Chapter – III Mix Design of Concrete and Property of Green Concrete
3.1 Workability and its test The strict definition of workability is the amount of useful internal work necessary to produce full compaction. The useful internal work is a physical property of concrete alone and is the work or energy required to overcome the internal friction between the individual particles in the concrete. In practice, however, additional energy is applied to overcome the surface friction between concrete and the formwork or the reinforcement. Also, wasted energy is consumed by vibrating the form and in vibrating the concrete which has already been compacted. Thus, in practice, it is difficult to measure the workability as define, and what we measure is workability which is applicable to the particular method adopted. Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated, without excessive bleeding or segregation. Workability is one of the physical parameters of concrete which affects the strength and durability as well as the cost of labour and appearance of the finished product.
Factors affecting Concrete Workability 1. Water Cement Ratio The main factor affecting the workability of concrete is the water content of the mix since by simply adding water, the interparticle lubrication is increased resulting in higher fluidity and hence higher workability. However, to achieve optimum conditions for minimum voids or for maximum density with no segregation, the influence of the aggregate type and grading has to be considered. 2. Amount and Type of Aggregates For a constant w/c ratio, the workability increases as the aggregate/cement ratio is reduced because the amount of water relative to the total surface area of solids is increased.
A rather high ratio of volumes of coarse to fine aggregates can result in segregation and a lower workability, so that the mix is harsh and not easily finished.
Too many fines lead to a higher workability, but such an oversanded mix makes less durable concrete.
3. Amount and Type of Cement Fineness of cement is of minor influence on workability but the finer the cement, the greater the water demand. 4. Weather Conditions (Temperature, Wind) A higher temperature reduces the workability (due to evaporation of water) and increases the slump loss.
If the concrete is exposed to the sun or wind, some of the water is lost by evaporation thus causing the stiffening of concrete and a loss of workability with time.
5. Chemical Admixtures Chemical admixtures can be used to increase workability. Use of air entraining agent produces air bubbles which act as a sort of ball bearing between particles and increases mobility, workability and decreases bleeding, segregation. The use of fine pozzolanic materials also has better lubricating effect and more workability.
Workability Tests 1. Slump Test
Tools and Apparatus used for Slump Test: 1.
Standard Slump Cone (100 mm top diameter x 200 mm bottom diameter x 300 mm height)
2. Bullet-nosed road (600 mm long x 16 mm diameter) 3. Slump Plate (500 mm x 500 mm) Procedure: i.
The mould in the form of a frustum of a cone is placed on a smooth surface with the smaller opening at the top and filled with concrete in three layers.
ii.
Each layer is tamped 25 times with a standard 16 mm φ steel rod.
iii.
The top surface is struck off by means of a screeding and rolling motion of the tamping rod.
iv.
The mould must be firmly held against its base during the entire operation; this is facilitated by handles or foot-rests brazed to the mould.
v.
Immediately after filling, the cone is slowly lifted and the unsupported concrete will now slump. The decrease in the height of the centre of the slumped concrete is called slump.
Precautions:
The inside of the mould and its base should be moistened at the beginning of every test.
The test should be repeated in case a shear slump is observed.
Slump Test is not suitable for too wet or too dry concrete. 2. Compacting Factor Test
This test uses the inverse approach i.e. while workability is defined as the amount of work necessary to achieve full compaction, in this test, the degree of compaction achieved by a standard amount of work is determined.
Tools and Apparatus required: 1.
Two hoppers, each a frustum of a cone
2.
One cylinder
The hoppers have hinged doors at the bottom. All inside surfaces are polished to reduce friction. Procedure: i.
The upper hopper is filled with concrete, this being placed gently so that, at this stage, no work is done on the concrete to produce compaction.
ii.
The bottom door of the hopper is then released and the concrete falls into the lower hopper. This lower hopper is smaller than the upper one.
iii.
The lower hopper is filled to overflowing and thus always contains approximately the same amount of concrete in a standard state.
iv.
The bottom door of the lower hopper is released and the concrete falls into the cylinder.
v.
The net mass of concrete in the known volume of cylinder is determined.
vi.
The density of the fully compacted concrete is obtained by actually filling the cylinder with concrete in four layers, each tamped or vibrated. Thus, the compacting factor can be calculated.
3. Vee Bee Consistometer Test
A – Slump Cone (20 cm lower dia., 10 cm upper dia., 30 cm height)
B – Container C – Clear Plastic Disk/Transparent Rider (2.75 kg) (held by moving vertical rod) D – Vebe Table (260 mm x 260 mm)
Developed by V. Bahrner of Sweden
The Vebe time test is a more scientific test for workability than the Slump test, in that it measures the work needed to compact the concrete. The freshly mixed concrete is packed into the slump cone in a standard manner. The cone stands within a special container on a platform, which is vibrated with an eccentric weight rotating at 50 Hz so that the vertical amplitude of the table is approximately ±0.35 mm. Vibration is performed after the cone has been lifted off the concrete. The time taken for the concrete to be compacted is measured. Compaction is assumed to be complete when the transparent rider is totally covered with concrete and all cavities on the surface of the concrete have disappeared. This is judged visually. It is assumed that the input of energy required for full compaction is a measure of workability of the mix, and this is expressed Vebe seconds. 4. Flow Table Test
Tools and Apparatus required: The apparatus consists essentially of a wooden board covered by a steel plate with a total mass of 16 kg. This board is hinged along one side to a base board (bottom plate), each board being a 700 mm square. The upper board can be lifted up to a stop so that the free edge rises 40 mm. Procedure:
The table top is moistened and a frustum of a cone of concrete is placed. The table top is lifted and allowed to drop, avoiding a significant force against the stop, 15 times, and each cycle taking approximately 4 seconds. In consequence, the concrete spreads and the maximum spread parallel to the two edges of the table is measured. The average of these two values represents the flow. A value of 400 indicates a medium workability and 500 a high workability. Workability
Vebe Test
Suitable Case
(in s) Very Low
12 – 20
Heavily vibrating
Compacting
Approximate
Factor
Sumps (in mm)
0.78
0 – 10
concrete Low
8 – 12
Medium vibrating
0.85
10 – 25
Medium
3–6
RCC works with easy
0.92
25 – 50
0.95
60 – 150
access High
1–3
RCC with congested section
3.2 W/C ratio in concrete Strength of concrete primarily depends upon the strength of cement paste. The strength of cement paste depends upon the dilution of paste or in other words, the strength of paste increases with cement content and decreases with air and water content. In 1918 Abrams presented his classic law in the form:
Where, x = water/cement ratio by volume and for 28-day result, the constants A and B are 14,000 lbs. /sq. in. and 7 respectively. Abrams water/cement ratio law states that the strength of concrete is only dependent upon water/cement ratio provided the mix is workable. The law has stood the test of time and is held valid even today as a fundamental truth in concrete-making practices. It was Feret, who formulated in as early as 1897, a general rule defining the strength of the concrete paste in terms of the volume fractions of the constituents by the equation: (
)
Where, S = Strength of concrete; c, e and a = volume of cement, water and air respectively; K = a constant
In this expression, the volume of air is also included because it is not only the water/cement ratio but also the degree of compaction, which indirectly means the volume of air filled voids in the concrete, is taken into account in estimating the strength of concrete. Thus, the lower water/cement ratio could be used when the concrete is vibrated to achieve higher strength, whereas comparatively higher water/cement ratio is required when concrete is handcompacted. In both cases when the water/cement ratio is below the practical limit, the strength of the concrete falls rapidly due to introduction of air voids.
The graph showing the relationship between the strength and water/cement ratio is approximately hyperbolic in shape. Sometimes it is difficult to interpolate the intermediate value. From geometry, it can be deduced that if the graph is drawn between the strength and the cement/water ratio, an approximately linear relationship will be obtained. This linear relationship is more convenient to use than water/cement ratio curve for interpolation.
3.3 Introduction to Nominal Mix Mix Design can be defined as the process of selecting suitable ingredients of concrete and determining their relative quantities with the purpose of producing an economical concrete which has certain minimum properties, notably workability, strength and durability. Mix Design is of three types:
(i) Nominal Mix
(iii) Standard Mix
(ii) Design Mix In the past, the specifications for concrete prescribed the proportions of cement, fine and coarse aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate strength are termed nominal mixes. Hence, nominal mix is prescriptive time aiming that adherence to such prescriptive specification will result in satisfactory performance. These offer simplicity and under normal circumstance, have a margin of strength above that specified. However, due to the variability of mix ingredients, the nominal concrete for a given workability varies widely in strength. M10 = 1 : 3 : 6 M15 = 1 : 2 : 4 M20 = 1 : 1.5 : 3 – 60% construction in Nepal M25 = 1 : 1 : 2 Employed for M5, M7.5, M10, M20 and M25. Design Mix is performance based concrete, where the properties like strength, workability, durability etc. are specified and ingredients ratio, maximum size of aggregate, type of cement, etc. are user’s choice. Each country has its own code for mix design. i.
DOE Method
ii.
ACI Method
iii.
IS Method
Standard Mix is designated by standard factories. E.g. Panchakanya = 1 : 1.23 : 1.30 : 0.53
3.4 Probabilistic Concept in Mix Design Approach Concrete, like most other construction processes, have certain amount of variability both in materials as well as in constructional methods. This results in variation of strength from batch to batch and also within a batch. It is not possible to have a large number of destructive tests for evaluating the strength of the end products and as such we have to resort to sample tests. The basis of acceptance of a sample is that a reasonable control of concrete work can be provided by ensuring that the probability of test result falling below the design strength is not more than a specified tolerance level.
The results of testing of a large sample of test specimens will show a scatter or a distribution of strengths about the mean strength. This can be shown by a histogram, which can be approximated by the dashed curve, which is called the frequency distribution curve. For the strength of concrete, this curve can be assumed to have a characteristic form called the normal or Gaussian distribution. This curve is described in terms of the mean strength fm and the standard deviation s, the latter being a measure of the scatter or distribution of strength about the mean. ∑ ( [
)
]
Or,
[
∑
(∑ ) ] )
(
Where, fi = strength of test specimen i ∑
And, n = number of test specimens The theoretical normal distribution is represented graphically in above figure. It can be seen that the curve is symmetrical about the mean value and extends to plus and minus infinity. In practice, these very low and very high values of strength do not occur in concrete but these extremes can be ignored because most of the area under the curve (99.6 %) lies within ±3s and
can be taken to represent all the strength values of concrete. In other words, we can say that the probability of a value of strength falling within ±3s from the mean value is 99.6 %. Likewise, the probability of a value falling between any given limits about the mean value (fm ± ks) can be stated.
Characteristic and Average Strength
Where, fck = characteristic strength, fm = average/mean strength (minimum 3 cube test), ft = target strength, k = probability factor, s = standard deviation
The value of ‘k’ is usually chosen as 1.64 or 2.33, i.e. there is a probability that 1 in 20 or 1 in 100, respectively, of the strength values will fall below the minimum strength. 1 in
%
Value of ‘k’
6
16
1
10
10
1.28
20
5
1.65
25
4
1.75
33
3
1,88
40
2.5
1.96
50
2
2.05
100
1
2.33
The values of ‘s’ for given quality control condition are given as below: Degree of Control
‘s’ in N/mm2
Lab
2.0 – 3.5
Excellent Site
2.5 – 4.5
Poor Site
7.0 – 8.0+
3.5 Concrete Mix Design by DOE, ACI and IS Method Information required for Mix Proportioning using any method: 1. Type of Mix 2. Grade Designation
3. Maximum nominal size of coarse aggregates 4. Type of cement 5. Placing methods of concrete 6. Minimum water content 7. Workability 8. Zoning of fine aggregates (sand) 9. Degree of supervision 10. Exposure conditions 11. Temperature while concreting 12. Type of work
British Mix Design Method
The Building Research Establishment Laboratory (BRE) of the Department of Environment (DOE), UK developed a method of concrete mix design (in 1975, revised in 1988).
This method replaces the traditional method of Road Research Laboratory (RRL), Road Note No. 4.
The DOE method outlines a procedure for design of normal concrete mixes having 28day cube compressive strength as high as 75 MPa for non-air entrained concrete.
It does not consider the use of combined aggregate grading curves, aggregate-cement ratio and type of aggregate (rounded, angular or irregular) as was the case with the RRL method.
The degree of workability ‘very low’, ‘low’, ‘medium’ and ‘high’ have also been replaced in terms of specific values of slump and Vee-Bee time.
The proportion of fine aggregate is determined depending on the maximum size of aggregate, degree of workability, grading of fine aggregate and the water/cement ratio. The mix proportions are presented in terms of quantities of materials per unit volume of concrete.
Steps: 1.
Determine Target Strength (fmean or ft).
2. Selection of minimum water/cement ratio based on target strength.
3. Calculate water content based on slump/Vee-bee time, maximum size of aggregates (m.s.a) and type of aggregate (crushed/uncrushed). Thereafter, calculate, with the help of the w/c ratio obtained from step 2 and the approximate water required.
Maximum size
Slump, mm ⇒
0 – 10
10 – 30
30 – 60
60 – 180
> 12
6 – 12
3–6
0–3
Uncrushed
150
180
205
225
Crushed
180
205
230
250
of Vee-Bee (s) ⇒
aggregate,
Type
mm
aggregate
of ↓
10
20
40
Uncrushed
135
160
180
195
Crushed
170
190
210
225
Uncrushed
115
140
160
175
Crushed
155
175
190
205
4. Calculate w/c ratio based on durability considerations. Use the highest value of cement content.
S.N
Exposure
.
Plain Concrete (PCC)
Reinforced Concrete (RCC)
Minimu
Maximu
Minimu
Minimu
m
m
cement
Water-
of
cement
Water-
of
content,
Cement
Concrete
content,
Cement
Concrete
kg/m3
Ratio
kg/m3
Ratio
Free m Grade m
Maximu m
Minimu
Free m Grade
1
2
3
4
5
6
7
8
i.
Mild
220
0.60
-
300
0.55
M20
ii.
Moderat
240
0.60
M15
300
0.50
M25
e iii.
Severe
250
0.50
M20
320
0.45
M30
iv.
Very
260
0.45
M20
340
0.45
M35
280
0.40
M25
360
0.40
M40
Severe v.
Extreme
5. Find relative density of combined aggregates and determine the wet density of fresh concrete mix. Hence, calculate the weight of coarse aggregates.
6. Determine the weight of fine aggregates after knowing the zones in which they lie. The zone is determined by passing the sand through IS 600 micron sieve and obtaining the data of passing. Zone I = 15 to 34 % Zone II = 35 – 59 % Zone III = 60 – 79 % Zone IV = 80 – 100 % 7. Finally, determine the mix proportion.
Field Data for Mix Design by British Method: (a) M20 grade concrete (b) Slump – 30 to 60 mm (c) Exposure – Mild (d) Maximum size of coarse aggregates – 20 mm (crushed) (e) Sand passing through 600 micron IS sieve – 54.576 (f) Specific gravity – coarse aggregates: 2.707, fine aggregates: 2.5 (g) Cement – OPC – 53 grade (h) Type of Work – RCC Steps: 1.
Target strength
2. Minimum water/cement ratio based on target strength From graph of water/cement ratio vs. 28-day cube compressive strength, W/C = 0.644
3. Approximate water content For, Maximum size of aggregates = 20 mm Type of aggregate
= crushed
Slump
= 30 to 60 mm
From table, Approximate water required = 210 kg for 1 m3 concrete Hence, Cement required = 210/0.644 = 326.1 kg for 1 m3 concrete 4. w/c based on durability consideration Maximum free w/c ratio for M20 concrete, mild exposure and RCC Walls: From table, Maximum free w/c
= 0.55
Hence, weight of cement
= 210/0.55 = 382 kg for 1 m3 concrete
Also, from table, Minimum cement content, kg/m3 = 300 kg Hence, final weight of cement to be taken = 382 kg (the maximum value) 5. Relative density of Combined Aggregates
Hence, wet density of fresh concrete mix from graph = 2355 kg/m3 Weight of aggregates required = 2355 – 210 – 382 = 1763 kg 6. For Sand The sand lies in second zone. From figure, For maximum size of aggregates = 20 mm % of fine aggregates = (42 + 32)/2 = 37% of fine aggregates Hence, weight of fine aggregates = 37 % of 1763 = 652 kg for 1 m3 concrete Weight of coarse aggregates = 1763 – 652 = 1111 kg for 1 m3 concrete
MIX PROPORTION: CEMENT
SAND
GRAVEL
WATER
382
652
1111
210
1
1.71
2.91
0.55
for 1 m3 concrete
1 : 1.71 : 2.91 @ 0.55
American Concrete Institute (ACI) method for Mix Design
This method was suggested by ACI committee no. 211.
This method takes into consideration the requirements for workability, consistency, strength and durability.
Uses dry rodded density of coarse aggregates.
Uses fineness modulus of fine aggregates.
One method is based on the estimated weight of concrete per unit volume whereas the other is based on calculation of the absolute volume occupied by concrete ingredients.
Differences between British Method and ACI Method: British Method
ACI Method
No adjustment of aggregate mixture is Adjustment is required. required. Based on strength and durability.
Based on balance between economy and requirements for strength, place ability, density, etc.
Volume based calculations cannot be done. Both volume and weight based calculations Only weight based calculation is possible.
can be done.
In mixing ingredients, air content isn’t Air content is considered. considered. It’s
used
for
producing
entraining concrete.
only
non-air It is used for producing both air entraining and non-air entraining concrete.
Field Data for Mix Design by ACI Mix Method: (a) Characteristic Strength – 20 N/mm2 (b) Exposure Condition – Mild (c) Slump required – 30 to 60 mm (d) Maximum size of aggregate grading – 20 mm (crushed) – from sieve analysis (e) Sand passing through 600 micron sieve – 54.578 (f) Specific gravity of (a) c.a. – 2.707, (b) f.a. – 2.507 (g) Type of work – RCC (h) Unit weight of dry rodded c.a. – 1600 kg/m3 (i) F.M. of f.a. (from sieve analysis) – 3.1 Steps: 1.
Target strength
2. Approximate water content (kg/m3) For slump = 30 – 60 mm, nominal size of aggregates = 20 mm (From Table 6) Water required for 1 m3 concrete = 179 kg (Adopted for slump = 25 – 50 mm, size of aggregates = 19 mm for non air-entrained concrete) 3. Minimum w/c ratio based on target strength and durability requirements Based on target strength, w/c ratio = 0.644 Hence, weight of cement required = 179/0.644 = 277.95 ≈ 278 kg/m3 Due to durability considerations, w/c ratio = 0.55 Hence, cement content = 300 kg/m3 Hence, final cement quantity = 326 kg for 1 m3 concrete
4. Relative density of combined aggregates = 2.60 5. To find amount of coarse aggregates For fineness modulus = 3.1 (generally < 3) from sieve analysis And maximum size of coarse aggregates = 20 mm
Weight of coarse aggregates = 0.60 x dry rodded density (Adopted for max. size of coarse aggregates = 19 mm and F.M. = 3.0) = 0.60 x 1600 = 960 kg for 1 m3 concrete 6. Wet density of fresh concrete For max. Size 20 mm,
Wet density of concrete = 2355 kg/m3 Hence, weight of fine aggregates = 2355 – 960 – 179 – 326 = 890 kg/m3 MIX PROPORTION: CEMENT
SAND
326
890
1
2.73
GRAVEL
WATER
960 2.95
for 1 m3 concrete
179 0.55
1 : 2.73 : 2.95 @ 0.55 (IS Method – study yourself)
3.6 Segregation and Bleeding SEGREGATION IN CONCRETE Segregation can be defined as separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform. In the case of concrete, it is the difference in the size of particles (and sometimes in the specific gravity of the mix constituents) that is the
primary cause of segregation, but its extent can be controlled by the choice of suitable grading and by care in handling. Segregation may be of two types: internal and external. In internal segregation, coarsest or heaviest part of aggregates accumulates on the bottom and lighter or finer ones accumulate at the top during compaction. External segregation, on the other hand, is caused by external forces due to say improper handling. Inadequate cohesiveness of cement (high w/c ratio) paste can cause wet segregation. Insufficient w/c can result in dry segregation (external). The conditions favourable for segregation are: 1.
Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates.
2. Insufficiently mixed concrete with excess water content. 3. Dropping of concrete from heights as in the case of placing concrete in column concreting. 4. When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades. 5. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete. Prevention of Segregation: (a) Choice of suitable grading of aggregates. (b) Employing correct methods of handling, transporting and placing. (c) The danger of segregation can be reduced by the use of air-entrainment.
CONCRETE BLEEDING Bleeding, also known as ‘water gain’, is a form of segregation in which some of the water in the mix tends to rise to the surface of freshly placed concrete. This is caused by the inability of the solid constituents of the mix to hold all of the mixing water when they settle downwards. As a result of bleeding, the top of every lift (layer of concrete placed) may become too wet, and, if the water is trapped by superimposed concrete, a porous and a weak layer of non-
durable concrete will result. If the bleeding water is remixed during the finishing of the top surface, a weak wearing surface will be formed. If bleeding is undisturbed (and the water evaporates), the effective w/c ratio may be lowered with a resulting increase in strength. Prevention of Bleeding: (a) Proper mix proportioning and uniform and complete mixing. (b) Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. (c) Air-entraining agent is very much effective in reducing the bleeding. (d) Bleeding can be reduced by the use of finer cement or cement mixed with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.
3.7 Quality Control in Site: Mixing, Handling, Placing, Compaction and Curing MIXING Thorough mixing of the materials is essential for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in colour and consistency. There are two methods adopted for mixing concrete: i. Hand Mixing ii. Machine Mixing Hand Mixing is practiced for small scale unimportant concrete works. As the mixing cannot be thorough and efficient, it is desirable to add 10 percent more cement to cater for the inferior concrete produced by this method. Hand mixing should be done over an impervious concrete or brick floor of sufficiently large size to take one bag of cement. Mixing of concrete is almost invariably carried out by machine, for reinforced concrete work and for medium or large scale mass concrete work. Machine mixing is not only efficient, but also economical, when the quantity of concrete to be produced is large. Mixers can be batch mixers or continuous mixers. Batch mixers produce concrete, batch by batch with time
interval, whereas continuous mixers produce concrete continuously without stoppage till such time the plant is working.
MIXING TIME It
is
important
to
know
the
minimum
mixing
time
necessary
to
produce
a concrete of uniform composition, and of reliable strength. The mixing time or period should be measured from time all the cementing materials and aggregates are in mixer drum till taking out the concrete. Mixing time depends on the type and size of mixer, on the speed of rotation, and on the quality of blending of ingredients during charging of the mixer. Generally, a mixing time of less than 1 to 1.25 minutes produces appreciable non-uniformity in composition and a significant lower strength; mixing beyond 2 minutes causes no significant improvement in these properties. Table: Recommended minimum mixing times
Capacity of mixer (yd³) Mixing time (Minutes) Up to 1
1
2
1.25
3
1.5
4
1.75
5
2
6
2.25
10
3.25
Prolong mixing: If mixing take place over a long period, evaporation of water from the mix can occur, with a consequent decrease in workability and an increase in strength. A secondary effect is that of
grinding of the aggregate, particularly if soft; the grading thus becomes finer and the workability lower. In case of air entrained concrete, prolong mixing reduces the air content.
Ready mixed concrete: If instead of being batched and mixed on site, concrete is delivered for placing from a central plant. It is referred to as ready-mixed or pre-mixed concrete. This type of concrete is used extensively abroad as it offers numerous advantages in comparison with other methods of manufacture: (a) Close quality control of batching which reduces the variability of the desired properties of hardened concrete. (b) Use on congested sites or in highway construction where there is little space for a mixing plant and aggregate stockpiles; (c) Use of agitator trucks to ensure care in transportation, thus prevention segregation and maintaining workability (d) Convenience when small quantities of concrete or intermittent placing are required. There are two categories of ready-mixed concrete: central-mixed and transit mixed or truck mixed. In the first category, mixing is done in a central plant and then concrete is transported in an agitator truck. In the second category, the materials are batched at a central plant but are mixed in a truck.
PLACING AND COMPACTION OF CONCRETE The operation of placing and compaction are interdependent and are carried out simultaneously. They are most important for the purpose of ensuring the requirements of strength, impermeability and durability of hardened concrete in the actual structure. As for as placing is concerned, the main objective is to deposit the concrete as close as possible to its final position so that segregation is avoided and the concrete can be fully compacted. The aim of good concrete placing can be stated quite simply. It is to get the concrete into position at a speed, and in a condition, that allow it to be compacted properly.
To achieve proper placing following rules should be kept in mind: 1.
The concrete should be placed in uniform layers, not in large heaps or sloping layers.
2. The thickness of the layer should be compatible with the method of vibration so that entrapped air can be removed from the bottom of each layer. 3. The rate of placing and of compaction should be equal. If you proceed too slowly, the mix could stiffen so that it is no longer sufficiently workable. On no account should water ever be added to concrete that is setting. On the other hand, if you go too quickly, you might race ahead of the compacting gang, making it impossible for them to do their job properly. 4. Each layer should be fully compacted before placing the next one, and each subsequent layer should be placed whilst the underlying layer is still plastic so that monolithic construction is achieved 5. Collision between concrete and formwork or reinforcement should be avoided. 6. For deep sections, a long down pipe ensures accuracy of location of concrete and minimum segregation. 7. You must be able to see that the placing is proceeding correctly, so lighting should be
available for large, deep sections, and thin walls and columns. Once the concrete has been placed, it is ready to be compacted. The purpose of compaction is to get rid of the air voids that are trapped in loose concrete. Why is compaction of concrete necessary? It is important to compact the concrete fully because:
Air voids reduce the strength of the concrete. For every 1% of entrapped air, the strength falls by somewhere between 5 and 7%. This means that concrete containing a mere 5% air voids due to incomplete compaction can lose as much as one third of its strength.
Air voids increase concrete's permeability. That in turn reduces its durability. If the concrete is not dense and impermeable, it will not be watertight. It will be less able to withstand aggressive liquids and its exposed surfaces will weather badly.
Moisture and air are more likely to penetrate to the reinforcement causing it to rust.
Air voids impair contact between the mix and reinforcement (and, indeed, any other embedded metals). The required bond will not be achieved and the reinforced member will not be as strong as it should be.
Air voids produce blemishes on struck surfaces. For instance, blowholes and honeycombing might occur.
Summing up, fully compacted concrete is dense, strong and durable; badly compacted concrete will be porous, weak and prone to rapid deterioration. Sooner or later it will have to be repaired or replaced. It pays, therefore, to do the job properly in the first place. Stiff mixes contain far more air than workable ones. That is one of the reasons why a lowslump concrete requires more compactive effort than one with a higher slump - the compaction needs to continue for a longer time, or more equipment has to be used. Even air-entrained concrete needs to be compacted to get rid of entrapped air voids. The difference between air voids and entrained air bubbles should be noted at this stage. The air bubbles that are entrained are relatively small and spherical in shape, increase the workability of the mix, reduce bleeding, and increase frost resistance. Entrapped air on the other hand tends to be irregular in shape and is detrimental to the strength of the mix. It is to remove this air that the concrete must be properly compacted. There is little danger that compaction will remove the minute air bubbles that have been deliberately entrained, since they are so stable.
CURING OF CONCRETE Curing can be described as the process of maintaining satisfactory moisture content and a favourable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service.
Curing serves two major purposes:
It prevents or replenishes the loss of moisture from the concrete.
It maintains a favourable temperature for hydration to occur for a definite period.
The most crucial time for strength gain of concrete is immediately following placement. In field conditions, heat and wind can dry out the moisture from the placed mixture. The accompanying figure shows how concrete strength varies with curing conditions. Concrete that is allowed to dry in air will gain only 50% of the strength of continuously moist-cured concrete. Lack of water also causes the concrete to shrink, which leads to tensile stresses within the concrete. As a result, surface cracking may occur, especially if the stresses develop before the concrete attains adequate tensile strength. Hydration is an exothermic chemical process, increasing the ambient temperature will increase the rate of hydration, and hence of strength development, while lowering it will have
the opposite effect. Too much heat reduces the final concrete strength. Selecting an appropriate curing process helps in temperature control during hydration. METHODS OF CURING: Concrete can be kept moist and often at a favorable temperature by any of three methods:
Maintaining the presence of mixing water during the early hardening period. Methods used include ponding or immersion, spraying (or fogging) and wet coverings. These methods will also cool the concrete as the water evaporates.
Preventing loss of mixing water from the surface by sealing. This may be achieved using impervious paper, plastic sheeting, applying membrane-forming compounds, or by leaving the forms in place.
Accelerating strength gain by supplying heat and additional moisture to the concrete. This may be accomplished using live steam (steam curing), insulating blankets or covers, and various heating techniques including coils and forms.
The method or combination of methods chosen will depend on which of the above-mentioned curing materials are available, size and shape of concrete members, in-situ versus plant production, economics and aesthetics. CURING UNDER DIFFERENT WEATHER CONDITIONS: Under normal weather, the key concerns in curing will be the maintenance of a moist environment around the concrete. Temperature variations are not a major problem, provided the concrete temperature is maintained above 5 degrees Celsius. Curing can therefore be achieved either through maintaining mixing water in the concrete during early hardening or by preventing moisture loss from the surface by sealing. The ultimate choice of the particular method to use will take into consideration factors such as economy, esthetics, member shape, etc. Under hot weather conditions, the high temperatures are likely to result in excessive moisture loss. Maintaining mixing water in the concrete is the major concern. Continuous moist curing should be done for the entire curing period. If this is not possible, the concrete surfaces should be protected from drying out using any of the previously mentioned methods and the surfaces kept damp. Surfaces should dry out slowly after curing to reduce possibility of surface cracking.
Curing in cold weather will be different as in this case the biggest concern will be the maintaining of an adequate and conducive temperature for hydration. For massive members, the heat generated by the concrete during hydration will be adequate to provide a satisfactory curing temperature. For non-massive members, a good alternative is steam curing, which provides both moisture and heat. In any case, a minimum favorable temperature in the range of 10 - 21º C must be maintained in the concrete for the minimum required curing period (see below). Where moist curing is not done, very low temperatures may be avoided by insulating the member appropriately. CURING PERIOD AND TEMPERATURE: The curing period depends upon the type of cement used, mixture proportions, required strength, size and shape of member, ambient weather, future exposure conditions, and method of curing. Since all desirable properties are improved with curing, the period should be as long as practical. For most concrete structures, the curing period at temperatures above 5º C (40º F) should be a minimum of 7 days or until 70% of the specified compressive or flexural strength is attained. The period can be reduced to 3 days if high early strength concrete is used and the temperature is above 10º C (50º F).
