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1¡¢ Characteristics of cement concrete
Cement concrete is neither as strong as steel nor as tough as steel. Why is it the most widely used engineering material? The main reasons are as follows:
(1) Concrete has very good water resistance
Unlike wood and ordinary steel, concrete can withstand the action of water without serious deterioration, making it an ideal material for the construction of structures for controlling, storing and transporting water. The use of concrete in dams, channels, water pipes and reservoirs is almost everywhere in the world. The resistance of concrete to some corrosive water makes its use extended to many harmful industries and natural environment. Structural elements exposed to humid conditions, such as piles, foundations, floors, beams, columns, roofs, external walls and pavements, are often made of concrete or reinforced concrete. In the design of reinforced concrete, it is assumed that reinforcement and concrete can bear the action of force together. Pre stressed concrete is a kind of steel bar or steel wire bundle in tensioned concrete. The introduction of a certain size or a certain distribution of pre stress can offset the tensile stress produced by the applied load to a certain extent. To be sure, a large amount of concrete is used to make reinforced concrete or prestressed concrete components.
(2) Concrete is easy to make structural members of various sizes
This is because the fresh concrete has good plasticity and consistency, which can be easily filled in the pre fabricated formwork. After a few hours, when the concrete has set and hardened, the formwork can be removed for reuse.
(3) Concrete is the easiest and cheapest material in engineering
The main ingredients in concrete - Portland cement and aggregate - are relatively cheap and easily available in most parts of the world. Compared with most other engineering materials, the energy consumption of concrete production is much less, and a large amount of industrial waste can be used as a substitute for cementitious materials or aggregates in concrete. Therefore, in the future, considering the protection of energy and resources, concrete as a structural material has its unique irreplaceable advantages.
When choosing a material, the professional judgment should not only consider the strength, dimensional stability and elastic properties of the material, but also consider the durability of the material. Because durability is of great economic significance to the maintenance and renewal cost of structures. Durability is defined as the service life of a material under given environmental conditions. Generally, dense or impermeable concrete has long-term durability. The 2700 age concrete in a reservoir on Rhode Island, Greece, and many hydraulic concrete structures built by the Romans are excellent examples of the durability of concrete in humid environments. The impermeability of concrete with poor durability depends not only on its mix proportion, tamping degree and curing, but also on the microcracks caused by normal temperature and humidity cycles. Generally speaking, there is a close relationship between the strength and durability of concrete. Progress in the field of materials mainly lies in the recognition that various properties of materials are determined by their internal structure; in other words, material properties can be modified by appropriately changing the structure or composition of materials. Although concrete is the most widely used structural material, its internal structure is uneven and highly complex. The relationship between structure and performance of concrete has not been well elucidated.
Concrete is different from other engineering materials, its structure is not stable (that is, its structure is not inherent characteristics of materials). This is because the two components of the structure, namely hardened cement paste and transition zone, change with time, ambient humidity and temperature. In theory, the relationship model between structure and performance is generally helpful to predict the behavior of engineering materials, but it is almost useless for concrete. The main reason lies in the high inhomogeneity of concrete structure and its dynamic characteristics. Knowledge of the important characteristics of the concrete component structure is still essential for understanding and controlling the properties of composite materials. The aggregate phase plays an important role in the bulk density, elastic modulus and dimensional stability of concrete. These concrete properties depend largely on the bulk density and strength of the aggregate, but also on the physical properties rather than the chemical properties of the aggregate structure. In other words, the chemical or mineral composition of the aggregate phase is usually less important than physical properties such as volume, size and pore distribution.
In addition to porosity, the shape and structure of coarse orthopedics also affect the properties of concrete. Generally, natural gravel is round with smooth surface structure. The surface of broken rock has a rough structure; the roughness depends on the rock type and the crushing equipment selected. Crushed aggregate can contain a considerable number of flat and long particles, which have adverse effects on many properties of concrete. Pumice lightweight aggregate with high honeycomb shape also has polygonal and rough structure, but ceramsite or shale lightweight aggregate usually has round and smooth structure. The degree of non-uniformity and non isotropy of concrete materials depends on the uniformity of raw materials, cement aggregate ratio and water cement ratio, as well as the construction operation technology of mixing, pouring, vibrating and curing. In addition, under the action of early hardening stress, the microcracks formed inside the concrete have certain directionality, which will have different responses to different stress states, development and deformation of micro cracks in the later stage of hardening, which is the non isotropic nature of concrete after stress.
2¡¢ Complex micro internal stress (deformation) state
If a block of concrete is enlarged in proportion, it can be regarded as a nonlinear, three-dimensional solid structure composed of coarse aggregate and hardened cement mortar. Before and after loading, there are very complex micro stress (strain) fields. This is the main reason for the great change of concrete material properties and the dispersion of performance indexes. In the course of concrete solidification, cement hydration produces gel, which makes cement mortar gradually thickened and hardened, and cemented together with coarse aggregate.
At the same time, the shrinkage of cement mortar is much larger than that of coarse aggregate. This shrinkage difference causes the coarse aggregate to be compressed and the mortar to be tensioned. Although the resultant stress on any section is zero, the local shrinkage stress may be large enough to form micro cracks at the interface of coarse aggregate. Similarly, due to the difference of the coefficient of linear expansion between the coarse orthopedic cement paste and the hardened cement slurry, even if their temperature changes are the same, the uneven three-dimensional stress field will be produced due to the inconsistent deformation and mutual restraint. What's more, concrete is a kind of warm material (thermal conductivity a = (0.81-1.86) w / m.k). Due to the influence of factors such as hydration heat, environmental temperature change or accident (fire) temperature rise, a large temperature difference will be formed between the surface and interior of concrete, and the internal micro temperature stress (stress) field is more complex and changeable. When the structure bears the action of external force, even if the macro stress of local concrete is even, there will be uneven micro stress field due to the random arrangement of coarse aggregate and the irregular shape of cement mortar, the difference of elastic (or deformation) modulus, tensile and compressive strength of the two, and the contact conditions around the coarse aggregate The stress distribution on the surface is not uniform.
As for the pores and cracks in the concrete, the local stress concentration area near the tip of the concrete changes greatly and the stress value is high, and it enters the plastic stage (refer to fracture mechanics theory for reference). All of these show that it is very complicated and uncertain to analyze concrete from the microscopic point of view. The three-dimensional stress (deformation) state has a great influence on the cracking, crack development, deformation, ultimate strength and failure mode of concrete.
3¡¢ Multivariate composition of deformation
Most of the concrete's elastic modulus is within the range of the concrete's elastic modulus In other words, the deformation is directly proportional to the stress value, and the deformation can be fully recovered after unloading, leaving no residual strain. The viscous flow of cement gel -- the gel formed by hydration of cement is not an absolutely fixed material (though its deformation is very small) for decades. Under the action of stress, in addition to the immediate deformation, there will also be a slow but gradually convergent viscous flow with the extension of time, which makes the deformation of concrete continuously increase, thus forming plastic deformation. When the stress is removed, the immediate recovery deformation is limited. Although the subsequent recovery deformation continues, there is still a large residual deformation. The greater the stress of concrete, the more plastic deformation and residual deformation increase.
Formation and propagation of microcracks -- under the action of tensile stress, microcracks are formed in the vertical direction of stress, and rapidly expand, which greatly increases the tensile strain. Under the action of compressive stress, longitudinal cracks are formed in the direction parallel to the stress direction, passing through the aggregate interface and cement mortar, weakening the connection between adjacent parts; the local concentrated stress at the crack end causes damage to the cement mortar. A weak area is formed, which makes the longitudinal deformation increase a lot. After the peak stress, although the stress of concrete decreases, the deformation will continue to increase. After unloading, the deformation can not be recovered. For different materials and components of concrete, in different stress stages, the proportion of deformation of these three parts has a great change.
Generally speaking, when the stress level is low, the elastic deformation of aggregate takes up the main component. With the increase of stress, the viscous flow deformation of cement gel gradually increases. When the ultimate strength of concrete is close to the value, the deformation of crack has obvious effect, but its deformation value is larger than that of the other two parts, and the descending section after peak strength becomes the main body of deformation. During the unloading process, the elastic deformation of aggregate can be restored completely, while the viscous flow deformation of cement gel presents hysteresis of strain recovery. The residual deformation of concrete after unloading is composed of crack deformation and viscous flow deformation. In addition, when the concrete begins to bear stress, the aggregate and cement mortar coordinate / share the stress and deformation. If the stress remains unchanged, the total deformation of the concrete will increase as the viscous flow deformation of the cement gel increases with the extension of time, and the stress will be redistributed between aggregate and cement mortar.
4¡¢ Influence of stress state and path on mechanical properties
The ratio of tensile strength to compressive strength of concrete is about 1; 10, and the corresponding peak strain ratio is about 1:20. This is in sharp contrast to the fact that the tension, pressure and deformation of structural materials such as steel and wood are nearly equal. Due to the great difference of mechanical properties under the basic tension and compression state, the strength, deformation and failure characteristics of concrete under multiaxial stress state vary greatly with the ratio of tension, compression and stress of principal stress. As for the more complex stress state, such as uneven stress (with strain gradient), repeated load (stress), boundary constraint, different ways to reach the same stress value, etc., due to the difference of deformation composition, the directionality of internal micro cracks and the accumulation of damage, different mechanical properties of concrete are formed, and some new concrete properties are brought about characteristic.
5¡¢ Influence of time and environmental conditions on mechanical properties
The hydration of cement and water develops from the surface of cement particles to the interior, and the concrete matures gradually. This process will continue for decades without ending. During this period, the environmental conditions around the concrete not only affect the degree of cement hydration (i.e. the maturity of concrete), but also have a variety of physical and chemical effects with concrete materials, which have a variety of favorable or adverse effects on the mechanical properties of concrete. As the age of concrete increases, the bond strength of cement gel increases and the mobility decreases, thus improving the strength and modulus of elasticity of concrete. On the other hand, under the long-term stress of concrete, the deformation of cement gel will increase with time (creep) and the long-term strength will decrease due to the continuous viscous flow of cement gel and the development of internal microfracture. The temperature change of the surrounding environment makes the internal temperature field of concrete uneven, which affects the hydration speed of cement, produces temperature deformation and internal stress, and even cracks. The humidity of the environment affects the migration speed and quantity of water in concrete, the distribution of water content, shrinkage deformation and internal stress state, as well as the appearance of micro cracks. These will make the strength and deformation of concrete change accordingly.
The CO2 gas in the atmosphere makes the surface layer of concrete carbonize, and the carbonation layer gradually thickens with time; some chemical media in the environment have corrosive effect on concrete, which all affect the micro crack propagation, strength and durability of concrete. The characteristics of concrete materials determine the complexity, variability and dispersion of its mechanical properties. In addition, the differences in the properties and mix proportions of concrete raw materials make it difficult to study the mechanical properties of concrete from the micro quantitative theoretical analysis. From the point of view of structural engineering, the concrete specimens with the size of ¡Ý 70mm or 3-4 times of coarse aggregate size are usually taken as the unit, which is regarded as continuous and isotropic homogeneous material, and its performance is stable in a short time (hour level). The average strength, deformation value and macroscopic failure mode are taken as the research standards. The mechanical properties of the specimens with the same size are measured After summarizing and analyzing, the strength criterion and constitutive relation are established, which can be applied to practical engineering with sufficient accuracy.
6¡¢ Basic situation of polymer modified cement concrete
Incorporation of polymer emulsion into new concrete can significantly improve the performance of concrete, which is called polymer modified cement concrete. Its English abbreviation is PMC (polywer modified concrete). The first patent of polymer emulsion modified cement was published in Lefebure in 1924. Since then, the research and development of various polymer emulsion modified mortars and concrete have been actively carried out in many countries. In recent years, polymer modified mortar and modified concrete have been widely used as building materials because of their high strength, good bending, bonding, waterproof and durability.
There are many kinds of polymers used in cement concrete modifiers at home and abroad, basically divided into three types, namely polymer emulsion, water soluble polymer and liquid resin. Most of the polymer emulsion used for modified cement materials has been specially used as additives for cement, such as polyvinyl acetate (PVAC), styrene butadiene latex (SBR), polyvinylidene chloride two (PVDC), acrylic acid and modified acrylic acid. The polymer should be in the form of powder or water dispersion. In the dispersed phase, the particles of solid polymer should be dispersed, uniform and stable. Therefore, appropriate emulsifier and stabilizer (protective colloid) should be used.
In 1930s, the first polymer used was natural rubber slurry, followed by polyvinyl acetate emulsion. After 70s, styrene butadiene latex, neoprene latex and acrylate copolymer emulsion were widely used. Polymers used as modified cement materials in the past few decades can be partially replaced or replaced by polymer emulsions as cement modifiers. After modification, the alkalinity of most mortar is the same as that of ordinary cement mortar, which can protect steel bar, and the thickness of mortar cover is only 12-15mm.
Polymer latex has the following characteristics:
(1) As a water reducing plasticizing penalty, the mortar has good workability and small shrinkage. It can reduce the water cement ratio;
(2) It can improve the bonding ability between mortar and old concrete;
(3) Moreover, it can improve the resistance to water and carbon dioxide;
(4) To some extent, it can be used as curing agent;
(5) Increase the flexural and tensile strength of mortar.
The type of monomer, emulsifier and protective colloid make the polymer dispersion have different characteristics. The influence of additives and different types of polymers. When polymer is selected as the modifier of concrete or mortar, many requirements must be met
(1) Improve workability and flexibility;
(2) Increase mechanical strength, especially bending strength, adhesive strength and elongation at break;
(3) Reduce contraction;
(4) Improve the anti-wear performance;
(5) Improve the resistance to chemical media, especially salt, water and oil;
(6) Improve durability.
The following problems should be paid attention to in the preparation of polymer dispersion system
(1) It has no adverse effect on hydration and cementation of cement
(2) It will not be hydrolyzed or destroyed in the alkaline medium of cement;
(3) It has no corrosion effect on reinforcement.
Considering the structural composition of polymer, PVDC latex is not recommended to be used as repair mortar for reinforced concrete, because free chloride may be released over time. PVAc copolymer latex is widely used in construction engineering as an adhesive, but it is better not to use it in wet conditions, because in wet alkaline conditions, the polymer may separate and reduce the performance of mortar. In order to make the polymer emulsion have the chemical stability of a large number of multivalent metals in the hydration products of cement and the mechanical stability of shearing force produced by stirring, and meet the requirements of construction and workability, it is necessary to add stabilizer, defoamer and other auxiliary materials, and add auxiliary materials to the emulsion product in advance.
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