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43966925 Concrete Mix Design

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    1

    Comments

    Concrete mix designs is best defined as a process in selecting suitable ingredients, which is

    cement, aggregate, sand and water, and determining their relative proportions to give the

    required strength, workability and durability. A design mix, which is a performancespecification stating required strength and minimum cement content but leaving the grading anddetails of the mix design to be work out.

    Objective of Mix Design

    Concrete Mixer Drum type 140L

    Two main objectives forconcrete mix design:

    To determine the proportions of concrete mix constituents of; Cement, Fine aggregate (ornormally Sand), Coarse aggregate, and Water.

    To produce concrete of the specified properties.

    To produce a satisfactory of end product, such as beam, column or slab as economically

    as possible.

    Theory of Mix Design

    http://www.civilcraftstructures.com/materials-testing/steps-of-concrete-mix-design/#commentshttp://www.civilcraftstructures.com/materials-testing/steps-of-concrete-mix-design/#commentshttp://www.civilcraftstructures.com/construction-materials/the-facts-about-concrete/http://www.civilcraftstructures.com/construction-materials/the-facts-about-concrete/http://www.civilcraftstructures.com/materials-testing/steps-of-concrete-mix-design/#commentshttp://www.civilcraftstructures.com/materials-testing/steps-of-concrete-mix-design/#comments
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    The Process of Mix Design

    The method of mix design applied here is in accordance to the method published by the

    Department of Environment, United Kingdom (in year 1988).

    There are two categories of initial information required:

    1. Specified variables; the values that are usually found in specifications.

    2. Additional information, the values normally available from the material supplier.

    Reference data consists of published figures and tables is required to determine the design values

    including;

    Mix parameters such as target mean strength, water-cement ratio and concrete density.

    Unit proportions such as the weight of materials.

    The design process can be divided into 5 primary stages. Each stage deals with a particular

    aspect of the mix design:

    Stage 1: Determining the Free Water/ Cement Ratio

    i) Specify the required characteristic strength at a specified age, fcii) Calculate the margin, M.

    M = k x s .. [ F1 ]

    where;k = A value appropriate to the defect percentage permitted below the characteristic strength. [ k

    = 1.64 for 5 % defect ]

    s = The standard deviation (obtained from CCS 1).

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    CCS 1: Approximate compressive strength (N/mm2) of concrete mixes made with a free-water/cement ratio of 0.5

    iii) Calculate the target mean strength, fm

    fm = fc + M .. [ F2 ]

    where;

    fm = Target mean strengthfc = The specified characteristic strength

    iv) Given the type of cement and aggregate, use the table of CCS 1 to obtain the compressive

    strength, at the specified age that corresponds to a free water/cement ratio of 0.5.

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    CCS 4: Relationship between compressive strength and free-water/ cement ratio.

    v) In figure CCS 4, follow the starting line to locate the curve which passes through the point

    (the compressive strength for water/cement ratio of 0.5). To obtain the required curverepresenting the strength, it is necessary to interpolate between the two curves in the figure. At

    the target mean strength draw horizontal line crossing the curve. From this point the required freewater/cement ratio can be determined.

    Stage 2: Determining the Free-water Content

    CCS 2: Approximate free-water contents (kg/m3) required to give various levels of workability.

    Given the slump or vebe time, determine the free water content from table CCS 2.

    Stage 3: Determining the Cement Content

    Cement Content = Free Water Content / Free-water or Cement Ratio .. [ F3 ]

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    The resulting value should be checked against any maximum or minimum value that may be

    specified. If the calculated cement content from F3 is below a specified minimum, this minimum

    value must be adopted resulting in a reduced water/cement ratio and hence a higher strength thanthe target mean strength. If the calculated cement content is higher than a specified maximum,

    then the specified strength and workability simultaneously be met with the selected materials; try

    to change the type of cement, the type and maximum size of the aggregate.

    Stage 4: Determining the Total Aggregate Content

    This stage required the estimate of the density of fully compacted concrete which is obtained

    from figure CCS 5. This value depends upon the free-water content and the relative density of

    the combined aggregate in the saturated surface-dry condition. If no information is availableregarding the relative density of the aggregate, an approximation can be made by assuming a

    value of 2.6 for un-crushed aggregate and 2.7 for crushed aggregate.

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    CCS 5: Estimated wet density of fully compacted concrete.

    With the estimate of the density of the concrete the total aggregate content is calculated using

    equation F4:

    Total Aggregate Content = D C W .. [ F4 ]

    where;

    D = The wet density of concrete ( in kg/m3)C = The cement content (in kg/m3)

    W = The free-water content (in kg/m3)

    Stage 5: Determining of The Fine and Coarse Aggregate Contents

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    This stage involves deciding how much of the total aggregate should consist of materials smaller

    than 5 mm, i.e. the sand or fine aggregate content. The figure CCS 6 shows recommended values

    for the proportion of fine aggregate depending on the maximum size of aggregate, theworkability level, the grading of the fine aggregate (defined by the percentage passing a 600 m

    sieve) and the free-water/ cement ratio. The best proportion of fines to use in a given mix will

    depend on the shape of the particular aggregate, the grading and the usage of the concrete.

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    CCS 6: Recommended proportions of fine aggregate according to percentage passing a 600 m

    sieve.

    The final calculation, equation F5, to determine the fine and coarse aggregate is made using theproportion of fine aggregate obtained from figure CCS 6 and the total aggregate content derived

    from Stage 4.

    Fine Aggregate Content = Total Aggregate Content x Proportion of Fines .. [ F5 ]

    Coarse Aggregate Content = Total Aggregate Content Fine Aggregate

    Procedures of Mixing

    Production of Trial Mix

    1. The volume of mix, which needs to make three cubes of size 100 mm is calculated. The

    volume of mix is sufficient to produce 3 numbers of cube and to carry out the slump test.2. The volume of mix is multiplied with the constituent contents obtained from the mix

    design process to get the batch weights for the trial mix.

    3. The mixing of concrete is according to the procedures given in laboratory guidelines.

    4. Firstly, cement, fine and course aggregate are mixed in a mixer for 1 minute.5. Then, water added and the cement, fine and course aggregate and water mixed

    approximately for another 1 minute.

    6. When the mix is ready, the tests on mix are proceeding.

    Slump Test apparatus for Concrete Workability

    Tests on Trial Mix

    1. The slump tests are conducted to determine the workability of concrete.

    2. Concrete is placed and compacted in three layers by a tamping rod with 25 times, in a

    firmly held slump cone. On the removal of the cone, the difference in height between the

    uppermost part of the slumped concrete and the upturned cone is recorded in mm as theslump.

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    3. Three cubes are prepared in 100 mm x 100 mm each. The cubes are cured before testing.

    The procedures for making and curing are as given in laboratory guidelines. Thinly coat

    the interior surfaces of the assembled mould with mould oil to prevent adhesion ofconcrete. Each mould filled with two layers of concrete, each layer tamped 25 times with

    a 25 mm square steel rod. The top surface finished with a trowel and the date of

    manufacturing is recorded in the surface of the concrete. The cubes are storedundisturbed for 24 hours at a temperature of 18 to 220C and a relative humidity of not less

    than 90 %. The concrete all are covered with wet gunny sacks. After 24 hours, the mould

    is striped and the cubes are cured further by immersing them in water at temperature 19to 21oC until the testing date.

    4. Compressive strength tests are conducted on the cubes at the age of 7 days. Then, the

    mean compressive strengths are calculated.

    The Calculations

    Here is one example of calculation from one of the mix design obtained from the laboratory. We

    have to fill in all particulars in the concrete mix design form with some calculations

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    CCS 3: Relationship between standard deviation and characteristic strength.

    Firstly, we specified 30 N/mm2 at 7 days for the characteristic strength. Then, we obtained the

    standard deviation, s from the figure CCS 3. So, s = 8 N/mm2.

    From the formula F1, k = 1.64 for 5 % defect. The margin, M is calculated as below:

    M = k x s = 1.64 x 8 = 13.12 N/mm2

    With the formula F2, target mean strength, fm is calculated as below:

    Target mean strength, fm = fc + M= 30 + 13.12 = 43.12 N/mm2

    The type of cement is Ordinary Portland Cement (OPC). For the fine and course aggregate, the

    laboratorys fine aggregate is un-crushed and for coarse aggregate is crushed before producing

    concrete.

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    Then, we obtain the free-water/ cement ratio from table CCS 1. For OPC ( 7 days ) using crushed

    aggregate, water/cement ratio = 36 N/mm2.

    After that, from the figure CCS 4, the curve for 42 N/mm 2 at 0.5 free-water ratio is plotted andobtained the free-water ratio is 0.45 at the target mean strength 43.12 N/mm2.

    Next, we specified the slump test for slump about 20 mm and the maximum aggregate size we

    used in laboratory is 10 mm. For the specified above, we can obtained the free-water content

    from table CCS 2 at slump 10 30 mm and maximum size aggregate 10 mm, the approximatefree-water content for the un-crushed aggregates is 180 kg/m3 and for the crushed aggregates is

    205 kg/m3. Because of the coarse and fine aggregates of different types are used, the free-water

    content is estimated by the expression:

    Free-water Content, W= 2/3 Wf +

    1/3 Wc= (2/3 x 180) + (

    1/3 x 205)

    = 188.33 kg/m

    3

    where,Wf = Free-water content appropriate to type of fine aggregate

    Wc = Free-water content appropriate to type of coarse aggregate

    Cement content also can obtained from the calculation with the expression at F3:

    Cement Content, C = Free Water Content / Free-water or Cement Ratio= 188.33 / 0.45 = 418.52 kg/m3

    We assumed that the relative density of aggregate (SDD) is 2.7. Then, from the figure CCS 5

    with the free-water content 188.33 kg/m

    3

    , obtained that concrete density is 2450 kg/m

    3

    . The totalaggregate content can be calculated by:

    Total Aggregate Content = D C W

    = 2450 418.52 188.33 = 1843.15 kg/m3

    The percentage passing 600 m sieve for the grading of fine aggregate is about 60 %. The

    proportion of the fine aggregate can be obtained from the figure CCS 6, which is 38 %. Then, thefine and course aggregate content can be obtained by calculation:

    Fine Aggregate Content

    = Total Aggregate Content x Proportion of Fines= 1868.74 x 0.38 = 700.40 kg/m3

    Coarse Aggregate Content = Total Aggregate Content Fine Aggregate

    = 1843.15 700.40 = 1142.75 kg/m3

    The quantity per m3 can be obtained, which is;

    Cement = 418.52 kg

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    Water = 188.33 kg

    Fine aggregate = 700.40 kg

    Coarse aggregate (10 mm) = 1142.75 kg

    The volume of trial mix for 3 cubes

    = [(0.1 x 0.1 x 0.1) x 3] + [25% contingencies of trial mix volume]= 0.006 + 0.00075

    = 0.00375 m3

    The quantities of trial mix = 0.00375 m3, in which is;

    Cement = 1.57 kg

    Water = 0.71 kgFine aggregate = 2.61 kg

    Coarse aggregate (10 mm) = 4.29 kg

    The Results of Mix Design

    Slump Test = True Slump of 55 mm

    All the 3 concrete cubes produced were then cured for 7 days. After that, the compressive cubetest is carried out. The results are as follows:

    Sample 1 2 3

    Compressive Strength 32.37 33.54 35.70

    Average (32.37 + 33.54 + 35.70) / 3 = 33.87

    For cubes after 7 days of curing, compressive strength should not be less than 2/3 target mean

    strength.

    = 2/3 43.12 = 28.75 N/mm2 < 33.9 N/mm2

    After 7 days of curing, the compressive strength of concrete cubes produced by the mix design

    method pass the specific strength requirements.

    Discussions Upon Mix Design

    Although our compressive strength passes the specific requirements, we still identified severalfactors which contribute to the lacking of compressive strength of concrete mixes produced in

    the experiment. However, the main factor is the condition of aggregates whether it is exposed tosunlight or rainfall.

    When the free water/cement ration is high, workability of concrete is improved. However,excessive water causes honey-comb effect in the concrete produced. The concrete cubes

    become porous, and hence its compressive strength is well below the design value. Other

    possible reasons include over compaction, improper mixing methods and some calculation

    errors.

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    Few suggestion upon several steps to avoid the problems previously faced:

    All the raw materials, which is cement, aggregates, and sand should be protected from

    precipitation or other elements which may affect its physical properties.

    The quantity of ingredients may be adjusted if necessary, theoretical values are not

    always suitable. For example, if the aggregates are wet or saturated, less amount of watershould be added, vice versa.

    Compaction should be done carefully, as either under or over-compaction will bringsignificant negative effect on the concrete produced.

    The Conclusion

    1. By using the concrete mix design method, we have calculated the quantities of allingredients, that is water, cement, fine and coarse aggregate according to specified

    proportion.

    2. The concrete produced did not fulfill the compressive strength requirements due to

    several reasons. Furthermore, some steps mentioned above should be taken intoconsideration to overcome this problem.

    Standard reference for the mix design is as accordance to British Standard;

    BS 5328: 1981 : Methods of Specifying Concrete including Ready-Mixed Concrete

    WHAT IS CONCRETE

    Concrete is the product of mixing, aggregate, cement and water.The setting of concrete is a chemical reaction between the cement and the water, not a dryingprocess.This reaction is called hydration, it evolves heat as does any chemical reaction, and the process isirreversible.There is an initial set when the concrete will cease to be liquid but have little strength (e.g. 6 to24hrs. old), thereafter the concrete will gradually gain strength over time until it achieves thestrength required.Differing mix proportions and cement types will achieve required strengths in differing timespans.

    CONSTITUENTS OF CONCRETE

    Cement, Aggregate and Water, (and sometimes additives).

    AggregateAggregates are usually distinguished between fine and coarse aggregate.Aggregates are classed as inert materials, such as washed natural sand (fine); and natural gravel,which can be crushed to produce the appropriate size and grading of aggregate, and similarlycrushed, quarried stone (coarse).

    http://www.bsigroup.com/http://www.bsigroup.com/
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    The aggregate must have a minimum inherent strength requirement for structural concrete, thecoarse aggregate must not be weaker than the concrete paste.All aggregate must be "clean", i.e. not contaminated with organic matter or clay/silty soils andoverburden during extraction and storage.

    CementBasically a material made by heating limestone and a suitable clay to produce a clinker rich incalcium silicates.This clinker is ground to produce a fine powder, this is cement.By using different clinkers, grinding them to differing degrees of fineness and the use of additivesmany different types of cement are produced with varied properties in their use, e.g. rapidhardening cement, sulphate resisting cement, etc..Generally speaking the more cement in a mix the stronger more durable the concrete producedwill be, but this does have to be related to other factors, primarily the amount of water used in themix, i.e. water/cement ratio.

    WaterWater is an extremely important part of concrete, and drinking quality water is usually required, orwater from an approved source free from impurities.

    AdditivesThe most commonly used additive is a "foaming" agent to produce air entrained concrete, mainlyfor carriageway concrete, but also other exposed situations.Another common use of an additive is to increase the workability of concrete without adding extrawater and thus increasing the water/cement ratio and decreasing the strength of the concrete.

    TIME TAKEN TO PLACE CONCRETE AFTER BATCHING

    From the time of adding water to the cement the chemical reaction has begun and you only have alimited amount of time to place and compact the concrete, this is usually specified as 90 minutes.The delivery ticket of the load of concrete will be stamped with the time of batching.

    ADDITION OF WATER

    Given a set amount of cement and aggregate there is an optimum amount of water to be added toproduce a chemical reaction to give the maximum obtainable strength, too little or too much waterwill produce a weaker concrete.Unfortunately as in all things, life is not that simple, and the workability of the concrete has to beconsidered when placing concrete, especially in difficult situations.These situations can be areas of high density of reinforcing bars, complicated formwork design,or where the concrete needs to be suitable for pumping.In these situations water content is increased to make the concrete more workable, BUT thisincrease in water content is calculated at the design stage and the cement content is increasedaccordingly to retain the strength of the mix.

    For every designed concrete mix with a specified strength there is a set WATER:CEMENT RATIO

    which must be retained in order to achieve the designed strength.

    WATER SHOULD NEVER BE ADDED TO CONCRETE, ESPECIALLY ON SITE, WITHOUT THECONSIDERATION OF ALL THE ABOVE POINTS,

    AND THE APPROVAL OF THE ENGINEER

    CONCRETE WORKABILITY

    An on site simple test for determining workability is the SLUMP TEST.

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    This consists of a conical mould 300mm. high, with an opening at the top of 100mm. diam., and atthe bottom of 200mm. diam..The mould is filled with concrete in 4 layers and rodded to remove air voids, with the smallerorifice uppermost.The "slump" is the difference in height between the height of the mould and the height of theconcrete column with the mould removed.The workability of the concrete will depend upon the situation into which the concrete is beingplaced.Low workability, i.e. stiff concrete, is needed for carriageway concrete which is laid by a "pavingtrain".High workability concrete is needed in situations of high density of reinforcing steel to enable theconcrete to flow around all the reinforcing without leaving any voids.

    SPECIFYING CONCRETE STRENGTH

    The strength/grade of concrete is specified and measured in newtons/sq. mm., meganewtons/sq.metre or even megapascals, in fact the numerical figure will be the same in each case.E.g. a strength of 20 newtons/sq.mm. is the same as 20 meganewtons/sq.metre.

    The strength/grade of concrete is normally specified by stating the strength you wish the concrete

    to achieve after a period of 28 days.

    The specifications governing the design, use and testing of concrete have undergone tremendouschanges in the lat few years, I will not go into this topic on this page other than to say you maylike to be aware of the introduction of,

    BS 8500-1:2002:Concrete - Complementary British Standard to BS EN 206-1Part 1 : Method of specifying and guidance for the specifierBS 8500-2:2002:Concrete - Complementary British Standard to BS EN 206-1Part 2 : Specification for constituent materials and concrete

    These are British Standards that have been published to help you understand, the currentstandard for concrete, which is,

    BS EN 206 - 1 : Concrete : Part 1 : Specification, performance, production and conformity and it is likely that you will need the help ofBS 8500, even then it may prove difficult tounderstand BS EN 206. It is not a "user friendly" document.

    MEASURING CONCRETE STRENGTH

    The strength is measured by crushing concrete cubes to failure and recording this strength.

    Concrete cubes are made from fresh concrete sampled at the time of pouring by placing correctlysampled concrete into a steel mould and compacting to remove air voids.The concrete is allowed an initial "set" period of 24 hours, the mould is then stripped and the cubeis cured in water at a temperature of 20 deg.c for 28 days prior to crushing.

    If you wish to strike shuttering before 28 days, extra cubes will be required to determine that thein-situ concrete has achieved the appropriate strength at the time you wish to strike theshuttering.This is usually an arrangement agreed by the contractor, the concrete supplier and the engineer.

    SAMPLE CONCRETE MIXES, FOR GUIDANCE ONLY (but you will be able to understand them)

    Below are a number of different types of concrete mixes showing batch weightsand cement contents when a particular source of clean crushed river gravel andsand where used, so they are only an indication of concrete composition and will

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    not be suitable for all aggregates / sand / cement.I have included this item because of what I believe is the lack of basic concrete information that isreadily available to young engineers and engineering technicians wishing to improve theirworking knowledge of concrete.These mixes are in fact concrete mixes produced to comply with the Specification for Road andBridge works of 1963, i.e. still current in 1969 to 1972 while I was working on the M6 Link for OwenWilliams and Partners.Back in this period, when I was beginning my career in highways materials, specifications andBritish Standards were written in such a way that basic information regarding concrete could beobtained from them, and they were my original learning tool.It is my opinion that this is no longer the situation with current specifications and standardshence the publication ofBS 8500, Parts 1 & 2, and I would not regard these as particular userfriendly for a "beginner".

    Basic concrete mixes are basic concrete mixes, and these are examples of basic concrete mixes.I know things have moved on, and there are now many types of cement, admixtures and fibresthat produce enhanced concrete for particular uses, and that you do need to take into account thedensity of the aggregates, and the workability of the produced concrete, the source/chemicalcomposition of the aggregate, etc., etc..But remember this is "The Idiot's Guide to Highways Maintenance", and this is some basic

    information to assist those who want to know a little bit more about concrete for basic on siteuses, and possibly minor structures.For most uses you will be buying your ready mixed concrete from an established supplier to anappropriate, modern, specification, but it does not hurt to have some knowledge of what you arebuying and how it was produced.

    I have used the original broad descriptions (e.g. A) used in this specification to describe thetypes of concrete, each type having a particular use with regard to required strength, workabilityand cost to produce.These mixes were used in the Longford Viaduct and the Bedworth Viaduct, and the many otherlarge structures on Contract 11, and the last time I looked they were all still standing, so I haveconfidence to reproduce them here.

    NOTE : I have quoted the quantities in LBS. and GALLONS (the original measures) as well asconverting them to metric quantities, and S.I. units for strength, although I remain withmeganewtons not megapascals, they are the same figures.

    If you do wish to use these quantities as a basis for determining your own batching weights, youMUST note that these figures will only give you a cubic yard of mixed concrete NOT a cubic metre ,and if the density of your aggregates are different to the aggregates used in these mixes you mayget slightly less or slightly more than a cubic yard.To save you looking it up, 1 cubic yard of concrete = 0.7646 cubic metres.

    As with any new concrete mix you must take cubes and crush them at appropriate dates todetermine the real strength of the concrete you have designed / produced, only then can youconsider using it in the works /structure, and you should expect to attain the Preliminary

    strengths in your trials to ensure the Works strength for normal production.

    The cement used in these "standard" mixes was OPC (Ordinary Portland Cement), and thestrengths quoted are for 150mm. (6 inch) cubes crushed at 28 days.

    And do not forget to weigh your cubes and determine the density, as density is a good indicationof a well designed concrete, and if you have crushed as many concrete cubes as I have, you willhave noted that for each "mix", cube making and curing being constant, the cubes with thehighest density will give the highest strengths.

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    Surface Dry Batch Weights for a number of Classes of Concrete

    Concrete

    Class

    Strength (Mn -lbs/in) Cement

    (kgs/lbs)

    Sand

    (kgs/lbs)

    Aggregate Agg./Cement

    Ratio

    Water/Cement

    RatioPreliminaryWorks 20mm.-5mm. 40mm.-20mm.

    A 38.5 / 560029.0 /4200

    327 /720

    408 /900

    975 /2150

    - 4.25 0.52

    B 34.5 / 500026.0 /3750

    281 /620

    458 /1010

    975 /2150

    - 5.1 0.49

    C 27.5 / 400020.5 /3000

    218 /480

    517 /1140

    975 /2150

    - 6.85 0.62

    E1 N/A N/A168 /370

    472 /1040

    454 /1000

    630 /1390

    9.3 0.78

    Y 52.0 / 7500 41.5 /6000 340 /750 395 /870 975 /2150 - 4.1 0.51

    10mm.-5mm.

    Y 52.0 / 750041.5 /6000

    371 /820

    1279 /2820

    840 /1850

    - 5.7 0.45

    The amount of "free" water added to the above aggregate is based on the water cement ration andthe moisture contents of the aggregates, especially the sand.

    That is why batching weights are initially given as "surface dry", you then determine the totalamount of "free" water by applying the water cement ration to the quantity of cement.

    E.g. the water cement ratio for the concrete mix below is 0.45,

    water = 0.45cement

    Therefore:- weight of water = weight of cement x 0.45 = 371 x 0.45 kgs. = 167kgs./litres(near enough for the purposes of demonstration)

    To better indicate this I include below a reproduction of a batching chart forYconcreteused ata concrete batching plant, i.e. as the moisture content of the aggregate, usually the sand,

    increases the added water decreases.However at times of heavy rainfall and with smaller aggregate the coarse aggregate can retainsufficient water that it needs to be taken into account, especially with high strength concrete.This indicates the importance of knowing the moisture content of the sand in the stockpiles, andoften in the different levels of the stockpile.

    Sand MoistureContent %

    10mm. CrushedGravel

    SandAdded "free"

    WaterCement

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    0840kgs. /1850lbs.

    1271kgs. /2801lbs.

    168litres /36.5galls

    371kgs. / 820lbs.

    1 "1284kgs. /2830lbs.

    155litres /34.0galls.

    "

    2 "

    1297kgs. /

    2860lbs.

    141litres /

    31.0galls. "

    3 "1309kgs. /2885lbs.

    130litres /28.5galls.

    "

    4 "1320kgs. /2910lbs

    118litres /26.0galls.

    "

    5 "1334kgs. /2940lbs.

    105litres /23.0galls.

    "

    6 "1347kgs. /2970lbs.

    91litres /20.0galls.

    "

    7 "

    1359kgs. /

    2995lbs.

    77litres /

    17.0galls. "

    8 "1372kgs. /3025lbs.

    66litres /14.5galls.

    "

    9 "1383kgs. /3050lbs.

    52litres /11.5galls.

    "

    10 "1397kgs. /3080lbs.

    40litres / 9.0galls. "

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