混凝土施工常见问题与防治

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The common problems and control of concrete construction

ZhaoHua

Heilongjiang heihe engineering quality test center

Abstract: the quality of the concrete engineering, in relation to the safety of the structure of the building and structures, this paper analyzes the concrete engineering accident prevention and quality control measures.

Keywords: the concrete construction quality management

Along with the vigorous development of the construction industry, increasing of the high-rise building, especially in the popularization and promotion of the concrete products after application, coagulation and technology in project construction is used more and more. Concrete is easy to be forming,. Carrying ability, speed, shorten the construction period, lower cost and can of continuous operation characteristics, especially for the high-rise buildings and large volume of concrete foundation construction, more shows its superiority. But, in the concrete engineering in practical application. Often appear abnormal of concrete sets, insufficient strength and cracks, and affected to some extent the quality of the project, and shall cause enough attention.

A, improper use additives

Admixture of improper use is the most common kind of accident

1, problem performance

(I) after concrete casting. Local or big minister time not condense sclerosis.

(2) has been pouring the concrete structure surfaces up bulge.

2, the prevention and control of problems

To avoid this kind of quality accident happened, we must pay attention to the following three problems:

(1) the adaptability of the cement and admixture. Admixtures comes into play, you must try to match, know the characteristics: slump and loss of time consuming, setting time and decrease water rate and so on, to determine whether use; For anhydrite of the cement of the agent to do, which is especially important, lest concrete mixing into, happen quickly or slump and loss of overheated.

(2) the admixture every time release, must be in strict accordance with the mixture ratio measurement. Measuring instruments must often check, ensure the sensitivity and accuracy.

(3) the powdery admixtures to keep dry state, prevent moisture agglomerate. Has agglomerate of powdered admixture, drying, should be crushed. 0.6 mm screen after use, in order to avoid including run into in the form of powder particles not in water is inflation, cause the concrete surface speed bulge.

Three, the remedy measures

(1) for the large area not condense sclerosis loose of the structures have to tear down to casting.

(2) because of the slow type water reducing agent use excessive cause concrete long time don't condense sclerosis, can extend its maintenance time, delay dismantling formwork, later the strength of concrete general not affected. When acceptance in concrete structure should be according to the requirements of the code field analysis.

Second, the strength of concrete problems

This kind of accident is reflected in two aspects: one is the strength of concrete itself has to meet the design requirements, and the second is the scene of the concrete test piece of sampling didn't meet the design requirements. The proportion of the latter is larger.

Put an end to this kind of accident, one is to ensure that the quality of raw materials of concrete, cement had better use of the giant or normal manufacturer cement, because of its quality control, management level is high, the stability of the product quality is far higher than the small factory. 2 it is strictly control the concrete mixture ratio, ensure accurate measurement, especially water mix dosage must foot, can't buckle dosage of cement. Influence factors of concrete strength is various, some in the laboratory to the index, in the field construction of who can achieve. Thus, the dosage of cement, must be taken into consideration the actual situation of the scene. Such as the use of bagged cement, cement bags of weight should be nuclear check, in case the cement short measure. Three is to build concrete mixing post responsibility system, be reasonable mixing, guarantee the concrete mixing time. Four is to prevent concrete and early. Five is serious production test blocks. Strengthen the management to try piece, according to the standard requirement of concrete test piece of standard maintenance, with dry structure acceptance of the block will try and the component with condition maintenance.

The national code, to determine whether the strength of concrete is cubic qualified compressive strength of representative value, relates to the method according to the standard production, with sides for 1 5 cm cube specimens, in standard maintenance condition 2 8 day age, using standard test method, the measurement of the compressive strength of concrete, the inspection for acceptance of, the minimum strength value of the strength of concrete requirements depending on the evaluating method.

Standard in total emphasized here "standard method production, standard method maintenance, standard size specimen and standard test method", there is any a is not standard measured the strength of concrete value is not accurate, is not fully represent the strength of concrete.

In the construction of reality, a lot of the concrete projects production, maintenance does not accord with a standard to regulation, to assess the strength of concrete brings certain difficulty.

From sampling method of speaking, rules and regulations from the same set of concrete samples or the same car extraction. In the unloading process appropriate in a quarter of the feed discharge-3/4 range between extraction.

From the maintenance of speaking, as the strength of concrete acceptance, must implement standard maintenance.

Some sites try mould serious deformation, rub Angle, some bolts missing base, some lateral plate deformation cracks. According to the statistics, because not accurate specimen, can make concrete test piece of actual strength reduce 2 above 0%.

Three concrete crack causes and prevention and control

Concrete crack main component is 3 kinds: one kind is the load (including construction and use phase of the static load and dynamic load) cause cracks, one kind is the deformation (including temperature, humidity deformation, the uneven settlement) cause of cracks. Another kind is the construction operation (such as production, parting, maintenance, piled up, transportation, hoisting, etc) cause of cracks. Here is only caused by deformation of the cracks of the concrete engineering is discussed.

1, cause deformation cracks main reasons

(1) temperature changes. The concrete temperature changes volume deformation, expand or contract, this is the intrinsic material physical properties. When this volume change restrictions will be produced when stress, the stress if more than concrete tensile strength, it will cause craze. Such as mass concrete casting, in during hardening cement give off a lot of the hydration heat, internal temperature rising, make the concrete surface and the internal temperature difference is very big, if its internal and surface temperature more than 2 5 ℃. Will crack; Some large size or a long concrete structure, in a few days or more days after construction appearance of a large number of crack; Some structure in roof corners appear inclined cracks. These are is because the temperature change of the crack.

(2) shrinkage crack. Concrete's shrinkage is divided into their own contraction, cement hydration role that cause the volume contraction has nothing to do with the outside world humidity. The plastic shrinkage, namely early in condensate occurred in the process of chemical contraction; Carbonized contraction, namely carbon dioxide and water cement hydrates through chemical reaction, cause shrinkage, drying shrinkage, humidity contraction, that extra water evaporation in concrete, as the temperature and reduce the volume reduce happened contraction, the shrinkage shrinkage of the most. Shrinkage concrete volume decreases, which in itself can also have the stress, when this kind of stress than concrete tensile strength, could cause concrete cracks. For example, some concrete structure volume is not big, but concrete final set, the surface there is large number of irregular crack, some floor after dismantling formwork, found that the board and beam appear horizontal crack junction, some longer structure, in winter or summer cool suddenly drop suddenly storm, these are mostly due to the shrinking crack.

(3) the uneven settlement. If structures based not firm, happened the uneven settlement, lead to structural deformation, also can be in the internal cause stress and cause cracking concrete structure. This is the case in daily work will also happen from time to tome.

(4) chemical reactions can also cause cracking. For example alkali aggregate reaction will cause coagulation and the expansion of volume cracks. Chloride caused by erosion of corrosion of reinforcement can also cause the concrete cracking.

2, the construction measures for the prevention and cure

(1) have to control the quality of materials, especially the sand, the content of the tensile strength of concrete and contraction deformation affected.

(2) the mass concrete, as far as possible in the guarantee under the premise of strength the dosage of cement reduce, can use high quality coal fly ash and slag, fine grinding material such as pumice, replace part of cement, it can reduce costs, and can reduce the hydration heat to reduce shrinkage of concrete, to prevent the cracks are very favorable.

(3) the strict control of concrete water cement ratio. The coagulation and the greater the water cement ratio, the volume contraction is bigger also, especially in the concrete of the first and second shape days, water cement ratio of concrete too, will appear a large number of irregular crack. The best concrete initial set before, with sand plate another rub pressure, to prevent the early concrete shrinkage crack.

(4) strengthen the maintenance, make the concrete surface keep wet state, constantly supply evaporated water. Such already can prevent concrete's shrinkage crack, and can accelerate the concrete hydration, improving the concrete tensile strength.

(5) to strengthen the maintenance of the mass concrete insulation. Is to reduce the temperature cracks the most effective measures. Mass concrete curing of heat preservation, the most common is the sack with plastic film joint use, with the sack for thermal insulation in plastic to protect wet, the thermal insulation layer should be demolished according to temperature measurement depends, to confirm below 2 5 ℃ temperature difference inside and outside when can dismantle, and should also layered gradually removed, should try to avoid cooling too fast and cause because concrete cracking.

(6) to important concrete engineering should control cement, and admixture and alkali amount of mixed with material, at the same time, to the aggregate should carry on the alkali activity determination, thus fundamentally avoid alkali aggregate reaction occurs.

Ensuring the quality of projects. Is a technical problem and a management problem, we must have the codes and specifications for the standard, strict operation, and scientific management.