Workability

Workability is that property of concrete which determines the case with which it can be placed in position and compacted. Workability is normally measured by the ‘slump test’ as a guide. In order to obtain concrete of maximum strength, good compaction is essential and this can only be achieved if the concrete has adequate degree of workability in relation to the method of compaction to be used. The workability if concrete should be just sufficient to enable the concrete to be compacted fully by whatever method is employed. Concrete that is to be placed in narrow forms congested with reinforcement will require a much higher degree of workability (i.e. fluidity) than that for unreinforced mass concrete. Concrete which is to be compacted by mechanical vibration may be much drier than that which is to be tamped by hand. This concrete needs about 20 percent less water and about 15 percent less cement. As such, a drier concrete which has been compacted by vibrations gives more strength and density for the same quantity of materials. The principle factors which effect the workability of concrete are :
Consistency

For RC works, concrete which will flow sluggishly into the forms and around the reinforcement without any segregation of coarse aggregate from the mortar, has to be used. The degree of consistency, which shall depend on the nature of the work and weather the concrete is to be vibrated or hand tamped, shall be determined by slump tests.

The quantities of materials may be regulated by carrying out regular slump tests. More water will be needed for dry conditions and less for wet and cold conditions. Structures in contact with water should be made of drier cement.
Rapid hardening cement needs about 4.5 liters more water per 50 kg of cement than ordinary cement.


Water cement ratio or water content of a concrete mix

Water cement ratio of the water in a mix (excluding water already absorbed by the aggregate) to the weight of cement therein, and this is the most important factor governing the strength of a concrete. The strength of a concrete depends mainly upon the amount of cement and the amount of water in it. The (the quantity of water varies with the size and shape of both the fine and coarse aggregate), method of compaction applied (whether with vibrations or hand tamping), and the weather conditions.



 Water content and workability

The workability of a concrete increases as the water content of the mix is increased, water lubricates the mixture. But increase in water content would cause a decrease in strength. Excess of water weakens a concrete, produces shrinkage, cracks (shrinkage under “contraction and expansion joints”). Water occupies space in concrete and as it evaporates it leaves voids and cracks. The volume of water voids may be as much as 10 percent of the total volume of concrete. An excess of 10 percent of water may reduce the strength by about 15 percent and an excess of 30 percent of water may reduce the strength by half. Generally speaking, lower the water content the stronger the concrete but the quantity of water must be sufficient to produce a workable mix required for the particular method of compaction to be adopted.

Concrete made with low water/cement ratio is un workable. If stiff or dry concrete is used honey-combing will result decreasing density and strength. An unworkable concrete results in incomplete compaction giving rise to air voids. Presence of 5 percent air voids will cause a 30 percent strength loss and 10 percent air voids may cause as much as 50 percent strength loss. Therefore, there is an optimum value of the water/cement ratio for every mix. The quantity of water has to be restricted within certain minimum limits. Concrete should be just plastic enough to be worked around the reinforcement rods.

Sometimes strength has to be sacrificed by adding more water to obtain a higher degree of workability where concrete has to be place in narrow and thin sections. The best mix is the one which gives the maximum workability with the minimum amount of water. An increase in water content must be accompanied by a proportionate increase of cement if strength is to be maintained.



Grading of aggregate

Other things being equal the workability of concrete is greater with aggregate of larger maximum sizes. For dry mixes workability is generally greater with rather coarse aggregate gradings but for wet mixes better results are often obtained with finer gradings.


Shape of aggregate

A smooth and rounded aggregate will produce a more workable concrete than sharp angular aggregate (crushed rock or crushed gravel). A flaky aggregate produces the harshest or most unworkable concrete. (aggregate producing more workable concrete need less water and hence give higher strength).


Cement content

The higher the cement the greater the workability and the less the effect of grading. As such, much greater latitude in grading can be permitted with a rich mix (high cement content) than with a lean mix (low cement content), but that is uneconomical. A slight increase in the quantity of cement increases correspondingly the concrete strength provided water/cement ratio is kept constant.


Segregation

Segregation is the separating of the coarse aggregate from the rest of the mix or the separating of the cement water paste from the aggregate. Segregation generally indicates poor aggregate grading or mix design. Segregation may occur in mixes which are too wet or too dry, and most frequently in under-sanded mixes.

Segregation can generally be reduced by altering the water or sand content or by using a finer sand. Even with a mix of satisfactory design, segregation may be caused by mishandling during transport, faulty placing or over-compaction. Segregation leads to lack of uniformity causing honey-combing which reduces the strength and durability of the structure.

If segregation occurs the larger particles of aggregate tend to move to the bottom and this causes undesirable variation of strength through the thickness of the slab.



Bleeding

Bleeding is the appearance of a watery scum (also called laitance) on the surface of a concrete after compaction. It is an indication that there is too much water or deficiency of fine material in the mix, or that too much tamping, floating or toweling has been done. The result is a porous, dusty and weak surface. This scum should be removed. Bleeding makes weak joints between successive lifts in structural work. Bleeding can be reduced by using less water, a finer sand, or by adding a finely ground inert material (stone dust).

The aggregate commonly used are seldom found in a perfectly dry state in the field. Moreover, aggregates have to be washed very often for removing impurities which further add to the moisture content. The weather conditions, and this is especially so in the case of sand. The aggregate when dry will absorb water from the concrete and when wet at the surface the mixture will have excess of water. Therefore, while computing the quantity of water due consideration must be given to the surface conditions of the aggregate that would exist at the time of preparing the mix.

Small size of aggregate need more water than big size and angular aggregate need more than rounded aggregate. In other words, a concrete containing a finely graded aggregate will require more water for a given workability than one containing an aggregate with a coarse grading. Consequently, the more finely graded aggregate, or that containing a larger proportion of fine aggregate (and similarly a concrete with angular aggregate) will produce a weaker concrete.



Hydration of cement

When a water is added to cement, the cement hydrates, calcium hydroxide or hydrated lime is liberated. During the chemical reaction which takes place while cement is setting and hardening an increase in temperature occurs and considerable quantity of heat is evolved. Shrinkage occurs on subsequent cooling resulting in cracks. Hydration of cement is incomplete without an adequate quantity of water. Less water impedes complete setting of cement and decreases strength. The amount of water required to hydrate cement is about 25 percent of the weight of the cement. The amount of mixing water is rarely less than twice this quantity.

If water/cement ratio is less than 0.4 to 0.5 complete hydration of cement will not occur. Roughly water/cement ratio is 0.60 for a 1:2:4 mix, 0.5 for 1:1.5:3 mix, and 0.45 for 1:1:2 mix.

Quantity of water and aggregate per 50 kg (one bag) – about 35 liters of cement for Hand Compaction.

Mix
Water litres
Fine Aggregate litres
Coarse Aggregate
litres

1:1:2
21 to 27
35.0
70.0
For vibrated concrete, the quantity of water can be reduced by about 20 percent
1:1.5:3
26 to 30
52.5
105.0
1:2:4
29 to 32
70.0
140.0
1:3:6
34 to 36
105.0
210.0
1:4:8
45 to 47
140.0
280.0

If sand is wet, increase its quantity by 25 percent and reduce quantity of water by 20 percent.


Litres of water per 50 kg of cement
Water/cement Ratio by Weight
Cubes Crushing Strength at 28 days in kg/sq.cm.

17.5
0.35
530
Mix too dry for hand compaction.
20.0
0.40
470
22.5
0.45
420
25.0
0.50
370
27.5
0.55
320
Mix workable for hand compaction.
30.0
0.60
280
32.5
0.65
250
35.0
0.70
220
37.5
0.75
200
40.0
0.80
180