Through field visits to dozens of commercial concrete mixing plant, we deeply feel that there are many hidden problems of commercial concrete, which will cause great impact once problems occur.
Although many research institutions have done a lot of commercial concrete ratio, performance tests, but the most urgent problem is how to solve the actual situation of commercial concrete stations and laboratory conditions do not match the serious problem. According to the situation examined, there are four main problems in commercial concrete stations.
The sand and gravel mixed pile, the material bottom pollution is serious, the mud content exceeds the standard
Many of the commercial concrete mixing station sand and gravel yard directly connected with the land, no concrete bedding, once it rains, all the sand and mud. Some commercial concrete mixing plants have concrete bedding, but there is no vehicle washing device at the entrance, and all vehicles entering the site do not have the procedures and measures to wash their wheels. Hundreds of vehicles enter and leave the concrete station every day, and the mud carried by them causes the mud content in the sand and gravel to exceed the standard. At the same time, the material bottom of the stockpile is not cleaned every day, and hundreds of vehicles crush it back and forth to make it a mixture of soil and stone, and when the loader passes the pile, it passes the soil and stone into the big stone pile.
Sand and gravel mixed pile phenomenon is serious. Many concrete stations have mixed piles of sand and gravel, some even have no retaining wall; some piles are higher than the retaining wall, resulting in mixed piles; some have serious mixed piles when unloading sand and gravel outside the retaining wall. Into the big hopper, due to the short barrier plate makes the two sides of the material mixed pile; or loaders repeatedly loaded with different materials, buckets often do not unload the net, resulting in another sand and gravel mixed pile, some hoppers have three kinds of material.
The cement batch is unclear, the logo is unclear, not waiting for the retest risk use
The cement tank of commercial concrete mixing station can only be loaded with one day’s consumption, so the incoming cement is put into use without waiting for 3 days to pass the retest report, and even some do not even wait for 1 day to retest the stability, which is completely at risk. Some commercial mixing stations do not even have the cement manufacturer’s 3-day report, but even more at risk.
A large number of bulk cement in the case of not clearing the tank has been loaded, so that the tank of different production dates, varieties, grades, manufacturers of cement has become a hodgepodge. Some people say "are a plant, a variety, a grade of cement, should not be very different", if this understanding is established, whether each plant as long as each cement to do a test can be done once and for all? The work of the national laboratory can be greatly reduced? Perhaps because of this, many cement tanks without identification, and the correct practice is the identification plate should be manufacturers, brands, varieties, grades, production dates, incoming date, on behalf of the batch, the inspection status of the eight identification.
Do not test the moisture content of sand and gravel, the impact is serious
About the moisture content test, many concrete stations are not testing but estimation. During the inspection, we saw some commercial concrete mixing plant open-air sand and gravel quarries, stone moisture content is 0.1% all year round, while some marked as 3% moisture content of the sand in the hopper even flowing outward.
How to measure the accurate moisture content, there must be corresponding measures. The sand in the upper part of the sand pile is relatively dry due to evaporation and sinking, and the lower part of the sand is often supersaturated due to the sinking of the upper water, while the loader is loaded from the bottom of the sand pile and poured into the mixer, resulting in a high water ratio easily, resulting in a large increase in the water-cement ratio.
If the water-cement ratio from 0.5 to 0.65, other materials remain unchanged, the concrete 28 days standard strength can only reach 69%. To more accurately measure the actual moisture content of sand and gravel, one is measured in the mixer on the mouth, one will be made of sand and gravel quarry backward drainage slope, to ensure that the front of the upper and lower parts of the sand and gravel moisture content close.
Must sign a technical contract, while doing process control analysis
Commercial concrete mixing plant and the construction unit should not only sign an economic contract, but also sign a technical contract. For example, the bulk concrete must be slow hydration reaction cement, then the contract should be marked to prohibit the commercial concrete mixing plant to use ordinary cement, high strength early strength cement, otherwise it will seriously affect the quality of the project construction.
Another example is that many projects do not allow the construction of cold joints, it is necessary to ensure the continuous supply of concrete, so the contract should be clear how long every (plus or minus how many minutes) must supply a truckload of concrete, in order to ensure the timely coverage of the last layer of concrete and pounding in the contract to determine the initial setting time of concrete.
In order to ensure the proper supply of concrete, the first truckload of concrete must bring all the information about the concrete mix ratio and material tests (except for the 28-day test results), which are verified to be correct on site before being poured. These copies of information is an important basis for each batch of concrete, to be included in each concrete certificate under the bound book, and should not just give the site a 28-day test certificate of conformity.
If it is a layer of concrete, such as 28-day test pressure report out, the site has often been constructed to the sixth and seventh floors, if a layer of concrete failed, by no means only one floor demolition, but to demolish from six or seven floors, this loss whether the commercial mixing station, construction units, or developers can not afford to bear.
There must be process control and analysis. In order to avoid this big loss, the commercial mixing station and construction units should be based on the accumulated process control information, in accordance with the "early strength derivation specification" in a timely manner to find the concrete strength growth malpractice, early processing, so as not to cause huge losses.
In order to have a clearer understanding of the quality of commercial concrete, there should be a detailed computer analysis of the whole process of concrete mixing, transportation, arrival, waiting, unloading, conveying, and pouring. The specific practice is to record the 4 moments on the small ticket of each truck of concrete (out of the station, arrival, unloading, pumping) and input it into the computer, it will be automatically analyzed, so that no matter in which time period the problem occurs, there is evidence, in addition to measures to deal with, and can trace the responsibility of the parties concerned, in order to prompt them to improve their work.
The technical information to be provided by the commercial concrete mixing plant includes: ready-mixed concrete certificate; concrete alkali content calculation (underground part only); concrete ratio application form; concrete opening identification; cement alkali content test report (underground part only); cement plant record certificate; cement 3d ex-factory quality certificate; cement 28-day ex-factory quality certificate (post-completion); cement quick test re-test report; cement 3d retest report; cement 28-day retest report (later supplement); sand alkali activity test report (underground part only); sand test report; stone alkali activity test report (underground part only)
Stone test report; concrete admixture alkali content test report (underground part only); admixture test report; concrete admixture certificate; concrete admixture alkali content test report (underground part only); concrete admixture record certificate; admixture use and performance manual; admixture factory certificate or test report (comparison table with national standard); concrete admixture retest report; concrete initial setting Test report; concrete admixture effect test report; concrete mixing plant concrete test block 28 days compressive test report and concrete 3d strength report.
The information required for each concrete pour is sent to the site at the same time with the first truck of concrete for verification, in triplicate. Among them, the staff receiving the small ticket analysis form of commercial concrete must carefully check: the name of the commercial concrete mixing plant manufacturer; the strength grade of commercial concrete and the comparison of technical contract and technical content; the receiving unit; the date and the time of leaving the station at the same time and carefully record the time of arrival, the moment of unloading and the moment of completion of pouring, if it is found that the time exceeds the difference too much, the person receiving the ticket should immediately report to the technical person in charge. If the time difference is too large, the ticket collector should report to the technical person in charge immediately.