There is no a simple, rapid and accurate determination method to completely reflect the indexes of workability because the connotation of concrete workability is complex. Mobility has great impact on the properties of mixture through actual analysis. Thus, mobility is often used to test concrete mixture assisted by observation of viscidity and water retention to judge whether fresh concrete can satisfy the demand of projects. The mobility of fresh concrete is expressed by slump and vebe consistometer.
(1) The Determination of Slump
Install the concrete mixture into the standard conical tube (slump cone) in a certain way, tamp it evenly, fill it fully and shave it smoothly; lift the cone vertically and the mixture slumps downwards under the role of deadweight;
measure the height difference (mm) between the cone and the peak of concrete specimen after slump, namely the slump (see Figure 5.6) which is usually expressed by T.
I4 200 A
Figure 5.6 The Determination of Slump (unit:mm)
The higher the slump is, the better the mobility will be. The cohesion and water retention can be also inspected when slump is determined. The inspection method of cohesion is: to tap the side of the cone full of slumped concrete with tamper slightly, and if the cone can fall down gradually after slight tapping, it means that the cohesion is good. If the cone collapses or cracks partly, it indicates that the cohesion is not good. The inspection of water retention is: that if the concrete mixture loses pulp and the aggregates appear, or more slurries flow out from the bottom of the cone, it means that the water retention is bad.
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Structure Categories
Cushion layer for foundation or ground, massive structure without steel bars (retaining walls, foundations, etc.) or reinforced sparse structure
Plates, beams and cross-section pillars of medium and large size Reinforced dense structure (thin walls, bins, silos, and pigtail post) Reinforced extreme-dcnse structure
According to different slumps, concrete mixtures can be divided into four grades: great mobile ( T 3 160mm); mobile ( T for 100-1 50mm); plastic ( T for 50 - 90mm); and low plastic (Tfor 10 - 40mm). The determination of slump is only limited to the concrete mixture of which .the aggregate’s biggest particle diameter is no more than 40mm and the slump is no less than 10mm.
In the plastic concrete projects, the slump should be tested in terms of the cross-section size of specimen, the density of steel bars and the tamping method. Slump should be in line with Table 5.10, Code for Construction and Acceptance of Concrete Structure Works (GB50204).
Slump 10-30 30-50 50-70 70-90 Table 5.10 The Slump of Concrete in Casting (mm)
(1) The Quantity of Cement and Water-cement Ratio
The mobility of concrete mixture depends on cement paste. In unit volume, the more the cement paste, the greater the mobility, if the water-cement ratio is
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the same. However, if cement paste is too much, pulp fluid will occur; and if cement paste is too little, the cohesion among aggregate particles will be small, which may lead to segregation and collapse.
If the using amounts of cement and aggregate are the same, increasing the water-cement ratio can strengthen the mobility; on the contrary, the mobility will be lowered. Still, if the water-cement ratio is too big, the cohesion and water retention of the mixture will decrease; if the ratio is too small, the mobility will decrease, which may impact projects. Thus, the ratio should be chosen reasonably according to the strength and durability of concrete. It should be noted that both the impact of cement paste and that of water-cement ration are basically the impact of water consumption. Therefore, the key factor to affect the workability of concrete mixture is the water consumption per unit volume. In the light of experiments, with fixed amount of aggregate, if the unit water consumption is certain and the amount of cement is within 50-100kg, the slump remains unchanged, which is usually known as the fixed water consumption law. It is very convenient to adopt this law to design mix proportion of concrete. Through the fixed water consumption, it can not only meet the requirements of workability by changing water-cement ratio, but also satisfy the requirements of design.
(2) Sand Percentage
Sand percentage refers to how large the quality of sand within concrete is relative to the total mass of sand and stone. Sand percentage has great influence on mixture. Figure 5.7 is the impact of sand percentage on slump.
On one hand, sand slurry can function as a lubricant among aggregates, and within a certain sand percentage, .the bigger the percentage is, the more obvious the lubricating effect will be, and thus the mobility can increase; on the other hand, the increase of sand percentage can lead to the increase of the total surface area of aggregates, and more water is needed. If the water consumption is fixed, the mobility of mixture will decrease. Thus,.if the percentage is beyond a certain range, the mobility will decrease with the increase of sand percentage. Otherwise, if the percentage is too small, the quantity of sand slurry will be small, which can reduce the cohesion and water retention of mixture. Then, segregation and pulp fluid will happen. Therefore, reasonable sand percentage should be chosen to ensure mobility, cohesion and water retention under the fixed quantity of water and cement.
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Figure 5.8 The Impact of Slump on Temperature
With the extension of time, concrete mixtures can become dry and dense and their mobility will decrease, because part of the moisture in mixtures is absorbed by aggregates, part of it evaporates, and part of it reacts with cement and turns into bound water of hydration product. Figure 5.9 is the relationship between slump and time.
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Figure 5.9 The Relation between Slump and Time
5.3.2 The Strength of Hardened Concrete
Ordinary concrete is the major building material and strength is its main technical property.