CHAPTER 3 MATERIAL PROPERTIES AND MIX PROPORTIONS

Transcription

1 45 CHAPTER 3 MATERIAL PROPERTIES AND MIX PROPORTIONS 3.1 GENERAL In the present investigation, it was planned to cast M40 & M50 grade concrete with and without supplementary cementitious material such as fly ash, silica fume and rice husk ash. The M40 and M50 grade concrete were made without supplementary cementitious material called as control concrete. The cement was replaced by 40% and 50% fly ash, 10% silica fume and 10% rice husk ash separately in the combination mix. Properties of the ingredients used to make M40 and M50 grade concrete mix were studied as per the relevant standards to asses its quality. 3.2 MATERIAL PROPERTIES Cement The 53 grade Ordinary Portland Cement (OPC) was used in this investigation. The physical properties of the cement which were tested are given in the Table Fine Aggregate In the present investigation, locally available river sand was used as fine aggregate. The sand was screened at the site to remove deleterious materials and tested as per the procedure given in IS 2386 Part 1. The

2 46 results of sieve analysis of typical aggregate sample are included in Table 3.2 and the properties are given in Table 3.3. Table 3.1 Physical Properties of Cement Physical Properties OPC 53 grade cement Recommendations (Sample considered) (IS 12269) Specific gravity Standard consistency (%) 31 - Initial setting time (min) 120 > 30 Final setting time (min) 200 < 600 Compressive strength at 28 days (MPa) Table 3.2 Sieve Analysis of Fine Aggregate Sample IS Sieve size Weight retained (gms) % Retained % Passing 4.75mm mm mm µ µ Table 3.3 Physical properties of Fine Aggregate Sample Description of Test Results obtained Permissible Limits as per IS Specific gravity 2.65 Min 2.55 Water absorption (%) 0.90 Max Fineness Modulus

3 Coarse Aggregate The locally available crushed granite coarse aggregate was used for the preparation of HPC concrete. The coarse aggregate of suitable gradation was obtained by blending 65% of 20mm size with 35% of 12.5mm. The coarse aggregates were tested as per the procedure given in IS 2386 part 1. The results of sieve analysis of typical aggregate sample are included in Table 3.4 and Table 3.5. Properties are given in Table 3.6. Table 3.4 Sieve Analysis of 20mm size Coarse Aggregate Sample IS Sieve size (mm) Weight retained (gms) % Retained % Passing Table 3.5 Sieve Analysis of 12.5mm size Coarse Aggregate Sample IS Sieve size (mm) Weight retained (gms) % Retained % Passing

4 48 Table 3.6 Physical Properties of Coarse Aggregate Sample Description of Test Results obtained Permissible Limits as per IS Specific gravity 2.72 Minimum 2.55 Water absorption (%) 1.20 Maximum 3.00 Fineness Modulus Bulk Density (kg/m 3 ) Water Water is being added to concrete mainly for hydration and for workability. The excess water would end up with large micro pores and capillary pores. The strength of the cement paste is inversely proportional to the dilution of the paste. Hence to improve impermeability adequate quantity of water should be added in the concrete. Water conforming to the requirements of IS: is found to be satisfactory for making concrete. For the present investigation the water drawn from drinking water source was used for making concrete and curing Admixtures An admixture is a material other than water, aggregates and cement added to the batch immediately before or during its mixing. In high performance concrete there are two types of admixtures are used. They are mineral and chemical admixtures. These admixtures are used for the following reasons.

5 49 To enable the concrete to meet requirements of specifications, namely permitted maximum water cement ratio, minimum early and 28 days strength and retention of workability. To increase the workability or water reduction at given consistencies due to deflocculation of cement particles and retard or accelerate time of initial setting, retard or reduce heat evolution during early hardening, improve finishing qualities, control bleeding and segregation, improve pump ability and reduce the rate of slump loss. To reduce the cost of concreting operations by affecting a reduction of the overall cost of concrete ingredients, permitting rapid mould turn over and ease of placing and finishing. To improve the properties of hardened concrete, such as increased early and long term strength and modulus of elasticity, decreases permeability (and hence inhibit corrosion of embedded steel reinforcement) and increases abrasion resistant, increase the resistant to chemical attack and increase bond with reinforcement. Control expansion caused by the reaction of alkalies with certain aggregates Supplementary Cementitious Materials or Finer Pozzolans The following supplementary cementitious materials are used in the present investigation Fly ash Silica fume Rice husk ash

6 50 The fineness (or) surface area of the Finer cement - 300m 2 /kg Fly ash - 340m 2 /kg Silica fume - 20, 000m 2 /kg Rice husk ash - 50, 000m 2 /kg Fly ash Fly ash is a by product of coal combustion. In this present investigation Class F fly ash which was collected from Ennore thermal power plant was used. The dosage of fly ash was 40 % and 50% replacement by mass of cement. The physical and chemical properties of fly ash were tested and the results are given in Table 3.7 and Table 3.8. Table 3.7 Physical Properties of Fly ash Properties Results obtained ASTM C 618 recommendation Fineness: Amount retained when wet Max. 34 sieve on 45 µm sieve (%) Strength activity Index: 7 days (%) Min days (%) Min. 75 Specific gravity

7 51 Table 3.8 Chemical Properties of Fly ash Chemical composition Content SiO Al 2 O Fe 2 O CaO N.D MgO N.D Na 2 O 0.02 SO MnO 0.03 LOI Silica Fume Silica fume is a by product resulting from the reduction of high purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. In the present investigation, the Elkem silica fume was used. The silica fume dosage used in cementitious mortar (1:3) was 10% of mass of the cement. The properties of silica fume tested are given in Table 3.9. Table 3.9 Physical Properties of Silica fume Properties Results obtained ASTM C 1240 recommend Fineness: Amount retained when wet sieve on 45 µm sieve (%) 4 Max. 10 Pozzolan activity Index: 7 days (%) Min days (%)

8 Rice Husk Ash The Rice husk ash dosage was 10% of mass of the cement in a cementitious mortar (1:3). Its physical and chemical properties were tested and the results are given in Table 3.10 and Table Table 3.10 Physical Properties of Rice Husk Ash Physical properties Results obtained Particle size (Micron) 25 Specific gravity 2.3 Compressive strength (MPa) at 3 days days days days Table 3.11 Chemical Properties of Rice Hush Ash Chemical properties Recommended SiO2 Silica (Min) 85 % Humidity (Max) 2 % Colour Grey Loss on ignition at 800 C (Max) 4 % ph value Chemical Admixture Originally, concrete was made using a mixture of three materials cement, aggregate and water. Almost invariably the cement was Portland

9 53 cement. Later on, in order to improve some of the properties of concrete, either in the fresh or in the hardened states very small quantities of chemical product were added in to the mix. These are called as chemical admixtures. The chemical admixture added in the concrete deflocculates the cement particles and thus made use of the entrapped water to enhance the fluidity of the mix. By adding super plasticizers to the concrete mix, it is possible to obtain workable mixes even at water binder ratio of 0.3 and less. In the present investigation, a high range water reducing admixture of sulphonated naphthalene formaldehyde superplasticizers named CONPLAST SP 430 was used as a chemical admixture and the properties are given in Table Table 3.12 Properties of CONPLAST SP 430 Properties of CONPLAST SP 430 Appearance Brown liquid Specific gravity 1.20 at 20 C Chloride content Nil to BS 5075 Air Entrainment Less than 2% additional air entrained at Normal dosages Alkali content Less than 72.0 g. Na 2 O equivalent / litre of admixture. 3.3 MIX DESIGN PROCEDURE FOR HIGH PERFORMANCE CONCRETE General The objective of any mixture proportioning method is to determine an appropriate and economical combination of concrete constituents that can be used for a first trial batch. In this present investigation, mix design of M40 and M50 was done based on ACI 211 4R-93.

10 Design Procedure ACI 211.4R Step 1: Choice of Slump and Required Concrete Strength a) Slump According to ACI 211.4R-93, the range of slump before adding HRWA will be 25 50mm. Suggested slump values needed to cast concrete for different type of construction are given in a Table These values can be used if the slump is not specified. Table 3.13 Recommended Slumps for Various Types of Structures Types of construction Slump mm Maximum Minimum Reinforced foundation wall and footings Plan footings, caissons and substructure wall Beams and reinforced walls Building columns Pavements and slabs Mass concrete Source: ACI (2002) b) Required Average Compressive Strength (f cr) ACI 318/318R -46 is silent about the required average compressive strength (f cr) to establish a standard deviation. As per ACI 211.4R, the required average compressive strength are as follows.

11 55 Table 3.14 Required Average Compressive Strength Specified compressive strength f c MPa Required average compressive strength f cr MPa Less than 20 f c to 35 f c Over 35 f c Step 2: Choice of Maximum Size of Coarse Aggregate Many studies have shown that crushed stone produces higher strengths than rounded gravel and smaller aggregate sizes are also considered to produce higher strength. And also based on strength requirement, the recommended maximum size for coarse aggregate are given in Table Table 3.15 Recommended Maximum Size for Coarse Aggregate Required concrete strength (MPa) Suggested maximum size coarse aggregate (mm) < to 25.4 > to 12.7 Step 3: Estimation of mixing Water and Air Content The quantity of water per unit volume of concrete required to produce a given slump is dependent on the maximum size, particle shape and grading of the aggregate, the quantity of cement and type of water reducing admixture used. It is given in Table 3.16.

12 56 Table 3.16 Approximate Mixing Water and Air Content Slump (mm) Mixing water Kg/m 3 Maximum size coarse aggregate (mm) to to to Entrapped air content Step 4: Selection of Water Cement Ratio The water cement ratio was taken from Table Table 3.17 Maximum W / B Ratio for Concrete Made with HRWR Average compressive strength f cr MPa days W/(C+P) Maximum size coarse aggregate (mm) Recommended maximum W/(C+P) is given as a function of maximum size aggregate to achieve different compressive strength at 28 days.

13 57 Step 5: Calculation of Cementitious Materials Content The weight of cementitious materials per m 3 calculated by dividing the mass of the free water by W/(C+P) ratio. of concrete was Step 6: Estimation of Coarse Aggregate Content The quantity of coarse aggregated is estimated from Table Table 3.18 Volume of Dry Rodded Coarse aggregate per unit volume of concrete Nominal Fineness modulus of sand maximum size of aggregate (mm) The mass of the coarse aggregates per unit volume calculated by the volume is multiplied by the dry rodded unit mass of the coarse aggregate.

14 58 Step 7: Estimation of Fine Aggregate Content The masses of all mixture constituents, expect sand are added, and it is subtract from the unit weight of concrete. This value is the mass of the sand. Step 8: Moisture Adjustments The mass of aggregates obtained in this procedure is in a saturated dry state. Therefore their mass, along with that as the mixing water, is adjusted for actual. Step 9: Trial Batches Trial batches are made and the mixture proportion is adjusted to meet the desired physical and mechanical characteristic of the concrete Concrete Mix Design and Proportions In this present investigation two grades of concrete mix were used. The M40 & M50 mix was designed by ACI report 211.4R 93. Finally the proportions were altered based on the water absorption in fine aggregate and coarse aggregate. The mix design was carried out for control concrete as well as concrete with cementitious material at different combination for both the grades of mix. Two different water cement ratios were used for control concrete and concrete with cementitious material in order to bring the compressive strength of both the concrete mixes at 28 days are at same level. Final quantities of materials required in terms of kg/m 3 and the mix proportions are presented in the Table 3.19 and Table 3.20.

15 59 Table 3.19 Concrete Mix with various Binder Composition Grade of Concrete M 40 M 50 Mix Identification ACC1 (1:2.136:2.513) AFS1 (1:1.841:2.387) AFS2 (1:1.811:2.387) AFR1 (1:1.839:2.387) AFR2 (1:1.812:2.387) BCC1 (1:1.521:2.067) BFS1 (1:1.248:1.953) BFS2 (1:1.218:1.953) BFR1 (1:1.242:1.953) BFR2 (1:1.211:1.953) Water/ Binder Ratio Binder Composition Quantities of Super plasticizer used (%) 0.40 Control Concrete % Cement + 40% Fly ash + 10% Silica fume 40% Cement + 50% Fly ash + 10% Silica fume 50% Cement + 40% Fly ash + 10% Rice husk ash 40% Cement + 50% Fly ash + 10% Rice husk ash Control Concrete % Cement + 40% Fly ash + 10% Silica fume 40% Cement + 50% Fly ash + 10% Silica fume 50% Cement + 40% Fly ash + 10% Rice husk ash 40% Cement + 50% Fly ash + 10% Rice husk ash

16 60 Table 3.20 Quantity of Materials Required for per m 3 of Concrete Grade of Concrete Mix Combination Weight (kg/m 3 Volume ) (Litre/m 3 ) Cement Sand Aggregate FA SF RHA Water W/B ratio ACC AFS M40 AFS AFR AFR BCC BFS M50 BFS BFR BFR FA Fly ash; SF Silica Fume; RHA Rice Husk Ash