The tensile strength of high-strength concrete is divided into three types: axial tensile strength, splitting tensile strength and bending strength. Very little is done because the axial tension test is more complicated; The specimens with splitting strength are cubes in China, and cylinders are often used in other countries. The bending test often uses a rectangular cross-section simply supported beam loaded at three points, the size of the beam is 150mm*150mm, and the span is 3 times that of the beam, and other countries also use the beam with a rectangular section of 102mm*102mm, and the bending strength has a great relationship with the cross-sectional size and maintenance conditions.
The tensile strength of high-strength concrete increases with the increase of compressive strength, but their ratios decrease with the increase of compressive strength, but the ratio relationship between the three tensile strengths has no obvious relationship with the concrete strength.
Below we give three empirical formulas for tensile strength for readers' reference.
1) Cleaved tensile strength
The empirical formula of the splitting strength f(t,s) of high-strength concrete given by the China Academy of Building Research.
f(t,s)=0.3f(cu)^(2/3);
Empirical formula for the split strength of high-strength concrete recommended by the Eurocode CEB FIP.
f(t,s)=0.3(f1)(c)^(2/3);
Empirical formulas for the splitting strength of high-strength concrete recommended by the ACI High Strength Concrete Committee of the United States;
f(t,s)=0.6(f1c)^(1/2);
There are many factors that affect the durability of structures, among which concrete carbonization is an important factor. Typically, early concrete is highly alkaline and its pH is generally greater than 125. The steel bar embedded in such a high alkaline environment is prone to passivation, which makes a passivation film on the surface of the steel bar, which can prevent the corrosion of the steel bar in the concrete. However, when carbon dioxide and water vapor enter the concrete from the surface of the concrete through the pores, and neutralize with the alkaline substances in the concrete, the pH value of the concrete will decrease. When the concrete is completely carbonized, the pH <9 appears, in this environment, the passivation film on the surface of the reinforcement embedded in the concrete is gradually destroyed, and when other conditions are met, the reinforcement will corrode. The corrosion of the steel bar will lead to a series of adverse consequences such as cracking of the concrete protective layer, the destruction of the bond between the steel bar and the concrete, the reduction of the stress section of the steel bar, and the reduction of the durability of the structure.
Therefore, it is of great significance to analyze the carbonization law of concrete, study the change of concrete chemical composition caused by carbonization and the state of internal carbonization of concrete, and study the durability of concrete structures.
1) Concrete carbonization mechanism
The basic composition of concrete is cement, water, sand and gravel, in which the cement reacts with water to produce a hydrate that has its own strength (called cement stone), and at the same time bonds the bulk sand and gravel into a hard whole. During the hardening of concrete, about one-third of the cement amount will be used to generate calcium hydroxide [Ca(OH)2], which crystallizes in the hardened cement slurry or exists in the form of a saturated aqueous solution in its voids. Because the saturated aqueous solution of calcium hydroxide has a pH of 126 alkaline substances, so fresh concrete is alkaline [2,3].
However, the carbon dioxide in the atmosphere is constantly diffusing to the interior of the concrete, interacting with the calcium hydroxide in the concrete to form carbonate or other substances, so that the original strong alkalinity of the cement stone is reduced, and the pH value drops to 85 or so, this phenomenon is called concrete carbonization. This is the most common form of concrete neutralization.
The main chemical reaction formula of concrete carbonization is [2,4].
co2+h2o→h2co3
ca(oh)2+h2co3→caco3+2h2o
Factors influencing the carbonization of concrete
The carbonization of concrete is a complex physicochemical process accompanied by the diffusion of CO2 gas into the concrete, the water dissolved in the pores of the concrete, and then carbonized with various hydration products. Studies have shown that the carbonization rate of concrete depends on the diffusion rate of CO2 gas and the reactivity of CO2 with concrete components. The diffusion rate of CO2 gas is affected by the structural compactness of the concrete itself, the concentration of CO2 gas, the ambient humidity, and the moisture content of the specimen. Therefore, the carbonization reaction is affected by the composition of the concrete inner pore solution, the morphology of the hydration products and other factors. These influencing factors can be boiled down to internal factors related to the concrete itself and external factors related to the environment. For Service Structure Five, since its internal factors have been identified, the main factors that affect its carbonization rate are external factors such as CO2 concentration, ambient temperature and humidity.