SPECIFIC GRAVITY AND ABSORPTION OF COARSE AGGRAGATE

PROGRAM: HK01 CIVIL ENGINEERING

COURSE INSTRUCTOR:

LOW CHEE WAI

HK2006-3316

NG VUI ON

HK2006-3520

RAYMOND RAYMOND RAJ A/L SUKUMARAN SUKUMARAN HK2006-40 HK2006-4056 56 RONALD CHEE KO KOH CHIEN

HK2006-4725

TEOH HOW MENG

HK2006-2983

YAN CHIN FEI

HK2006-3263

EXPERIMENT AGG-1 : SPECIFIC GRAVITY AND ABSORPTION OF COARSE AGGRAGATE

Objective: •

To determine the specific gravity (bulk and apparent) of coarse aggregate.

•

To determine the absorption of coarse aggregate.

•

To understand the uses of balance and oven.

Introduction: There are many types of aggregate that have been studied. Coarse aggregate and fine aggregate are the most common in the uses of making concrete. From this experiment, we are determining the specific gravity and absorption of coarse aggregate. In theory, the size of the coarse aggregate is equal or more than 4.75mm. Coarse aggregates should be strong, clean and free from adherent coating and do not have harmful element. To ensure that this experiment can well be performed, some appropriate instruments are essential. For this experiment, equipments that have been explored are  balance, volumetric flask of 500cm³, oven and some laboratory instruments such as  beaker and mechanical tanks. There are some definitions of the terms that are related to this experiment:

•

Specific gravity is the ratio of weight in air of a unit volume of a material to the weight of an equal volume of water.

•

Bulk specific gravity is the ratio of the weight in air of a unit volume of  aggregate (including the permeable and impermeable voids in the particles,  but not including the voids between the particles) to the weight of an equal volume of water.

•

Apparent specific gravity is the ratio of the weight in air of a unit of volume of  the impermeable portion of aggregate to the weight of an equal volume of  water.

•

Absorption is the increase in weight of aggregate due to the water in pores, but not including water adhering to the outside surface of the particles, expressed as a percentage of the dry weight.

Equipments: The equipments need for this experiment: balance, wire basket (of 3.35 mm or fine wire mesh), absorbent cloth, water tank, aluminium tray and oven. The balance is used to determine the weight of the sample. The load of the balance can be manipulated. Oven is used to dry the test sample at a certain temperature. The temperature of the oven can be controlled by adjusting the temperature indicator. The function of the wire basket is to place the test sample into it. The wire basket consist a lot of tiny holes where the water can be flow in or out from the test sample. Absorbent cloth: The absorbent cloth is used to roll the test sample which consist water and to remove the visible films of water from it.

Sample: A minimum of 4000g test sample for aggregate of maximum nominal size 25mm. The sample should not have particles of size less than 4.75mm.

Procedure: 1. The test sample: A (g) is weighted. 2. The aggregate is immersed in the water at room temperature for a period of  24+4hr. 3. The sample is removed from the water after 24+4 hours. The sample is rolled in a large absorbent cloth until all visible films of water are removed. The sample now is in saturated surface dry (SSD) condition. 4. The sample is weighted and its saturated surface dry weight: B (g) is obtained. 5. The SSD sample is placed in wire basket and its weighted in water: C is determined and sample during weighting is tested. 6. The sample is removed from the wire basket.

7. The sample is dried to constant weight at a temperature of

10+5˚C

(approximately 24 hours) and weighted 8. The specific gravity and absorption value are calculated.

Result and Analysis: There are two kinds of specific gravity used with aggregate particles. Bulk specific gravity is based on oven dry weight and saturated, surface dry volume of the aggregate particles. Pores in particles are considered part of the volume. Apparent specific gravity is based on oven dry and solid volume of the particles. Either one of  these can be considered as a true specific gravity, or each has its own use.

 No 1. 2. 3. 4.

Sample weight (g) A B (SSD) C (sample + water) D (oven-dry sample)

Bulk specific gravity, dry = Bulk specific gravity, SSD = Apparent specific gravity = Absorption

Result 4000.00 g 3880.00 g 2397.00 g 3660.00 g

Result 4000.00

 A  B − C   B

3880.00 − 2397.00 3880.00

 D

3880.00 − 2397.00 3660.00

 D − C 

3660.00 − 2397.00

 B − C 

 B −  D  D

× 100%

3880.00

−

3660.00

3660.00

=

2.70

=

2.62

=

2.90

× 100% =

6.01%

Discussion: It is known that water absorption and shrinkage are related phenomena especially in the context materials’ research and construction works. It should be noted that the moisture absorption capacity of aggregates must be determined before mixing. This is due to the fact that the surface absorption property of aggregate effects on workability, water /cement ratio, strength and durability of concrete. It is true that the absorption of excessive water in the aggregate will produce shrinkage in the concrete, which in turn leads to cracking, and therefore unsoundness. Therefore, a clear correlation can be shown in between the drying shrinkage of  concrete and water absorption of aggregates that are being used. In this experiment, it is necessary for us to follow the procedure in the  prepared lab sheet and the reading of sample is obtained as stated in the procedure. Likewise, the aggregate sample must be immersed into the water at room temperature for at least 24 hours before it is tested. In fact, this is to ensure that the voids of  aggregate sample are filled with water. This means that in this condition, such aggregates are known as wet sample. It is true that the sufficient dry clothes must be used to wipe away the surface water of the sample in the process of drying the aggregate sample. At this time, this condition is known as saturated surface dry. It is important to take a good care during this process in order to avoid the aggregates from falling to the ground. It is consistent with the fact that ignoring it will affect the total weigh of saturated surface dry (SSD). On the other hand, it would be better if the aggregates are wiped individually instead of wiping together. This step is of great use in making sure that each aggregate is in saturated surface dry condition, without disturbing the water content in pores of  aggregate. If done otherwise, the result will be inaccurate due to some of the aggregate not in saturated surface dry condition. It is always better to immerse the wire basket to a sufficient depth that just only covering it during weighing SSD sample. This is to ensure that the weight of  SSD sample in water is as accurate as possible. In addition, it is recommended to wait until the reading of weight of the balance stop moving while taking the reading of  weight of aggregate with wire basket in water. The purpose is to reduce the parallax error as it may be occurred. Moreover, it should be made sure that there is no deviation error when using the digital balance and all the fans are switched off before

weighing the samples. Beside that, make sure the sample has dried at temperature of  110 ± 5 ºC (approximately 24 hours) before carrying out the experiment.

Conclusion: Bulk specific gravity (dry) of coarse aggregate is 2.70. Bulk specific gravity (saturated surface dry) of coarse aggregate is 2.62. Apparent specific gravity of coarse aggregate is 2.90. Water absorption of coarse aggregate is 6.01%.

The Coarse aggregate consists of rounded river gravel, crushed stone, or  manufactured aggregate. Water absorption or absorbed moisture (also called absorption capacity) is defined as the weight of water absorbed by dry aggregate  particles in reaching a moisture level or condition called saturated surface dry condition (SSD). The bulk specific gravity is computed as the ratio of the weight in air of unit volume of a permeable material. The bulk specific gravity (bulk SG) in SSD condition for the coarse aggregate is 2.62. Absorption in aggregates is the ability to retain water. This is directly dependant on porosity. The aggregate consist of pores, and due to this the pores will change the amount of moisture in the material. If the moisture content is more than the absorption capacity, the aggregate is termed – wet. Meaning that there is free moisture on the aggregate surface, and if it is less the aggregate is termed – dry. The absorption capacity for coarse aggregate in this experiment is 6.01%. However coarse aggregates have higher moisture contain due to its surface tension and particle size.