External Environmental Factors

The cement temperature was too high, causing rapid loss of concrete slump.

A project in June during the pouring of C30 beam slab, found that the loss of concrete slump very quickly, resulting in concrete caking in the drum.

Cause analysis: The temperature of the incoming cement was found to be 80℃, and the cement was generally fine, resulting in an increase in water demand, which caused rapid slump loss when the water consumption was insufficient.

Preventive measures: In the summer and autumn months of May-October, the cement directly from the cement plant or grinding station for short-distance transport into the cement, must be less than 65 ℃ per car to measure the temperature of cement.

The admixture crystallizes during winter and clogs the pipe.

From November to February, due to the rapid drop in temperature, more crystallization occurs when the admixture pump pumps the material to the weighing hopper, blocking the pipeline and butterfly valve and causing slow metering.

Cause analysis: the company in the use of naphthalene high efficiency water reducing agent Na2SO4 in the lower temperature, reaching supersaturation and precipitation of crystals.

Preventive measures: Consult with the water reducing agent supplier to adjust the water reducing agent formula, from the solid content of 32% to 20% solid content, has basically solved the problem of crystal precipitation.

rain or flushing water into the admixture tank, resulting in lower concentration, affecting the concrete slump.

A day began to supply the project of the concrete factory by the test bench check before the slump was found to be small.

Cause analysis: check the water reducing agent tank above the feed opening was not covered after being opened, under the rainstorm and may wash the top of the tank water into the tank, resulting in water reducing agent concentration decreased.

Preventive measures: The feeding supplier should cover the upper lid tightly in time after pumping the water reducing agent into the tank, and the material officer should strengthen the inspection.

misuse of wood calcium water reducing agent as naphthalene water reducing agent caused by slow setting.

In the supply of a project of the second floor beam slab, the construction unit reflected that the setting time was too long, 24 hours after pouring still had not solidified.

Reason analysis: After checking the ratio of material records, ordinary water reducing agent was used as high efficiency water reducing agent into the computer, high efficiency water reducing agent mainly naphthalene system, according to the powder dose of 0.5% to 0.8%, while the main component of ordinary water reducing agent is calcium lignosulfonate, the dose of 0.25% to 0.3%. When the wood calcium exceeds the amount of binder material 0.4% will appear serious slow condensation.

Corrective measures: the use of wood calcium water reducing agent admixture should be strictly controlled, with the ratio input must have another operator review.

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different brands of waterproofing agent mixed use.

A project basement exterior wall C40P8 construction, found that a truckload of concrete slump is large returned.

Reason analysis: After checking the purchase records, the same period of purchase of two kinds of waterproofing agent used in two projects, one has a significant water-reducing effect, the other less water reduction rate, workers in the handling of bags of waterproofing agent mixed use.

Corrective measures: each batch of waterproofing agent (including expansion agents, etc.) into the factory must be retested before use, warehouse materials stacked should be isolated and marked, not mixed.

sand contains large pieces of mud and pebbles.

In a site pumping construction found more large pebbles and mud blocks on the pump truck off the hopper.

Cause analysis: The sand farm supplying river sand broke the screen during sand pumping, and the pumping mouth was inserted too deep, which pumped large stones and mud masses from the bottom of the river bed into the sand.

Corrective measures: The sand farm should regularly check whether the screen of the sand pumping machine is broken, and the company’s sand and gravel inlet and the host building waiting tank should be equipped with a spacer to prevent debris from entering the concrete.

The mud content of crushed stone exceeded the standard.

The mud content of the crushed stone fed after the rain exceeded 1%, causing the water requirement of concrete to increase and the strength of concrete to decrease.

Cause analysis: The crushing process of crushed stone after rain contained a large amount of sand and soil, which could not be separated cleanly during vibratory screening.

Corrective measures: Add water flushing equipment at the loading dock to control the mud content within the standard range. Unqualified crushed stone is strictly prohibited to enter the warehouse.

Unqualified quality of fly ash and slag.

The company purchases Class F primary ash, and the indicators of fineness, water demand ratio and burn loss when it comes into the plant occasionally fail to meet the requirements; the purchased S95 grade ground fine mineral powder, the 7-day and 28-day activity indicators occasionally fail to meet the requirements, but the testing period is long.

Cause analysis: the quality of coal combustion in the power plant fluctuates, and the fly ash sorting equipment fails, causing quality fluctuations; the source of slag is unstable, and the specific surface area is small.

Corrective measures: each car of fly ash and slag powder into the plant should check the fineness or water demand ratio qualified before storage. Each car test slag specific surface area ≥ 400 ㎡ / kg qualified before entering the warehouse, regular sampling to the supply plant.

Cement, fly ash and mineral powder and other powders shall be filtered by screen before entering the warehouse.

The operator of the main building reflected that the screw transport machine was stuck by debris, causing the motor to burn up.

Cause analysis: When bulk materials are fed into the silo, they contain iron nails, steel balls and other debris in the ball mill, which are stuck into the gap of the screw conveyor.

Corrective measures: Install a φ10mm aperture screen in front of all feed pipe openings to prevent large pieces of debris from entering the tank.

Raw material mixing bin.

A project supply process on-site quality inspector found poor concrete adhesion, white color, and significantly low strength by rapid testing, the concrete that had been poured was cleaned up and re-poured.

Reason analysis: The supplier driver was not familiar with the company’s piping arrangement, connected the wrong pipe opening and punched the fly ash into the cement tank by mistake.

Corrective measures: The mouth of all powder tanks must be locked and clearly marked, and the material officer should strengthen the inspection and supervision of each material store.Concrete quality lessons 35 examples

Matching ratio management

Pumping height and length change without timely adjustment of the ratio.

A project basement floor C40P8 supply process occurred serious blockage phenomenon, the site pump pipe length of more than 200 meters.

A project construction of the twenty-first floor column wall, the phenomenon of pipe plugging occurred several times.

Cause analysis: the construction unit in the issuance of production tasks, did not write the pumping height or pumping horizontal pipe length, but according to the ordinary 5-31.5mm crushed stone and ordinary pumping slump opening ratio, resulting in blocking burst pipe.

Corrective measures: 1) pumping height of more than 20 layers or 60 meters, must specify 5-25mm gravel particle size, and slump should be ≥ 160mm. 2) pumping class laying pipe should be laid horizontally and vertically, to minimize the number of elbows and variable pipe, such as horizontal pipe length of more than 150 meters, slump should be ≥ 160mm.

Sand fineness modulus changes affect the concrete slump.

A project construction process appears slump is large, the concrete appears to be segregated.

Cause analysis: The fineness modulus of river sand becomes larger and the specific surface area decreases, causing the slump to be large.

Corrective measures: The incoming river sand should be mixed evenly, and the construction ratio should be adjusted in time if the fineness modulus is detected to change significantly.

Production process management

Scheduling work failure.

When supplying concrete to a site, the quality inspector found that the concrete markings of the delivery order did not match the strength grade.

Reason analysis: When entering the production task list, the dispatcher on duty mistakenly entered the strength grade as C15, while the host production was indeed produced according to C25P6, which was an input error.

Corrective measures: When dispatching the first truckload of concrete, the dispatcher must check the correctness with the production task list before printing the “Delivery Inspection Sheet”.

A project was constructing independent foundation bearing C30 and independent foundation mat C15 at the same time. When the site constructor guided the vehicle to the unloading place, he unloaded the material without reading the C15 delivery order and strength grade signboard, and only found the strength grade was wrong after dispatching inquiry.

Cause Analysis: The site constructor unloaded the material without checking the delivery order and the identification plate carefully when pouring different strength grades of concrete at the same site.

Corrective measures: The dispatcher must check the strength grade and construction part with the construction unit before dispatching. The information on the delivery order must be accurate and the mixer driver should be confirmed by the site before unloading.

One day, when the salesman answered the temporary order, the concrete grade C35 was mistakenly filled in as C30.

Cause Analysis: The salesman was notified verbally by phone at the site to give the production task order, and the dispatcher did not carefully confirm the delivery information was correct.

Corrective measures: The monthly production schedule must specify site requirements in detail, and temporary orders must be confirmed by site material clerks or construction workers that the relevant information is correct, or sent to the dispatching center in written fax form to avoid the occurrence of part or strength grade errors.

Host operator’s work error.

One day when inquiring the host report, the host operator found that the C15 concrete of a site, selected to C30 mix ratio was mixed out.

one day the dispatcher shipping A site C25 with car number A # loading, B # car loading B site C35, but A # car is not loaded in time, the dispatcher did not contact the driver, called after a B # car replacement loading, and the host operator did not carefully check the delivery order directly into the order, resulting in B # driver actually loaded A site concrete but sent to B site.

Cause Analysis: The host operator did not check the delivery information carefully and cast the order in advance.

Corrective measures: Before the industrial control system was fully networked, the delivery order was first printed by the dispatcher and then handed over to the host operator in writing, who carefully checked the mixer truck number, project name, construction site, strength level and mix ratio number before mixing, and signed to confirm after mixing before casting the order. At present, through ERP system development, the plan entry production task system and scheduling system and industrial control system are linked together to effectively prevent human errors of host operators and schedulers.

Out-of-control measurement and over-weighing of sand and gravel.

The host operator found that the gravel waiting tank was seriously overloaded with gravel and notified the vehicle that the gravel measurement was wrong.

Cause analysis: gravel scale discharge gate action is not in place, stone metering gate began to release material.

Corrective measures: each shift focus on checking the cylinder, solenoid valve, limiters, the metering scale again regularly self-calibration qualified before production.

When the host operator mixed the nine-layer beam slab of a project, he did not observe the data in the display without the variety of cement but mixed the material to produce, resulting in the scrapping of the material.

Reason analysis: When the technical shift transmits the ratio to the IPC, the host does not have the variety of cement and the cement variety is not matched.

Corrective measures: Ask the software company to modify the program in time, the host operator should pay attention to the weighing situation of each scale, such as the abnormal situation shall not mix.

The concentration of admixture is not uniform, resulting in some concrete slow setting.

One day when pouring a site 14-story wall column, 15-story beam slab concrete, the slump is large, the initial setting time of more than 20 hours phenomenon.

Cause analysis: water reducing agent tank mixer failure, resulting in uneven density of water reducing agent, water reducing agent density in the lower part of the storage tank is large, the setting time is too long.

Corrective measures: The water reducing agent must be stirred evenly at regular intervals, and the material officer should strengthen inspection, and if the mixer stops, it should be reported for repair immediately.

The gravel bin was not fed in time, resulting in poor concrete compatibility.

One day at 5:00 a.m. when supplying concrete to the basement floor bearing platform, two trailer pumps blocked the pipe at the same time.

Cause analysis: gravel feed was not timely, and the material level of gravel bin was low, causing pipe blockage.

Corrective measures: strengthen the air bin and feeding inspection, the feeding loader should be loaded in time, and the air gravel bin must be kept full. The night technical duty, material clerk and dispatcher must check the material feeding situation and gravel quality every two hours.

Large error in small measurement of fly ash and slag powder.

A day fly ash and slag metering scales appear to overload the situation, a single plate metering error greater than 2%

Cause and analysis: the speed of the screw transport machine is too fast, resulting in too large a drop, part of the impulse can not be recorded.

Corrective measures: The equipment department changed the gearbox gear ratio of the screw conveyor, and installed a gate valve between the screw conveyor and the weighing hopper to control the material drop.

Volume difference

A site complained about the difference between the calculated quantity according to the drawing and the actual quantity poured, which exceeded the national standard range.

Reason analysis: Only the actual apparent density was measured when calculating the apparent density of the mix ratio, without multiplying the density coefficient.

Corrective measures: The density of the production mix should be determined by multiplying the apparent density measured by the concrete test by the concrete compacting factor (generally taken as 1.01). Each shift, each machine should sample three cars over the pound to review, to prevent the measurement of runaway.

Transportation and pumping management

The driver sent the wrong site.

A mixer truck driver loaded C25 concrete to another site C30.

Cause Analysis: The driver did not see the site name and construction unit when filling out the vehicle tracking form, and worked sloppily and carelessly.

Corrective measures: Each concrete mixer truck must be clearly marked with strength level, and the driver should fill in the project name and construction unit on the vehicle tracking sheet in time after loading concrete to avoid sending the wrong site. The dispatch center should pay close attention to the GPS satellite positioning system vehicle operation, and correct the problems found in time.

Water accumulation in the drum of driver’s transport vehicle.

A driver loading concrete, the concrete slump in the drum water accumulation is large.

Cause Analysis: The previous vehicle was loaded with pump water and arrived at the site without using it all, but did not unload the remaining water in the drum in time, and did not ask for confirmation on loading.

Corrective measures: The dispatching center must control the mixing truck loaded with pumping water and notify the loading only after receiving the driver’s confirmation of unloading water. After the driver washes the truck and after the rainstorm, the water inside the drum must be unloaded.

Leakage and knotting of material.

One day the driver was delivering concrete to the site, the drum reversed and leaked material, causing large area of road pollution.

Reason analysis: The anti-rotation pin was not inserted according to the process before loading, which caused material leakage.

Corrective measures: The driver of the mixer truck was required to check whether the anti-rotation pin was inserted before loading.

(2) One day, the driver of the mixer truck delivered C40 concrete to a construction site, and did not check whether the concrete was finished after unloading, and did not check the empty weight of the empty truck.

Cause Analysis: The driver of the mixer truck was careless and did not check whether the concrete in the drum was unloaded according to the prescribed process.

Corrective measures: Strengthen the quality awareness training of the mixer truck driver, check whether there is any accumulation of concrete in the drum after unloading, and conduct regular spot checks on the weight of the empty truck back to the plant by the transportation department to avoid mixing leftover materials and concrete of different strength grades.

(3) One day, the reducer screw of the mixer truck broke and the drum could not rotate, resulting in concrete condensation in the drum.

Cause and effect: The speed reducer screw broke and the drum could not rotate.

Corrective measures: The driver of the mixer truck should do the daily vehicle inspection and focus on checking whether the drum reducer screws are tightened or have bends and cracks to avoid the screws from breaking during high-speed rotation, resulting in concrete scrapping.

Pumping burst pipe plugging.

the end of a site pumping concrete, cleaning the concrete pipe, the use of air washing, due to excessive pressure led to the pump pipe wall rupture, resulting in injury to workers.

Cause analysis: the pump truck’s own water tank water is not enough, the use of water washing can not all wash out the concrete, and the use of gas washing, gas washing pressure is three times the pressure of water washing, connected to the tail of the pump pipe wall is thin, can not withstand the pressure of gas washing and rupture, resulting in concrete flying out and injury.

Corrective measures: strictly limit the air washing method, disable the thin old pump pipe. The number of hoses at the outlet of the pump pipe should not exceed two and the bend should not exceed 90 degrees. The water for pump washing is specially filled by mixing truck from the mixing station and delivered to the site for use. Strengthen personnel safety operation training

A site construction of fifteen-story column wall, pumping when the water and mortar of the wet pipe failed to pump out, resulting in the return of all shipped concrete.

Cause analysis: After the construction of the fourteenth floor beam slab, the drag pump cutting ring leaked seriously and could not wash out all the concrete by water washing, leaving some concrete and cement bags in the pipe.

Corrective measures: Regularly check and replace the cutting ring and other perishable accessories. When washing the pipe, the fabric rod should be folded and cleaned separately first, and the cement bag must be checked after washing the pipe, and the pipe must be checked to see if it is smooth before taking over.Concrete quality lessons 35 examples

Site construction management

Pumping mortar pouring into structural parts.

After demoulding of the second floor beam slab at a site, it was found that the bottom of the local beam showed the phenomenon of anti-sand falling off.

Cause analysis: When using clear water and mortar with the same ratio to lubricate the pipe before pumping concrete, the mortar was concentrated to the bottom of the beam, which caused the phenomenon of anti-sand and peeling due to the reduction of strength after mixing water and mortar.

Corrective measures: Before pumping concrete, the cement mortar for lubrication should be dispersed fabric and not concentrated in the same place.

different admixtures mixed.

A project supply with the ratio of polycarboxylic acid water reducing agent, and the site quality inspector did not pay attention to the admixture species in the ratio, when the site slump is small, according to the naphthalene water reducing agent adjustment amount to add naphthalene water reducing agent 8kg, resulting in poor concrete and ease of return.

Reason analysis: The quality inspector on site did not have enough knowledge of new technology, and the polycarboxylic acid water reducing agent and naphthalene water reducing agent were not compatible when mixed, resulting in poor concrete compatibility.

Corrective measures: Strengthen the training of quality inspectors on site by the laboratory, and different admixtures shall not be mixed.

column demoulding peeling.

In November, a plant layer of column construction after the removal of the formwork, found that the column surface concrete off.

Cause analysis: November encountered a sudden drop in temperature, the concrete setting time is long, the demoulding time is too early, resulting in surface delamination.

Corrective measures: often pay attention to the weather forecast, in the event of weather cooling, the admixture formula should be adjusted in the slow-setting ingredients to avoid setting time is too long.

concrete pavement shelling, cracking.

A day of construction of municipal roads flexural 4.5MPa, the side of the cut joints appear several thin cracks. A site construction pavement, the upper part of the concrete shelling, while the lower part of the concrete has not yet hardened.

Cause analysis: the construction of the pavement was completed, and the cut joint was too late to pull the concrete that had hardened on the surface. Concrete setting time is long, and the surface is not covered and watered.

Corrective measures: pay attention to the construction of the smear surface maintenance, the cutting time should be cut when the concrete reaches 25% to 30% of the design strength, and the concrete setting time should not be too long.

The surface layer of the ground is powdered.

A site construction basement surface layer of C20 concrete, the surface of the phenomenon of powdering.

Cause analysis: the construction unit issued a task order for the basement leveling layer, not specified for the garage wear-resistant flooring, the mix ratio of fly ash accounted for 15% of the cementing material, the phenomenon of fly ash floating.

Corrective measures: by the construction party in the production plan and task orders issued when writing clear pouring parts, the ratio of the part should be mixed with fly ash doping less than 8%.

Underwater pile construction plugging, broken pile.

Underwater pile construction, due to long waiting tail number time, poor road conditions caused by trapped, and can not be rescued in time, the pump truck failure is not replaced in time or the reinforcement cage stuck conduit caused by broken pile.

Analysis of the causes: underwater pile construction, must ensure continuous pouring, to prevent concrete stratification and discrete and interruption time is too long.

Corrective measures: Underwater pile construction must prepare sufficient vehicles, pump truck equipment intact to ensure continuous supply, and develop contingency measures to prevent broken piles caused by equipment failure.

Low concrete strength of pile foundation.

During the construction of a site, it was raining heavily when the pump was blocked, causing a large amount of rainwater to flow into the pile, and the core strength of the pile was low, while the strength of the standard maintenance specimen met the requirements.

Cause analysis: the pile is hand-dug pile, the core sample strength of the upper and middle qualified, the lower low, should be piles have water not pumped clean caused by the water-cement ratio is large, the strength is reduced.

Corrective measures: When supplying hand-dug piles, check whether the water at the bottom of the pile is pumped dry, and try to avoid constructing the pile foundation when it rains heavily.

The strength of same-condition specimen failed.

The compressive strength C35 of same-condition specimen of the first floor mezzanine wall column of a project, the strength of standard maintenance specimen meets the requirement, but the same-condition maintenance specimen only reaches 85%.

Reason analysis: the same condition maintenance specimen of winter construction site was sent for inspection when it did not reach 600℃-days, and the strength was low.

Corrective measures: The site is required to strengthen the management of the same condition maintenance specimens, record the daily average temperature and cumulative temperature, and send them for inspection only when they reach the requirements.

Floor slab cracks and maintenance.

A site 18-story beam slab C30 pumped concrete, surface plastic cracks.

Cause analysis: When the air humidity is <100%, the evaporation of water inside the concrete will produce dry shrinkage. After pouring the concrete, the construction staff did not timely carry out the second smear and cover before the initial set and water maintenance, and the outside weather is hot and the wind speed at high level is great, causing the concrete tensile stress is greater than the early tensile strength of concrete and cracks.

Corrective measures: strengthen the construction unit publicity, for the construction team requirements in the concrete before the initial set of the second plastering pressure, eliminate early plastic shrinkage cracks, early watering or spray maintenance to prevent the surface drying shrinkage, and should be track cover plastic film or maintenance blanket. Conditional construction units can build a wind-blocking wall or awning, and do a good job of heat preservation and moisturizing.

Cracks in the external wall.

A project basement exterior wall strength grade C40P10 pumping construction, a month after demolition of the mold multiple vertical penetration cracks, located in the column side of a meter, span parts and opening parts below.

Cause analysis: check the external wall horizontal tendons spacing up to 200mm, long-term exposure to air after mold removal, not timely waterproofing and backfill.

Corrective measures: the wall is easy to appear vertical shrinkage cracks, the horizontal tendon spacing should be less than 150mm, the horizontal tendon spacing within 300 ~ 400mm in the middle and end of the wall should be 50 ~ 100mm, the wall and column connection parts should be inserted into 1500 ~ 2000mm, φ8 ~ 10mm reinforcement bars, inserted into the column 200 ~ 300mm, inserted into the side wall 1200 ~ 1600mm; structure opening parts, variable cross-section parts and entrance/exit parts increase additional reinforcement in appropriate amount. It is not easy to maintain the wall after demoulding, the humidity should be maintained by spraying with flower pipe to reduce the shrinkage of concrete, and waterproofing and backfilling should be done as soon as possible.

Ground beam bearing platform sinking shrinkage.

The ground beam of a site appears along the direction of steel sink shrinkage cracks.

Cause analysis: the construction process of concrete using pumping construction, the slump is large, after pouring sand and gravel and other materials with high aggregate density migrate downward, encounter the reinforcement block, it will produce cracks along the direction of the reinforcement.

Corrective measures: concrete slump should not be too large, after pouring with a vibrating rod pounding, about an hour before the initial set should be pounded again and smoothed.

The bottom of the column rotten root appears honeycomb pockmarked.

A project pouring a layer of column non-pumped C30 concrete, after demoulding in the bottom and corners of the column appeared honeycomb pockmarked.

Cause analysis: the height of the column is more than 4 meters, the reinforcement is denser, and the concrete produces segregation when it is filled from the top.

Corrective measures: require the construction unit to pour the same strength grade of mortar bedding first, and then pour the concrete, with appropriate increase in the sand ratio.