PROPOSED MIX DESIGN METHOD FOR HPC AND VALIDITY

Concrete mix design is a process of proportioning the ingredients in right proportions. Though it is based on sound technical principles and heuristics, the entire process is not in the realm of science and precise mathematical calculations. This is because of impreciseness, vagueness, approximations and tolerances involved. The objective of any mix design method is to determine an appropriate and economical combination of concrete ingredients that can be used for a first trial batch to produce a certain concrete which is close to that can achieve a good balance between various desired properties of concrete at the minimum cost. A mixture proportioning only provides a starting mix design that will have to be more or less modified to meet the desired concrete characteristics. In spite of the fact that mix design is still something of an art, it is unquestionable that some essential scientific principles can be used as a basis for calculations. Mix design of high performance concrete (HPC) is different from that of usual concrete because of the following reasons (Laskar and S. Talukdar, 2008): • Waterbinder ratio is very low. • Concrete quite often contains cement replacement materials that drastically change the properties of fresh and hardened concrete. • Slump or compaction factor can be adjusted using high range water reducing admixture (HRWRA) without altering water content. Concrete is required to exhibit performance in the given environment. However, there has been no established method whereby the mixture proportions of concrete can be optimized according to the required performance. The conventional mix design methods are no longer capable of meeting the stringent multiple requirements of HPC (Wong and Kwan, 2005). These methods are not directly applicable to HPC mixes. Several methods have been proposed over the years for the proportioning of mineral admixture – based HPC mixes. The methods proposed byChapter 6 Proposed Mix Design Method for HPC and Validity Mix Proportioning of High Performance Concrete for Indian Environment 230 ACI, modified ACI, Mehta and Aitcin, DOE among other methods are popular and are found to be suitable for designing HPC mixes especially in cold countries where temperature hardly goes beyond 25oC (Kumbhar and Murnal,2011). These methods have been in use successfully by the engineers over the years. However, the British or American methods will not be applicable for our country, as the specific relationships constituting figures and tables are based on their materials. Also the climatic conditions of our country are different from the climates of these countries. India, being a tropical country has different environment in its different parts. Tropical countries usually receive significant rainfall during only some part of the year leading to substantial variation in the level of humidity in many parts of the tropics. High temperatures, low humidity and wind cause rapid evaporation of water from the concrete mix during summer. This drying of concrete leads to cracking and crazing of the surface (Gambhir, 2005). The variation in temperature and humidity has profound effect on the properties of HPC such as strength and durability since the mix proportions are usually decided at laboratory conditions. Therefore, a new modified method of mix design procedure has been proposed and is discussed in subsequent sections for design of HPC mixes taking into account the effect of environmental conditions such as varying humidity and temperatures on the properties of HPC mixes

Chapter Proposed Mix Design Method for HPC and Validity Chapter PROPOSED MIX DESIGN METHOD FOR HPC AND VALIDITY 6.1 INTRODUCTION Concrete mix design is a process of proportioning the ingredients in right proportions Though it is based on sound technical principles and heuristics, the entire process is not in the realm of science and precise mathematical calculations This is because of impreciseness, vagueness, approximations and tolerances involved The objective of any mix design method is to determine an appropriate and economical combination of concrete ingredients that can be used for a first trial batch to produce a certain concrete which is close to that can achieve a good balance between various desired properties of concrete at the minimum cost A mixture proportioning only provides a starting mix design that will have to be more or less modified to meet the desired concrete characteristics In spite of the fact that mix design is still something of an art, it is unquestionable that some essential scientific principles can be used as a basis for calculations Mix design of high performance concrete (HPC) is different from that of usual concrete because of the following reasons (Laskar and S Talukdar, 2008): • Water-binder ratio is very low • Concrete quite often contains cement replacement materials that drastically change the properties of fresh and hardened concrete • Slump or compaction factor can be adjusted using high range water reducing admixture (HRWRA) without altering water content Concrete is required to exhibit performance in the given environment However, there has been no established method whereby the mixture proportions of concrete can be optimized according to the required performance The conventional mix design methods are no longer capable of meeting the stringent multiple requirements of HPC (Wong and Kwan, 2005) These methods are not directly applicable to HPC mixes Several methods have been proposed over the years for the proportioning of mineral admixture – based HPC mixes The methods proposed by Mix Proportioning of High Performance Concrete for Indian Environment 229 Chapter Proposed Mix Design Method for HPC and Validity ACI, modified ACI, Mehta and Aitcin, DOE among other methods are popular and are found to be suitable for designing HPC mixes especially in cold countries where temperature hardly goes beyond 25oC (Kumbhar and Murnal,2011) These methods have been in use successfully by the engineers over the years However, the British or American methods will not be applicable for our country, as the specific relationships constituting figures and tables are based on their materials Also the climatic conditions of our country are different from the climates of these countries India, being a tropical country has different environment in its different parts Tropical countries usually receive significant rainfall during only some part of the year leading to substantial variation in the level of humidity in many parts of the tropics High temperatures, low humidity and wind cause rapid evaporation of water from the concrete mix during summer This drying of concrete leads to cracking and crazing of the surface (Gambhir, 2005) The variation in temperature and humidity has profound effect on the properties of HPC such as strength and durability since the mix proportions are usually decided at laboratory conditions Therefore, a new modified method of mix design procedure has been proposed and is discussed in subsequent sections for design of HPC mixes taking into account the effect of environmental conditions such as varying humidity and temperatures on the properties of HPC mixes 6.2 GENERAL INFORMATION Based on the results of experimentation it is stated that the following material specifications must meet to develop HPC mixes of desired workability and target compressive strengths 6.2.1 Cement: The ordinary Portland cement (OPC) of 53 grade satisfying the IS Specifications The cement content obtained from the existing IS Code method of mix design (IS10262-1982) works out to be more than the maximum specified limit of 450kg/m3 for mixes with low water-cement ratios Hence, the quantity of cement needs to be altered and should be used in combination with suitable mineral Mix Proportioning of High Performance Concrete for Indian Environment 230 Chapter Proposed Mix Design Method for HPC and Validity admixture such as micro silica The total quantity of materials (called as binders) thus will include both cement and desired quantity of mineral admixture (micro silica) 6.2.2 Fine Aggregates: From the investigations carried out to study the effect of sand zones on various properties of HPC mixes, it is recommended to use fine aggregates conforming to Zone-I for obtaining better strength properties 6.2.3 Coarse Aggregates: The cubical shaped coarse aggregates of two fractions are recommended for use in developing HPC mixes of grades M50-M90 Fraction: I – passing through 20mm IS sieve and retained on 12.5mm IS sieve (60%) Fraction: I – passing through 12.5mm IS sieve and retained on 10mm IS sieve (40%) 6.2.4 Mineral Admixtures: Several mineral admixtures such as fly ash, micro silica, GGBS etc are being used in making HPC mixes of different grades However, micro silica is a highly pozzolanic mineral admixtures and the addition it in concrete will start contributing to strength in about days (Bagade and Puttaswamy, 2007) A micro silica content of 5% to15 % leads to an increase of concrete strength However, the desirable content of micro silica needed from point of view of workability and 28 day compressive strength is to be determined by trials 6.2.5 Chemical Admixtures: The research and the experience indicate that the admixtures based on the poly carboxylic ethers (PCE) are the best suited as they have a water reducing capacity of 18%-40% in reference to the control concrete These admixtures assist in achieving higher slump at much lesser w/c ratios (< 0.30) 6.3 PROPOSED MIX DESIGN METHOD FOR HPC The proposed mix design method for HPC mixes is based on the principles of existing IS Code method of mix design (IS 10262-1982 and IS 10262-2009) In the development of this proposed method, the basic mix proportions were obtained for Mix Proportioning of High Performance Concrete for Indian Environment 231 Chapter Proposed Mix Design Method for HPC and Validity making HPC mixes using w/c ratio’s worked out by extrapolating the established relationships between free water cement ratio and concrete strength for different cement strengths as shown in figure 6.1 (IS:10262-1982) Different curves indicating 28 day strength range of cement, when tested according to IS 4031-1968 is given Table 6.1 The curves indicating relationship between free water-cement ratio and 28 days compressive strength for different cement strengths were extrapolated and modified to generate smooth curves (figure 6.2) The equations used in developing the smooth curves are given in Table 6.2 The quantities of fine aggregate and coarse aggregate were determined using the equation given in IS Code method (IS: 102621982) The basic mix proportions thus obtained by following the guidelines of existing IS Code method were used in making trial HPC mixes by incorporating desirable contents of micro silica and superplasticizer in view of achieving the desired workability and strength properties 100 28 Days Comp St., MPa 90 F-Curve 80 E-Curve 70 D-Curve 60 C-Curve B-Curve A-Curve 50 40 30 20 10 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Water-Cement Ratio Figure 6.1: Relation between free Water-Cement Ratio and Concrete strength for different cement strengths (Extrapolated curves-original) Table 6.1: 28 days strength of cement, tested according to IS: 4031-1968 A=31.9-36.8 N/mm2 D=46.6-51.5 N/mm2 B=36.7 – 41.7 N/mm2 E=51.5 -56.4 N/mm2 C=41.7 – 46.6 F=56.4-61.3 N/mm2 Mix Proportioning of High Performance Concrete for Indian Environment 232 Chapter Proposed Mix Design Method for HPC and Validity 100 28 Day Comp St., MPa 90 F-Curve E-Curve 80 70 D-Curve 60 C-Curve B-Curve 50 A-Curve 40 30 20 10 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 Water-Cement Ratio Figure 6.2: Relation between free W/C Ratio and Concrete St for different cement strengths (Modified Extrapolated-smooth curves) Table 6.2: Equations used in developing smooth curves for relationship between free water-cement ratio and concrete strength Curve Exponential Equation of curve A y = 92.85e-3.05x B y = 107.6e-3.09x C y = 119.5e-3.07x D y = 131.8e-3.06x E y = 145.0e-3.02x F y = 159.1e-3.06x Where, y= Comp St., in MPa and x= w/c ratio Further, based on experimental observations and the results of compressive strengths of various grades of HPC mixes, the curves given in IS Code method are modified so as to arrive at water-binder ratios best suited to different grades of HPC mixes (Figure 6.3 to 6.5) The curve indicating generalized relationship between free water cement ratio and compressive strength of concrete is also extrapolated and modified so as to arrive at water-binder ratios best suited to different grades of HPC mixes (Figure 6.6 to 6.8) The equations used to develop smooth curves for the relationship between free water-binder ratio and compressive strength of concrete and Mix Proportioning of High Performance Concrete for Indian Environment 233 Chapter Proposed Mix Design Method for HPC and Validity also to arrive at water-binder ratios best suited for different grades of HPC mixes are 28 days Comp St., MPa given in Table 6.3 and Table 6.4 respectively 100 90 80 70 60 50 40 30 20 10 F-Curve E-Curve D-Curve C-Curve B-Curve A-Curve 0.2 0.25 0.3 0.35 0.4 Water- Cement Ratio 0.45 0.5 Figure 6.3: Relationship between free W/C Ratio and Concrete St for 28 Day Comp St., MPa different Cement Strengths (Extrapolated to suit HPC) 120 110 100 90 80 70 60 50 40 30 20 10 F-Curve E-Curve (y = 235.4e-4.15x) D-Curve C-Curve B-Curve A-Curve 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 Water-Binder Ratio F-CURVE D-CURVE B-CURVE E-CURVE C-CURVE A-CURVE Figure 6.4: Relationship between free W/C Ratio and Concrete St for different Cement Strengths (Extrapolated-Smooth Curves) Mix Proportioning of High Performance Concrete for Indian Environment 234 Chapter Proposed Mix Design Method for HPC and Validity Table 6.3: Equations used in developing smooth curves for relationship between free water-cement ratio and concrete strength Curve Exponential Equation of curve A y = 156.8e-4.28x B y = 176.5e-4.26x C y = 195.0e-4.22x D y = 216.1e-4.19x E y = 235.4e-4.15x F y = 267.4e-4.25x 28 Days Comp St., MPa Where, y= Comp St., in MPa and x= w/c ratio 130 120 110 100 90 80 70 60 50 40 30 20 10 F-Curve E-Curve (y = 235.4e-3.85x) D-Curve C-Curve B-Curve A-Curve 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 Water-Binder Ratio F-CURVE E-CURVE D-CURVE C-CURVE B-CURVE A-CURVE Figure 6.5: Relationship between free W/C Ratio and Concrete St for different Cement Strengths (Extrapolated Modified Curves to Suit HPC) Table 6.4: Equations used in developing curves for relationship between free w/b ratio and concrete St best suited to HPC Curve Exponential Equation of curve A y=156.8e-4.28x B y=176.5e-4.26x C y =195e-4.22x D y =216.1e-4.19x E y = 235.4e-3.85x F y = 267.4e-4.025x Where, y= Comp St., in MPa and x= w/c ratio Mix Proportioning of High Performance Concrete for Indian Environment 235 Chapter Proposed Mix Design Method for HPC and Validity 28 Days Comp St., MPa 80 70 y = 146.3e-3.46x 60 50 40 30 20 10 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 Water-Cement Ratio Figure 6.6: Extrapolated Generalized Relation between free W/C ratio and Comp St of Concrete (Original Curve as per IS: 10262-1982) 28 days Comp St., MPa 130 y = 297e-4.84x 110 90 70 50 30 10 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 Water-Binder Ratio Figure 6.7: Extrapolated Modified (1st) generalized relation between free W/C ratio and Comp St of Concrete to suit HPC mixes Mix Proportioning of High Performance Concrete for Indian Environment 236 28 days Comp St., MPa Chapter Proposed Mix Design Method for HPC and Validity 120 110 100 90 80 70 60 50 40 30 20 10 y = 297e-4.84x y = 146.3e-3.46x 0.2 0.225 0.25 0.275 0.3 0.325 0.35 0.375 0.4 0.425 0.45 0.475 0.5 Water-Binder Ratio Figure 6.8: Extrapolated Generalized relation between free W/C ratio and Comp St of Concrete to suit HPC Mixes (Original & Modified) From the experimental observations, the basic mix proportions adopted for making trial HPC mixes were modified by altering coarse aggregate to fine aggregate ratio and incorporating appropriate micro silica and superplasticizer contents so as to get desired workability and compressive strengths for different combinations of humidity and temperature The proposed mix design method for HPC thus provides the final mix proportions taking into account the parameters or variables necessary to be incorporated in achieving the desired workability and strength properties for different grades of HPC mixes The various variables or parameters considered in the proposed mix design method for HPC mixes are as given below: Grade of the HPC mix under consideration Desired workability for the mix Prevailing Relative Humidity in the atmosphere Prevailing Temperature in the atmosphere Total binder content Total cement content Desired micro silica content Desired water-binder ratio Desired coarse aggregate to fine aggregate ratio 10 Desired superplasticizer dose (by weight of cement) Mix Proportioning of High Performance Concrete for Indian Environment 237 Chapter Proposed Mix Design Method for HPC and Validity The stepwise procedure for the proposed method of mix design is outlined in the following sections 6.3.1 Stepwise Procedure of Proposed Mix Design Method for HPC The mix design procedure consists of a series of steps, which when completed provide a mixture meeting strength and workability requirements based on the properties of selected and proportioned ingredients Following are the necessary steps 6.3.1.1 Step I: Test data for Materials The test data of ingredients of HPC mixes namely specific gravity, fineness modulus; water absorption, moisture content etc should be obtained 6.3.1.2 Step II: Target Mean Compressive Strength of HPC The target mean compressive strength at 28 days curing period for HPC mix is determined using the relationship given below: where, f'’ck = target mean compressive strength , fck = characteristic strength of concrete (grade of concrete) and S = standard deviation (as per IS 456-2000) As the strict quality control is necessary in making HPC mixes, the standard deviation (SD) is not likely to exceed MPa (Mullick, 2006) Hence, a standard deviation value of 5MPa (as per IS 456 code) is assumed for arriving at target mean strength 6.3.1.3 Step III: Determination of Water-Binder Ratio The determination of water-binder ratio is done by referring to the plotted relationships between the 28 day compressive strength of concrete and water-binder ratios for different humidity and temperature conditions as given in the figure 6.9 to 6.11 The w/b ratios for specific compressive strengths (grades of HPC mixes) and for different humidity levels at 30oC, 35oC, 40oC temperature along with the equations used for development of curves indicating relationship between w/b ratio and corresponding compressive strengths are given in Table Mix Proportioning of High Performance Concrete for Indian Environment 6.5 to 6.10 238 Chapter Proposed Mix Design Method for HPC and Validity 115 RH-30% 105 RH-40% 28 Day Comp St, MPa RH-50% 95 RH-60% RH-70% 85 RH-80% RH-90% 75 65 55 45 0.15 0.2 0.25 0.3 W/B Ratio 0.35 0.4 Figure 6.9: Relation between 28 day Comp St and Water-Binder for different humidity at 30OC Temp Mix Proportioning of High Performance Concrete for Indian Environment 239 0.45 Chapter Proposed Mix Design Method for HPC and Validity Table 6.5: Comp St and W/B ratio for different RH at 30OC Temp 28 Day Comp St., MPa 58.25 68.25 78.25 88.25 98.25 Humidity Levels,% (Temp-30oC) 30 40 50 60 0.382 0.374 0.365 0.365 0.338 0.331 0.324 0.318 0.300 0.294 0.289 0.278 0.267 0.261 0.258 0.242 0.237 0.232 0.230 0.211 70 0.357 0.311 0.271 0.236 0.204 80 0.354 0.306 0.264 0.227 0.195 90 0.350 0.302 0.261 0.225 0.193 Table 6.6: Equations used for development of curves for relationship between Comp St and W/B ratio for different RH at 30OC Temp Rh 30 40 50 60 70 80 90 Where, Exponential Equations (Temp 30oc) y = 230.7e-3.6x y = 230.1e-3.67x y = 239.8e-3.88x y = 201.2e-3.4x y = 198e-3.43x y = 186.5e-3.29x y = 186.3e-3.32x y =Compressive Strength in MPa, x = w/b ratio and e =exponent, the approximate value of e is 2.718282 Mix Proportioning of High Performance Concrete for Indian Environment 240 Chapter Proposed Mix Design Method for HPC and Validity RH-30% 115 RH-40% RH-50% 28 Day Comp St MPa 105 RH-60% RH-70% 95 RH-80% RH-90% 85 75 65 55 0.1 0.15 0.2 0.25 0.3 0.35 0.4 W/B Ratio Figure 6.10: Relation between 28 day Comp St and Water-Binder for different humidity at 35OC Temp Mix Proportioning of High Performance Concrete for Indian Environment 241 0.45 Chapter Proposed Mix Design Method for HPC and Validity Table 6.7: Comp St and W/B ratio for different RH at 35OC Temp Humidity Levels,% (Temp-35oC) 28 Day Comp St., MPa 30 40 50 60 70 80 90 58.25 0.404 0.404 0.403 0.402 0.393 0.391 0.390 68.25 0.347 0.342 0.336 0.334 0.327 0.321 0.316 78.25 0.298 0.289 0.277 0.275 0.270 0.262 0.253 88.25 0.254 0.243 0.226 0.224 0.220 0.209 0.197 98.25 0.216 0.201 0.181 0.178 0.175 0.162 0.147 Table 6.8: Equations used in development of curves indicating relationship between Comp St and W/B ratio for different RH at 35OC Temp Rh Exponential Equations (Temp 35oC) 30 y = 179e-2.78x 40 y = 165.1e-2.58x 50 y = 150.2e-2.35x 60 y = 148.8e-2.335x 70 y = 149.5e-2.40x 80 y = 142.5e-2.29x 90 y = 134.7e-2.15x Mix Proportioning of High Performance Concrete for Indian Environment 242 Chapter Proposed Mix Design Method for HPC and Validity 145 RH-30 28 Day Comp St., MPa RH-40 125 RH-50 RH-60 RH-70 105 RH-80 RH-90 85 65 45 0.1 0.15 0.2 0.25 0.3 0.35 0.4 W/B Ratio Figure 6.11: Relation between 28 day Comp St and Water-Binder for different humidity at 40OC Temp Mix Proportioning of High Performance Concrete for Indian Environment 243 0.45 Chapter Proposed Mix Design Method for HPC and Validity Table 6.9: Comp St and W/B ratio for different RH at 40OC Temp 28 Day Comp Humidity Levels,% (Temp-40oC) St., MPa 30 40 50 60 70 80 90 58.25 0.378 0.371 0.372 0.369 0.369 0.363 0.348 68.25 0.339 0.331 0.323 0.312 0.310 0.302 0.290 78.25 0.305 0.297 0.281 0.262 0.258 0.249 0.240 88.25 0.275 0.267 0.244 0.218 0.213 0.203 0.196 98.25 0.249 0.240 0.210 0.179 0.172 0.162 0.156 Table 6.10: Equations used in development of curves for relationships between Comp St and W/B ratio for different RH at 40OC Temp i Rh Exponential Equations (Temp 40 OC) 30 y = 269.9e-4.06x 40 y = 256.8e-4.00x 50 y = 193.8e-3.23x 60 y =160.9e-2.75x 70 y =155e-2.65x 80 y =149.6e-2.6x 90 y = 150.6e-2.73x Selection of Water-Binder Ratio The maximum w/b ratio for different exposure conditions from view point of durability is to be adopted as per IS 456-2000 The values of w/b ratio obtained from the developed relationships taking into account the ambient RH and Temperature is compared with the values specified in IS 456-2000 for different exposure conditions and the value whichever is smaller is selected for designing the HPC mixes 6.3.1.4 Step IV: Determination of Binder Content From the w/b ratio obtained for the target mean compressive strength and for the specified humidity and temperature condition, the required binder content is determined based on the proposed relationship between binder content (cement + micro silica) and w/b ratio (Figure 6.12) Mix Proportioning of High Performance Concrete for Indian Environment 244 Binder Content, Kg/m3 Chapter Proposed Mix Design Method for HPC and Validity 850 800 750 700 650 600 550 500 450 400 0.15 y = -1665x + 1078 0.2 0.25 0.3 W/B Ratio 0.35 0.4 Figure 6.12: Proposed Relationship between Binder Content and W/B ratio (Linear equation: y = -1665x + 1078, where y=binder content and x =w/b ratio) From the selected w/b ratio and the obtained binder content, the total water content is Micro Silica Content, Kg/m3 calculated using the following relationship: 100 y = 1.146x - 33.80 90 80 70 60 50 40 30 20 50 55 60 65 70 75 80 85 90 95 100 105 28 Days Comp St of HPC, MPa Figure 6.13: Proposed variation of Micro Silica content with 28 days Comp St of HPC 6.3.1.5 Step V: Determination of Desirable Contents of Mineral Admixture (Micro Silica) and Cement Content The desirable contents of micro silica required for making different grades of HPC mixes can be obtained from the established relationship of micro silica content Mix Proportioning of High Performance Concrete for Indian Environment 245 Chapter Proposed Mix Design Method for HPC and Validity and compressive strength of HPC (Figure 6.13) Thus, knowing the micro silica content, the required quantity of cement can be worked out by subtracting the micro silica content from the total binder content 6.3.1.6 Step VI: Determination of Desirable Contents of Superplasticizer (SP) The type and desired dosage of superplasticizer needs to be decided by trials to produce and maintain reasonable workability and enhance the strength of concrete when micro silica is used as a mineral admixture Though, in market different varieties or brands of superplasticizers are available, the research and experience have indicated that the admixtures based on the poly carboxylic ethers (PCE) are the best suited as they have a water reducing capacity of 18%-40% in reference to the control concrete In the present research work, HPC mixes have been developed using PCE based superplasticizers The dosage of superplasticizer was determined by weight of the cement used for the HPC mix In the proposed mix design method the approximate superplasticizer dosages for different grades of HPC mixes (M50-M90) corresponding to different water-binder ratios can be obtained using the plotted relationship between superplasticizer content and the cement content required for the SP Content, Kg/m3 specified grade of HPC mix (Figure 6.14) 4.5 3.5 2.5 1.5 0.5 y = 0.012x - 2.83 400 425 450 475 500 525 550 575 600 Cement Content, Kg/m3 Figure 6.14: Proposed variation of SP for different Cement contents Mix Proportioning of High Performance Concrete for Indian Environment 246 Chapter Proposed Mix Design Method for HPC and Validity 6.3.1.7 Step VII: Determination of Coarse and Fine Aggregate Contents Taking into account the adopted volume of coarse aggregate in the total aggregate volume during the experimentation, a relationship between ratio of volume of coarse aggregate to the volume of total aggregate per unit volume of concrete (figure 6.15) is established for corresponding 28 days compressive strengths obtained From the established relationship the ratio of volume of coarse aggregate to the volume of total aggregate is determined for the specified 28 days compressive Ratio of Vol of Coarse Aggt to Vol of Total Aggt, m3 strength of the concrete 0.8 y = 0.433e0.004x 0.75 0.7 0.65 0.6 0.55 0.5 0.45 50 55 60 65 70 75 80 85 28 Days Comp St, MPa 90 95 100 Figure 6.15: Ratio of Volume of Coarse to Total Aggregate and 28 days Compressive Strength (Exponential Eq.) Thus, the following variables required for mix design process are determined: 1) Water-Binder Ratio 2) Binder content = ( Cement + Micro silica) content, Kg/m3 3) Water Content = (water-binder ratio x total binder content), kg/m3 4) Superplasticizer (by weight of cement) 5) Total Aggregate Content = Volume of Concrete – (water + binder + superplasticizer) content 6) CA / FA Ratio 7) Volume of Fine and Coarse aggregate Mix Proportioning of High Performance Concrete for Indian Environment 247 Chapter Proposed Mix Design Method for HPC and Validity 6.4 ILLSUTRATIVE EXAMPLE FOR MIX DESIGN OF M50 GRADE HPC MIX An illustrative example for mix proportioning of M50 grade HPC mix using the Proposed Mix Design Method is presented below 6.4.1 STIPULATIONS FOR MIX PROPORTIONING Characteristic comp strength : 50 N/mm2 Maximum size of aggregate : 20mm (angular) Degree of workability – (slump) : 50-100 Degree of quality control : Good Type of Exposure : Severe Temperature : 30oC Relative humidity : 50% 6.4.2 TEST DATA OF MATERIALS Cement : OPC – 53 Grade Specific gravity of cement : 3.15 Specific gravity of coarse aggt : 2.90 Specific gravity of fine aggt : 2.80 Water absorption,% Coarse aggregate : 2.03 Fine aggregate : 1.48 Free (surface) moisture,% Coarse aggregate : 1.98 Fine aggregate : 1.33 (Confirming to grading zone I of table of IS: 383-1970) 6.4.3 TARGET STRENGTH FOR MIX PROPORTIONING Where, =target average compressive strength at 28 days = characteristic compressive strength at 28 days, Mix Proportioning of High Performance Concrete for Indian Environment 248 Chapter Proposed Mix Design Method for HPC and Validity = Standard deviation (Table of IS456-2000) = N/mm2, Therefore, target strength, = 50 + 1.65 x = 58.25 N/mm2 6.4.4 SELECTION OF WATER-BINDER RATIO Referring to the plotted relationship between the 28 day compressive strength of concrete and water-binder ratio (Figure 6.9), for a target mean compressive strength of 58.25MPa and for specified humidity level of 50% and 30oC temperature a water–binder ratio 6.4.5 of 0.371 is obtained DETERMINATION OF BINDER CONTENT, MICRO SILICA AND CEMENT From the proposed relationship between binder content and w/b ratio (Figure 6.12), for a compressive strength of 58.25 MPa binder content (cement + micro silica) of 460.28 Kg/m3 is obtained Also, referring to the established relationship between micro silica and 28 days compressive strength of HPC mixes (Figure 6.13), a micro silica content of 32.94 kg/m3 (7.71%) is obtained Thus, knowing the total binder content and the amount of micro silica in it, the quantity of cement required can be calculated by subtracting the micro silica content from the total binder content Thus, the quantity of cement is calculated as given below: 6.4.6 DETERMINATION OF DESIRABLE CONTENTS OF SUPERPLASTICIZER (SP) The desirable content of superplasticizer required for the desired workability is determined by weight of cement The superplasticizer dosage is obtained from the established relationship between superplasticizer dosage and the cement content required to attain the specified compressive strength under given humidity and temperature conditions (Figure 6.14) Thus, from the calculated quantity of cement a Mix Proportioning of High Performance Concrete for Indian Environment 249 Chapter Proposed Mix Design Method for HPC and Validity superplasticizer dose of 0.54% is determined The quantity of superplasticizer per m3 of concrete is obtained as given below: 6.4.7 DETERMINATION OF WATER CONTENT From the obtained w/b ratio and binder content, the required water content is calculated as given below: 6.4.8 PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT The estimation of volume of coarse aggregate in the volume of total aggregates is determined using the established relationship between 28 days compressive strength and ratio of volume of coarse aggregate to the volume of total aggregate per unit volume of concrete (Figure 6.15) Thus, for M50 grade HPC the ratio of volume of coarse aggregate to the volume of total aggregate per unit volume of concrete as obtained from the established relation is 0.55m3 Hence the volume of fine aggregate is obtained as given below: Volume of fine aggregate = (1- 0.55) = 0.45m3 6.4.9 MIX CALCULATION Mix Calculations per unit volume of concrete shall be as follows: Mix Proportioning of High Performance Concrete for Indian Environment 250 Chapter Proposed Mix Design Method for HPC and Validity 6.4.9.2 Final Quantities of Ingredients and Mix Proportion Cement = 427.34 kg/m3 Micro Silica = 32.94kg/m3 Water = 170.76 kg/m3 Fine Aggregate = 856.80 kg/m3 Coarse Aggregate =1084.60 kg/m3 Superplasticizer = 2.56 kg/m3 Mix Proportioning of High Performance Concrete for Indian Environment 251 Chapter Proposed Mix Design Method for HPC and Validity Mix Proportion obtained is : 0.37:1 (0.93: 07):1.86:2.36 The mix proportions so obtained are adjusted for field conditions as per usual procedure before preparing trial mix 6.5 EXPERIMENTAL VALIDATION OF PROPOSED MIX PROPORTIONING METHOD FOR HPC The mix proportions for different grades of HPC mixes are obtained by using the established relationships (curves) developed through experimental studies The proposed method of mix proportioning of HPC mixes provides the mix proportions for different levels of humidity and temperature combinations for the grades of M50 to M90 HPC mixes To verify the validity of the proposed mix design method, a trial HPC mix of M50 grade was prepared using the mix proportion obtained by the proposed method The mix was designed for a slump range of 75-100mm considering a humidity of 80% with a temperature of 30oC The mix proportion, expressed as parts of water : binder content (cement+micro silica) : fine aggregate : coarse aggregate, adopted for making the HPC mix was 0.36:1(0.93:0.07):1.81:2.29 A superplasticizer content of 0.56% by weight of cement was used as obtained from the mix design The trial mix produced using the above proportion showed a very good quality with a slump of 90mm and a flow value of 23.67% and a compressive strength of 54.07MPa at 28 days curing The slump test and the flow test conducted on the trial mix are shown in figures 6.17(a) and 6.17(b) respectively Since the trial HPC mix prepared was found to give satisfactory workability with good flow property and also in a single trial for the mix proportion obtained by the proposed method, it can be stated that the proposed method of mix design is valid for proportioning HPC mixes for specified humidity and temperature conditions Figure (a): Slump Test Figure (b: Flow Test mixture Figure 6.16: Workability of Trial HPC Mix (M50) Mix Proportioning of High Performance Concrete for Indian Environment 252 Chapter Proposed Mix Design Method for HPC and Validity The Software based on the Artificial Neural Network (ANN) approach is also developed to get the mix proportions for the given grade of HPC and any given humidity and temperature conditions In order to check the validity of the software, a trial mix of M50 grade of HPC was prepared for the ambient humidity (80%) and temperature (30OC) conditions by adopting the mix proportion provided by the developed software (ANN) The prepared trial HPC mix showed good results in respect of workability (slump & flow values) and compressive strength The results of the slump (210mm) and compressive strength test (56.70MPa) were found to be close to the expected results for the mix The figure 6.18 shows the various stages during slump test The flow of the trial mix concrete indicating the spread on the table is shown in figure 6.19 (a) (b) (c) (d) Figure 6.17: Stages in the Slump Test on Trial Mix of HPC Figure 6.18: Flow Test on Trial HPC mix Mix Proportioning of High Performance Concrete for Indian Environment 253