The \'British Method\' of Mix Design PDF

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48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

The “British Method” of Mix Design (or, “DOE Method”)

1 Introduction Mix design is the method used to calculate the proportions of raw materials (e.g. cement, sand, coarse aggregate, etc) that, when properly mixed and properly placed, will produce an economical concrete mix that meets the requirements for placeability, consistency, strength, durability, and appearance. There are many methods of designing concrete mixtures; for example, ACI Method, Optimum Sand Method, and the “British Method”. The “British Method” is restricted to designing concrete mixes to meet workability, strength, and durability requirements using Portland cements and natural aggregates, however it does not deal with special materials or special concretes such as lightweight aggregate concrete, self-compacting, or pumped concrete. The Practice Class will focus on the design of a concrete mix that meets the strength and workability (slump) requirements that have been specified for each group in Table 1. The factors influencing compressive strength, considered in this design method, are freewater/cement ratio, cement type, aggregate type and concrete age. The other factors, which are not directly considered in this design method includes aggregate to cement ratio, degree of compaction and curing. 2 Principles The problem of designing a concrete mix consists of selecting the correct proportions of cement, fine and coarse aggregate and water to produce concrete having the specified properties. There are many properties of concrete that can be specified (e.g. workability, strength, density, thermal

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines characteristics. elastic modulus, durability, etc.), however the most commonly properties specified relate to: (i) The workability of the fresh concrete; (ii) The cornpressive strength at a specified age; (iii) The durability (commonly, for example, by means of specifying the minimum cement content and/or the maximum free-water/cement ratio, and in some cases, requiring the use of selected types of materials). The mix design process must take account of those factors that have a major effect on the characteristics of the concrete. The principle behind the “British Method” is that from the restricted data usually available at the mix design stage, mix proportions are derived in an attempt to produce a concrete having the required workability and strength. A trial mix is then made, but because of the assumptions made at this stage in the design it is probable that this trial mix will not completely comply with the requirements. If necessary it is possible, from the trial mix results to adjust the mix proportions and to use these for actual production or to prepare a revised trial mix. In reality, the procedure is iterative until the mixture is optimised, however in your case, you will be designing a concrete mix for the purpose of your laboratory class and undertaking testing to compare the measured properties with your design. 2.1 Strength The strength developed by a concrete made with given materials and given proportions increases for many months under favourable conditions due to ongoing chemical reactions (termed “hydration”) between the cement and water within the concrete. In the majority of specifications the strength is specified at an age of 28 days. Other key factors that can influence the strength include the temperature and humidity conditions during curing. Higher temperatures increase the speed of the chemical reactions within the hydrated cement and thus the rate of strength development, and in order to achieve higher strengths at later ages loss of water from the concrete must be prevented.

Standard Concrete Test Cylinder (100 mm diameter x 200 mm length)

Figure 1

Test Cylinder during Compressive Strength Testing

Test Cylinder After Failure

Concrete Test Cylinders – Compressive Strength

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines Your mix will consider the compressive strength of the concrete at 28 days. In the laboratory, the compressive strength test samples will be made to Australian Standard AS 1012.8.1—2000 Method 8.1, consisting of test cylinders of 100 mm diameter and 200 mm length. The samples will be made during your laboratory classes and, after hardening, removed from the moulds and cured until the samples reach 28 days age. AS1012.1 specifies the type of curing of test cylinders as follows:

Kuala Lumpur

2.2 Workability The ease with which concrete is mixed, transported, placed, and compacted is extremely important in executing successful concrete construction. “Workability” can be defined as the ease with which the fresh concrete can be handled, placed and compacted without excessive segregation (i.e. – separation of the fine aggregate and cement paste from the coarse aggregate particle), or air voids.

High Workability Concrete (high flow)

Low Workability Concrete (zero flow)

Figure 2 Concrete Workability: “high” and “low”

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

Workability is affected by: the grading, particle shape, and proportions of aggregate; the amount and qualities of cement and other cementitious materials; free water content; cementitious paste content; and the presence of chemical admixtures. There are many test methods that are used to measure workability: we will consider the Slump Test for our mix design, however during your laboratory classes you will also have the opportunity to measure workability by two alternative methods for comparison. You will test workability by the slump test, in accordance with the Australian Standard AS1012.3.1. Put simply, a cone of fresh concrete is made within a steel mould of dimensions 200 mm diameter base, 100 mm diameter top, and 300 mm length (Figure 4). During testing, the cone is Figure 3 Slump Test filled in 3 layers of equal volume, with (CIV2226 laboratory each layer compacted by rodding 25 class in 2012) times. Following placement and compaction of the 3rd (top) layer, the top surface is levelled off to the top surface of the mould (Figure 5). The cone mould is then lifted upwards, clear of the concrete, and placed, upside-down next to the concrete. The fresh concrete then “slumps” due to gravity and the height difference between the steel cone and the slumped concrete is measured (Figure 6). The height difference is termed the “slump”. In general terms, a higher measured slump signifies a concrete with higher workability (although there are exceptions and we will cover these during the lectures).

Figure 4 Mould (cone) for Standard Slump Test

slump cone rod

concrete

Figure 5

Figure 6

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines 2.3 Type of cement There are various types of cement that differ in composition, fineness, and fresh and hardened properties they impart to concrete, as will be covered during lectures. For your mix design, assume the type of cement is Type GP General Purpose Cement that complies with Australian Standard AS3972. 3 Mix Design Methodology The aim is to design the mix proportions to create a cubic metre of concrete (1 m3) that has the specified strength and workability (slump) that has been assigned to your group (refer to Table 1). The methodology of the British Method can be summarised as follows: Stage 1: Design for strength, leading to determination of the free water to cement ratio of the concrete mixture Stage 2: Design for workability, leading to the free water content Stage 3: Combines the results of Stages 1 and 2 to give the cement content Stage 4: Deals with the determination of the total aggregate content Stage 5: Deals with the selection of the fine and coarse aggregate contents ⇒ the mix proportions to create your laboratory mixture 4 Strength and Free Water/Cement Ratio Concrete strength is affected by many factors, however for a given set of materials and conditions a key parameter that is used for concrete mix design is the free water to cement ratio (or, w/c). Water is necessary in the concrete mixture because it reacts with the cement in a series of chemical reactions termed “hydration”. Theoretically, very little water is necessary to react with the cement and a free water to cement ratio as low as 0.27 would be adequate – however, the concrete mixture would be very stiff and unworkable. Additional water (and cement) is needed to coat the coarse and fine aggregate particles with cement paste to

Low w/c

High w/c

Cement Particles Suspended in mixing Water

Fully Hydrated Cement

Low w/c = Less Voids ⇒ Higher Strength

Air and/or Water-filled voids

High w/c = More Voids ⇒ Lower Strength

Figure 7 Relationship between w/c & air voids in hardened concrete

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines achieve suitable workability. Excess water that is not consumed by the hydration reactions may leave the concrete as it hardens, resulting in microscopic pores that will reduce the final strength of the concrete. This is shown in Figure 7, where low w/c concrete has less voids than a higher w/c concrete: resulting in differences in concrete strength. 5 Free Water Coarse and fine aggregates are porous and absorb water. Aggregates that are soaked in a water tank will be “fully saturated” and surface-wet whereas oven-dried aggregates will be dry and prone to absorb water. The dilemma for the mix designer is: w/c is needed to achieve strength, however my laboratory mix will be affected by: • w/c too high if “wet” aggregates are used because the aggregates will be fully saturated and the excess water will be introduced into the mix as additional mixing water ⇒ weaker concrete (and workability too high because of the excess water) • w/c too low if “dry” aggregates are used because the aggregates will absorb some of the mixing water ⇒ higher strength but lower workability We therefore design concrete mixtures that are in the Saturated and Surface Dry (SSD) condition (Figure 8).

Absorbed Moisture

Fully Dry Condition Figure 8

Air Dry (partially dry)

Free Moisture

Saturated and Surface Dry

Saturated and Surface Wet

Moisture conditions and concrete aggregates, showing saturated and Surface Dry Condition (SSD)

Saturated Surface Dry (SSD) describes the condition of the aggregates (sands and crushed rock) in which all the pores of the aggregate are filled with water and no excess water is on the aggregate surface. When the aggregate is in SSD condition, it would neither absorb nor add to the free water available in concrete mix for cement hydration. When the aggregate is in unsaturated condition, it would absorb the free water until it is saturated, and therefore the water added to concrete mix should be increased to compensate for this aggregate absorption. When the aggregate is in surface wet condition, the excess water on the surface of the aggregate would be available for cement hydration, and therefore the water added to concrete mix should be reduced to compensate for the aggregate surface water amount. When reporting concrete mix proportions, it is normal practice to state the aggregates in SSD condition.

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines The strength of the concrete is related to the free-water in the mix, and is not dependent on the absorption properties of the aggregates. 6 Mix Design Step 1: Free water/cement ratio (w/c) We commence the mix design by estimating the concrete strength at w/c of 0.5. Table 1 sets out the approximate compressive strengths of concrete mixes made with a freewater/cement ratio of 0.5. The data is applicable to concretes cured in water at 23oC. Note that selection of the right type of cement is important. Our lectures will cover the six major types of cement: Type GP (General Purpose Portland cement) Type GB (General Purpose Blended cement) Type HE (High Early Strength cement) Type LH (Low Heat cement) Type SR (Sulfate Resisting cement) Type SL (Shrinkage Limited cement) Table 1 provides data for Types GP, SR, and HE cements, and can be used for mix design strengths at 3, 7, 28, and 91 days.

Table 1: Approximate compressive strengths (MPa) of concrete mixes made with a freewater/cement ratio of 0.5 Type of Type of coarse Compressive Strengths (MPa) Age (days) Cement aggregate 3 7 28 91 Type GP or Uncrushed 17 24 38 45 Type SR Crushed 22 31 45 54

Type HE

Uncrushed Crushed

24 31

36 39

47 52

52 60

With all other mix variables held constant, values of compressive strength given in Table 3 show that an uncrushed coarse aggregate generally produces a concrete with lower strength than one made with crushed coarse aggregate. If local knowledge indicates that this is not the case, values in Table 3 should be modified accordingly. Factors such as the type of fine aggregate, the maximum size of aggregate and the overall grading have relatively small effect on compressive strength. Figure 9 shows a family of curves for the relationship between compressive strength and the freewater/cement ratio. They have been obtained for a large number of different concrete mixes using different Portland cements and different types of aggregates. During mix design, the value of compressive strength is obtained from Table 1 for a mix made with a free-water/cement ratio of 0.5 for the specified age, type of cement, and aggregate to be used.

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines This value is then plotted on Figure 9 (refer to example shown below) and a curve is drawn from this point, parallel to the adjacent curves until in intercepts a horizontal line passing through the ordinate representing the specified strength for your mix. The corresponding value of freewater/cement ratio is then read – this is the free w/c that is used in your particular mix design. An example follows and is illustrated by Figure 10.

Figure 9

Relationship between compressive strength and free water/ cement ratio

EXAMPLE Estimate the free w/c that is needed for the design of a concrete mix that is made with Type HE cement, crushed rock coarse aggregate, and 28 day strength of 30MPa. Refer to Table 2 and Figure 9 (reproduced below for this example) and follow the procedures discussed earlier: (i) Type HE cement and crushed rock aggregate. Table 2 provides 28 day strength of 52 MPa for w/c=0.5 (ii) Refer to Figure 10. Find the intersection point for w/c=0.5 and 52 MPa. (iii)Through the intersection point, a curve is drawn from this point, parallel to the adjacent curves (Figure 10). In our example, the design strength is 30MPa: follow the curve until in intercepts a horizontal line passing through 30MPa. (iv) The intersection point provides the w/c for your mix: in this example, w/c=0.68.

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines Table 2: Approximate compressive strengths (MPa) of concrete mixes made with a freewater/cement ratio of 0.5 Type of Type of coarse Compressive Strengths (MPa) Age (days) Cement aggregate 3 7 28 91 Type GP or Uncrushed 17 24 38 45 Example: Type SR Crushed 22 31 45 54 Type HE cement Type HE

Uncrushed Crushed

24 31

36 39

47 52

52 60

Table 3: 28 day strength = 52 MPa

Step1

Sketch a curve through the point where w/c = 0.5 intersects the Table 2 value. The curve should be parallel to the adjacent curves. Follow the curve until you reach your design strength ⇒ free w/c=0.68

Step 2

Step 4

Type HE cement 28 day strength = 52 MPa (Table 2)

Example: Design strength = 30MPa

Step 5

w/c = 0.5

Design w/c = 0.68

Step 3 Figure 10 Example: Utilization of Table 2 and Figure 9 to Estimate w/c

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

7 Mix Design Step 2: Free-Water Content The free water content required for your concrete mix is selected primarily to obtain the particular workability of a given concrete mix. The free-water content is read from Table 3 and depends on the type and maximum size of aggregate to give a concrete of specified workability. Table 3: Approximate free-water content (kg m-3) for various levels of workability Maximum size of aggregate (mm)

Type of aggregate

10

uncrushed crushed uncrushed crushed uncrushed crushed

20 40

Slump (mm) 0-10

10-30

30-60

60-100

100-160

135 160 120 150 100 140

160 185 140 170 125 155

195 220 170 200 155 180

210 235 185 210 170 195

225 235 185 210 180 200

The values provided in Table 3 will vary according to the specific characteristics of the coarse and fine aggregates (for example, aggregate shape, surface texture, grading). The data in Table 3 has been adjusted to include the laboratory test results compiled by CIV2226 students in previous years. 8 Cement Content The cement content is simply calculated by dividing the free-water content by the free-w/c ratio. 9 Total Aggregate Content In order to be able to calculate the total aggregate content, an estimate of the wet density of the fully compacted concrete is required. This can be read from Figure 11. It is also necessary to know or to be able to assume the relative density of the aggregate. An approximation can be made by assuming an average value of specific gravity of 2.6 for uncrushed aggregate and 2.7 for crushed aggregate. The specific gravity of Australian aggregates generally ranges from 2.5 to 2.9, depending on the source. However, in your case, the specific gravity of the fine and coarse aggregates has been tested and are provided (at the back of this guide for fine-tuning your calculations). Based on aggregates being in the saturated and surface dry (SSD) condition, Total aggregate content = D − C− W where, D = wet density of concrete, kg m-3 C = cement content, kg m-3 W= free-water content, kg m-3

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

Figure 11 Wet density of fully compacted concrete

10 Fine and Coarse Aggregate Contents The proportion of fine aggregate for use in a given mix is selected from the curves in Figure 12. The best proportion to use in a given mix will depend: • Free-water/cement ratio • Maximum size of aggregate • Slump • Fineness modulus of the fine aggregate or sand (i.e. based on sieve test data: the fineness modulus is the sum of the total percentages retained on each of a specified series of sieves, divided by 100. The coarser the aggregate, the higher the fineness modulus). An example calculation is provided below. Figure 12 shows recommended values for the proportion of fine aggregate depending on the maximum size of coarse aggregate, the workability level (slump), the grading of the fine aggregate (defined the fineness modulus), and the free-water cement ratio. The proportion of fine aggregate estimated from Figure 12 will generally produce a satisfactory concrete in the first trial mix, which can then be adjusted as required to meet the prevailing conditions.

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

Figure 12 Proportions of fine aggregate determined from Fineness Modulus

48352 Construction Materials Spring 2018 Mix Design: Background and Guidelines

The method to calculate the Fineness M...