Permissible Stresses In Concrete

Permissible Stresses In Concrete (IS 456 – 2000) (ALL Values IN N/mm2)

Grade of Concrete	Permissible Stress in Shear		Permissible Stress in Compression		Permissible Stress in Bond (Avarage) for deformend bars in Tension
	For 100 AS bd =0.25	For 100 AS bd =0.50	Bending	Direct
M 10	----	-----	3.0	2.5	----
M 15	0.22	0.29	5.0	4.0	0.96
M 20	0.22	0.30	7.0	5.0	1.28
M 25	0.23	0.31	8.5	6.0	1.44
M 30	0.23	0.31	10.0	8.0	1.60
M 35	0.23	0.31	11.5	9.0	1.76
M 40	0.23	0.32	13.0	10.0	1.92
M 45	0.23	0.32	14.5	11.0	2.08
M 50	0.23	0.32	16.0	12.0	2.24

 Note: - The bond value shall be increased by 25% for bars in compression

NOMINAL CONCRETE MIXES REQUIREMENT OF MATERIALS PER M3 OF CONCRETE

Proportion	Cement in bags of 50 Kg	Sand m3	Coarse aggregates  m3
1 : 1 : 2	11.25	0.40	0.79
1 : 1.5 : 3	8.10	0.43	0.86
1 : 2 : 4	6.35	0.45	0.90
1 : 2.5 : 5	5.20	0.46	0.92
1 : 3 : 6	4.50	0.48	0.95
1 : 4 : 8	3.45	0.49	0.98
1 : 5 : 10	2.80	0.49	0.98

Note: - 20mm MSA & 30 to 80mm Slump ± 0 to 30mm Slump

Assumed Specific Gravity	:  2.69 (Sand) & 2.80 (Coarse aggregate)
Assumed Loose Density kg/m3	:  1450 (Sand) & 1400 (Coarse aggregate)
Bulk age for wet sand and wastage for sand & coarse aggregate has not been considered

AREA PERIMETER AND WEGHT OF STEEL ROUNDS

B Bar Diameter (mm)	Area of Cross Section (Sq.cm)	Perimeter (cm)	Weight in kg per sq Meter
32	8.04	10.05	6.314
28	6.16	8.80	4.836
25	4.91	7.86	3.855
22	3.80	6.91	2.985
20	3.14	6.29	2.467
16	2.01	5.03	1.579
12	1.13	3.77	0.888
10	0.79	3.14	0.617
8	0.50	2.51	0.395
6	0.28	1.89	0.222

Note: - Density of steel is 7850 kg/m³

Concrete Work Ability Tests At Site & Recommended Values

What is workability of concrete? Which test is generally performed at site for its determination? And what are its recommended values for different purposes?

Workability of concrete describes the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity.

Workability is important because, if

• The concrete mixture is too wet, coarse aggregates settle at the bottom of concrete mass and as a result concrete becomes non-uniform composition,
• the concrete mixture is too dry, it will be difficult to handle and place it in position

Workability of concrete mixture is measured by:

• Vee-bee consistometer test
• Compaction factor test
• Slump test

The first two tests are laboratory tests while the third test is the field test.

Slump Test

This test is carried out with a mould called slump cone whose top diameter is 10cm, bottom diameter is 20 cm and height is 30 cm. the test may be performed in the following steps:

1.  Place the slump mould on a smooth flat and non-absorbent surface.
2.  Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
3.  Place the mixed concrete in the mould to about one-fourth of its height.
4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.
5.  Place the concrete in the mould about half of its height and compact it again.
6. Place the concrete upto its three fourth height and then upto its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second subsequent layers, the tamping rod should penetrate into underlying layers.
7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top.
8.  Remove the mould immediately, ensuring its movement in vertical direction.
9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.

Figure: Measurement of slump

SL NO	TYPES OF CONCRETE	SLUMP
1	Concrete for road construction	20 to 40 mm
2	Concrete for tops of curbs, parapets, piers, slabs and wall	40 to 50 mm
3	Concrete for canal lining	70 to 80 mm
4	Normal RCC work	80 to 150 mm
5	Mass concrete	20 to 50 mm
6	Concrete to be vibrated	10 to 25 mm

Concrete Slump Test

Concrete slump test is to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during the progress of the work. Concrete slump test is carried out from batch to batch to check the uniform quality of concrete during construction.

The slump test is the most simple workability test for concrete, involves low cost and provides immediate results. Due to this fact, it has been widely used for workability tests since 1922. The slump is carried out as per procedures mentioned in ASTM C143 in the United States, IS: 1199 – 1959 in India.

Generally concrete slump value is used to find the workability, which indicates water-cement ratio, but there are various factors including properties of materials, mixing methods, dosage, admixtures etc. also affect the concrete slump value.

FACTORS WIHICH INFLUENCE THE CONCRETE SLUMP TEST:

1. Material properties like chemistry, fineness, particle size distribution, moisture content and temperature of cementitious materials. Size, texture, combined grading, cleanliness and moisture content of the aggregates,
2. Chemical admixtures dosage, type, combination, interaction, sequence of addition and its effectiveness,
3. Air content of concrete,
4. Concrete batching, mixing and transporting methods and equipment,
5. Temperature of the concrete,
6. Sampling of concrete, slump-testing technique and the condition of test equipment,
7.  The amount of free water in the concrete, and
8. Time since mixing of concrete at the time of testing.

EQUIPMENT REQUIRED FOR SLUMP TEST:

Mould for slump test, non porous base plate, measuring scale, temping rod. The mould for the test is in the form of the frustum of a cone having height 30 cm, bottom diameter 20 cm and top diameter 10 cm. The tamping rod is of steel 16 mm diameter and 60cm long and rounded at one end.

SAMPLING OF MATERIALS:

A concrete mix (M15 or other) by weight with suitable water/ cement ratio is prepaid in the laboratory similar to that explained in 5.9 and required for casting 6 cubes after conducting Slump test.

Figure-1: Measuring Slump of Concrete

PROCEDURE FOR CONCRETE SLUMP TEST:

• Clean the internal surface of the mould and apply oil.
•  Place the mould on a smooth horizontal non- porous base plate.
• Fill the mould with the prepared concrete mix in 4 approximately equal layers.
• Tamp each layer with 25 strokes of the rounded end of the tamping rod in a uniform manner over the cross section of the mould. For the subsequent layers, the tamping should penetrate into the underlying layer.
• Remove the excess concrete and level the surface with a trowel.
• Clean away the mortar or water leaked out between the mould and the base plate.
• Raise the mould from the concrete immediately and slowly in vertical direction.
•  Measure the slump as the difference between the height of the mould and that of height point of the specimen being tested.

Figure-2: Concrete Slump Test Procedure

Figure-2: Concrete Slump Test Procedure

NOTE:

The above operation should be carried out at a place free from Vibrations or shock and within a period of 2 minutes after sampling.

SLUMP VALUE:

The slump (Vertical settlement) measured shall be recorded in terms of millimeters of subsidence of the specimen during the test.

RESULT OF CONCRETE SLUMP TEST:

Slump for the given sample=

When the slump test is carried out, following are the shape of the concrete slump that can be observed:

Figure-3: Types of Concrete Slump Test Results

True Slump – True slump is the only slump that can be measured in the test. The measurement is taken between the top of the cone and the top of the concrete after the cone has been removed as shown in figure-1.

Zero Slump – Zero slump is the indication of very low water-cement ratio, which results in dry mixes. These type of concrete is generally used for road construction.

Collapsed Slump – This is an indication that the water-cement ratio is too high, i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.

Shear Slump – The shear slump indicates that the result is incomplete, and concrete to be retested.