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1 CONCRETE MIX DESIGN1 IntroductionConcrete mix design is the process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the conc...

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1.0 CONCRETE MIX DESIGN

1

1.1 Introduction Concrete mix design is the process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. 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 but usually contain more initial strength than the more workable concrete. Higher workability concrete tend to be used more to fill in narrow gaps to ensure that the gap is completely filled up as higher workability concrete tends to ‘’flow’’ easily into the gaps. Concrete is mixture of aggregates, sand and gravel held together by a hardened paste of Portland cement and water. The compressive strength is one of the properties of concrete and it is used to determine the capacity of a structure to withstand loads. These are the main reason why the concrete mix design are designed carefully that is to ensure that the structure can withstand the amount of load and stress that it is supposed to. The compressive strength of concrete can be influenced by the water/cement ratio because of hydration process.

Figure 1: The proportion of the concrete mix design. In this experiment, the characteristic strength given is 25N/mm2. It is used to assume the concrete strength at day 28. The requirements that need to be taken into consideration for the concrete mix: 1. The ingredients and proportions must be correct in mixing. 2

2. Water-cement ratio for the mix must be correct. 3. Mix the concrete thoroughly. 4. Use proper finishing techniques.

1.2 Theory The overall procedures for concrete mix design can be divided into five (5) stages: Stage 1 – Determine the free water/cement ratio for the targeted mean strength. Stage 2 – Determine the free water content for the targeted workability. Stage 3 – Analyse the results in stage 1 and 2 and get the cement content. Stage 4 – Determine the total aggregate content. Stage 5 – Selection of the fine and coarse aggregates content.

1.3 Objectives 1. To ensure the most optimum proportions of the constituent materials to achieve desired minimum strength, workability and durability in the given environment. 2. To produce concrete with the most economical manner.

1.4 Procedure 1. At 28 days, the characteristic strength of the concrete,

f ck resulted as 25 N /mm2

. 2. The standard deviation,

s is determined from Figure 1 based on the characteristic

strength and the value is 8 N /mm2 .

3

Figure 2: The relationship between the standard deviation, s and characteristic strength.

3. The margin is calculated by giving the values, which is risk factor,

k

and standard

deviation, s :

Margin=Risk factor , k × standard deviation , s

4. The target mean strength,

f t has been calculated through the formula:

Target mean strength , f t =Characteristic strength , f ck +margin

5. From this design, the cement strength class is 42.5 which the type of cement used is Ordinary Portland (OPC), coarse-crushed aggregate and fine-uncrushed aggregate will be used for concrete mix design. The information is used to obtain the

compressive strength / age from the Table 1 and the value is 49

N mm2 . days

4

Cement

Type

Strength

coarse

class

aggregate

of Compressive strength (N/mm2) Age (days) 3

7

28

91

Uncrushed

22

30

42

49

Crushed

27

36

49

56

42.5

52.5

Uncrushed

29

37

48

Crushed 34 43 55 1N/mm2 = 1MN/m2 = 1MPa (see footnote on earlier page). Table 1: The Approximate compressive strengths

(

54

61

N ) of concrete mm2

mix made with a free-water/cement ratio of 0.5

6. With the compressive strength/ age given, the Free-water/ cement ratio can be

determined from the Figure 2. The 49

N mm2 days

is marked as a point at the y-axis

which is corresponding to strength at water-cement 0.5. A curve is drawn parallel to the nearest curve through the point and the target mean strength,

ft

is marked at

the y-axis. The value of free-water/cement ratio is determined at x-axis corresponding to the target mean strength,

ft

via the curve.

5

49

38.12

0.58

Figure 3: The relationship between the compressive strength and free-water/cement ratio.

6

7. The value of slump is obtained with the range 30mm to 60mm and the maximum aggregate size is used as 20mm. 8. The free-water content is obtained by referring the value of slump and the maximum aggregate size from Table 2. Slump (mm)

0-10

10-30

30-60

60-180

Vebe time (s) Maximum Type of

>2

6-12

3-6

0-3

size

Aggre gate

aggregate (mm) 10

Uncrushed

150

180

205

225

20

Crushed Uncrushed

180 135

205 160

230 180

250 195

40

Crushed Uncrushed

170 115

190 140

210 160

225 175

Crushed 155 175 190 205 Table 2: The Approximate free-water contents (kg/ m 3 ¿ require to give various level of workability. 9. The cement content is determined by using the formula:

Cement content=

free−water content free −water ratio cement

10. The relative density of aggregate (SSD) is assumed as 2.6 kg /m2 . 11. The value of concrete density has been identified from Figure 3 once the value of Free-water content is obtained.

7

Figure 4: The estimated wet density of fully compacted concrete. 12. Total aggregate content is calculated by the following equation and the unit is 2

kg /m

.

Totalaggregate content=concrete density−free water content −cement content

13. The grading of fine aggregate is determined by the percentage of aggregate passing 600 μmm , which is 40%. 14. The proportion of the fine aggregate is obtained by referring the maximum aggregate size, value of slump, and free-water/cement ratio from the Figure 4.

8

37 %

Figure 5: The recommended proportions of fine aggregate according to percentage passing 600μm sieves.

9

15. The value of fine aggregate content is calculated as the following: Fineaggregate content=Total aggregate content × grading of fine aggregate 16. The value of coarse aggregate content is obtained from the equation: Coarse aggregate content=total aggregate content −fine aggregate content

17. 5 concrete cubes are used during this experiment and the size of concrete cubes is taken as 150mm x 150mm x 150mm. The total quantities of concrete needed in this experiment are calculated as the following and the unit is

m

3

.

long x width x height ） x 5 Total quantities of concrete needed=¿ 18. The total amount of ingredients needed for concrete mix design is recorded in the form below.

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1.5 Results and Calculation Concrete mix design form Job title………………………………………….. Stage

1

Item

Reference or calculation

1.1 Characteristic strength

Specified

1.2 Standard Deviation 1.3 Margin

Fig 1 C 1 or Specified

25 N/mm2 at 28 days Proportion defective 5 % 8 N/mm2 or no data N/mm2 (k = 1.64) 1.64 x 8 = 13.12 N/mm2

1.4 Target mean strength 1.5 Cement type

Specified

25 + 42.5/52.5

Table 1 Fig 2 Specified

Crushed / uncrushed Crushed / Uncrushed 0.58 0 use the lower value

1:6 Aggregate type: coarse Aggregate type: sand 1.7 Free-water/cement ratio 1.8 Maximum freeWater / cement ratio 2

3

value

13.12

2.1 Slump or Vebe time 2.2 Maximum aggregate size 2.3 Free-water content

Specified Specified Table 2

Slump 30-60

3.1 Cement content 3.2 Maximum cement Content 3.3 Minimum cement Content

C3

210

= 38.12 N/mm2

mm or Vebe time 20

s mm

210 kg/m3 ÷

0.58

Specified

kg/m3

Specified

kg/m3 Use if ≤ 3.2 Use if > 3.1

=

kg/m3

362.07

Kg/m3

3.4 Modified free-water/cement ratio 4

5

4.1 Relative density of Aggregate (SSD) 4.2 Concrete density 4.3 Total aggregate content 5.1 Grading of sand 5.2 Proportion of sand 5.3 Sand content 5.4 Coarse aggregate content

Quantities Per m3 (to nearest 5kg) Per trial mix of 0.016 m3

Cement (kg) 362.07 5.79

2.6 Fig 3

2400 kg/m3 2400 – 210 − 362.07 = 1827.93 kg/m3

Percentage passing 600µm sieve Fig 4 1827.93 × 0.40 C5 1827.93 − 731.172 Water (kg or L) 210 3.36

known/assumed

Sand (kg) 731.172 11.70

40 % 40 % = 731.172 kg/m3 = 1096.76 kg/m3 Coarse aggregate (kg) 10mm 20mm 40mm 1096.76 17.55

Item in italics are optional limiting values that may be specified OPC = ordinary Portland cement; SRPC = sulphate-resisting Portland cement; RHPC = rapid-hardening Portland cement SSD = based on a saturated surface-dry basis.

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1.6 Discussion Concrete mix design is economically proportioning to the concrete ingredients for better strength and durability based on construction site. This is because proportional amount of concrete contribute toward the strength and workability of the concrete mix. The concrete must be mixed equally and evenly at a constant speed. This is to avoid the segregation from occurring and to maintain its workability and strength. When finding the free-water/cement ratio, we must choose to use the standard value of 0.50 for water/cement ratio, if the reading of water cement ratio that we find in Figure 2 is bigger than standard value. This is because low water/cement ratio will lead to high strength and high durability of concrete. All the apparatus must be wash quickly after using for the concrete mix to avoid concrete stick to the apparatus and spoiling the apparatus. The water must be poured slowly and gradually into the mix without any loss. This is for the hydration process to be complete. While the nominal concrete mix may have higher amount of cement, when it is designed mix, the cement requirement may be low for the same grade of concrete for a given site. This might because due to weather effect that lowering the workability of the concrete, making it to harden faster and the setting time start earlier than expected.

1.7 Conclusion The strength and workability of the concrete is dependent on the amount of water content and cement paste. The proportion that we use will produce 3 cube of sides 150mm and will be tested with slump test, compacting factor test, concrete density test and compression strength test.

1.8 Reference

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1. Slidesharenet. (2017). Slidesharenet. Retrieved 11 July, 2017, from

https://www.slideshare.net/gauravhtandon1/concrete-mix-design-46415349 In-text citation: (Slidesharenet, 2017)

2.0 CONCRETE SLUMP TEST

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2.1 Introduction Slump test is a method used to determine the consistency or workability of a fresh concrete. The consistency indicates how much water has been used in the mix. Consistency is very closely related to workability that describes the state of fresh concrete. It indicates the degree of wetness and refers to the ease with which the concrete flows. Concrete slump test is popular due to the simplicity of apparatus used and simple procedure. The test is very simple. That is why it often allows a wide variability in the manner that the test is performed and able to be conducted on site. The application of slump test is used to ensure the uniformity for different batches of similar concrete under field conditions and to ascertain the effects of plasticizers on their introduction. This test is very useful on site as a check on the day-to-day or hour-to-hour variation in the materials being fed into the mixer. An increase in slump might indicate that the moisture content of aggregate has unexpectedly increases. Other cause would be a change in the grading of the aggregate, such as deficiency of sand. Too high or too low a slump gives immediate warning and enables the mixer operator to remedy the situation.

2.2 Theory The slumped concrete sample may be in various shapes. According to the profile of slumped concrete, the slump can be termed as collapse slump, shear slump, and true slump.

Degree

of Slump

Workability

(mm)

Very low

0 -25

Low

25 – 50

Description Very dry mixes, used in road making. Road vibrated by power operated machines. Low workability mixes, used for foundations with 14

light reinforcement. Roads vibrated by hand operated machines. Medium workability mixes, manually compacted flat Medium

50 -100

slabs using crushed aggregates. Normal reinforced concrete manually compacted and heavily reinforced sections with vibrations. High workability concrete,

High

100 – 175

for

sections

with

congested reinforcement. Not normally suitable for vibration.

Figure 1: Types of slump: collapse slump, shear slump and true slump. Factors that will affect the workability of concrete: 1. Water cement ratio 2. Material properties 3. Size, texture, grading, cleanliness of aggregate 4. Aggregate cement ratio 5. Use of admixtures 6. Temperature of concrete 7. Condition of equipment

2.3 Objective To determine the workability slump for a given volume of wet concrete sample so that its properties to be placed, compacted and finished is suitable for the specific purpose using the Slump Test Method.

2.4 Apparatus and materials 1. Coarse aggregate 2. Fine aggregate

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3. Ordinary Portland cement (OPC) 4. Water 5. Shovel 6. Trowel 7. Wheelbarrow 8. Standard slump cone (300mm) 9. Tamping rod with 16mm diameter and 600mm long 10. Large plate 11. Metric ruler

2.5 Procedure 1. Based on the concrete mix design, the weight Ordinary Portland Cement (OPC), coarse aggregate, fine aggregate and water are measured through electronic balance. 2. These ingredients are mixed together on the flat plate after the weight is measured. A small hole is dig at the centre of the mix ingredients by using shovel. 3. The water is poured slowly into the hole and mixed together with the ingredients to perform chemical reaction between water and cement. 4. The slump test is conducted. The slump cone is placed in a flat condition and held down by foot rests. 5. 1/3 of concrete sample is filled into slump cone and 25 times of compaction is compacted by using tamping rod. 6. Step 5 is repeated for another two layers. From the top of the cone, the surplus concrete is strip off by trowel. The cone is removed vertically upward from the wet concrete carefully. 7. The side of the slumped concrete is placed with the tamping rod and metric rule is used to measure the slump from the top surface of concrete to the tamping rod. 8. The slump test result is recorded into the forms provided.

2.6 Result The recorded slump height is 48mm, which is within the range of 30mm – 60mm. Therefore, the slump is a true slump.

2.7 Discussion

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The Slump Test Method for the workability of wet concrete sample must be within 45 minutes after the mixing. The slump test was conducted 5 minutes after the concrete has been mixed. The slump value we got from the experiment is 48mm which is a true slump. The slump value given is 30-60mm. Therefore, the concrete has achieved the slump value specified in the concrete mix design. The workability of the concrete is low. It is because the coarse aggregates are too dry under the hot sun. Thus, aggregates absorb the water content when concrete mix is conducted. This can be avoided by immersing the aggregate into the water tank for a few minutes before using it. So that the temperature of aggregates will be decreased and it will not absorb too much water during the mixing period. Throughout the experiment, all materials which produce the concrete should be kept indoor to prevent changes in quality and quantity which will affect the concrete mix.

2.8 Precaution Precautions that need to take into consideration: 1. The slump cone has to place on the flat surface and held down it by foot rests to prevent the leaking of concrete when compact it inside the slump cone. 2. Make sure that the tamping rod is equally freely fall and when compacting the fresh concrete. 3. When the compacting task finished, the slump cone should be removed immediately by raising it slowly and carefully in vertical direction.

2.9 Conclusion The higher the water content of concrete, the higher the workability of concrete. However, the higher the water content of concrete, the lower the strength of the concrete. Throughout the experiment, we noticed that the materials that produce concrete are important and should be kept indoor to prevent modification on the concrete mix.

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The type of concrete produced in our experiment can be used for sections with congested reinforcement. Not suitable for vibration. Therefore, the conclusion for the concrete in our slump test is high workability and low strength.

2.10 Reference 1. Civil Engineers PK. (2017). Retrieved from http://civilengineerspk.com/plainreinforced-concrete-experiments/exp-7-slump-test/ 2. Mishra, G. (2017). CONCRETE SLUMP TEST – PROCEDURE AND RESULTS.

Retrieved

from

The

Constructor

-

Civil

Engineering

Home:

https://theconstructor.org/concrete/concrete-slump-test/1558/

2.11 Appendix

Figure 2: Weight the materials are measured.

Figure 3: Cement and aggregates are mixed.

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Figure 4: Water is added and mixed

Figure 5: 25 times of compaction is done by using tamping rod.

Figure 6: Slump value is measured.

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3.0 COMPACTING FACTOR TEST

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3.1 Introduction These tests were developed in the UK in the year 1947 and it is measure the degree of compaction. It has been used continuously after that as the test is highly reliable and does not require a lot of work and money. The test require measurement of the weight of the partially and fully compacted concrete and the ratio the partially compacted weight to the fully compacted weight, which is always less than one, is known as compacted factor . For the normal range of concrete the compacting factor lies between 0.8 - 0.92. When the compacting ...