One, preface
Concrete or concrete, is the main material of civil engineering. It is composed of cement, aggregate and water according to a certain proportion, stirring and forming with vibration, under a certain condition maintenance form of artificial stone. Concrete with abundant raw materials, low price, simple production process characteristics, thus their consumption is increasing; at the same time concrete also has a high compressive strength, good durability, strength grade range, the range of application is very wide, not only in various civil engineering use, is the shipbuilding industry, machinery industry, ocean development, Geothermal Engineering, concrete is an important material.
Concrete in ancient western had been used, the Romans used the volcano ash mixed with lime, sand made of natural concrete in ancient times some buildings in use. Natural concrete cohesive strength, firmness and durability, impermeability characteristics, which are widely used in Rome, promoted Rome greatly the development of construction of building, and the arch and dome span continuous breakthrough, created a large number of remains for people indulge in elaborating on in public building. First Century BC, natural building in the arch structure is almost completely rejected stone. Since the eighteen twenties appeared Portland cement, because it made the concrete with the engineering needed for strength and durability, and easily available raw materials, low cost, especially the low energy consumption, so the use is very extensive. At the beginning of twentieth Century, water cement ratio, concrete strength theory laid the theoretical foundation. Later, appeared in succession of lightweight aggregate concrete, concrete and other concrete, all kinds of concrete admixture also started using. Since 60 time, wide application of water-reducing agent, and the emergence of high efficiency water reducing agent and the corresponding concrete; polymer material into the concrete material field, appeared a variety of polymer concrete; fiber was used to disperse reinforcement fiber concrete. Modern testing technology is also increasingly used in concrete materials science research.
1.1types of concrete:
According to the cementitious material is: cyclophos phamide inorganic cementitious material such as concrete, cement concrete, gypsum concrete, Portland concrete, water glass concrete; the organic binder such as concrete, asphalt concrete, polymer concrete. According to the density of points are: 1heavy concrete, bulk density of 2600~ 5500 kg / M 3 or even more; the ordinary concrete, the bulk density of2400 kg / M3 the left and right sides; the light concrete, bulk density of 500 ~1900 kg / M3lightweight aggregate concrete, porous concrete, porous concrete etc.. By using the function mainly has: the structure of concrete, concrete, decorative concrete, waterproof concrete, concrete, concrete, concrete, concrete road, radiation protection concrete. According to the construction process points are: centrifugal concrete, concrete, concrete, vacuum grouting jetting concrete, roller compacted concrete, extruded concrete, pumping concrete, concrete and other. Raw materials of concrete pouring concrete cement, lime, gypsum and other inorganic cementitious material mixing with water to make the concrete mix is plasticity; then through chemical and physical and chemical effects of hardening and strength. Generally speaking, drinking water can meet the requirements of concrete mixing water. Water excess acid, alkali, salt and organic matter will produce harmful influence on concrete. Aggregate is not only filling the role, but also to the bulk density, strength and deformation of concrete properties have important implications.
1.2properties of concrete
1.2.1 and workability of concrete mixture: the most important performance. It 's said mixture consistency, fluidity, plasticity, anti segregation of bleeding property and easy finishing etc.. Determination and said the workbility of method and index of many Chinese, mainly adopts truncated cone cylinder determination slump slump ( mm ) and Vebe instrument determination Vebe time (seconds), as the main indexes of consistency.
1.2.2: after the concrete has hardened, the strength of the most important mechanical performance, refers to the concrete resistance pressure, tensile, bending, shear stress ability. Water cement ratio, cement varieties and dosage, aggregate the variety and the quantity or mixing, molding, curing, have a direct impact on the strength of concrete. The tensile strength of concrete is only its compressive strength of 1/ 8 to1 /13. Improve the concrete tensile strength, compressive strength ratio are important aspects of modification of concrete.
1.2.3, deformation: concrete in load or temperature and humidity under the effect of the deformation may be produced, including elastic deformation, plastic deformation, shrinkage and temperature deformation. Concrete under short time load elastic deformation elastic modulus expressed mainly by. Under long term loading produced by plastic deformation, is called creep. Due to the hydration of cement, cement carbide and water loss caused by volume deformation, called contraction.
1.2.4, durability: in general, the concrete has good durability. But in the cold area, especially in the engineering site and the variation of water level under the condition of water saturation is subject to frequent alternation of freezing and thawing effect, easy damage of concrete. The concrete should have certain durability requirements. For impervious engineering, concrete has a good impermeability and corrosion resistance.
1.3concrete development
Along with the development of modern science and technology and the development of production, a variety of super-long, super, super large concrete structures, as well as in the harsh environment of great concrete structure, such as a building, bridge, tunnel, offshore oil platform, nuclear reactor, toxic and hazardous waste disposal project construction need is increasing in. These concrete engineering construction difficulty, the use of environment, repair difficulties, thus requiring concrete not only construction performance is better, as far as possible in the pouring without defects, more want good durability, long service life.
Concrete construction is the construction of one of the most important types of engineering. No matter the quantities, material, or engineering cost accounts for the proportion of large construction project. May cause quality accident are also more likely to. Therefore, in the actual construction management process, must attach great importance to the quality of concrete engineering, concrete engineering and to understand the possible quality accident. Focuses on the practical work of the concrete engineering may exist in the quality accident analysis, in order to avoid quality accident in construction process, improve the construction of concrete structure project quality theory.
In recent years, with the construction of market reform, the frame structure engineering because of its various aspects performance gradually become construction commonly used in the form of the building structure, so the concrete construction quality directly affects the engineering use function and effect. In order to work in a better control of concrete project quality, must understand the concrete project quality accident may occur, in order to facilitate early prevention.
Two. Concrete construction process and the causes of common quality defects
Common quality accidents of concrete engineering are: insufficient concrete strength; concrete cracks; concrete structure or component surface defect; deformation; concrete appearance perception quality poor quality accident or quality defect.
2.1main causes of insufficient concrete strength
Concrete strength deficiency on the structure of the greatest influence, may cause the structure or component bearing capacity decreased, crack resistance, impermeability, frost resistance and durability reduction, as well as the structural strength and stiffness degradation.
The emergence of insufficient strength is the main reason:
( a ) the material quality: some construction enterprises value benefit, do not pay attention to quality, low price to buy some unqualified materials, cement quality difference; aggregate ( sand, stone ) of poor quality; mixing water quality standards; the admixture of poor quality.
( two) improper concrete mix. Concrete mixture ratio is an important factor determining concrete strength of water cement ratio, which directly affects the strength of concrete, such as water, sand ratio, mass ratio, also will affect the strength of concrete and other properties, resulting in insufficient concrete strength. In the construction of the main performance is: arbitrarily applied mix; use too much water; the insufficient amount of sand, stone cement; metric forbid; admixture with the wrong.
( three): poor concrete construction technology of concrete mixing, mixing uneven; traffic condition is poor; concrete pouring is improper, vibrating compaction, not timely, not uniform, leading to concrete loose, with bubbles or hole, causing local coagulation force is reduced; template leakage is serious; concrete curing deficiency leads to concrete consolidation of water and not fully hydrated, caused the concrete quality overall decline, greatly reduce the strength.
( four) the concrete block management. Not in accordance with the provisions of the production test block, block mold management difference and not in accordance with the standard curing test block, in the same condition curing early dehydration or external damage etc..
2.2classification and causes of concrete crack
Cracks in the concrete structure or component is widespread, many reinforced concrete structure or component failure is from the crack begins. Therefore, we must attach great importance to the analysis of concrete crack. Concrete is a kind of heterogeneous brittle material, consisting of aggregate, cement, sand and retained therein a gas and water. The changes in temperature and humidity conditions, the volume strain hardening. As a result of all sorts of material deformation is inconsistent, mutual constraints arising from the initial stress, resulting in aggregate and cement paste or cement stone between appear invisible to the naked eye fine crack. This kind of fracture distribution is irregular, must not connected, but under the action of load or further to generate a temperature difference, dry shrinkage cases, cracks began to expand, and gradually collude with each other, larger visible cracks ( generally crack width in 0.03-0.05mm ), called macro crack, namely what we say normally crack.
Some of the crack of concrete is very difficult to avoid. So in the current design and construction specification of concrete cracks problems were made certain provisions.
Concrete cracks, deformation and cracks caused by temperature changes: such as, contraction, expansion, uneven subsidence caused by fracture; external loads and cracks caused by improper maintenance; environmental and chemical effects induced by cracks and so on
2.2.1. Dry shrinkage crack
Shrinkage cracks in the concrete curing after the end of a period of time or concrete pouring completed after a week or so. Slurry evaporation will produce dry shrinkage, and the shrinkage is not reversible. Shrinkage cracks of concrete and is mainly due to water evaporation degree to the different deformation with different results: concrete affected by extraneous factors, surface moisture loss too fast, large deformation, internal humidity changes small deformation, large surface shrinkage deformation of concrete produced by internal constraints, large tensile stress crack. Relative humidity is low, the cement slurry drying shrinkage is larger, dry shrinkage crack more easily. Shrinkage cracks for the surface of parallel linear or reticulated shallow thin cracks, multiple of the width of0.05 ~ 0.2mm, large volume of concrete plane more common parts, thin Liang Banzhong along its short to the distribution. Shrinkage cracks usually affect the impermeability of concrete, rebar corrosion effect caused by the durability of the concrete in the water, under the action of the pressure will produce hydraulic fracturing effect of concrete bearing capacity etc. The drying shrinkage of concrete and concrete water cement ratio, cement concrete plastic shrinkage refers to in the condensation surface before, because dehydrate faster and produced contraction. 2plastic shrinkage cracks in dry or windy weather, cracks were wide in the middle, two fine and vary in length, are not mutually coherent state. Short crack the general length of 20~ 30cm, long cracks of up to 2 ~3M,1~ 5mm wide. The main reason is : the concrete in the final setting before almost no strength or intensity is very small, or just the final setting and strength of concrete under high temperature is small, or large wind effects, concrete surface water loss too fast, resulting in larger negative pressure produced in the capillary tube and the volume of concrete is a sharp contraction, while the strength of concrete and unable to resist the contraction, resulting in cracking. The effect of concrete plastic shrinkage cracking of the main factors of ratio of water to cement, the setting time of concrete, temperature, wind speed, relative humidity and so on.
Composition, amount of cement, aggregate, admixture dosage nature and amount of the.
2.2.2settlement cracks
Subsidence crack is due to the structure of soil is not uniform, soft, or backfill untrue or flooding caused by uneven settlement caused by the insufficient stiffness; or because the template, template support spacing is too large or support the loosening and other causes, especially in the winter, template support on permafrost, permafrost thaw after uneven subsidence, resulting in cracks in concrete structure. Such a fracture to the deep or penetrating cracks, its trend and subsidence conditions, generally along and perpendicular to the ground or in the form of30degrees to 45 degrees angle direction, a larger settlement crack, often have certain dislocation, crack width is often associated with settlement directly proportional relationship. Crack width is influenced by the temperature change is small. Foundation deformation stability, subsidence cracks are basically stable.
The temperature cracks of2.2.3.
Temperature cracks occurred in the large volume concrete surface or the larger changes in temperature in concrete structure. After pouring concrete, in the hardening process of cement hydration, generates a large amount of heat of hydration, ( when the dosage of cement in 350 ~550 kg / m3, per cubic meter of concrete will release a 17500~ 27500kJ heat, so that the concrete internal temperature rise up to about 70 DEG C or higher ). Due to the large volume of concrete, a lot of hydration heat accumulation in the concrete internal and not easy to emanate, causing the internal temperature rises sharply, and the concrete surface cooling rapidly, so that the formation of the large temperature difference inside and outside, the larger temperature difference caused by internal and external heat expansion and cold contraction degree is different, make the concrete surface to produce a certain tensile stress ( practice the concrete temperature reached 25 degrees to 26degrees, the concrete will generate roughly around the 10MPa tensile stress). When the stress exceeds the tensile strength of concrete limit, concrete surface cracks will occur, this kind of cracks occurred in concrete construction period. In the construction of concrete when the larger changes in temperature, or concrete subjected to cold wave attack, will lead to the concrete surface temperature decreased dramatically, shrinkage, shrinkage of the concrete surface by internal concrete binding, will generate a lot of tensile stress and cracks, the cracks in the concrete surface is usually only a shallow range produce. 5chemical reaction caused by cracks
Alkali aggregate reaction and the corrosion of reinforcing steel caused by cracks in reinforced concrete structure crack is the most common due to the chemical reaction of cracks caused by.
Concrete mixing will produce some alkaline ions, the ions with some reactive aggregate to produce chemical reaction and the absorption of the surrounding environment of water and the volume increased, resulting in crisp, expansion cracking of concrete. The crack appears generally in concrete structure during use, once appear very difficult to remedy, and should therefore be taken in the construction of effective measures for prevention.