21:23:29 PM 1/22, 2021

35 Cases of Lessons From Commercial Concrete Quality

2020-08-181Thomas Huang

 Raw material quality and management
[Example 1] The temperature of cement is too high, causing the loss of concrete slump too fast.
During the process of pouring C30 beam slab in June, a project found that the concrete slump loss was very fast, causing the concrete in the drum to aggregate
Analysis of the cause: The temperature of the incoming cement reached 80℃ after investigation, and the cement is generally too fine, resulting in an increase in water demand. When the water consumption is insufficient, the slump loss will be too fast.
Preventive measures: In the summer and autumn season from May to October, for the cement purchased directly from the cement plant or grinding station for short-distance transportation, the temperature of the cement must be measured to be less than 65℃ per vehicle.
[Example 2] The admixture crystallizes in winter and blocks the pipeline.
From November to February of the following year, due to the rapid temperature drop, more crystals will appear when the admixture pump is pumped to the weighing hopper, which will block the pipeline and butterfly valve and cause slow metering.
Reason analysis: Na2SO4 in the naphthalene-based superplasticizer used by the company reaches supersaturation and precipitates crystals when the temperature is low.
Preventive measures: negotiate with the water-reducing agent supplier to adjust the water-reducing agent formula, and reduce the solid content from 32% to 20%, which has basically solved the crystallization problem.
[Example 3] Rainwater or flushing water enters the admixture tank, causing a decrease in concentration and affecting the slump of concrete.
The concrete supplied to the project on a certain day was inspected by the inspection platform before leaving the factory. It was found that the slump was too small.
Reason analysis: check that the feed port on the top of the water-reducing agent tank is opened and the lid is not closed. The water that may wash the top of the tank flows into the tank due to heavy rain, causing the concentration of the water-reducing agent to decrease.
Preventive measures: After the feed supplier pumps the water reducing agent into the tank, the upper cover should be tightly closed in time, and the material staff should strengthen inspections.
[Example 4] Misuse of wood-calcium water-reducing agent as naphthalene-based water-reducing agent to cause retardation.
When supplying a two-story beam slab for a project, the construction unit reported that the setting time was too long, and it had not been solidified 24 hours after pouring.
Reason analysis: After checking the mixing ratio and cutting records, the ordinary water reducing agent is input into the computer as a high-efficiency water-reducing agent. The high-efficiency water reducing agent is mainly based on naphthalene, and the powder content is 0.5% to 0.8%, while the ordinary water reducing agent The main component of the water agent is calcium lignosulfonate, with a mixing amount of 0.25% to 0.3%. When wood calcium exceeds 0.4% of the amount of cementing material, severe retardation will occur.
Corrective measures: The dosage of wood calcium water reducing agent should be strictly controlled, and another operator must review the mix ratio input.
[Example 5] Mixed use of different brands of waterproofing agents.
During the construction of the C40P8 exterior wall of the basement of a certain project, it was found that the slump of a truck of concrete was too large and returned.
Reason analysis: After checking the purchase records, the two water repellents purchased at the same time were used in two projects, one has obvious water reduction effect, and the other has a lower water reduction rate. Workers mixed use when handling bagged water repellents Case.
Corrective measures: each batch of waterproofing agent (including expansion agent, etc.) must be re-inspected before it can be used when it enters the factory. The warehouse materials should be isolated and marked, and mixed.
[Example 6] The sand contains large pieces of mud and pebbles.
During pumping at a construction site, it was found that there were many large pebbles and mud on the lower hopper of the pump truck.
Reason analysis: the screen of the river sand supplied by the sand yard was broken during the sand pumping, and the suction port was inserted too deep, which pumped the large stones and mud masses at the bottom of the river bed into the sand.
Corrective measures: The sand yard should regularly check whether the screen of the sand pump is damaged, and the company's sand and gravel inlet and the waiting trough of the main building should be installed with grilles to prevent debris from entering the concrete.
[Example 7] The mud content of the gravel exceeds the standard.
After a rainy day, the mud content of the crushed stone exceeds 1%, which increases the water demand of the concrete and reduces the strength of the concrete.
Reason analysis: the crushing process of gravel after rain contains a lot of sand, which cannot be separated cleanly during vibrating screening.
Corrective measures: Add flushing equipment to the loading dock to control the mud content within the standard range. Unqualified gravel is strictly prohibited from entering the warehouse.
[Example 8] The quality of fly ash and slag is unqualified.
When the company purchases Class F grade one ash, the fineness, water demand ratio and loss on ignition etc. occasionally fail to meet the requirements when entering the factory; the purchased S95 grade fine ore powder, the 7-day and 28-day activity indicators occasionally fail to meet the requirements , But the detection cycle is long.
Cause analysis: the quality of coal-fired power plants fluctuates, and the fly ash separation equipment fails, causing quality fluctuations; the source of slag is unstable and the specific surface area is small.
Corrective measures: After each truck of fly ash and slag powder enters the plant, the fineness or water demand ratio shall be checked before being put into storage. Only when the specific surface area of slag tested by each vehicle is ≥400m2/kg is qualified, it can be sent to the warehouse for sampling regularly.
[Example 9] Cement, fly ash and mineral powder must be filtered through a screen before entering the warehouse.
The operator of the main engine building reported that the screw conveyor was jammed by debris, causing the motor to burn.
Reason analysis: when bulk materials enter the warehouse, they contain iron nails, steel balls and other debris in the ball mill, which jam into the gap of the screw conveyor.
Corrective measures: install a φ10mm aperture screen before all the feed nozzles to prevent large debris from entering the tank.
[Example 10] Mixed warehouse of raw materials.
The on-site quality inspector in the supply process of a project found that the concrete was poor in viscosity, white in color, and the strength was obviously low after rapid testing, so the poured concrete was cleaned up and poured again.
Reason analysis: The supplier's driver was not familiar with the company's pipeline layout, connected the wrong nozzle, and mistakenly injected fly ash into the cement tank.
Corrective measures: all powder tank openings must be locked and clearly marked, and the material staff shall strengthen the inspection and supervision of each material warehouse.
2. Mix ratio management
[Example 11] The mixing ratio was not adjusted in time for changes in pumping height and length.
1) A serious pipe blockage occurred during the supply of C40P8 basement floor of a project, and the length of the pump pipe on site exceeded 200 meters.
2) During the construction of the 21st-story column wall in a certain project, the phenomenon of pipe blockage occurred many times.
Analysis of the cause: The construction unit did not specify the pumping height or the length of the pumping horizontal pipe when it issued the production task. Instead, the mixing ratio was set according to the ordinary 5-31.5mm crushed stone and the ordinary pumping slump, which caused the pipe to block and burst.
Corrective measures: 1) When the pumping height exceeds 20 layers or 60 meters, the 5-25mm gravel particle size must be indicated, and the slump should be ≥160mm. 2) When laying the pipes of the pump truck, the pipes should be laid horizontally and vertically, and the number of elbows and pipes should be minimized. If the length of the horizontal pipe exceeds 150 meters, the slump should be ≥160mm.
[Example 12] Changes in sand fineness modulus affect concrete slump.
During the construction of a certain project, the slump was too large and the concrete segregated.
Reason analysis: the fineness modulus of river sand becomes larger and the specific surface area decreases, which causes the slump to be larger.
Corrective measures: The river sand entering the factory should be mixed evenly. If the detection fineness modulus changes greatly, the construction mix ratio should be adjusted in time.
3. Production process management
[Example 13] Scheduling mistakes.
1) When supplying concrete at a certain construction site, the quality inspector found that the concrete marking on the delivery order did not match the strength grade.
Reason analysis: When the shift dispatcher entered the production order, he mistakenly entered the intensity level as C15, and the main engine production was indeed produced according to C25P6, which was an input error.
Corrective measures: When dispatching the first batch of concrete, the dispatcher must verify the correctness with the production task list before printing the "Inspection Form".
2) A certain project constructs the independent foundation cap C30 and the independent foundation cushion C15 at the same time. When the site constructor guides the vehicle to the unloading location, the material is unloaded without seeing the C15 delivery order and the strength grade sign. The strength is found only after dispatching inquiry Wrong grade.
Reason analysis: when the on-site construction workers poured concrete of different strength grades on the same construction site, they unloaded the material without carefully checking the invoice and the identification plate.
Corrective measures: The dispatcher must check the strength level with the construction unit and the construction site before dispatching the vehicle. The delivery order information must be accurate, and the mixer truck driver should first be confirmed by the construction site before unloading.
3) When a clerk took a temporary order on a certain day, the concrete number C35 was mistakenly filled in as C30.
Reason analysis: the salesman received the verbal call from the construction site to place the production order, and the dispatcher did not seriously confirm that the delivery information was correct.
Corrective measures: The monthly production plan must specify the site requirements in detail, and the temporary order must be confirmed by the site material staff or construction staff, or sent to the dispatch center in written fax to avoid errors in position or strength level.
[Example 14] The host operator made a mistake.
1) One day, when querying the report of the host, it was found that the operator of the host had mixed the C15 concrete at a certain construction site to C30.
2) On a certain day, the dispatcher used vehicle No. A to load material for site A C25, and B# vehicle installed C35 on site B. However, the A# vehicle was not in time for the goods, and the dispatcher did not contact the driver, so he called the latter B# vehicle to replace it. The operator of the main engine did not seriously check and submit the inspection order directly, causing the concrete from the actual armored site of the driver B# to be delivered to the B site.
Reason analysis: the host operator did not carefully check the delivery information to place the order in advance.
Corrective measures: Before the industrial control system is fully networked, the dispatcher prints the delivery note first, and submits it to the host operator in writing to carefully check the truck number, project name, construction location, strength level, and mix ratio number. Only after mixing, sign for confirmation after mixing, and then put the order. At present, through the development of the ERP system, the plan input production task system is connected with the scheduling system and the industrial control system to effectively prevent the human error of the host operator and the dispatcher.
[Example 15] The measurement is out of control, and the sand is overweight.
1) The operator of the main engine found that there was too much sand in the sand and gravel waiting chute, and notified the vehicle that the weighed gravel was measured incorrectly.
Reason analysis: the gravel weighing gate starts to discharge if the action of the gravel weighing gate is not in place.
Corrective measures: each shift focuses on checking the cylinder, solenoid valve, and limiter, and re-calibrates the weighing scale regularly before production.
2) When the host operator mixed the nine-layer beam slab of a certain project, he did not observe and showed that there was no cement of this type in the data, but the mixed material was produced, which caused the mixed material to be scrapped.
Reason analysis: when the technical duty sends the mix ratio to the industrial computer, the host has no cement of this type, and the cement type is not matched.
Corrective measures: The software company is requested to modify the program in time, and the host operator should pay attention to the weighing conditions of each weighing scale, and do not stir if there is an abnormal situation.
[Example 16] The concentration of the admixture is uneven, causing some concrete to retard.
When the 14-story wall column and 15-story beam slab concrete were poured on a certain construction site on a certain day, the slump was too large and the initial setting time exceeded 20 hours.
Reason analysis: the failure of the agitator of the water-reducing agent tank caused uneven density of the water-reducing agent, the density of the water-reducing agent in the lower part of the storage tank was too large, and the setting time was too long.
Corrective measures: The water reducing agent must be stirred evenly at regular intervals, and the material staff should strengthen inspections. If the mixer stops rotating, report for repairs immediately.
[Example 17] The sand and gravel warehouse was not fed in time, resulting in poor workability of the concrete.
When concrete was supplied to the basement floor cap at 5:00 a.m., two trailer pumps blocked the pipes at the same time.
Reason analysis: the crushed stone is not fed in time, and the material level of the crushed stone warehouse is low, causing pipe blockage.
Corrective measures: Strengthen the inspection of aerial silos and feed materials, the feed loader should load materials in time, and the aerial sand and gravel silos must be kept full. The night technical duty, material staff and dispatcher must check the material feeding and the quality of sand and gravel every two hours.
[Example 18] Large errors occur in small measurement of fly ash and slag powder.
On a certain day, the fly ash and slag metering scale was overweight, and the measurement error of a single plate was greater than 2%
Reason analysis: the speed of the screw conveyor is too fast, causing the drop to be too large, and part of the impulse cannot be recorded.
Corrective measures: The equipment department changes the gearbox gear ratio of the screw conveyor and installs a gate valve between the screw conveyor and the weighing hopper to control the material drop.
[Example 19] Quantity difference
The amount of complaints on a construction site is poor, and the difference between the calculated amount according to the drawings and the actual pouring amount exceeds the scope of the national standard.
Reason analysis: when calculating the apparent density of the mix ratio, only the actual apparent density is measured, and the compaction coefficient is not multiplied.
Corrective measures: The production mix density should be determined by multiplying the apparent density measured by the concrete test by the concrete compactness coefficient (usually 1.01). Each machine in each shift shall be randomly checked by three vehicles for weighing and rechecking to prevent loss of measurement.
4. Transportation and pumping management
[Example 20] The driver sent the wrong site.
The driver of a mixer truck loaded C25 concrete and sent it to another construction site C30.
Reason analysis: The driver did not see the site name and construction unit clearly when filling in the vehicle tracking form, and he was careless in his work.
Corrective measures: each concrete mixer truck must be clearly marked with its strength level. After loading the concrete, the driver should fill in the project name and construction unit on the vehicle tracking form in time to avoid sending it to the wrong site. The dispatch center should pay close attention to the operation of GPS satellite positioning system vehicles, and promptly correct any problems found.
[Example 21] Water accumulated in the roller of the driver's transport vehicle.
When a driver loaded concrete, the slump of concrete in the drum was too large.
Reason analysis: After the previous vehicle was loaded with washing pump water, it was not used up at the construction site, but the remaining water in the drum was not discharged in time, and the material was loaded without confirmation.
Corrective measures: The dispatch center must control the mixer trucks that are loaded with pump washing water, and can only notify the loading after receiving the driver's unloading confirmation. After the driver washes the car and after heavy rain, the accumulated water in the drum must be drained.
[Example 22] Leakage and coagulation.
1) One day the driver was transporting concrete to the construction site, and the drum reversed and leaked, causing a large area of road pollution.
Reason analysis: The anti-reverse pin was inserted before the installation process was not followed, causing material leakage.
Corrective measures: It is stipulated that the driver of the mixer truck must check whether the anti-reverse pin is inserted before loading the truck.
2) One day, the mixer truck driver transported C40 concrete to a construction site. After unloading, it was not checked whether the concrete was unloaded. The empty truck was weighed and the empty weight was not checked. There were 2 left in the drum and brought back to the company.
Reason analysis: The driver of the mixer truck was sloppy and did not check the concrete in the drum according to the prescribed process.
Corrective measures: Strengthen the quality awareness training of the mixer truck drivers. After unloading, check whether there is any accumulation in the drum. The transportation department regularly checks the weight of the empty truck back to the factory to avoid mixing of leftover materials and concrete of different strength levels.
3) One day, the screw of the reducer of the mixer truck broke and the drum could not rotate, causing the concrete to condense in the drum.
Reason analysis: the screw of the reducer is broken, causing the drum to fail to rotate.
Corrective measures: The mixer truck driver should do a daily vehicle inspection, and focus on checking whether the drum reducer screws are tightened or have bends and cracks, so as to prevent the screws from breaking during high-speed rotation and causing concrete scrap.
[Example 23] Pumping burst pipe to block the pipe.
1) When the concrete pumping at a construction site is finished, air washing is used when cleaning the concrete pipeline. The excessive pressure causes the wall of the pump tube to rupture, causing injuries to workers.
Reason analysis: the pump truck’s self-provided water tank has insufficient water, and the concrete cannot be completely washed out by the water washing method. However, when the air washing method is used, the air washing pressure is 3 times the washing pressure, and the wall of the pump tube connected to the tail is thin and cannot bear The air-wash pressure ruptured, causing the concrete to fly out and hurt people.
Corrective measures: Strictly restrict air washing methods, and prohibit thin and old pump tubes. The number of hoses at the outlet of the pump tube should not exceed two and the bend should not exceed 90 degrees. The water for washing the pump is specially filled and transported from the mixing station to the site by a mixer truck. Strengthen personnel safety operation training.


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