INTRODUCTION
Concrete is obtained by mixing cement, fine aggregate, coarse aggregate, water and admixtures in required proportions. The mixture when placed in forms and allowed to cure becomes hard like stone.
The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age.
It is the most widely-used man-made construction material in the world.
CONCRETE MAKING MATERIALS
• Cement• Aggregates • Water
CEMENT
Cement and water forms the „active‟ component of concrete, while the inactive group comprises the fine and coarse aggregates. The cement and water form a paste that hardens and bonds the
aggregates together.
Types of Cement
Although around 18 types of cements are recognized by BIS, more commonly used ones are:
• Ordinary Portland Cement 33, 43, & 53 grade OPC, • Blended Cements (PPC and PSC).
• Sulphate Resisting Cement (SRC), • Low Heat Portland Cement (LHPC), • Hydrophobic Portland Cement,
AGGREGATES
• They are cheaper than cement and impart greater volume
stability and durability to concrete.
• The main purpose is to provide bulk to the concrete. • Some of the aggregates may be chemically active.
CLASSIFICATION
• Geological origin – natural and artificial • Size – fine, coarse and all-in
• Shape – rounded, irregular, angular, flaky and elongated • Unit weight – normal-weight, heavyweight and lightweight
WATER
• The most important and least expensive ingredient of concrete. • One part of the water is used in the hydration of cement to form
the binding matrix.
• The remaining water affords lubrication and workability to the
concrete.
• Water-cement ratio depends on the grade of concrete, its
workability, durability, nature and type of aggregates etc.
• Potable water is ideal for concreting. • Seawater may be used in PCC.
ADMIXTURES
Added to the concrete immediately or during mixing to modify its properties in the fresh or hardened state.
Types:
• Accelerators - speed up the initial set of concrete. • Retarders – delay the setting time of concrete mix. • Plasticizers and Super-plasticizers - water reducers. • Air entraining admixtures
• Water proofers • Pigments
• Corrosion-inhibiting chemicals • Antifungal admixtures
TYPES OF CONCRETE MIXES
1. NOMINAL MIX
Mixes of fixed proportions, IS:456-2000 permits nominal mixes for concretes of strength M20 or lower
2. DESIGN MIX
Designed on the basis of requirements of the concrete in fresh and hardened states.
TRIAL MIXES
Prepared to verify whether the Design Mix would perform as per the assumptions. If appreciable variation exists, the
available alternatives are:
1. directly employ the trial mix proportions at the site
2. modify the trial mix proportions on the basis of intuition and employ the revised proportions at the site
3. prepare further trial mixes incorporating changes in the proportions based on the feedback generated from the previous mix.
CONCRETE MIX DESIGN
DEFINITION
“ Mix Design is the science of determining the relative proportions of the ingredients of concrete to achieve the desired properties in the most economical way.”
PRINCIPLES OF MIX DESIGN
1 The environment exposure condition for the structure 2 The grade of concrete, their characteristic strength‟s and
standard deviations 3 The type of cement
4 The types and sizes of aggregates and their sources of supply 5 The nominal maximum sizes of aggregates
6 Maximum and minimum cement content in kg/m3
7 Water cement ratio
9 Air content inclusive of entrained air
10 The maximum/minimum density of concrete
11 The maximum/minimum temperature of fresh concrete 12 Type of water available for mixing and curing
FACTORS DEFINING THE CHOICE OF MIX
PROPORTIONS
1. Compressive Strength 2. Workability
3. Durability
4. Type, size and grading of aggregates 5. Aggregate-cement ratio
COMPRESSIVE STRENGTH Abram‟s Law
log F = log A1 – x log B1
where F is the compressive strength A1, B1 are constants and
WORKABILITY
“that property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed,
consolidated and finished.”
DURABILITY
“the resistance to weathering action due to environmental conditions such as changes in temperature and humidity, chemical attack, abrasion, frost and fire.”
REQUIREMENTS AND TESTS OF MATERIALS REQUIRED
FOR MIX DESIGN
ASSIGNMENT # 1 TO BE SUBMITTED BY 07-02-2011
Roll #s 01-21 – Cement
Roll #s 22-42 – Aggregates
METHODS OF CONCRETE MIX DESIGN
1. American Concrete Institute Committee 211 method
2. Bureau of Indian Standards Recommended method IS 10262-82 3. Road note No. 4 (Grading Curve) method
4. Department Of Environment (DOE - British) method 5. Trial and Adjustment Method
6. Fineness modulus method 7. Maximum density method
American Concrete Institute Method of Mix Design
(a) Data to be collected :
(i ) Fineness modulus of selected F.A.
(ii ) Unit weight of dry rodded coarse aggregate.
(iii ) Sp. gravity of coarse and fine aggregates in SSD condition (iv ) Absorption characteristics of both coarse and fine
aggregates.
(v ) Specific gravity of cement.
(b) From the minimum strength specified, estimate the average design strength by using standard deviation.
(c) Find the water/cement ratio from the strength and durability points of view. Adopt the lower value.
(d) Decide the maximum size of aggregate to be used. Generally for RCC work 20 mm and pre-stressed concrete 10 mm size are used.
(e) Decide workability in terms of slump for the given job.
(f ) The total water in kg/m3 of concrete is determined,
corresponding to the selected slump and selected maximum size of aggregate.
(g ) Cement content is computed by dividing the total water content by the water/cement ratio.
(h) Select the bulk volume of dry rodded coarse aggregate per unit volume of concrete, for the particular maximum size of coarse aggregate and fineness modulus of fine aggregate.
( j ) The weight of C.A. per cubic meter of concrete is calculated by multiplying the bulk volume with bulk density.
(k ) The solid volume of coarse aggregate in one cubic meter of concrete is calculated by knowing the specific gravity of C.A.
(l ) Similarly the solid volume of cement, water and volume of air is calculated in one cubic meter of concrete.
(m) The solid volume of FA is computed by subtracting from the total volume of concrete the solid volume of cement, CA, water and entrapped air.
(n) Weight of fine aggregate is calculated by multiplying the solid volume of fine aggregate by specific gravity of F.A.
ACI METHOD: DESIGN EXAMPLE
1 Design a concrete mix for the construction of an elevated water tank. The specified design strength of concrete (characteristic strength) is 30 MPa at 28 days measured on standard cylinders. Standard deviation can be taken as 4 MPa. The specific gravity of FA and C.A. are 2.65 and 2.7 respectively. The dry rodded bulk density of C.A. is 1600 kg/m3, and fineness modulus of FA is 2.80. Ordinary Portland cement (Type I) will be used. A slump of 50 mm is necessary. C.A. is found to be absorptive to the
extent of 1% and free surface moisture in sand is found to be 2 per cent. Assume any other essential data.
1 Mean Cylinder Compressive Strength
Assuming 5 per cent of results are allowed to fall below specified design strength,
The mean strength,
fm= fmin + ks = 30 + 1.64 x 4 = 36.5 MPa 2 Water/cement ratio Strength criteria Durability criteria
3 Mixing Water Content
Slump = 50 mm
Maximum size of aggregate = 20 mm Concrete is non air-entrained
The mixing water content is 185 kg/m3 of concrete
The approximate entrapped air content is 2%.
The required cement content =185/0.47 = 394 kg/m3
4 Bulk Volume of C.A.
Maximum size of C.A = 20mm Fineness Modulus of F.A. = 2.80
The dry rodded bulk volume of C.A. = 0.62 per unit volume of concrete
The weight of C.A. = 0.62 x 1600 = 992 kg/m3
The weight of all the known ingredients of concrete Weight of water = 185 kg/m3 Weight of cement = 394 kg/m3 Weight of C.A. = 992 kg/m3 Weight of F.A. = 2355 – (185 + 394 + 992) = 784 kg/m3 6 Weight of FA
Alternatively, the weight of F.A. can be determined by the more accurate absolute volume method
Total absolute volume = 697 x 103 cm3
Absolute volume of F.A. = (1000 – 697) x 103
= 303 x 103
Weight of FA = 303 x 2.65 = 803 kg/m3
7 Estimated quantities of materials per cubic meter of concrete are: Cement = 394 kg F.A = 803 kg C.A = 992 kg Water = 185 kg
8 Proportions
C : F.A : C.A : water 394 : 803 : 992 : 185 1 : 2.04 : 2.52 : 0.47
Weight of materials for one bag mix in kg = 50 : 102 : 126 : 23.5
9 The above quantities is on the basis that both F.A and C.A are in saturated and surface dry condition (SSD conditions).
FA has surface moisture of 2%
Total free surface moisture in FA = (2/100 x 803) = 16.06 kg/m3
Quantity of water absorbed by C.A. = (1/100 x 992) = 9.92 kg/m3
Weight of C.A in field condition = 992 – 9.92 = 982 kg/m3
C.A absorbs 1% water
Water contributed by F.A = 16.06 kg Water absorbed by C.A. = 9.92 kg
Extra water contributed by aggs. = 16.06 – 9.92 = 6.14 kg
Total water content = 185.00 – 6.14
= 179 kg/m3
Cement = 394 kg/m3
F.A. = 819 kg/m3
C.A. = 982 kg/m3
Water = 179 kg/m3
Field density of fresh concrete = 2374 kg/m3
Quantities of materials to be used in field, corrected for free surface moisture in F.A and absorption characteristic of C.A
2. A mix with a mean 28-day compressive strength of 35 MPa and a slump of 50 mm is required, using OPC. The maximum size of well shaped, angular aggregate is 20mm, its bulk density is 1600 kg/m3 and its specific gravity is 2.64. The available fine
aggregate has a fineness modulus of 2.60 and a specific gravity of 2.58. No air-entrainment is required.
Water/Cement ratio
As durability criteria is not
Water/cement ratio =0.48
Slump = 50 mm
Max size of aggregate = 20 mm
Composition of concrete (kg/m3) Water = 185 Cement = 386 CA = 1020 FA = 738 Density of Concrete = 2329 (kg/m3)
ROAD NOTE No. 4 METHOD OF MIX DESIGN
Proposed by the Road Research Laboratory, UK (1950) Procedure:
1. The average compressive strength of the mix to be designed is obtained by applying control factors to the minimum
compressive strength
2. Water/cement ratio is read from compressive strength v/s w/c ratio graph
3. Proportion of combined aggregates to cement is determined from tables, for maximum size 40 mm and 20 mm
4. If the aggregate available differs from the standard gradings, combine FA and CA so as to produce one of the standard gradings
5. The proportion of cement, water, FA and CA is determined from knowing the water/cement ratio and the
aggregate/cement ratio.
6. Calculate the quantities of ingredients required to produce 1 m3 of concrete, by the absolute volume method, using the
DRAWBACKS OF ROAD NOTE NO. 4 METHOD
1. Cannot be used directly for the design of air - entrained concrete
2. No recommendations for durability or strength, regarding the water/cement ratio
3. The design tables refer to mixes in which the FA and CA are of the same shape
4. In selecting aggregate/cement ratio, only 3 shapes of aggregates and 4 gradings are recommended.
DOE METHOD OF MIX DESIGN
Can be used for concrete containing fly ash. PROCEDURE
1. Target mean strength is calculated
2. Select water/cement ratio, from the type of cement and CA. Compare this with the ratio from durability conditions.
3. Decide the water content for required workability
4. Compare the cement content with the minimum cement content value and adopt the higher value
5. Find out the total aggregate content
6. Determine the proportion of FA using the appropriate FA% v/s CA size graph, and find the weight of CA and FA
BIS RECOMMENDED MIX DESIGN METHOD
The BIS recommended mix design procedure is covered in IS 10262-82.
In line with IS 456-2000, the first revision IS 10262-2009 was published, to accommodate some of the following changes:
• Increase in strength of cement
• Express workability in terms of slump, rather than the
compacting factor
MIX DESIGN BASED ON IS RECOMMENDATIONS
Based on IS 10262:1982
Procedure:
1. Target mean strength for mix design:
fck* = fck + tS
where fck = characteristic compressive strength at 28 days S = standard deviation
5. Calculation of Cement Content:
cement by mass = Water content/Water cement ratio
To be checked against the minimum cement content for the requirement of durability and the greater of the two values to be adopted.
7. Actual quantities required for mix
Adjust the mix for deviations from assumed conditions
DESIGN EXAMPLE: BIS RECOMMENDED METHOD
Grade M20
(a ) Design stipulations
(i ) Characteristic compressive strength required in the field at 28 days - 20 MPa
(ii ) Maximum size of aggregate - 20 mm (angular) (iii ) Degree of workability - 0.90 compacting factor (iv ) Degree of quality control - Good
(b) Test data for Materials
(i ) Specific gravity of cement - 3.15
(ii ) Compressive strength of cement at 7 days - Satisfies the requirement of IS: 269–1989
(iii ) 1. Specific gravity of coarse aggregates - 2.60 2. Specific gravity of fine aggregates - 2.60
(iv ) Water absorption:
1. Coarse aggregate - 0.50% 2. Fine aggregate - 1.0%
(v ) Free (surface) moisture:
1. Coarse aggregate - Nil 2. Fine aggregate - 2.0%
Design Procedure
1. Target mean strength of concrete
fck* = fck + tS
fck = 20, t = 1.64 , S = 4
W/C ratio from strength considerations = 0.50 W/C ratio from durability considerations = 0.55
4. Determination of cement content
Water-cement ratio = 0.50 water = 191.6 kg/m3
Cement = 191.6/0.50 = 383 kg/m3
5. Determination of coarse and fine aggregate contents
Specified max. size of aggregate = 20 mm Corresponding entrapped air = 2%
fa = 546 kg/m3,
Final Mix Proportions
Water Cement FA CA
191.6 383 546 1188
