushed stone exceeds the standard.
When the mud content of crushed stone exceeds 1% in rainy days, the water demand of concrete increases and the strength of concrete decreases.
Cause analysis: there is a large amount of sand in the process of crushed stone crushing after rain, which cannot be separated completely during vibration screening.
Corrective measures: add flushing equipment in the loading dock to control the mud content within the standard range. Unqualified gravel is not allowed to enter the warehouse.
The quality of fly ash and slag is unqualified.
The fineness, water demand ratio and loss on ignition of class F grade I ash purchased by the company occasionally fail to meet the requirements; The activity index of S95 grade ground ore powder purchased in 7 days and 28 days can not meet the requirements occasionally, but the detection cycle is long.
Cause analysis: the coal quality of the power plant fluctuates, and the failure of the fly ash separation equipment causes the quality fluctuation; The source of slag is unstable and the specific surface area is small.
Corrective measures: each vehicle of fly ash and slag powder should be checked for fineness or water demand ratio after entering the plant, and then they can be put into storage. Only qualified slag with specific surface area of more than or equal to 400 m2 / kg can it be put into the warehouse and sampled at the supply plant regularly.
Cement, fly ash, mineral powder and other powder materials must be filtered through the screen before entering the warehouse.
The operator of the main engine building reported that the screw conveyor was stuck by debris, causing the motor to burn down.
Cause analysis: when the bulk materials are put into the warehouse, there are impurities in the ball mill such as iron nails and steel balls, which are stuck into the clearance of screw conveyor.
Corrective action: all feed pipe orifices are equipped with φ The screen with a diameter of 10 mm can prevent large debris from entering the tank.