If the sand rate is too low, the concrete workability will be poor, and the pump and pipe will be blocked easily
.
Workability is a general concept for evaluating the fluidity, stability and other comprehensive technological properties of concrete mixture in terms of consistency of concrete mixture, which cannot be expressed quantitatively
.
For each higher grade, more than 10% can be added
.
It contains Cao, but the amount is less, which is more than fly ash
.
The test shows that the water consumption of ± 5-8KG / m3 can affect the slump of concrete about ± 20mm and the strength of concrete about ± 1MPa
.
When the alkali content in cement is low and high, the adaptability to superplasticizer is poor and the sulfate resistance is weakened
.
The results show that the two materials play a super superposition effect of pozzolanic effect, morphology effect, filling effect and micro aggregate effect
.
Under the best sand rate, the concrete strength will be reduced by more than 4% for every 1% sand rate
.
3.4 due to the influence of cement varieties, the cement with unstable quality can not be used for commercial concrete, and P · I, P · II, P · o cement should be preferred; the cement with less water demand is better, which is related to its mineral composition, fineness, C3S, C3a content or early strength
.
When the slump tube is lifted, the concrete collapses or shears at one side, which indicates that the workability of the concrete is not good; when the slump tube is lifted, it indicates that the workability of the concrete is good
.
(2) Cohesion of concrete: it is also qualitative and cannot be expressed quantitatively
.
The strength of concrete mixed with slag powder is higher than that of concrete mixed with fly ash, and the strength of concrete mixed with slag powder is almost equal to that of pure cement concrete
.
(1) Workability of concrete: at present, there is no accurate definition of workability of concrete, so it is difficult to accurately express it with a certain technical quantitative index
.
Generally, the medium sand in zone II is better for pumping concrete, and the fineness modulus of sand is above 3.6
.
If three stones are added together, the diameter of the pipe is just stuck, and the slurry can not pass through during pumping, which is easy to cause blockage
.
If the cone gradually sinks, it means that the cohesiveness is good
.
The “three properties” of concrete should be considered comprehensively, not only to improve the fluidity of concrete and increase the water consumption, but also to reduce the cohesion and water retention
.
The pumping resistance is small, and the pipe and pump are not blocked
.
There are many factors affecting the working performance of concrete, mainly in the following aspects: 3.1 specification for the influence of unit water consumption: there are provisions on the maximum w / B ratio of concrete and the minimum amount of cementitious materials
.
Therefore, concrete slump is the external performance of internal quality of concrete, which is extremely important
.
Through the test, the best sand ratio of pumping concrete should be 38% ～ 45%, which is about 3% ～ 5% higher than that of ordinary concrete
.
Therefore, to keep the w / b unchanged, the amount of cementitious materials should be increased while increasing the water consumption
.
Tap gently on the side of the collapsed concrete cone with a tamping rod
.
(3) Water holding capacity of concrete: refers to the ability of concrete mixture to have certain water holding capacity in the construction process, so that the concrete will not produce more serious water separation – bleeding phenomenon
.
Therefore, do not try to be cheap, but buy the crushed stones broken by impact crusher
.
Two kinds of admixtures can significantly improve the workability of concrete
.
The standards of concrete and the requirements of concrete workability are expressed by slump
.
The practice has proved that the slump loss in 30min and 60min should not be more than 20 ~ 30mm
.
The maximum particle size of the stone should not exceed 5%, and it should not be greater than 1 / 3 of the conveying pipe diameter
.
Workability of concrete: workability, cohesiveness and water retention are the three main characteristics of workability of concrete
.
If the sand rate of concrete is too high, the concrete strength will be reduced
.
The content of C3A should be 4% ～ 6%, and it has good adaptability with additives
.
Through the actual measurement, the static loss of pumping concrete slump is about 20 mm larger than the dynamic loss
.
The specific gravity of fly ash is about 2.2, which is 1 / 3 lighter than that of cement
.
The content of cement slurry in the concrete is large, the concrete has good workability and is easy to construct
.
When pumping concrete, it can flow as a whole no matter how high and how far away it is
.
The slump of concrete with fly ash is 170mm, which is better than that of pure cement concrete without fly ash 190mm
.
Generally, for a given w / B and aggregate, the reduction of aggregate cement ratio will improve the workability of concrete, but it should not be too small, and the influence on the reduction of concrete strength should also be considered
.
3.3 the aggregate cement ratio is the ratio of aggregate to cement slurry, which has a great influence on the workability of concrete
.
Cohesiveness refers to that the concrete mixture should have certain cohesiveness in the process of transportation and pouring, and it can flow forward as a whole when pumping, and the wrapping force and cohesive force of concrete should be strong; the concrete does not produce stratification and segregation phenomenon, so that the concrete can obtain the overall uniform performance, and ensure that the pumping is unobstructed and there is no blockage phenomenon
.
Therefore, the concrete truck keeps turning (3-6 or 4-8 RPM)
.
Pumping concrete is not suitable for use, and the super fine sand is not suitable for 30%, and the sand passing 0.16mm sieve is not less than 5%
.
Compound mixing is not a simple mixing, not 1 + 1 = 2, but a technical and economic effect of 1 + 1 > 2.
.
If the cone collapses, cracks or segregation, it means that it is not good
.
The needle flake stones broken by jaw crusher exceed the standard, the particle shape is not good, the large particles are very large, and the grading is poor, which is most likely to cause the pumping concrete blockage and affect the service life of the pump truck
.
Under the same cement slurry conditions, the concrete has good workability and easy construction operation
.
The appearance state of concrete can be observed and judged by experience
.
Keeping w / b constant, keeping the content of cement slurry in concrete constant, and making the absolute volume of cement slurry in concrete about 0.3 ~ 0.35m3, can not only ensure the quality of concrete, but also improve the working performance of concrete, which is easy to construct
.
If more slurry precipitates from the bottom and the aggregate is exposed, it indicates that the water retention of the concrete is poor; after the slump tube is lifted, no slurry or a small amount of slurry precipitates from the bottom, it indicates that the water retention of the concrete is good
.
It is a comprehensive requirement that we require concrete to maintain certain working performance
.
In order to facilitate the quality control, the concrete slump can be divided into five levels, and the expansion can be divided into six levels (when the concrete slump is greater than 220mm, the expansion should be measured in mm), as shown in Table 1
.
When C3a is increased by 1%, the standard consistency is increased by 1%, and the water consumption is increased by about 5% ～ 8%
.
When the particle size of aggregate is large and the grading is good (the gap is small, the total surface area is small, and the large and small particles fill the gap with each other), the cement slurry used is relatively small
.
“R” type cement can be selected in spring and autumn and winter, ordinary type cement can be selected in summer, or cement mixed with admixture or increase the amount of admixture
.
About 30% fly ash, 40% ground slag and more than 50% compound slag can be added
.
When measuring the slump, observe the cohesiveness and water holding capacity of the concrete sample at the same time
.
If the unit water consumption is constant, the best sand ratio can make the concrete have the maximum fluidity, and can maintain good cohesion and water retention
.
Commercial concrete must use fly ash above grade II and ground slag above grade S75
.
3.2 the particle size, gradation and water absorption of aggregate have great influence on the working performance of concrete
.
Different from slag cement, slag powder can reach a certain fineness, exert its potential, and will not bleed
.
3.5 influence of concrete admixtures the most common concrete admixtures are fly ash and slag powder
.
Generally, the aggregate cement ratio is about 60:40 or 65:35, and the influence of W / B, water consumption, maximum particle size and particle shape of aggregate should be considered comprehensively
.
The water retention is evaluated by the degree of slurry precipitation of concrete mixture
.
The workability, cohesiveness and water retention of concrete can be judged by concrete slump test
.
1
.
3
.
Concrete slump refers to the pouring slump at the time of pouring
.
