Aggregates perform different purposes

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Lab 4

Introduction

Aggregates perform different purposes in Civil Engineering, including functioning as component materials for Portland cement concrete, hot mix asphalt, and bounding foundation layers beneath buildings and pavements. Natural aggregates consist of individual granular materials obtained from natural deposits, such as river run deposits, quarrels, or gravel pits. Particle shape and texture; absorption; bulk gravity; soundness and durability; bulk unit weight’ and gradation and maximum size form the most essential properties of aggregate investigated in the experiment. Concrete is a mixture of cement, water, and aggregate. 60-70 percent of the volume of concrete and approximately 80 percent of the weight of concrete are made up of aggregate (Prowell, Jingna, and Brown 12).

Engineers have come up with different equations used to calculate the properties of aggregate. Laboratory tests have been used to prove these equations, and the following report conducts several tests to investigate different properties of aggregate as mentioned above.

Sieve analysis of fine and coarse aggregate

            Sieve analysis forms the basic essential test for determining the gradation of aggregates. The main properties investigated in this test are the particle size. The particle size of the aggregate is given as a percentage of weight retained between consecutive sieves used in the test as shown in equation 1.

Percentage particle size =   ……………………………… 1

Dry-rodded unit weight of aggregate

Dry rodded unit weight of aggregate is determined by compacting dry aggregate into a test container of a known volume as per ASTM C 33 gradation number. The dry rodded weight is obtained by dividing the weight of the aggregate by the volume of the container. Equation 2 shows how dry rodded weight is calculated theoretically.

Dry Rodded unit weight =   ………………………………….2

Where; W2 – Total weight of the proctor mole and the base plate

W1 – sum of weights of the mold, the base plate and dry rodded coarse aggregate.

Moisture content of stored aggregate

Moisture content is the percentage of water found in a given volume of aggregate. The moisture content of an aggregate helps in developing the perfect water/cement ratio to use when making concrete. Each aggregate contains a specific percentage of moisture that depends on the porosity of particles making up the aggregate and the moisture condition of the storage area. Moisture content of aggregate is given by equation 3 below:

Moisture content of Aggregate (%) =  …………………….………..3

Where: W1– Weight of dry pan

W2 – Weight of pan and moist aggregate

W3 – Dry weight of aggregate sample and pan

Bulk Specific Gravity and Water Absorption of Fine Aggregate

            Bulk unit weight represents the dry weight of compacted aggregate occupying a specific bulk volume. Water absorption of the aggregate is the rate at which an aggregate draws water from a container of a specific volume. Water absorption is important in the construction because it provides an Engineer with the idea of the specific dry-time of concrete. It is calculated using the formula shown in equation 4 below:

Water absorption =   ………….4

The bulk specific gravity is calculated as:

Bulk Specific gravity =  ………5

Objectives

The experiment was conducted in two parts. Part I involved sieve analysis and bulk rodded unit weight determination while part II investigated the moisture content, bulk specific gravity and absorption capacity of the aggregate. The objectives of part I was to determine the gradation and dry rodded unit weight of coarse aggregate of coarse and fine aggregate to be used in making concrete mix using. The objective of part II was to determine the bulk specific gravity, moisture content of stored material, and absorption of coarse and fine aggregate to be used in making a concrete mix.

PART I: Sieve analysis and bulk rodded unit weight test

Equipment and materials

  • Dry, coarse and fine aggregate
  • Weigh balance
  • Proctor mold with base plate and extension shovel
  • (5/8) Inch) Tamping Rod with hemispherical tip
  • Sieves and an electric sieve shaker

Procedure

Test 1: Sieve analysis of coarse aggregate

  1. Approximately 2.5 Kg of air dry aggregate was weighed (test sample).
  2. The appropriate sieve sizes that represented all particle sizes were selected. The selected sieves were 1”, ¾”, 3/8”, ¼”, and #4 sizes and a pan. All the sieves were pre-weighed and the results recorded in table 1. The sieves were sorted and arranged in a descending order with the pan at the bottom to hold the last particles.
  3. The pre-weighed sample of the aggregate was placed in the upper sieve and sieve-shaker operated for ten minutes.
  4. The weights of retained aggregate in each sieve and in the pan were determined and recorded in table 1.
  5. The total sum of the retained weights was checked to correspond to the original sample weight. Difference between the weights showed that a correction factor would be applied
  6. The percentage of aggregate retained in each sieve was calculated using the equation 1 above
  7. The cumulative percentage aggregate retained and aggregate passing for each sieve was also calculated
  8. A graph of finer versus grain size was plotted, and ASTM C 33 scale used to identify the size number of the course aggregate used in the sieve analysis test.

Test 2: Sieve analysis of the fine aggregate

  1. A test sample of the fine aggregate was weighed as shown in step 1 in test 1
  2. The sieve sizes used for this test were #4, #8, #16, #30, #50, and #100. The sieves were arranged in an ascending order so as to compute the finest modulus.
  3. Steps 3 to 8 in test I were followed and the determined values recorded in table 2.

Test 3: Dry Rodded Unit Weight of Coarse Aggregate

  1. Ten ponds of air-dry coarse aggregate were obtained from the sample
  2. The proctor mold of 3.07 liters with a base plate and an extension was obtained
  3. The weights of the proctor mode and the base was measured and recorded in table 3. (W1)
  4. An extension was attached to the top of the proctor mold that seated in the base plate as shown in figure 1

Figure 1: A mould with base plate and extensions used for test 3

  1. The coarse aggregate was placed in three equal layers. Each layer was rodded with a 0.625 inches (0.016m) diameter rod, with an hemispherical tip, 25 times
  2. The extension was removed from the mode and a straight edge used to trim the excess aggregate above the mold
  3. The sum weights of the mold, the base plate and the dry rodded coarse aggregate were measured (W2). The figures obtained were recorded in table 3.
  4. The dry rodded weight was calculated using equation 2

PART II: Moisture content, bulk specific gravity & absorption capacity

Apparatus

  • Absorption cone (mold) and corresponding tamping rod
  • Absorbent Towels/blankets
  • Aspirator and Fine aggregate
  • Coarse and Fine Aggregate
  • Weighing balance
  • Distilled water
  • 500 ml flask
  • Metal frame and basket for submerging aggregate samples
  • Shovel
  • Water tank (5 gallon basket)
  • Oven

Procedure

Test 4: Moisture Content of stored aggregate

  1. The pan was weighed and its weight recorded (W1)
  2. A sample of aggregate was places in the pan, and the weight of the pan together with a sample taken (W2)
  3. The pan and the moist aggregate were placed in an oven for 24 hours. After the end of 24 hours, the dry weight of the aggregate sample plus the pan was taken (W3)
  4. The moisture content of the aggregate sample was calculated using equation 3.
  5. The procedure was repeated using coarse aggregate sample and its moisture content calculated

Test 5: Bulk Specific Gravity and water absorption of coarse aggregate

  1. A coarse aggregate was obtained and soaked in water for 24 hours to ensure it was fully saturated. The excess water was carefully decanted in order not to lose any aggregate. The sample was placed on a flat surface exposed to a gently moving current of warm air and stirred frequently
  2. The sample was dried to the saturated surface-dry condition (SSD) using a towel. Care was taken not to over-dry the sample
  3. 2 kg of SSD aggregate was obtained from the dried sample (A)
  4. The weight of metal frame with the basket submerged in water without aggregate was obtained and recorded (D)
  5. The weight of SSD aggregate and metal frame submerged in water was obtained and recorded (E)
  6. The entire sample of coarse aggregate was removed from the metal frame and emptied into a pan. The sample was then placed in the oven for 24 hours. The weight of the pan was recorded. In addition, the weight of the oven dry aggregate was taken and recorded (B)
  7. The weight of SSD aggregate submerged in water (C) was taken as follows: C = (E – D)
  8. The bulk specific gravity was calculated using equation 5
  9. The absorption at SSD condition was calculated using equation 4.

Test 6: Bulk specific gravity and water absorption of fine aggregate

  1. A sample of fine aggregate was obtained and soaked in water for 24 hours to ensure saturation. The excess water was carefully decanted in order not to lose any aggregate. The sample was placed on a flat surface exposed to a gently moving current of warm air and stirred frequently
  2. The sample was dried to the saturated surface-dry condition (SSD) taking care not to over-dry. The SSD condition was determined by a cone test as follows:
    1. The cone was filled with fine aggregate
    2. A tamping rod was used to lightly rod the fine aggregate 25 times with a drop height of 2 inches
    3. The cone was then gently lifted from the sample
    4. SSD condition was the point at which enough moisture had evaporated from the drying process that allowed the fine aggregate sample to fail when the cone was removed
  3. After identifying the SSD condition of fine aggregate, two samples were obtained. The first sample weighing 250 grams was used to determine the absorption capacity of fine SSD aggregate while the second sample weighing 100 g was used to determine bulk specific gravity.

Absorption:

  1. The 250 grams sample of SSD fine aggregate (W4) was taken and the sample emptied into a pan and placed in the oven for 24 hours. The weight of dry oven was pre-determined (W5)
  2. The absorption at SSD condition was calculated as Absorption (SSD) = (W4 –W5)/ W5

Test 8: Bulk Specific Gravity

  1. A 500 ml flask was filled with distilled water and its weight recorded as (W6)
  2. The water was emptied into another container
  3. The 100 grams sample of SSD fine aggregate was placed into the flask and the bulb of the flask filled to two thirds full with distilled water
  4. An aspirator was used to remove all air from the sample for 15 minutes until all air bubbles disappeared. The process termed as de-aeration
  5. The flask was then filled with distilled water to the 500 ml mark and its weight recorded (W7) W7 = (weight of de-aired material + weight of distilled water to 500 ml mark + weight of the flask)
  6. The entire content was emptied into the pan. A squeeze bottle was used to wash out all remaining particles adhered to the flask into the pan
  7. The pan and its components were placed in the oven for 24 hours. After the drying period the weight of the dry aggregate together with the pan was taken (W8)
  8. The bulk specific gravity of fine aggregate was calculated as

Bulk specific gravity =   …………………………..6

Results and calculations

Table 1: Course aggregate sieve analysis

Sieve No.

Size (mm)

Sieve Weight (lb)

Sieve weight

(W/Agg)

Aggregate

Weight (lb)

Corr. factor

Corr. weight

% Retained

Cum. % Retained

% Cum. Finer

l”

25

1.258

1.522

0.264

0.264

4.80

4.8

95.2

3/4″

19

1.287

5.578

4.291

4.291

77.99

82.79

17.211

3/8″

9.5

1.176

2.072

0.896

0.896

16.28

99.13

0.873

1/4″

6.3

1.166

1.204

0.038

0.038

0.69

99.82

0.182

#4

4.75

1.132

1.736

0.604

0.604

10.98

99.89

0.109

Pan

0.812

0.818

0.006

0.006

0.11

100.00

0

Sample initial weight

Sample final weight

Sample corrected weight

W0 = 5.502

Wf = 5.502

Wc = 5.502

 

Figure 1: A graph of percentage finer versus grain size of the coarse aggregate sample


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