The shrinkage is large and the tensile strength is lower than that of concrete. When the shrinkage causes the tensile stress inside the concrete to exceed the tensile strength of the concrete, the concrete is cracked. It is generally considered that the fracture is the biggest drawback of concrete, which will lead to moisture, such as the invasion of harmful media, to accelerate the corrosion of steel bars and reduce the durability, and cause leakage is the phenomenon of structural use of the function of the reduction or loss. In order to solve the shortcomings of shrinkage of concrete, at home and abroad has developed a alunite expansive agent, anhydrous calcium sulfate expansion agent, lime expansion agent and calcium aluminate expansion agent. Concrete expansion agent is developed on the basis of the expansion of cement, so that after the hardening of the concrete to produce a certain volume of expansion, reduce the concrete hardening of the early shrinkage of concrete cracks. Long-term studies suggest that sulfur-aluminum compounds, calcium oxide and magnesium oxide can act as chemical constituents of expanded cement or concrete, but from controllable expansion and chemical stability, from the perspective of compensatory shrinkage and objectivity to produce greater self-stress In terms of domestic and foreign advocates calcium alum as the main hydration products of sulfuric aluminate as expansion agent. In the recommended dosage (6% -8%), 3d and 7d age, the mortar strength of the blending agent is slightly lower than that of the reference concrete, and the smoothness is too low The risk of early cracking, on the other hand delay the possible cracking time, so that the concrete strength to further improve, to avoid or reduce the purpose of concrete cracking. The addition of expansion agents gives us a better understanding of high performance concrete, from high-strength concrete to high-performance concrete (HPC) today. band knife cutting machine for sale inexpensive of pavement cutting machine asphalt mixing machinery suppliers how does a pipe expansion joint work