In this article we will discuss about:- 1. Introduction to Workability of Fresh Concrete 2. Factors Affecting the Workability of Fresh Concrete 3. Measuring the Workability 4. Effect 5. Segregation 6. Bleeding.
Introduction to Workability of Fresh Concrete:
The strength of concrete of a given proportion is affected very much by the degree of its compaction. Therefore it is desirable that the fresh concrete can be transported and placed without segregation and bleeding, compacted and finished easily. This property of concrete is known as workability. The workability of concrete depends upon a number of properties of concrete, which cannot be satisfactorily measured.
Furthermore, workability is a relative thing, a workability of concrete suitable for mass concrete is not necessarily sufficient for thin or heavily reinforced or inaccessible sections. Therefore workability should be defined as a physical property of concrete alone without reference to the circumstances of a particular type of construction.
To obtain such a definition, it is necessary to consider the phenomenon while compacting the concrete. Whatever may be the mode of compaction, whether it is achieved by ramming or vibration, the essential feature of the process is to eliminate the entrapped air from the concrete until it has achieved as close a configuration as possible for a given mix.
Thus the work done in compaction can be divided in the following three parts:
1. To overcome the friction between the individual particles in the concrete. This is called internal friction.
2. To overcome the friction between the concrete and the surface of the mould or the reinforcement. This is called surface friction.
3. In addition to the above two works some work done is used in vibrating the mould or in shock or vibrating those parts of concrete which have already been fully compacted. Actually this work done is a waste.
Thus the work done in overcoming the friction of individual particles in the concrete is called internal friction and surface friction i.e., work done in- (1) and (2) is useful work. As the internal friction is an intrinsic property of the mix, the workability can be best defined as the amount of useful internal work necessary to produce the full compaction. In other words the property of concrete which determines the amount of useful internal work necessary to produce complete compaction is called workability of concrete.
It relates to the degree of wetness of concrete within limits. Wet concretes are more workable than dry concretes, but concretes of the same wetness (consistency) may vary in workability.
Factors Affecting the Workability of Fresh Concrete:
Following factors affect the Workability of concrete:
1. Water content,
2. Size of aggregate particles,
3. Coarse and fine aggregate ratio,
4. Particle interference,
5. Particle interlocking, and
1. Water Content:
It is the main factor which influences the workability of concrete. More the water content, higher the water/cement ratio, resulting in better workability, but the strength of concrete is reduced. Further approximately it can he assumed that for a given type and grading of aggregate and workability of concrete, the water content is independent of the aggregate/cement ratio.
On the basis of this assumption, the mix proportions of concrete of different richness can be determined. Typical values of water content for different slumps and maximum sizes of aggregate are shown in Table 7.1. These values are applicable only for non-air entrained concretes. For air entrained concrete the reduction of water content may be done according to the amount of air entrained as shown in Fig.7.1.
2. Size of Aggregate Particles:
Increase in the maximum sizes of aggregate, without any change in the mix, improves the workability of mix as smaller quantity of water is needed to wet the surface area of the aggregate particles and more water is available to act as lubricant in the mortar. Further as the size of aggregate particles increases, voids decrease and require less mortar to fill the voids. Thus more mortar is available to cause lubrication of the particles.
3. Coarse and Fine Aggregate Ration:
For a constant aggregate/cement ratio, if the quantity of coarse aggregate is increased and fine aggregate decreased to maintain the total aggregate/cement ratio constant, the total surface area of the aggregate is reduced. Thus for a constant water/cement ratio the quantity of water available per unit, area of aggregate surface is increased resulting in improvement of the workability.
4. Relative Quantities of Paste and Aggregate:
For any given paste (a quantity of cement with its definite proportion of water) decreasing the amount of paste with respect to the quantity of aggregate stiffens the mixture and increasing the amount of paste makes the mix more fluid. Further if the quantity of the paste is reduced to such an extent that there is not enough paste to fill the voids and actually float the aggregate Particles, then the mix becomes granular or harsh and its placement will be impossible.
5. Particle Interference:
Particle interference occurs when the aggregate particles of one size are present in excess in the mix and the average clear space between the particles is less than the diameter of the particles in the size group next lower to it. Where there is no particle interference, smaller grains can pass freely between the large particles to distribute themselves evenly in the mass.
When grains of one size are in excess they form bunches and the lower size particles cannot pass through them. This causes local voids in the concrete from which water can leak. The mixing water will fill up these voids and quantity of water needed for lubrication of aggregate particles will decrease, resulting in lower workability as well as density.
Thus the aggregate should be so graded in descending order of size and in quantity that the voids in coarse aggregate in its coarser fraction is filled up by finer coarse fraction, this in turn leaving individual void of sufficient size to accept the coarse particles of the fine aggregate and still leave room necessary for the cement paste and fine sand.
6. Particles Interlock:
The surface texture of the aggregate also affects the workability. Smoothed surfaced round aggregate have higher workability for the same water/cement ratio than crushed aggregate due to the mechanical interlock between the crushed aggregate particles and their relatively high coefficient of friction.
7. Grading of Aggregates:
A well graded aggregate is one, which has least voids in a given volume. Other factors being the same, when the total voids are less, more paste is available to produce better lubricating effect. With excess amount of paste, the mix becomes cohesive which prevents segregation of particles and aggregate particles will slide over each other with minimum compaction. The better the grading, the less is the void content, and higher the workability. Thus it is one of the factors which have the maximum influences on the workability.
8. Use of Admixtures:
Use of certain admixtures improves the workability of concrete.
Measuring the Workability of Fresh Concrete:
Truly speaking there is no acceptable test which can measure directly the workability. Numerous attempts have been made to correlate workability with some easily determinable physical measurements, but none is found fully satisfactory. However they may provide useful information within a range of variation in workability.
Effect of Time and Temperature on Workability of Fresh Concrete:
It has been observed that freshly mixed concrete stiffens with time due to the fact that some water from the mix is absorbed by the aggregates and some is lost by evaporation, particularly if the concrete is exposed directly to the sun or wind and some water is consumed by initial chemical reactions. The compacting factor has been found decreasing upto 0.1 during a period of one hour from mixing.
However the exact value of the loss of workability depends upon the following factors:
1. Richness of the mix
2. Type of cement
3. Temperature of concrete
4. Ambient temperature
5. Initial workability of concrete
6. Moisture condition of aggregate. (Loss being greater for dry aggregate owing to absorption of water by aggregate) and grading of aggregate.
Evans has suggested the following curve correlating slump with time. From the laboratory tests it has been found that on a hot day the water content of the mix would have to be increased for a constant workability to be maintained as shown in Fig. 7.13, Fig. 7.14 shows the effect of temperature on the slump of concrete made with different sized aggregate. Thus it is evident that on a hot day the water content of the mix would have to be increased for a constant workability to be maintained. Fig. 7.15 shows that as the concrete temperature increases the percentage increase in water required to effect a 2.5 cm change in slump also increases.
However Shalon did not observe any loss of workability with increase in temperature in a hot climate by site tests. Upto a temperature of 104°F (40°C) and a relative humidity between the ranges of 20% to 70% no effect of temperature on slump was observed. Only above 122°F (50°C) effect of temperature or below 20% humidities, the slump falls off rapidly. Actually full phenomenon is not known as yet and it is recommended that for any new conditions actual site tests should be made. Further it has also been observed that if the temperature rises from 40°F to 100°F, the slump falls from 16 cm to 7.0 cm as shown in Fig.7.16.
Segregation of Fresh Concrete:
Segregation can be defined as the separation of the constituent materials of concrete of a heterogeneous mixture so that their distribution is no longer uniform, in a good concrete all the ingredients should be properly distributed to make it a homogeneous mixture. The concrete which shows the tendency of separation say coarse aggregate separates from rest of the ingredients, such a concrete is said to be showing the tendency of segregation.
The segregation of concrete will not only produce weak, but also non homogeneous concrete which would develop undesirable properties in the hardened concrete. The difference in the size of aggregate particles and the specific gravity of the mix constituents are the main cause of segregation, but the extent can be controlled by the choice of suitable grading and by careful handling.
A well-made concrete taking various parameters into consideration such as grading, size, shape and surface texture of aggregate with optimum quantity of water makes a cohesive mix. Such concrete will not show 2.5 any tendency of segregation. The cohesive and fatty characteristics of matrix do not allow the different constituents to fall apart, causing segregation.
Types of Segregation:
Segregation can be divided into the following groups:
1. Coarse Aggregate Segregation:
In this case, the coarser particles tend to separate from the rest of the matrix. This type of segregation occurs in lean mixes when a too dry mix is prepared with same grading of aggregate. Addition of water will improve the workability.
2. Paste or Matrix Separating Out from the Mix:
This type of segregation occurs in wet-mixes. The paste separates out from the rest of the mix. This type of segregation occurs, if excessive water is used in lean mixes.
3. Water Separation:
When water separates out from the rest of the mix, then it is called water segregation. This is known as bleeding.
Favourable Conditions for Segregation:
Following conditions are favourable for segregation to occur:
1. Badly proportioned mix, where sufficient matrix is not there to bind and contain the aggregates.
2. Insufficiently mixed concrete with more water content.
3. Dropping of concrete from a height more than one metre.
4. Concrete discharged from a badly designed mixer or from a mixer of worn out blades.
5. Conveyance of concrete by conveyer belts, wheel barrow, long distance haul by dampers, long lift by skip and hoist etc.
6. Excessive vibration used for spreading concrete.
The tendency of segregation can be checked by the following measures:
1. By correctly proportioning the mix
2. Proper handling
3. Proper transporting
4. Proper placing
5. By proper compacting and finishing
Bleeding of Fresh Concrete:
It is also known as water gain. It is a form of segregation in which some of the water in the mix tends to rise to the surface of the freshly placed concrete, being of the lowest specific gravity of the all ingredients of the concrete. The phenomenon of bleeding is caused due to the inability of the solid materials of the mix to hold the mixing water when they settle down.
Causes of Bleeding:
Bleeding is observed under the following conditions:
1. In highly wet mixes
2. Badly proportioned mixes
3. Insufficiently mixed concrete
4. In thin members like roof slab or road slab etc. t
5. When concrete is placed in sunny weather.
Effects of Bleeding:
Due to bleeding following effects are observed:
1. Due to bleeding, water comes up and accumulates at the surface. Along this water certain quantity of cement also comes to the surface. When the surface is worked up with a travel and floats, the aggregate and cement goes down and water comes up on the surface, forming a cement paste at the surface, known as ‘Laitence’.
In such cases the top surface of slab and pavement will be of poor wearing quality. The laitence formed on the pavement will form dust in summer and mud in rainy season. This can be avoided by delaying the finishing operations till the bleeding water has evaporated and avoiding over working on the surface and using wooden floats.
2. As the top surface of the slab has higher water content and less aggregate matter, develops higher shrinkage cracks.
3. Due to bleeding, the top of every lift i.e. layer of concrete placed may become too wet and if this water is trapped by placing another layer over it a porous and weak layer of non-durable concrete will result. The bond with the next lift or layer will be poor due to the formation of a plane of weakness. This can be avoided by removing the laitence fully before pouring the next lift or layer.
4. If the rate of evaporation from the surface of bleeding water is faster than the bleeding rate, the plastic shrinkage cracks would develop.
5. Water rising from bottom to top makes continuous capillary channels. If the water/cement ratio used is more than 0.7, the bleeding channels remain as such and do not get filled by the development of cement ‘gel’. These capillary channels are responsible for the development of impermeable concrete structure.
6. The bleeding water in the process of coming up may be intercepted or blocked by aggregate particles and reinforcement. In such situations water is likely to accumulate below aggregate and reinforcement. This accumulation of water produces water voids below aggregates and reinforcement and reduces the bond between cement paste and aggregate and also between cement paste and reinforcing bars. The ill effect of poor bond between cement paste and aggregate or cement pate and reinforcement may be remedied by re-vibration of concrete.
Prevention of Bleeding:
The amount of bleeding depends largely on the properties of cement.
It can be decreased by the following measures:
1. It can be reduced by increasing the fineness of cement.
2. It can be reduced by adding high contents of C3A in the cement.
3. It can also be reduced by adding calcium chloride to the cement.
4. Rich mixes are less prone to bleeding than lean mixes.
5. Addition of pozzolanic or air entrainment admixture reduces the bleeding of concrete.
Determination of Bleeding Water Content:
The amount of bleeding water is determined by the following method.
Following equipments are used for this test:
(a) Cylindrical container of inside diameter of 250 mm and inside height 280 mm. The capacity of the container is about 0.01 m3.
(b) Tamping rod of steel 600 mm long and 16 mm in diameter having one end round as a bullet.
(c) A pipette for drawing free water from the surface of the concrete.
(d) A graduated jar of 100 cm3 capacity.
A sample of freshly mixed concrete is taken and filled in the cylindrical container upto a height of about 50 mm layer and tamped by the tamping rod 25 times. In this way 5 layers in all giving a total depth of concrete 250 ± 3 mm are filled and each layer is tamped before adding the next layer. After filling the concrete upto 250 ± 3 mm depth the top layer is made smooth by trowelling.
Now the test specimen is weighed and its weight is noted.
Let the weight of empty container = w1
the weight of container + concrete = w2
∴ weight of concrete = w2 – w1
Now the cylindrical container is placed on a level surface free from vibrations at a temperature of 27°C ± 2°C and covered with a lid. Water accumulated at the levelled surface of the concrete is withdrawn with the help of pipette at 10 minutes interval for the first 40 minutes and at 30 minutes interval after wards till the bleeding ceases. All the water is collected in the 100 c.c. capacity jar. To facilitate the collection of bleeding water, the container may be tilted slightly. Knowing the total water content in 1 m3 of concrete, the water content in the cylindrical container may also be calculated.
Thus bleeding water percentage = (Total quantity of bleeding water collected/Total quantity of water in the sample of concrete) x 10