Concrete Crack Repair Technology System (II) - An analysis of the six types of shrinkage and types and hazards of cracks produced by concrete

Heat expansion and cold contraction are the inherent properties of all substances, and concrete is no exception. This article will give you an analysis of the six types of cracks in concrete, and the types of cracks and hazards.
concrete shrinkage can be divided into the following six types:
1
driding shrinkage
in unsaturated air, the internal water of the concrete after the loss of water to produce contraction, the contraction process as shown in Figure 1

.
the concrete drying shrinkage process
2
since the contraction
hydration reaction, the average density of the gel material production increased, resulting in a reduction in the macro volume of concrete. The process can be divided into three stages: the first stage, self-shrinking is equal to chemical contraction, which is directly related to the degree of hydration; Self-shrinking cracks are independent of changes in external humidity, and the lower the water ash ratio, the greater the self-shrinking.
the development process of self-shrinking
3
plastic shrinkage
cement concrete within a few hours of its pouring, when the water evaporation rate of its surface is greater than the internal.
When the water migrates to the surface, a complex capillary system will be formed between particles close to the surface, as shown in Figures 3 and 4, and capillary pressure Pc is generated on the capillary wall, causing shrinkage between the capillary walls or between the two particles separated by the liquid membrane Fa, causing the two particles to be compressed close, at which point the concrete has not yet formed strength, when the shrinking driving force is greater than the particle pull strength, plastic contraction occurs mainly in the early stages.
pc-σ (1/R1-1/R2)
σ: surface pressure of the liquid
(N/m);
stress between two particles caused by capillary pressure
the process of plastic shrinkage of concrete
4
chemical contraction
the chemical contraction of concrete is the volume contraction of the cement hardening process. The mineral components of cement are tricalculum silicate: 3CaO.SiO2, calcium 2CaO.SiO2, and calcium aluminum acid: 3CaO.Al2O3. When they meet water, they catch the water molecules chelating into their crystalline water, the crystals continue to develop, forming strength, releasing heat while the volume shrinks, a process that is chemically changed and produces a contraction called chemical contraction.
tricalcification reaction equation for silicate:
3CaO SiO2, nH2O, xCaO. SiO2 yH2O plus (3-x) Ca (OH) 2
dicalculphate hydration reaction equation:
2CaO SiO2, nH2O, xCaO. SiO2 yH2O plus (2-x) Ca (OH) 2
5
Temperature Shrinking
When the concrete temperature increases, the concrete shrinks due to the inherent properties of the material itself (line expansion coefficient is approximately 1×10-5/C), while the stress of the capillary tube of the concrete also causes additional expansion of the concrete, which is caused by the concrete contraction when the concrete fluid migrates. The thicker the concrete structure, the slower its central temperature drops, causing a large contraction, which causes the temperature contraction stress to exceed the concrete's limit pull strength, causing the temperature to shrink and crack.
The process of increasing the temperature of concrete
the change of the center temperature increase with the size of the section of the structure
6
seed shrinkage
the subsiding contraction crack occurs about half an hour after the concrete is poured and stops when hardened. The reason for its formation is that the slurry after pouring uneven subsidion, coarse bone sinking, cement slurry floating, when the subsidion is inhibited (such as rebar or pre-buried parts block), so that the concrete is cut and cracked. In addition, the float layer formed on the surface can also be cracked by water. This kind of crack appears in the concrete surface, and distributes along the main rib or hoop length direction, the middle width is narrow at both ends, is a common early crack, especially in pumping construction is more common.
types and hazards of concrete cracks:
1
Types of concrete cracks
according to the position of the cracks
, cracks can be divided into:
through the cracks, deep cracks and surface cracks
three kinds.
of the types of cracks
according to the cause
, can be divided into:
plastic shrink cracks, subsiding shrink cracks and temperature cracks
.
2
1.
damage to
Lack of protection of rebar:
concrete produces cracks, the inside of the steel bar exposed, easy to rust, which affects the construction quality of the entire

; Reduce the overall strength of the building:
the presence of cracks, cut off the overall continuity of the concrete structure, damage the overall strength of the

structure; Easy to cause leakage:
, especially in the underground space part, concrete cracks can easily cause water seepage. The four elements of water seepage are water, pressure, path and long-term effect. Cracks provide a path.