The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age.

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1 CONCRETE MIX DESIGN

2 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. 2

3 CONCRETE MAKING MATERIALS Cement Aggregates Water Admixtures 3

4 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, Colored Cement (White Cement). 4

5 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 5

6 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. 6

7 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 7

8 TYPES OF CONCRETE MIXES 1. NOMINAL MIX Mixes of fixed proportions, IS: 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. 8

9 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. 9

10 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. 10

11 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/m 3 7 Water cement ratio 8 The degree of workability of concrete based on placing conditions 11

12 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 13 The source of water and the impurities present in it. 12

13 IS 456:

14 IS 456:

15 IS 456:

16 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 16

17 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. 18

18 METHODS OF CONCRETE MIX DESIGN 1. American Concrete Institute Committee 211 method 2. Bureau of Indian Standards Recommended method IS 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 8. Indian Road Congress, IRC 44 method 19

19 BIS RECOMMENDED MIX DESIGN METHOD The BIS recommended mix design procedure is covered in IS In line with IS , the first revision IS 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 Extend the W/C ratio v/s compressive strength graph 20

20 Modifications in IS

21 MIX DESIGN BASED ON IS RECOMMENDATIONS Based on IS 10262:1982 Procedure: 1. Target mean strength for mix design: f ck * = f ck + ts where f ck = characteristic compressive strength at 28 days S = standard deviation t = a statistical value depending on the risk factor. 22

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24 2. Selection of Water/Cement ratio 25

25 3. Estimation of Entrapped Air 26

26 4. Selection of Water Content and Fine to Total Aggregate ratio 27

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28 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. 29

29 6. Calculation of aggregate content: 30

30 7. Actual quantities required for mix Adjust the mix for deviations from assumed conditions 8. Check the calculated mix proportions 31

31 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 compacting factor (iv ) Degree of quality control - Good (v ) Type of Exposure - Mild 32

32 (b) Test data for Materials (i ) Specific gravity of cement (ii ) Compressive strength of cement at 7 days - Satisfies the requirement of IS: (iii ) 1. Specific gravity of coarse aggregates Specific gravity of fine aggregates (iv ) Water absorption: 1. Coarse aggregate % 2. Fine aggregate - 1.0% (v ) Free (surface) moisture: 1. Coarse aggregate - Nil 2. Fine aggregate - 2.0% 33

33 Design Procedure 1. Target mean strength of concrete f ck * = f ck + ts f ck = 20 N/mm 2, t = 1.65, S = 4 f ck * = MPa 34

34 2. Selection of Water/Cement Ratio 35

35 Durability Criteria: Mild Exposure Conditions 36

36 W/C ratio from strength considerations = 0.50 W/C ratio from durability considerations = 0.55 Adopt the lower value 37

37 3. Selection of water and sand content 38

38 Adjustments in Water and Sand Contents 39

39 4. Determination of cement content Water-cement ratio = 0.50 water = kg/m 3 Cement = 191.6/0.50 = 383 kg/m 3 Is this satisfactory for mild exposure condition? For mild exposure condition, minimum cement content = 300 kg/m 3 40

40 5. Determination of coarse and fine aggregate contents Specified max. size of aggregate = 20 mm Corresponding entrapped air = 2% Volume of concrete, V = = 0.98 m 3 f a = 546 kg/m 3, C a = 1188 kg/m 3 41

41 Final Mix Proportions Water Cement FA CA

42 Adjustments required for water absorption 43

43 Mix proportions for the trial mix W:C:FA:CA 44