Concrete water secretion and segregation
When formulating fluid concrete, if the concrete cohesion and water retention is poor, the homogeneity of the composition of the materials and stability of the equilibrium state will be broken, the concrete in its own gravity and other external forces under the action of separation, that is, segregation. If the mixing water precipitation out of the surface, that is, water secretion. Usually, water secretion is a prelude to segregation, which inevitably leads to delamination and increases the possibility of pump blockage. A small amount of water secretion is allowed in the project, and is beneficial to prevent the production of concrete surface cracks.
The causes and countermeasures of concrete water secretion and segregation.
- Low sand rate or sand in the content of fine particles, so that the concrete water retention is low, the sand contains a large amount of mud easy to produce slurry settlement, that is, “Catch the bottom.
Solution: Increase the sand rate, reduce the amount of mud in the sand, a reasonable sand rate can ensure the workability and strength of concrete.
- The total amount of cementitious materials is less, the volume of the slurry is less than 300l/m3.
Solution: Add fly ash, especially for the preparation of low strength grade of large fluidity concrete, the amount of fly ash admixture should be appropriately increased. Thus improve its water retention.
- Poorly graded stones or a single grain size of stone.
Solution: Adjust the stone gradation, single grain size stone should be increased sand rate.
- The water consumption is large, so that the concrete mix cohesion is reduced.
Solution: Increase the water reduction rate of admixture or increase the admixture mixture, reduce the water consumption.
- The admixture admixture is too large, and the admixture contains the ingredients of water secretion.
Solution: Reduce the admixture or add thickening components and air-entraining components in the admixture to improve the cohesiveness of concrete and prevent water secretion and segregation.
- Due to excessive storage time, the clinker in the cement has been partially hydrated, making the cement poor water retention.
Solution: Use compound thickening component and early strength component in the admixture.
- Use slag powder or slag silicate cement, which has poor water retention, easy to water secretion and segregation.
Solution: Increase the amount of cement or fly ash, reduce the amount of slag powder, or replace the cement species.
Hysteresis waterlogging of concrete
Lagging water is the phenomenon that the concrete works as required at the beginning, but produces a large amount of water secretion only after a period of time (for example, 1h). The possible reasons for its generation are: Low sand rate, more admixture slow-setting components, etc.
The causes and countermeasures of lagging water secretion.
- The real sand rate is low, sand containing too much stone
Solution: Increase the sand rate, increase the real sand content.
- Low content of fine particles in sand
Solution: Increase the amount of admixture and make necessary supplement.
- Irrational stone gradation, single grain size
Solution: Increase sand rate by 2~5%
- Cement, admixture water secretion rate is large
Solution: Replace cement and admixture; increase thickening component in admixture.
- Coarse fly ash particles, high carbon content.
Solution: Replace the fly ash.
- Low strength grade concrete is used
Solution: Use air-entraining agent or increase the amount of cementitious materials.
- For high strength grade concrete
Solution: Reduce the amount of admixture or reduce the slow setting component in the admixture.
- There is water in the tanker
Solution: Reverse the mixing tank before loading ash and discharge the stored water.
- Unknown reason
Solution: Change the admixture formula or take the above comprehensive measures.
Abnormal condensation of concrete
- Rapid setting: Concrete sets rapidly after mixing. This phenomenon is rarely encountered in daily work, generally is: The factory temperature of cement is too high, cement in the gypsum is seriously insufficient, admixtures and cement serious misfit, hot water and cement direct contact, etc..
- 2. the condensation time is too long: This phenomenon is often encountered, it can be divided into two cases: The
- the overall serious slow setting.
- local serious slow setting.
The first case is mostly caused by the admixture, due to the addition of inappropriate slow-setting components (there are many slow-setting components by the temperature and other effects of the significant changes in its setting time), or admixtures beyond the normal amount of admixture, resulting in excessive slow-setting of concrete.
The second situation, such as the majority of the floor or wall concrete set normally, the local concrete slow-setting, the reasons may be.
- admixtures used after the admixture method, concrete mixing is not uniform, resulting in local enrichment of admixtures.
- the site with water, concrete cohesion is reduced, water secretion or segregation, pouring vibration so that the local slurry concentration, water-cement ratio becomes large and the admixture is relatively excessive;.
- the admixture pool with slow-setting components of the sediment is not easy to mix evenly, resulting in local excessive concrete slow-setting.
Concrete “Hard shell” Phenomenon
After pouring concrete, the concrete surface has been “Hardened”, but the internal is still unset state, forming a “Sugar core”, let’s call it the “Hard shell” Phenomenon. And often accompanied by cracks of varying degrees, the cracks are difficult to smooth out with a trowel. This phenomenon often occurs in the hot weather and dry climate of the season. In fact, the surface is not really hardened, to a large extent due to the rapid evaporation of water so that the concrete loss of water drying caused. The strength of the surface layer of concrete will be reduced by about 30%, and then pouring water maintenance will not help.
In addition to climatic factors, the composition of the admixture ingredients and the type of concrete admixture also have a certain relationship, admixtures containing sugar and similar slow-setting components are easy to form a hard shell. When using mineral powder is more obvious than fly ash.
- the admixture formula to make appropriate adjustments, slow setting components such as phosphate, avoid the use of sugar, calcium wood, glucose, sodium gluconate, etc..
- the use of fly ash as admixture, its water retention performance is superior to that of mineral powder.
- such as the surface produces fine cracks, can be used in the concrete before the initial setting of the second vibration to eliminate cracks, so as to avoid further formation of penetrating cracks.
- the most effective way should be construction maintenance measures, that is, try to avoid direct sunlight on the concrete, just after the concrete is poured can be sprayed and sprinkled with water and other maintenance methods.
Concrete field slump is larger than out of the machine
When formulating concrete with higher strength grade, sometimes there is a phenomenon that the field slump is larger than the outgoing slump, which may be caused by.
the use of amino sulfonate or its admixtures with similar properties.
more slow-setting components in the admixture or more violent late reaction.
inappropriate matching ratio (such as small sand rate, too much admixture, etc.) leading to late water secretion.
there is water in the concrete tank.
Solution: The first three types of problems can be found and adjusted through trial matching in the laboratory (slump loss and setting time, etc.), and the actual production should be strictly controlled in terms of admixture admixture and water consumption.
Concrete production process slump loss suddenly accelerated
Concrete in the production process suddenly found slump loss faster, the possible causes are.
change of water-reducing component of admixture.
less admixture in the pool, mostly precipitated sodium sulfate and other early strength components.
cement composition changes, etc. These problems can be solved by adjusting the admixture components or its dosing.
“Salt precipitation” Phenomenon
Winter or spring, autumn test blocks or build surface sometimes appear “Salt” Phenomenon. Its external because of temperature changes, its internal because the concrete in the sodium sulfate (purity of not less than 98%) dosing more than 0.8% of the weight of cement that will appear on the surface of the salt phenomenon, it is not conducive to surface decoration. High alkali content of concrete can also lead to the above phenomenon. In addition, it may also be related to the setting time of cement (heat of hydration peak), early strength cement generally does not appear the phenomenon of salt precipitation.
Volcanic ash cements are not suitable for dry environments
If volcanic ash cement is used in a dry environment, the internal water will evaporate quickly, the hydration reaction will be suspended and the strength will stop growing, and the formed hydrated calcium silicate gel will gradually dry out, resulting in large volume shrinkage and internal stresses, thus forming micro cracks.
On the surface, due to the carbonation of the hydrated calcium silicate gel powder into a powdered mixture of calcium carbonate and silicon oxide, so that the surface of the concrete has been hardened to produce “Powder” Phenomenon. Therefore, for the concrete on the ground in a dry environment, volcanic ash cement should not be used.
A project has used volcanic ash cement preparation of concrete, the structure of the same conditions for the production of specimens than the standard 28d strength of 10 ~ 30% lower, and specimens broken inside the different degrees of “Fall powder” Phenomenon, this example fully illustrates the hydration reaction of volcanic ash cement in a dry environment is not sufficient.
The use of expansion agents need to pay attention to the problem
Although the use of expansion agents to control the concrete cracking method has achieved very significant results, but the application of expansion agents engineering cracking and seepage accidents are also on the rise.
Research data show that calcium sulfoaluminate expansion agent added to the cement after hydration of the formation of calcium alumina, the adsorption and detachment of crystalline water is a reversible process. In dry conditions, it is easy to take off and form intermediate hydrates, so the dry shrinkage is larger. Furthermore, calcium sulfoaluminate is unstable at high temperature, and the high hydration temperature rise of bulk concrete will make the calcium alumina generated at the early stage of hydration decompose, and there is a possibility of delaying the generation of calcium alumina, which will not only fail to produce expansion but also produce larger cold shrinkage, and cannot achieve the purpose of compensating temperature shrinkage.
In addition, the formation of hydrated calcium sulfate also requires a large amount of water, when the water supply is not sufficient it constantly consumes the water inside the concrete and produce self-shrinkage. The introduction of mgo in the expansion component has a unique effect on inhibiting the late shrinkage of concrete and preventing cracking. Mgo has better late expansion performance, which to a certain extent makes up for the defects of volume shrinkage of cement after hardening, enhances its anti-cracking ability in the bulk concrete project, and improves the integrity, safety and durability of the project.