Concrete Mix Design DOE PDF

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Design for concrete to DOE standard...

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TABLE OF CONTENT

1.0 INTRODUCTION 1.1 Mix Design 1.2 Importance of concrete mix design 1.3 Concrete design properties 1.4 Factors That Affect the Choice of Mix Design

2.0 Design mix methods 2.1 British Standard method of mix design (DOE) 2.2. Required Design Mix Data and Test 2.3. DOE Design Procedure

3.0 How British Mix design can be used to determine the various constituent of concrete and how a mixed proportion can be batched from the result using it to construct a stair of ten flights.

4.0 Using the table below to find the free water ratio for 20mm, crushed, slum of 0-10 for workability. 5.0 Conclusion

1.0 Introduction

Concrete is the basic engineering material used in most of the civil engineering structures. Its popularity as basic building material in construction is because of, its economy of use, good durability and ease with which it can be manufactured at site. The ability to mould it into any shape and size, because of its plasticity in green stage and its subsequent hardening to achieve strength, is particularly useful. Concrete like other engineering materials needs to be designed for properties like strength, durability, workability and cohesion.

1.1 Mix Design Concrete mix design is the process of selecting the proportions of cement, water, fine and coarse aggregates and, if they are to be used, additions and admixtures to produce an economical concrete mix with the required fresh and hardened properties. According to Bansal (2007), Concrete mix design is basically the process of selecting suitable ingredients and determining their relative proportions with the objective of having minimum workability, strength and durability as economically as possible. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance. The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing.

1.2 Importance of concrete mix design I. Quality concrete means • Better strength • Better imperviousness and durability • Dense and homogeneous concrete

II. Economy a) Economy in cement consumption It is possible to save up to 15% of cement for M20 grade of concrete with the help of concrete mix design. In fact higher the grade of concrete more are the savings. Lower cement content also results in lower heat of hydration and hence reduces shrinkage cracks. (Decorate, n.d)

b) Best use of available materials: Site conditions often restrict the quality and quantity of ingredient materials. Concrete mix design offers a lot of flexibility on type of aggregates to be used in mix design. Mix design can give an economical solution based on the available materials if they meet the basic standard requirements. This can lead to saving in transportation costs from longer distances. c) Other merits: Mix design can help us to achieve form finishes, high early strengths for early days, concrete with better flexural strengths, and concrete with lower densities.

1.3 Concrete design properties Concrete needs to be designed for certain properties in the plastic stage as well as in the hardened stage. Properties desired from concrete in plastic stage: • Workability • Cohesiveness • Initial set retardation Properties desired from concrete in hardened stage: • Strength • Imperviousness • Durability

1.4 Factors That Affect the Choice of Mix Design Grade of concrete Types of cement and its strength Desired workability of concrete Maximum nominal size of aggregate Minimum cement content and Maximum cement- ratio to satisfy durability requirement

2.0 Design mix methods American Concrete Institute Method of Mix Design (ACI) The Indian Standard Recommended Method Concrete of Concrete Mix Design British Standard method of mix design (DOE) Road note 4 mix design method etc.

2.1 British Standard method of mix design (DOE) The method of concrete mix design applied here is in accordance to the method published by the Department of Environment, United Kingdom (in year 1988).

The design involves; selection of the water/cement ratio appropriate for the required target mean strength from the code after which the free water content is selected relative to specified slump value. The ratio of the free water content to the water/cement ratio gives the cement content. Subtracting the sum of free water content and cement content all in kg/cm from the concrete density gives the aggregate content. A derived standard code provides the proportion of fine aggregate for different water/cement ratios. With this proportion the quantity of fine aggregates is estimated from the total aggregate content and the coarse aggregate content is also gotten from the difference between the aggregate content and fine aggregate content. (Aginam et al, 2013)

2.2 Required Design Mix Data and Test The following basic data are required: a) Grade of concrete along with characteristic strength (fck) b) Degree of workability desired (Slump or Vebe test) c) Degree of quality control expected to be exercised at the construction d) Exposure condition at the construction site e) Type and maximum size of aggregate to be used. f) Standard deviation (S) of compressive strength of concrete samples g) Specific gravity of aggregate.

2.3 DOE Design Procedure;

Find the target mean strength from the specified characteristic strength Target mean strength = (specified characteristic strength + standard deviation * risk factor)

Where fm = fc + ks = the target mean strength Fc = the specified characteristic strength ks = the margin, which is the product of: s = the standard deviation, and k = a constant

The constant k is derived from the mathematics of the normal distribution and increases as the proportion of defectives is decreased, thus: K for 10% defectives = 1.28 K for 5% defectives = 1.64 K for 2.5% defectives = 1.96 K for 1% defectives = 2.33 For the 5% defective level specified in BS 5328, k = 1.64 and thus f m = f + 1.64s. (Marsh, 1997) design of normal concrete mixes Calculate the water cement ratio

For a particular cement and aggregate type, the concrete strength at a given age is assumed to be governed by the free water: cement ratio only. The first step is to obtain a value of strength at water: cement ratio of 0.5 according to the 1988 British method.

Next decide water content for the required workability, expressed in terms of slump or vebe time, taking into consideration the size of aggregate and its type from required BS 1988 table. Find the cement content by simply dividing the known water content by the water/cement ratio.

Wet density of concrete & total aggregate content Wet density can be achieve from required standard BS 1988 table with known free water content and the specific gravity of aggregate. With known wet density, total aggregate = wet density - cement content – free water content. (Ta, = Wd - Cc - FWc)

Determination of FA based on aggregate size, w/c and grading zone of sand from curves acquired from a BS table. Then finally coarse aggregate content when fine aggregate volume is deducted from the total aggregate content.

3.0 How British Mix design can be used to determine the various constituent of concrete and how a mixed proportion can be batched from the result using it to construct a stair of ten flights.

Required data for design: Cement type = Ordinary Portland cement (OPC) Aggregate type = crushed from “Shai-Hills” quarries Maximum size aggregate size = 20mm Workability of paste , Slump/Vebe 0-10 mm (0-3s)

Sieve analysis shows that 40 passes through 600 micron sieve (percentage of fine agg.) Bulk specific gravity of aggregate = 2.75u (assumed for crush agg.) Moderate site condition with 25mm concrete cover Standard deviation of 4 (proper quality control and monitoring) Using a compressive strength of age 28 days with its 0.5 w/c ratio, BS 1988.

Steps: I. Choosing required compressive strength from table Cement type

Type C. A

OPC type I

uncrushed

Sulphate

crushed

Compressive 7 strength at age (Mpa) 3 22 30 26

28

91

42

49

36

49

resisting cement (t. V)

Cement = sulphate resisting, crushed, c strength at 28 days = 49Mpa II. Calculate target mean strength = fm= fc + k * s =49+ (1.64*4) = 55.6 MPa… Water cement ratio for 55.6 MPa can achieve from the BS table below.

56

Table source, (Marsh, 1997)

New w/c for the design = 0.46

111. Using the table below to find the free water ratio for 20mm, crushed, slum of 0-10 for workability. There required FWR = 170 kg/m3

M. agg size

Agg type

Slum(mm)

10-30

30-60

60-180

Vebe

6-12

3-6

0-3

time(s) 0-10, >12

20mm

uncrushed

135

160

180

195

crushed

170

190

210

225

IV. Find the cement content, Cc = FWR/WCR, Cc = 170/0.46 = 369.6

V. Determine Wet density of concrete. This can be achieved from table below base on 0.46 w/c and 2.7 specific density of aggregate. From table = 2490 kg/m3

VI. Finding Total volume of aggregate = Wet density – Cc – FWR = 2500 – 369.6 – 170 = 1960.4

VII. Determination of volume of FA based on aggregate size 20mm, w/c 0.46 and grading zone of sand (40% passing through 600 micron sieve) from curves acquired from a BS table below. From table 33% of Fine aggregate in Total aggregate volume FA= 0.33 * 1960.4(TA) = 646.93 Therefore coarse aggregate volume = 1960.4 – 646.93 = 1313.47

Total volume of various constituent of mix design is; Cement = 369.56 kg/m3

369.56/369356 = 1

Fine Agg. = 646.93 kg/m3

646.93/3693.56 = 1.75

Coarse Agg = 1313.47 kg/m3

1313.47/369.56 = 3.55

Free water ratio = 170 kg/m3

giving a mix ratio of (1: 2: 4)

4.0 How a mixed proportion can be batched from the result using it to construct a stair of ten flights. Speculations for stair construction; Assume total volume of concrete of a stairs of ten flight to be 8m3 Mix type = medium tilting type concrete mixer (200-750 lit) on site. Batching type weight batching Well erected steel prefab formwork for stairs with fixed M.S bar reinforcement of 12mm diameter and its required cover of 25mm. Drinkable water of neutral pH would be used as free water ratio in mixing. Procedure Batching : using a weighing hopper with the help of a gauge box to acquire a 1:2:4 mix Meaning 1bag of cement which is weights 1440kg/m3 requires 2*1440kg/m3 of sand and a 1440kg/m3 * 4 for coarse aggregate There for total of concrete (8m3) in stair needs a total weight of; Cement = (1440 * 1* 8) = 11520 kg Sand = (1440 * 2 * 8) = 23040 kg Coarse Agg = (1440 * 4 *8) = 46080 kg Since all weight of materials cannot be mix at once, mixing with the drum would be done in sections. Mode of transporting Transportation of concrete is an important activity in the production of concrete. The time taken in transit would be a design parameter as it depends on the initial setting time as well as the requirement of workability. Concrete pump would be used to transport concrete to staircase formwork, since using head pan or wheel barrow might be inefficient and time consuming.

Placing Placing is the process of positioning the concrete in the formwork, these operations employs precautions which includes; 1. Dropping concrete vertically at a mean height from pump 2. Using a constant slop and at a uniform layers 3. Since stair is sloppy in nature placing would start from the lower end of the form work. Compacting Compaction is the process of removing voids and air bubbles in concrete paste. Mechanical compaction mode (internal vibrator) would be adopted in this construction and the following precautions would be adhered; 1. Moderate vibration to avoid segregation which might weaken strength of concrete 2. Using strong vibrator throughout operation to acquire uniform vibration.

Curing Curing is a procedure of promoting the hydration of cement for development of concrete strength and controlling the temperature. As a result of curing, higher strength can be achieved and permeability would be reduced which is very vital for the long term strength or durability. Method of curing includes the combination of saturated coverings (jute bag) and water sprinkling. The following precautions would be employed; 1. Curing would be started early as possible. 2. For the portion of concrete which is covered with formwork, the curing should be started as soon as the formwork is removed.

3. Ensuring uninterrupted curing, if it is discontinue for any reason, the reaction of hydration will be stopped permanently. The partial hydration makes the capillary pores discontinuous and water can„t enter the concrete even if the curing is started again.

Suggestions Striking of formwork can be done seven (7) days after casting Hydration of concrete would start from day one to 28th day to ensure a full strength.

5.0 Conclusion

References

Ghansham Bansal (2007), Conceret Technology, Indian Railways Institute of Civil Engineering, Pune – 411 001 Durocrete (n.d) Mix Design Manual, Construction Quality Assurance Durocrete Engineering Servisec Pvt. Ltd. Aginam C. H., Umenwaliri S. N. and Nwakire, C. (2013) Influence of Mix Design Methods on the Compressive Strength of Concrete, ARPN Journal of Engineering and Applied Sciences. B K Marsh (2007) Design of normal concrete mixes , Building Research Establishment Ltd Garston Watford WD2 7JR, 2nd edition, Pp7, 12 (tables)...