Reasons Why Concrete Strength Does Not Increase In 7 To 28 DaysWhat happened to the concrete that barely grew in strength from 7 days to 28 days? The reasons can be broadly categorized as follows.

  1. Conservation conditions: Do they meet the requirements? Because 7d, 28d of the proportional relationship is in the standard maintenance conditions (constant temperature and humidity) under the empirical data, if not the standard maintenance conditions, talk about no comparison.
  2. The influence of 7d, 28d proportional relationship of admixture: Early strength agent, excessive retarder.
  3. The later strength of the admixture has an impact on the air-entraining agent.
  4. Cement composition, if the fruit of cement in the alkali content is too high, will reduce the later strength.
  5. Admixture and the adaptability of the cement. Must be tested to prove the degree of impact on the cement.
  6. Excessive early strength additive.
  7. The surplus strength of the cement itself is not high, the late-stage strength growth rate is small.

Concrete compressive strength test link test block destruction state

  1. Causes and treatment of insufficient strength of engineering concrete

“The strength grade of structural concrete must meet the design requirements.”

This is a mandatory provision stipulated in the construction code for engineering and construction and must be strictly enforced. However, there are still some projects where the concrete has caused many quality problems due to low strength. The consequences of low strength of concrete are mainly manifested in the following two aspects.

First is a reduction in the load-bearing capacity of structural elements.

Second, it is impermeability, anti-frost performance and durability decline. Therefore, the problem of insufficient strength of concrete must be carefully analyzed and dealt with.

(1). The common reasons for insufficient strength of concrete

Raw material quality issues

1). Poor quality cement

Low actual cement activity (strength).

There are two common situations, one is the poor quality of the cement leaving the factory, and when applied in the actual project, the cement strength grade is estimated before the 28d strength test result is measured to configure the concrete, when the measured strength of 28d cement is lower than the original estimated value, it will result in insufficient strength of the concrete; the other is the poor storage condition of the cement, or the storage time is too long, resulting in cement agglomeration, and the activity is reduced to affect the strength.

2). Failure of cement stability

The main reason for this is that cement clinker contains too much free calcium oxide (cao) or free magnesium oxide (mgo), which can sometimes be caused by too much gypsum blending. Because the cao and mgo in cement clinker are burned, the maturation is very slow after encountering water, and the volume expansion caused by maturation continues for a long time. When too much gypsum, gypsum and hydrated calcium aluminate reaction in cement to generate hydrated calcium aluminate sulfate, but also the volume expansion. These volume changes, if they occur after the concrete has hardened, can damage the cement structure, mostly leading to concrete cracking and also reducing the strength of the concrete. In particular, it should be noted that the strength of some concrete made of unqualified cement is extremely low, although there are no obvious cracks on the surface.

(2). Poor quality of aggregate (sand, stone)

1). Low stone strength.

In some concrete test block test pressing, many stones can be seen to be crushed, indicating that the strength of the stones is lower than the strength of the concrete, leading to a decrease in the actual strength of the concrete.

2). Poor volumetric stability of the stones.

Some gravel made of porous flint, shale, limestone with swelling clay, etc., under the action of alternating wet and dry or freeze-thaw cycles, often shows poor volume stability, which leads to a decrease in concrete strength.

3). Poor stone shape and surface condition.

The high content of needle-shaped stones affects the strength of concrete. On the other hand, stones have a rough and porous surface, which has a favourable effect on the strength of concrete, especially its flexural and tensile strength, due to its better combination with cement. One of the most common phenomena is that crushed concrete is about 10% stronger than pebble concrete, given the same cement and water-cement ratio. Test and inspection exam review material plus wechat: 17507557988

4). High content of organic impurities in aggregates (especially sand).

Such as aggregate containing decaying plants and animals and other organic impurities (mainly tannins and their derivatives), which adversely affects the hydration of cement, and reduces the strength of concrete.

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5). High clay and dust content.

The resulting decrease in the strength of concrete is mainly manifested in the following three aspects, first, these very small particles wrapped around the surface of the aggregate, affecting the bond between the aggregate and cement; second, increasing the surface area of the aggregate, increasing water consumption; third, clay particles, volume instability, dry shrinkage and wet swelling, which has a certain destructive effect on the concrete.

6). High sulphur trioxide content.

Aggregate contains sulfide or sulphate such as sulphide iron ore (fes2) or raw gypsum (caso4-2h2o), and when its content is high in sulphur trioxide (e.g. >1%), it may interact with hydrates of cement to produce calcium sulphur aluminate, which undergoes volumetric expansion, leading to cracks and reduced strength in hardened concrete.

7) high mica content in sand.

Due to the smooth surface of mica, the adhesion with cement stone is extremely poor, and it is easy to crack along the joints, so the high content of mica in sand has a negative impact on the physical and mechanical properties (including strength) of concrete.

(3). Unqualified mixing water quality

The use of swamp water with high content of organic impurities, sewage and industrial wastewater containing humic acid or other acids and salts (especially sulfate) in mixing concrete may result in the deterioration of the physical and mechanical properties of concrete.

(4). Poor quality of admixtures

At present, some small plants produce additives of substandard quality is quite common, due to additives caused by insufficient strength of concrete, and even concrete does not set the accident happens from time to time.

  1. Concrete with improper ratio

Concrete ratio is one of the important factors in determining strength, where the size of the water-cement ratio directly affects the strength of concrete, and other factors such as water consumption, sand rate, ash ratio, etc. Also affect various properties of concrete, resulting in under-strength accidents. These factors are generally manifested in the following aspects in the construction of the project.

(1). Arbitrary application of complex ratios.

Concrete mix ratio is determined by the site application to the laboratory after test mix according to the project characteristics, construction conditions and raw material. However, many sites are disregarding these specific conditions, only according to the concrete strength level of the index, randomly apply the ratio, resulting in many accidents of insufficient strength.

(2). Increased water consumption.

The more common are mixing equipment on the inaccurate measurement of water device; not deducted from the water content of sand and gravel; and even in the pouring location of arbitrary water. After increasing the amount of water, the water-cement ratio and slump of the concrete will increase, resulting in accidents of insufficient strength.

(3). Insufficient amount of cement.

In addition to the inaccurate measurement before mixing, the insufficient weight of the packaged cement also occurs repeatedly, resulting in the insufficient amount of cement in the concrete, resulting in low strength.

(4). Inaccurate measurement of sand and stone.

More commonly, the measuring tools are old or poorly maintained and managed, and the accuracy is substandard.

(5). Misuse of additives.

There are two main types; first, the wrong species, in the additive is not clear early strength, slow-setting, water-reducing properties, such as before, blindly mixed with additives, resulting in concrete does not meet the expected strength; second is not mixed. Test exam review material wechat:17507557988

(6). Alkali-aggregate reaction.

When the total alkali content of concrete is high, but also the use of coarse aggregate containing carbonate or active silicon oxide components (opal, chalcedony, obsidian, zeolite, porous flint, rhyolite, andesite, tuff, etc. Made of aggregate), may produce an alkali-aggregate reaction, that is, the formation of alkaline oxides after hydrolysis of sodium hydroxide and potassium hydroxide, they react chemically with the active aggregate, generating constant water absorption, swelling of the mixed gel. In japan, the reaction of alkali-aggregate resulted in concrete cracking or loss of strength. In japan, there is information that, under the same other conditions, the strength of concrete after the reaction of alkali aggregate is only about 60% of the normal value.

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  1. Problems in concrete construction process

(1). Poor concrete mixing.

The order of adding material to the mixer is reversed, and the mixing time is too short, which causes the mixture to be uneven and affects the strength.

(2). Poor transport conditions.

Concrete segregation is found during transportation, but no effective measures (such as re-mixing, etc.) are taken, and slurry leaks from transportation tools affect the strength.

(3). Improper pouring method.

If the concrete has been set at the time of pouring, or segregated before concrete pouring, etc., it may result in insufficient strength of concrete.

(4). Severe slurry leakage from formwork.

In a project, the steel mold was seriously deformed, the slab seam was 5~10mm, and the slurry leakage was serious, and the strength of the concrete 28d measured was only half of the design value.

(5). Forming vibration is not dense.

The porosity of the concrete after entering the mold reaches 10% to 20%, if the vibrating is not solid, or the formwork leaks slurry will certainly affect the strength.

(6). Poor maintenance system.

Mainly inadequate temperature and humidity, early water shortage and drying, or early freezing, resulting in low strength of concrete.

  1. Poor test block management

(1). The specimen has not been maintained to standard.

Up to now, some construction sites and many construction and testing personnel do not know that concrete specimens should be cured under standard conditions in a humid environment or water at a temperature of (20±2)°c and relative humidity of 90% or more, but the specimens are cured under the same conditions of construction, and some specimens have been hit and smashed, so the strength of specimens is low.

(2). Poor management of the specimen.

Test mold deformation not repaired or replaced in a timely manner.

(3). Failure to make specimen blocks as specified.

For example, the size of the test mold and the size of the stone are not suitable, the stone in the test block is too small, and the test block is not vibrated with the corresponding machine.

The impact of insufficient strength of concrete on different types of structural components

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 varies considerably, with the following general rule.

(1). Axially pressurized members.

Concrete is usually designed to carry all or most of the load. Therefore, insufficient concrete strength has a significant impact on the strength of the member.

(2). Axial tensile members.

The design specifications do not allow the use of plain concrete as a tensile member, and the role of concrete is not considered in the strength calculation of reinforced concrete tensile members, so the lack of concrete strength has little impact on the strength of the tensile member.

(3). Bending members.

The strength of the positive section of reinforced concrete bent members is related to the strength of concrete, but the magnitude of the influence is not large. For example, when the reinforcement ratio of longitudinally strained hrb335 grade steel reinforcement is 0.2% to 1.0% of the members, when the concrete strength is reduced from c30 to c20, the decrease in the strength of the positive section generally does not exceed 5%, but the lack of concrete strength has a greater impact on the shear strength of the oblique section.

(4). Eccentrically stressed members.

For members with small eccentric compression or with a large number of tensile bars, the concrete section is fully or largely compressed and may be damaged by compression, so the effect of insufficient concrete strength on the strength of the member is obvious. For the member with large eccentric compression and little tensile reinforcement, the effect of insufficient concrete strength on the strength of the positive section of the member is similar to that of the bent member. Test exam review material wechat: 17507557988

(5). Effect on punching strength.

Punching bearing capacity is directly proportional to the tensile strength of concrete, and the tensile strength of concrete is about 7% ~ 14% of the compressive strength (average 10%). Therefore, the punching resistance decreases significantly when the concrete strength is insufficient.

Before dealing with the accident of insufficient strength of concrete, it is necessary to distinguish the force performance of the structural components, correctly estimate the impact on the bearing capacity after the reduction of concrete strength, and then comprehensively consider the requirements of anti-cracking, stiffness, impermeability and durability to choose appropriate treatment measures.

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Commonly used treatment method for concrete under-strength accident

(1). Determination of the actual strength of concrete.

When the results of test block compression fail and the actual strength of concrete in the structure is estimated to possibly meet the design requirements, the actual strength of concrete can be determined by non-destructive testing methods, or by drilling samples, etc., as the basis for accident handling.

(2). Using the post-strength of concrete.

The strength of concrete increases with age, and the strength of 3 months in a dry environment can be about 1.2 times that of 28d, and 1.35~1.75 times that of one year. If the actual strength of the concrete is not much lower than the design requirements, and the time of loading the structure is late, it can adopt the principle of strengthening the curing and using the later strength of concrete to deal with the accident of insufficient strength.

(3). To reduce the load of the structure.

When the load-bearing capacity of a structure is significantly reduced due to insufficient strength of concrete, and it is inconvenient to deal with it by means of reinforcement and strengthening, the method of reducing the load of the structure is usually adopted. For example, the use of efficient and lightweight insulation materials instead of ash slag or cement slag and other measures to reduce the building’s dead weight, such as reducing the total height of the building.

(4). Structural reinforcement.

When the concrete strength of the column is insufficient, it can be reinforced by outsourcing reinforced concrete or steel, or by the spiral restraint column method. When the concrete strength of the beam is low, resulting in insufficient shear capacity, it can be reinforced by outsourcing reinforced concrete and pasting steel plate. When the concrete strength of the beam is seriously insufficient, resulting in the strength of the positive section not reaching the specification requirements, reinforced concrete can be used to raise the beam, or pre-stressing tie rod reinforcement system can be used for reinforcement.

(5). Analysis and calculation to tap the potential.

When the actual strength of the concrete does not differ much from the design requirements, generally through analysis and verification, most of them can be treated without special reinforcement. Because the insufficient strength of concrete has less influence on the strength of the positive section of the bent member, so this method is often used: If necessary, on the basis of verification, do load tests to further confirm the safety and reliability of the structure, without treatment. Insufficient strength of concrete in the core area of the nodes of the assembled frame beams and columns may lead to insufficient seismic safety, but as long as the strength meets the requirements under the action equivalent to the design seismic level after verification according to the seismic code, and the cracks and deformation of the structure can continue to be used without repair or after general repair, it is not necessary to adopt special measures to deal with it. It should be noted that the conclusion of the analysis and verification that no treatment is required is valid only with the consent of the design visa. At the same time, it should be emphasized that this treatment is in effect a design potential.