Raw material quality issues
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.
- 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 some unqualified cement made of concrete without obvious cracks on the surface of the concrete, but the strength is extremely low.
Poor quality of aggregate (sand, stone)
- 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.
- 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.
- 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.
- High levels of organic impurities in aggregates (especially sand).
Such as aggregate containing decaying plants and animals and other organic impurities (mainly tannic acid and its derivatives), which adversely affects the hydration of cement, and reduces the strength of concrete.
- 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.
- 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.
- High mica content in sand.
Due to the smooth surface of mica, the adhesion to 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 of concrete (including strength).
Mixing water quality is not up to standard
The use of high organic impurities in swamp water, sewage and industrial wastewater containing humic acid or other acids and salts (especially sulfate) may result in the deterioration of the physical mechanical properties of concrete.
Poor quality of admixture
At present, some small plants produce additives failed to meet the quality of the phenomenon is quite common, due to additives caused by the lack of concrete strength, or even concrete does not set the accident happens from time to time.
The improper concrete mix 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 expressed in the following aspects in the construction of the project.
Randomly apply the compound ratio.
Concrete mix ratio is determined by the site application to the laboratory for test mix according to the project characteristics, construction conditions and raw materials. 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.
Increased water consumption.
The more common 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 water consumption, the water-cement ratio and slump of concrete will increase, resulting in accidents of insufficient strength. Click>>Project data free download
Inadequate cement use.
In addition to inaccurate measurement before mixing, insufficient weight of packaged cement also occurs repeatedly, resulting in insufficient amount of cement in concrete, resulting in low strength.
Inaccurate measurement of sand and stone.
More common is the measurement of old tools or poor maintenance management, accuracy failure.
the wrong additive.
There are two main types; one is the wrong species, in the admixture is not clear early strong, slow-setting, water-reducing properties, such as blindly mixed admixture, resulting in concrete does not reach the expected strength; second is not mixed.
an aggregate reaction of alkali.
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. Japan has information, in other conditions, the same situation, the alkali aggregate reaction of concrete strength is only about 60% of the normal value.
the concrete construction process there are problems
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.
Poor transport conditions.
Concrete segregation is found during transportation, but no effective measures are taken (such as re-mixing, etc.), and slurry leakage from transportation tools affects the strength.
Improper pouring method.
If the concrete has been set at the time of pouring; if the concrete has been segregated before pouring, etc., it may result in insufficient strength of concrete.
Serious leakage of slurry from formwork.
In a project, the steel mold was seriously deformed, the slab seam was 5-10mm, the slurry leaked badly, and the strength of the concrete 28d measured only reached half of the design value.
Forming vibration is not compact.
The porosity of concrete is 10%~20% after entering into the mould, if the vibration is not solid, or the formwork leaks, it will certainly affect the strength. Click>>Project data free download
Poor conservation regime.
Mainly insufficient temperature and humidity, early water shortage and drying, or early freezing, resulting in low strength of concrete.
poor management of test block
The specimen has not been maintained to standard.
Some construction sites and testers still don’t 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.
Poor management of the specimen.
Test mold deformation is not repaired or replaced in a timely manner.
Failure to make test blocks as specified.
For example, the size of the test mold and the size of the stone are not suitable, too few stones in the test block, and the test block is not vibrated with the corresponding machine.