Concrete Keeps Cracking, And The Cause Is Finally Found

Causes of ordinary concrete cracks

Load-induced cracks

The cracks produced by conventional static and dynamic loads and secondary stresses on concrete are called load cracks, which are mainly divided into direct stress cracks and secondary stress cracks. Direct stress cracks refer to the cracks caused by direct stress caused by external loads, while secondary stress cracks refer to the cracks caused by secondary stress caused by external loads.

The characteristics of load cracks are different depending on different loads. These cracks are usually found in the tensile zone, shear zone or severe vibration area. However, it must be pointed out that the appearance of peeling or short cracks along the direction of pressure in the pressurized area is often a sign that the structure has reached its load-bearing capacity limit and a precursor of structural damage, which is often caused by the small cross-sectional size.

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Cracks caused by temperature changes

Concrete has the nature of thermal expansion and contraction, when the external environment or the internal temperature of the structure changes, the concrete will be deformed, if the deformation is restricted, stress will be generated in the structure, when the stress exceeds the tensile strength of concrete, temperature cracks. In some large-span bridges, the temperature stress can reach or even exceed the live load stress. The main feature that distinguishes temperature cracks from other cracks is that they will expand or close with temperature changes.

 

Cracks caused by shrinkage

In practice, cracks in concrete caused by shrinkage are the most common. Among the types of concrete shrinkage, plastic shrinkage and shrinkage (dry shrinkage) are the main reasons for the occurrence of concrete volume deformation, in addition to autogenous shrinkage and charring shrinkage.

Plastic shrinkage. Occurred in the construction process, about 4 to 5 hours after the concrete pouring, when the cement hydration reaction is intense, the gradual formation of molecular chains, water secretion and rapid evaporation of water, water loss of concrete contraction, while the aggregate sinks by its own weight, so when the concrete has not hardened, known as plastic shrinkage. Plastic shrinkage is generated by a large magnitude, up to about 1%. In the aggregate sinking process, if blocked by the reinforcement, it will form cracks along the direction of the reinforcement. In the members of the vertical variable cross-section, such as T beams, box girders, webs and top and bottom plate joints, due to uneven compaction before hardening will occur on the surface of the direction of the cracks along the web. In order to reduce the plastic shrinkage of concrete, the water-cement ratio should be controlled during construction, avoid long time mixing, the material should not be too fast, the vibrating should be dense, and the vertical variable cross-section should be poured in layers.

Shrinkage contraction (dry shrinkage). After hardening of concrete, with the gradual evaporation of surface water, humidity gradually decreases, the concrete volume is reduced, known as shrinkage contraction (shrinkage). Surface water loss due to fast concrete, slow internal loss, resulting in large surface contraction, small internal contraction of uneven contraction, surface contraction deformation by the internal concrete constraints, resulting in the surface of the concrete to withstand tension, when the surface of the concrete to withstand tension more than its tensile strength, shrinkage cracks. After the concrete hardening contraction is mainly shrinkage contraction. Such as reinforcement rate of the larger components (more than 3%), reinforcement on the concrete shrinkage constraints are more obvious, the concrete surface is prone to cracking cracks.

Autogenous shrinkage. Autogenous shrinkage is the hydration reaction between cement and water during the hardening process of concrete, which has nothing to do with external humidity and can be positive (i.e. shrinkage, such as ordinary Portland cement concrete) or negative (i.e. expansion, such as slag cement concrete and fly ash cement concrete).

Charring shrinkage. The shrinkage and deformation caused by the chemical reaction of atmospheric carbon dioxide with the hydrides of cement. Charring shrinkage occurs only at about 50% humidity and accelerates with increasing carbon dioxide concentration. Charring shrinkage is generally not calculated.

Most of the shrinkage cracks in concrete are surface cracks with fine widths and crisscrossed, cracked and irregular shapes.

Concrete Keeps Cracking, And The Cause Is Finally Found

Cracks caused by deformation of the foundation

Due to the uneven vertical settlement or horizontal displacement of the foundation, additional stresses are generated in the structure, which exceed the tensile capacity of the concrete structure and lead to structural cracking.

Cracks caused by corrosion of reinforcing steel

Due to the poor quality of concrete or insufficient thickness of the protective layer, the protective layer of concrete by carbon dioxide erosion charring to the surface of the reinforcing steel, so that the alkalinity of the concrete around the reinforcing steel, or due to chloride intervention, the reinforcing steel around the high content of chloride ions, can be caused by oxidation film damage to the surface of the reinforcing steel, iron ions in the reinforcing steel and the invasion of oxygen and moisture in the concrete corrosion reaction, the volume of rust iron hydroxide than the original growth of about 2 to 4 times, thus producing expansion stress on the surrounding concrete, leading to cracking and peeling of the protective layer of concrete, producing cracks along the longitudinal direction of the reinforcement, and rust penetrating into the concrete surface. As a result of corrosion, the effective cross-sectional area of the reinforcement is reduced, the holding and wrapping force of the reinforcement and concrete is weakened, the bearing capacity of the structure is decreased, and other forms of cracks will be induced to aggravate the corrosion of the reinforcement, leading to structural damage.

To prevent the corrosion of steel reinforcement, the design should control the crack width according to the specification and adopt sufficient thickness of protective layer (of course, the protective layer should not be too thick, otherwise the effective height of the components will be reduced and the crack width will be increased when under stress); the construction should control the water-cement ratio of concrete, strengthen tamping to ensure the compactness of concrete and prevent the intrusion of oxygen, and at the same time strictly control the amount of admixtures containing chlorine salts. Caution should be taken especially in areas with corrosive air and groundwater. Click>>Project Information Free Download

Cracks caused by frost expansion

When the atmospheric temperature is below zero, the water-absorbing saturated concrete freezes, the free water turns into ice, volume expansion 9%, thus producing expansion stress in concrete; at the same time, the supercooled water in the gel pores of concrete (icing temperature below -78 degrees) migrates and redistributes in the microstructure causing osmotic pressure, increasing the expansion force in concrete, reducing the strength of concrete, and leading to cracks. In particular, the concrete is most severely affected by freezing when it first sets, and the loss of concrete strength can reach 30% to 50% when it reaches maturity. If no heat preservation measures are taken after grouting the prestressing tunnel during winter construction, freezing and swelling cracks may also occur along the pipeline direction.

Cracks caused by the quality of construction materials

Concrete mainly consists of cement, sand, aggregates, mixed water and admixtures. Substandard materials used in configuring concrete may lead to cracks in the structure.

Cracks caused by the quality of construction workmanship

In the process of concrete structure pouring, component production, moulding, transportation, stacking, assembling and lifting, if the construction technology is unreasonable and the construction quality is poor, it is easy to produce longitudinal, transverse, oblique, vertical, horizontal, surface, deep and penetrating cracks, especially in long thin-walled structures are more likely to appear. The location, direction and width of the cracks vary according to the reasons.

Treatment method of common concrete cracks

Surface repair

Commonly used methods are compaction and leveling, application of epoxy adhesive, spraying cement mortar or fine stone concrete, pressure smearing epoxy mastic, epoxy resin paste on the silk cloth, increase the overall surface layer, steel anchor stitching.

Surface coating and surface patching method Surface coating is applicable to fine and shallow cracks that are difficult to be filled by the slurry, hairline cracks that are not deep enough to reach the surface of the reinforcing steel, watertight cracks, non-stretchable cracks and cracks that are no longer active. Surface patching (geomembrane or other waterproof sheet) method is suitable for large areas of water leakage (honeycomb pitting surface, etc. or difficult to determine the specific location of leakage, deformation joints) seepage plugging.

Partial repair method

Commonly used methods include filling method, pre-stressing method, and partial excavation and re-pouring of concrete.

Filling the cracks directly with repair materials is generally used to repair wider cracks, which is simple and inexpensive. For cracks with width less than 0.3mm, shallow depth, or cracks with filling material, it is difficult to achieve the effect by grouting, and for simple treatment of small-scale cracks, a V-shaped slot can be opened and then filled.

Cement pressure grouting method

It is suitable for patching stable cracks with width ≥0.5mm.

This method has a wide range of applications, from small cracks to large cracks can be applied, and the treatment effect is good. Using pressure feeding equipment (pressure 0.2~0.4Mpa) to inject the caulking slurry into the concrete crack to achieve the purpose of occlusion, this method is a traditional method with good effect. It can also use the elastic caulking device to inject the caulking glue into the cracks, without electricity, which is very convenient and has ideal effect.

Chemical grouting

It can penetrate cracks with width ≥0.05mm.

Reduce the internal force of the structure

Commonly used methods include unloading or load control, setting up unloading structure, adding support points or bracing. Change simple support beams into continuous beams, etc.

Structural reinforcement

Commonly used methods include adding reinforcement, thickening slabs, outsourcing reinforced concrete, outsourcing steel, pasting steel plates, and pre-stressing reinforcement systems.

Cracks caused by overload, cracks that are left untreated for a long period of time, cracks caused by fire, etc. can affect the strength of the structure by using structural reinforcement methods. Including cross-sectional reinforcement method, anchor reinforcement method, prestressing method, etc. The inspection of concrete crack treatment effect includes repair material test; drilling core sampling test; compressed water test; compressed air test and so on.

Concrete Keeps Cracking, And The Cause Is Finally Found

Change the structural scheme, strengthen the overall stiffness

For example, cracks in the frame are treated by adding partitions and deep beams.

Concrete replacement method.

Concrete replacement is an effective method of dealing with severely damaged concrete, in which the damaged concrete is removed and then replaced with new concrete or other materials. Commonly used replacement materials are: ordinary concrete or cement mortar, polymer or modified polymer concrete or mortar.

Electrochemical protection method

Electrochemical corrosion protection is the use of applied electric field in the medium to change the state of the environment in which the concrete or reinforced concrete is located, passivating the steel reinforcement in order to achieve the purpose of corrosion protection. The cathodic protection method, chlorine salt extraction method, and alkaline recovery method are the three methods commonly used and effective in chemical protection method. The advantage of this method is that the protection method is less affected by environmental factors, suitable for long-term corrosion protection of steel, concrete, both for the cracked structure can also be used for new structures.

Bionic self-healing law

Bionic self-healing law is a new crack treatment method, which mimics the function of biological tissue to automatically secrete certain substances to the traumatized area, so that the traumatized area can be healed, adding some special components to the traditional components of concrete (such as liquid-core fibers or capsules containing binder), forming an intelligent bionic self-healing nerve network system within the concrete, when cracks appear in the concrete, some of the liquid-core fibers can be secreted to make the cracks heal again.

Other methods

Commonly used methods include dismantling and redoing, improving the use conditions of the structure, and leaving it untreated through testing or analysis. Click>>Project data free download

The causes of cracks in bulk concrete

In large-volume concrete structure, due to the large structural section and large amount of cement, the heat of hydration released by cement hydration will produce large temperature changes and shrinkage, and the resulting temperature shrinkage stress is the main cause of cracks in reinforced concrete. There are two kinds of cracks: surface cracks and penetration cracks. Surface cracks are caused by the different heat dissipation conditions on the concrete surface and inside, and the temperature is low outside and high inside, forming a temperature gradient, which causes compressive stress inside the concrete and tensile stress on the surface, and the tensile stress on the surface exceeds the tensile strength of the concrete. Penetration cracks are caused by the gradual cooling of the concrete as the strength of the mass concrete develops to a certain level, and the deformation caused by this cooling difference, together with the volume shrinkage deformation caused by water loss of the concrete, and the tensile stress caused by the constraint of the foundation and other structural boundary conditions, exceeds the tensile strength of the concrete, which may result in cracks that penetrate the entire section. Both types of cracks are, to varying degrees, harmful cracks.

Early shrinkage of high-strength concrete is larger, which is due to the replacement of cement with 30% to 60% mineral fine admixtures in high-strength concrete, the admixture of high-efficiency water-reducing agent is 1% to 2% of the total amount of cementitious materials, and the water-cement ratio is 0.25 to 0.40, which improves the microstructure of concrete and brings many excellent characteristics to high-strength concrete, but the most prominent negative effect is the increase in the probability of concrete shrinkage and cracking. The shrinkage of high-strength concrete is mainly dry shrinkage, temperature shrinkage, plastic shrinkage, chemical shrinkage and self-shrinkage. The time of the first appearance of cracks in concrete can be used as a reference to determine the cause of cracks: plastic shrinkage cracks appear about a few hours to ten hours after pouring; temperature shrinkage cracks appear about 2 to 10 days after pouring; self-shrinkage mainly occurs a few days to tens of days after the concrete set and hardened; dry shrinkage cracks appear close to one year of age.

dry shrinkage

When concrete loses internal capillary and gel pores in unsaturated air to adsorb water, it will shrink, and the porosity of high-performance concrete is lower than ordinary concrete, so the dry shrinkage rate is also low.

Plastic shrinkage

Plastic shrinkage occurs in the plastic stage before concrete hardening. High-strength concrete has low water to glue ratio, less free water, fine mineral admixtures have higher sensitivity to water, high-strength concrete basically does not secrete water, the surface water loss is faster, so high-strength concrete plastic shrinkage is more likely to occur than ordinary concrete.

self-shrinking

The relative humidity inside closed concrete decreases as the hydration of the cement progresses, known as self-drying. Self-drying causes negative pressure due to unsaturation of water in the pores, thus causing self-shrinkage of the concrete. High-strength concrete due to the low water-cement ratio, the early development of faster strength, will make the free water consumption fast, resulting in pore system in the relative humidity below 80%, and high strength concrete structure is more dense, it is difficult for outside water to penetrate the supplement, resulting in self-shrinkage of concrete. In the total shrinkage of high-strength concrete, dry shrinkage and self-shrinkage are almost equal, and the lower the water-adhesive ratio, the greater the proportion of self-shrinkage. It is completely different from ordinary concrete, which mainly shrinks by dry shrinkage, while high-strength concrete mainly shrinks by self-shrinkage.

temperature shrinkage

For higher strength requirements of concrete, the amount of cement is relatively large, the heat of hydration, the rate of temperature rise is also large, generally up to 35 ~ 40 ℃, plus the initial temperature can make the maximum temperature of more than 70 ~ 80 ℃. Generally, the thermal expansion coefficient of concrete is 10×10-6/℃, when the temperature drops 20~25℃, the cold shrinkage amount is 2~2.5×10-4, while the ultimate tensile value of concrete is only 1~1.5×10-4, thus the cold shrinkage often causes concrete cracking.

chemical shrinkage

After hydration of cement, the solid phase volume increases, but the absolute volume of the cement-water system decreases, forming many capillary pore joints, high strength concrete water to glue ratio is small, external mineral fine admixtures, the degree of hydration is restricted, so the chemical shrinkage of high strength concrete is less than ordinary concrete.

When the concrete shrinks and is externally or internally constrained, tensile stresses will occur and may cause cracking. For high-strength concrete has a high tensile strength, but also high modulus of elasticity, in the same contraction and deformation, will cause a higher tensile stress, and because of the low creep capacity of high-strength concrete, stress relaxation is small, so poor anti-cracking performance.

Criteria for determining harmful and harmless cracks in mass concrete

In principle, cracks are not allowed to appear in reinforced concrete related to nuclear safety, especially in important parts such as the floor of the reactor plant, the containment vessel body and dome, and the turbine pump of the turbine plant, but cracks in other parts should be controlled as much as possible. However, cracks are unavoidable due to various reasons, so how to determine whether cracks are harmful or not? To this end, all the units of Fuqing Nuclear Power (owners, supervisors, engineering companies and construction units) have carefully discussed and determined the following criteria for identifying concrete cracks.

Harmless cracks.

δf≤0.3mm Depth h≤0.5H

δf≤0.2mm Penetration (self-healing)

1.0mm ≥ δf>0.3mm L≤0.1B

harmful cracks (meeting one of the following conditions).

(a) δf>0.3mm deep cracks, h>0.5H.

δf>0.2mm throughout the cross-section.

(a) Cracks that affect the function of the use (requirements such as permeability, ventilation, radiopaque, etc., where one of these is met, are sufficient).

(a) δf>0.3mm non-penetrating, which may cause corrosion cracks in the steel.

Cracks that reduce the load bearing capacity of the structure.

meaning of each symbol.

Δf – fracture width L – fracture length

h – fissure depth H – fissure depth

 

B – the width of the structure along the length of the crack, e.g. subsidence after pouring (plastic cracking)

Harmless fracture treatment methods

Secondary pressure surface method

For shrinkage cracks in freshly poured concrete, which mostly appear on the surface of newly poured and air-exposed structural members, there are plastic shrinkage, settlement shrinkage, dry shrinkage, carbonation shrinkage, condensation shrinkage and other shrinkage cracks, which are neither deep nor wide, and are treated as follows.

(1). If the concrete still has plasticity, the method of pressure smearing can be taken once and curing can be strengthened.

(2). If the concrete has been hardened, you can infiltrate cement paste into the cracks and then use iron trowel to flatten and compact it.

Surface mortar coating method

When handling, chisel the surface of the concrete near the crack or chisel along the crack into 15-20mm deep grooves 100-200mm wide, sweep clean and sprinkle water to moisten. Brush the cement slurry (interface agent approved by the owner) for one time, then use 1:1~2 cement mortar in 2~3 layers, and apply the total thickness of 10~20mm for polishing. When there is leakage, the application of cement slurry (2mm thick) and 1:2.5 cement mortar (4-5mm thick, can be miserable into 1-3% of the weight of cement ferric chloride waterproofing agent) alternately wiping 4-5 layers, 3-4 hours after smearing to cover and sprinkling maintenance.

Simple, refreshing title

Surface epoxy mastic (or epoxy glass cloth) method.

Before application, the surface near the crack should be cleaned (oil stains should be cleaned with acetone or xylene) and dried. If the grass-roots level is difficult to dry, epoxy coal tar mastic can be used. When epoxy glass cloth needs to be pasted, the glass cloth should be de-sodiumed and dried first, depending on the specific situation, it can be made into one cloth and two oil (or two cloth and three oil, the surrounding area of the second layer of cloth should be 10~15mm wider than the next layer).

Surface chisel groove mending method

When the cracks are rare but deep, a V-shaped or U-shaped groove should be cut along the concrete cracks, and the inside surface of the groove should be repaired and cleaned, and the groove should be kept dry. Embed rigid materials such as cement mortar, epoxy mud, or fill in flexible materials such as PVC mud, asphalt ointment and other sealing materials in the groove. Before embedding the sealing material, brush the dilution paint with the concrete material of the embedded material (the surface can be used as a protective layer of mortar or not), see Figure 1 for specific practice.

Figure 1 surface groove repair crack treatment methods

(a) General Crack Treatment (b) Seepage Crack Treatment (c) Active Crack Treatment (d) After Active Crack Extension 1-Cracks; 2-Cement mortar or epoxy mastic; 3-Polyoxyethylene; 4-1:2.5 Cement mortar or rigid waterproofing five-ply practice; 5- Sealing material; 6- Isolation buffer; B- Trough width; δ-Crack active distance

 

Note: For the cracks on the surface of the construction joint, the treatment can be connected with the construction section of the concrete before pouring, according to the requirements of the surface chisel groove in the crack location V or U-shaped groove, the groove is not filled with other filler, by the connection of the construction section of the concrete poured structure to ensure that the construction joint at the concrete.

Striping method on the surface

For cracks moving range is not limited to a plane and have waterproof requirements inconvenient to chisel groove repair of live cracks, a flexible polybutadiene rubber sealing strip placed on top of the crack, with polybutadiene rubber binder will be peripheral bonded to the concrete (see Figure 2), so that the middle of the sealing strip can move freely with the crack activity, long cracks can be sub-segmented for bonding, sub-segmented for the connection of the sealing strip using polybutadiene rubber paste lap, lap the upper and lower pressure rubbing should be cut into bevel lap, the length of 100mm.

Figure 2 flexible sealing tape surface paste

1 – cracks 2 – linoleum or plastic barrier; 3 – polybutadiene seals; 4 – binders

Concrete Keeps Cracking, And The Cause Is Finally Found

Treatment of harmful cracks

Cement grouting method

–Drilling: Drill holes by pneumatic drill, with a distance of 1-1.5m, except for shallow holes, which use stitching holes, the axis of holes and cracks are generally at a 30-45-slant angle (see Figure 3), and the depth of holes should be more than 0.5m through the crack surface, when there are two or more rows of holes, they should be crossed or arranged in the shape of a plum.

Figure 3 Drill hole schematic

1-crack, 2-flush, 3-beveled hole

–Rinsing: After drilling, rinse with water and proceed from top to bottom according to the vertical arrangement for each hole.

–Sealing: After rinsing the seam surface, apply 1:1-2 cement mortar or epoxy mastic on the crack surface.

–Bury pipe: Generally use steel pipe of ø19-38 as grouting pipe (steel pipe upper end processing wire buckle), before installation, wrap tightly with raw tape on the outer wall of steel pipe, and then screw into the hole, the gap around the pipe wall in the hole is blocked with cement mortar or sulfur mortar, to prevent slurry or grouting pipe from punching out of the hole.

–Test pressure: Use 0.1-0.2MPa pressure water for water penetration test, take grouting hole pressure water, drainage hole drainage method to check cracks and pipeline smooth situation, and then close the exhaust hole to check the effect of stopping slurry plugging, and wet the surface of the seam to facilitate bonding.

–Grouting: Qualified injectable cement is used for filling joints after approval of design, the net water-cement ratio of cement is 0.4, and the grouting pressure is 0.3-0.5MPa; after the whole crack is treated, the hole should be filled with net slurry, and filled with net sand and tamped with rods.

Chemical grouting method

–Drilling: pneumatic drill is used to drill holes at a distance of 1-1.5m, except for shallow holes, which use slit holes and generally account for the hole axis and the crack at a 30-45-slant angle (see Figure 3), the depth of the hole should be more than 0.5m through the crack surface, and when there are two or more rows of holes, they should be crossed or arranged in the shape of a plum.

–Sealing: After the seam surface rinse, in the crack surface with 1:1 ~ 2 cement mortar or epoxy mastic coating.

–Buried pipe: Generally use steel pipe of ø19-38 as grouting pipe (steel pipe upper end processing wire buckle), before installation, wrap tightly with raw tape on the outer wall of steel pipe, and then screw into the hole, the gap around the pipe wall in the hole is blocked with cement mortar or sulfur mortar, to prevent slurry or grouting pipe from punching out of the hole.

-Pressure testing: Use 0.2-0.3MPa compressed air for pressure experiments.

–Grouting: use epoxy resin slurry for grouting.