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

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

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

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)

6.2.4 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 3 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

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: 10262-1982) 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

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/mm2B=36.7 – 41.7 N/mm2 E=51.5 -56.4 N/mm2

C-Curve

B-Curve

A-Curve

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

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

C-Curve

B-Curve

A-Curve

also to arrive at water-binder ratios best suited for different grades of HPC mixes are given in Table 6.3 and Table 6.4 respectively

Figure 6.3: Relationship between free W/C Ratio and Concrete St for

different Cement Strengths (Extrapolated to suit HPC)

Figure 6.4: Relationship between free W/C Ratio and Concrete St for different Cement Strengths (Extrapolated-Smooth Curves)

C-Curve

A-Curve B-Curve

Table 6.3: Equations used in developing smooth curves for relationship

between free water-cement ratio and concrete strength

Where, y= Comp St., in MPa and x= w/c ratio

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

C-Curve

A-Curve B-Curve

Figure 6.6: Extrapolated Generalized Relation between free W/C ratio and Comp St of Concrete (Original Curve as per IS: 10262-1982)

W/C ratio and Comp St of Concrete to suit HPC mixes

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:

1 Grade of the HPC mix under consideration

2 Desired workability for the mix

3 Prevailing Relative Humidity in the atmosphere

4 Prevailing Temperature in the atmosphere

5 Total binder content

6 Total cement content

7 Desired micro silica content

8 Desired water-binder ratio

9 Desired coarse aggregate to fine aggregate ratio

10 Desired superplasticizer dose (by weight of cement)

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 5 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 6.5 to 6.10

Figure 6.9: Relation between 28 day Comp St and Water-Binder for different humidity at 30 O C Temp

Table 6.5: Comp St and W/B ratio for different RH at 30 O C Temp

Table 6.6: Equations used for development of curves for relationship

Figure 6.10: Relation between 28 day Comp St and Water-Binder for different humidity at 35 O C Temp

Table 6.7: Comp St and W/B ratio for different RH at 35 O C Temp

Table 6.8: Equations used in development of curves indicating relationship

Figure 6.11: Relation between 28 day Comp St and Water-Binder for different humidity at 40 O C Temp

Table 6.9: Comp St and W/B ratio for different RH at 40 O C Temp

Table 6.10: Equations used in development of curves for relationships

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)

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 calculated using the following relationship:

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

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 specified grade of HPC mix (Figure 6.14)

Figure 6.14: Proposed variation of SP for different Cement contents

y = 0.012x - 2.83

0.5

11.5

22.5

33.5

44.5