The reasons can be roughly divided into the following categories:
1. Conservation conditions: Does it meet the requirements? Because the ratio between 7d and 28d is empirical data obtained under standard curing conditions (constant temperature and humidity), if it is not standard curing conditions, it is impossible to talk about comparison.
2. Admixtures that affect the ratio between 7d and 28d: early strength agent, excessive retarder.
3. The admixtures that affect the later strength include air-entraining agents.
4. Cement composition: If the alkali content in cement is too high, it will reduce the later strength.
5. The adaptability of admixtures and cement. The degree of influence on this type of cement must be proved by tests.
6. Excessive early strength agent.
7. The excess strength of cement itself is not high, and the later strength growth rate is small.
Causes and Treatments of Insufficient Strength of Engineering Concrete
"The strength grade of the structural concrete must meet the design requirements."
This is a mandatory provision stipulated in the construction code of engineering construction and must be strictly implemented. However, there are still some engineering concretes that have caused many quality problems due to insufficient strength. The consequences of low concrete strength are mainly manifested in the following two aspects:
First, the bearing capacity of structural members is reduced;
Second, the impermeability, frost resistance and durability decrease. Therefore, the problem of insufficient concrete strength must be carefully analyzed and dealt with.
Causes and Treatments of Insufficient Strength of Engineering Concrete
1. Common causes of insufficient concrete strength
1. Raw material quality problems
(1) Poor cement quality
1) The actual activity (strength) of cement is low: there are two common situations. One is that the quality of cement is poor when leaving the factory, and when it is applied in actual engineering, before the 28d strength test results of cement are measured, the cement strength grade is estimated to configure concrete. , when the measured strength of 28d cement is lower than the original estimated value, the concrete strength will be insufficient; the second is that the cement storage conditions are poor, or the storage time is too long, resulting in cement agglomeration, reduced activity and affecting strength.
2) Unqualified cement stability:
The main reason is that the cement clinker contains too much free calcium oxide (CaO) or free magnesium oxide (MgO), and sometimes it may also be caused by the addition of too much gypsum. Because the CaO and MgO in the cement clinker are all burned, the curing is very slow after contact with water, and the volume expansion produced by the curing lasts for a long time. When the amount of gypsum is too much, the gypsum reacts with the calcium aluminate hydrate in the hydrated cement to form calcium aluminum sulfate hydrate, which also expands the volume. If these volume changes occur after the concrete hardens, they will destroy the cement structure, most of which will lead to cracking of the concrete and reduce the strength of the concrete. In particular, it should be noted that although the concrete surface prepared by some unqualified cement has no obvious cracks, its strength is extremely low.
(2) Poor quality of aggregate (sand, stone)
1) The strength of stones is low: In some concrete test blocks, many stones were crushed, indicating that the strength of stones is lower than that of concrete, resulting in a decrease in the actual strength of concrete.
2) Poor volume stability of stones:
Some crushed stones made of porous chert, shale, limestone with expanded clay, etc., often show poor volume stability under the action of alternating wet and dry or freeze-thaw cycles, resulting in a decrease in concrete strength.
3) Poor shape and surface condition of stones:
The high content of needle-like stones affects the strength of concrete. Stones, on the other hand, have a rough and porous surface, which has a favorable effect on the strength of concrete, especially the flexural and tensile strength, due to its better bonding with cement. The most common phenomenon is that under the same cement and water-cement ratio, the strength of crushed stone concrete is about 10% higher than that of pebble concrete.
4) High content of organic impurities in aggregate (especially sand):
If the aggregate contains rotten animals and plants and other organic impurities (mainly tannic acid and its derivatives), it will adversely affect the hydration of cement and reduce the strength of concrete.
5) High content of clay and dust:
The decline in concrete strength caused by this reason is mainly manifested in the following three aspects. First, these very fine particles are wrapped on the surface of the aggregate, which affects the bonding of the aggregate and cement; second, the surface area of the aggregate is increased to increase water consumption; It is clay particles, the volume is unstable, and it shrinks and swells when it dries, and has a certain destructive effect on concrete.
6) High content of sulfur trioxide:
Aggregate contains pyrite (FeS2) or raw gypsum (CaSO4 2H2O) and other sulfides or sulfates. When the content is high in terms of sulfur trioxide (eg >1%), it may interact with cement hydrates. In the production of calcium sulfoaluminate, volume expansion occurs, resulting in cracks and loss of strength in hardened concrete.
7) High content of mica in sand:
Because the surface of mica is smooth, the bonding performance with cement stone is extremely poor, and it is easy to crack along the joints, so the high content of mica in sand has adverse effects on the physical and mechanical properties (including strength) of concrete.
(3) The quality of mixing water is unqualified
If swamp water with high content of organic impurities, sewage and industrial wastewater containing humic acid or other acids and salts (especially sulfate) are used for mixing concrete, the physical and mechanical properties of concrete may be reduced.
(4) The quality of the admixture is poor
At present, the quality of admixtures produced by some small factories is not up to standard. It is quite common that the admixtures cause insufficient concrete strength, and even accidents where the concrete does not condense occur from time to time.
2. Improper concrete mix ratio
Concrete mix ratio is one of the important factors that determine the strength. The water-cement ratio directly affects the concrete strength. Others such as water consumption, sand ratio, and bone-ash ratio also affect various properties of concrete, resulting in insufficient strength accidents. These factors are generally manifested in the following aspects in engineering construction:
(1) Randomly apply the mix ratio:
The concrete mix ratio is determined by the construction site after applying to the laboratory for trial mixing according to the characteristics of the project, construction conditions and raw materials. However, many construction sites ignore these specific conditions and randomly apply the mix ratio according to the index of concrete strength grade, thus causing many accidents of insufficient strength.
(2) Increased water consumption:
The more common ones are inaccurate measurement of the water adding device on the mixing equipment; not deducting the water content in the sand; even adding water arbitrarily at the irrigation site. After the water consumption increases, the water-cement ratio and slump of the concrete will increase, resulting in insufficient strength accidents.
(3) Insufficient amount of cement:
In addition to inaccurate measurement before mixing, insufficient weight of packaged cement also frequently occurs, resulting in insufficient cement in concrete, resulting in low strength.
(4) Inaccurate measurement of sand and stone:
It is more common that the measurement tools are outdated or the maintenance management is not good, and the accuracy is not up to standard.
(5) Wrong use of admixture:
There are two main types; one is that the species is used wrongly, and the admixture is blindly mixed with the admixture before the performance of the admixture is clear, such as early strength, retarding, and water reduction, so that the concrete cannot reach the expected strength; the other is that the dosage is not correct. allow.
(6) Alkali-aggregate reaction:
When the total alkali content of concrete is high, coarse aggregate containing carbonate or active silica (opal, chalcedony, obsidian, zeolite, porous chert, rhyolite, andesite, tuff, etc.) material), may produce alkali-aggregate reaction, that is, sodium hydroxide and potassium hydroxide formed after the hydrolysis of alkaline oxides, which chemically react with active aggregates to form a mixed gel that continuously absorbs water and expands, causing concrete cracks or decrease in intensity. According to information from Japan, under the same other conditions, the strength of concrete after the alkali-aggregate reaction is only about 60% of the normal value.
3. There are problems in concrete construction technology
(1) Poor concrete mixing;
The order of adding materials to the mixer is reversed, and the mixing time is too short, resulting in uneven mixture and affecting the strength.
(2) Poor transportation conditions:
Concrete segregation was found during transportation, but effective measures were not taken (such as re-mixing, etc.), and the strength was affected by the leakage of transport tools.
(3) Improper pouring method:
If the concrete has been initially set during pouring; the concrete has been segregated before pouring, etc., which can cause insufficient concrete strength.
(4) Severe slurry leakage of the formwork:
The steel formwork of a certain project was seriously deformed, the slab gap was 5~10mm, and the grout was seriously leaked. The measured strength of the concrete at 28 days was only half of the design value.
(5) Forming vibration is not dense:
The porosity of the concrete after it is put into the mold reaches 10%~20%. If the vibration is not solid, or the formwork leaks, the strength will inevitably be affected.
(6) Poor maintenance system:
The main reason is that the temperature and humidity are not enough, the early water shortage and drying, or the early freezing, resulting in low concrete strength.
4. Poor management of test blocks
(1) Test block without standard maintenance:
So far, there are still some construction sites and many construction and test personnel who do not know that the concrete test block should be cured under standard conditions in a humid environment or water with a temperature of (20±2)°C and a relative humidity of 90% or more, and the test block should be maintained in a humid environment with a relative humidity above 90%. Under the same conditions of construction and maintenance, some test blocks have poor temperature and humidity conditions, and some test blocks have been smashed, so the strength of the test blocks is low.
(2) Poor mold trial management:
The deformation of the test mold is not repaired or replaced in time.
(3) Failure to make test blocks according to regulations:
For example, the size of the test mold does not match the particle size of the stone, there are too few stones in the test block, and the test block is not vibrated with the corresponding equipment.
Second, the impact of insufficient concrete strength on different types of structural members
According to the analysis of the design principles of reinforced concrete structures, the degree of influence of insufficient concrete strength on the strength of different structures is quite different, and the general rules are as follows:
(1) Axial compression member:
It is usually designed for concrete to bear all or most of the load. Therefore, insufficient concrete strength has a great influence on the strength of components.
(2) Axial tension members:
The design code does not allow the use of plain concrete as tension members, and the effect of concrete is not considered in the calculation of the strength of reinforced concrete tension members, so the strength of concrete is insufficient and has little effect on the strength of tension members.
(3) Bending members:
The normal section strength of reinforced concrete flexural members is related to the concrete strength, but the influence range is not large. For example, for members with a longitudinal tensile HRB335 steel reinforcement ratio of 0.2%~1.0%, when the concrete strength is reduced from C30 to C20, the strength of the normal section generally decreases by no more than 5%, but the concrete strength is insufficient for the shear strength of the oblique section. Greater impact.
(4) Eccentric compression member:
For members with small eccentric compression or tension reinforcement, all or most of the concrete cross-section is under compression, and concrete compression damage may occur. Therefore, insufficient concrete strength has a significant impact on component strength. For members with large eccentric compression and few tension reinforcements, the impact of insufficient concrete strength on the normal section strength of members is similar to that of flexural members.
(5) Influence on punching strength:
The punching shear capacity is directly proportional to the tensile strength of concrete, which is about 7% to 14% (average 10%) of the compressive strength. Therefore, when the strength of concrete is insufficient, the punching shear resistance will decrease significantly.
Before dealing with accidents of insufficient concrete strength, it is necessary to distinguish the mechanical properties of structural components, correctly estimate the impact of reduced concrete strength on the bearing capacity, and then comprehensively consider the requirements of crack resistance, stiffness, impermeability, durability, etc., and select appropriate treatment measures .
5. Common treatment methods for accidents of insufficient concrete strength
(1) Determination of the actual strength of concrete:
When the pressure test results of the test block are unqualified, and it is estimated that the actual strength of the concrete in the structure may meet the design requirements, the actual strength of the concrete can be measured by non-destructive inspection methods or drilling sampling, as a basis for accident handling.
(2) Utilize the late strength of concrete:
The strength of concrete increases with age. In a dry environment, the strength can reach 1.2 times that of 28 days in 3 months, and 1.35~1.75 times in a year. If the actual strength of the concrete is not much lower than the design requirement, and the loading time of the structure is relatively late, intensive maintenance can be adopted, and the principle of late strength of concrete can be used to deal with insufficient strength accidents.
(3) Reduce structural load:
When the bearing capacity of the structure is significantly reduced due to insufficient concrete strength, and it is inconvenient to use reinforcement methods to deal with it, the method of reducing the structural load is usually used to deal with it. For example, measures such as replacing lime slag or cement slag with high-efficiency and light-weight insulation materials can reduce the self-weight of buildings, and reduce the overall height of buildings.
(4) Structural reinforcement:
When the concrete strength of the column is insufficient, it can be reinforced by outsourcing reinforced concrete or outsourcing steel, and can also be reinforced by the spiral restraint column method. When the strength of beam concrete is low and the shear resistance is insufficient, it can be reinforced by outsourcing reinforced concrete and pasting steel plates. When the concrete strength of the beam is seriously insufficient, resulting in that the strength of the normal section cannot meet the specification requirements, the reinforced concrete can be used to increase the height of the beam, and the prestressed tie rod reinforcement system can also be used for reinforcement.
(5) Analysis and verification mining potential:
When the actual strength of concrete is similar to the design requirements, it is generally checked through analysis, and most of them do not need to be specially reinforced. Because the lack of concrete strength has little effect on the strength of the normal section of the flexural member, this method is often used to deal with it: if necessary, on the basis of checking calculation, do a load test to further prove that the structure is safe and reliable, and there is no need to deal with it. Insufficient concrete strength in the core area of the prefabricated frame beam-column joints may lead to insufficient seismic safety. As long as the strength meets the requirements under the equivalent design magnitude after checking and calculation according to the seismic code, structural cracks and deformations are not repaired or undergo general repairs If it can still be used, special measures are not necessary. It should be pointed out that the conclusion of not processing after analysis and calculation must be approved by the design visa to be valid. At the same time, it should be emphasized that this approach is actually tapping the design potential.
