A brief story of Concrete

Presented by: Dr. H.M.A.Mahzuz

Assistant Professor,

Department of Civil and Environmental Engineering

Shahjalal University of Science and Technology, Sylhet

Water+ Cement+ Fine aggregate+ Coarse aggregate (+ Admixtures)

Concrete=

Mix ratio (by volume or by weight)

Cement: Fine aggregate: Coarse aggregate

1 : 2 : 4

Weight of 1 bag cement=50 kgVolume of 1 bag cement= 1.25 cftUnit weight of cement= 90 lb/cftSand density= 95 lbs/ft3Gravel density= 105 lbs/ft3

Some useful information:

Types of concrete based on weight:

• Light weight concrete has an density (unit weight) on the order of 90 to 115 lb/ft3

• Normal weight concrete with a density in the range of 140 to 150 lb/ft3

Some special term related to concrete

• Segregation on concrete: Separation of aggregate caused by concrete placing from a height more than 5’, excess water usage, excess vibration.

• Bleeding in concrete: It is for excess water in concrete. Here, water comes out to the surface of the concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete.

• Laitance in concrete: After hardening, formation of crust of weak mortar or cement paste on the surface of concrete. Excess water is the main cause of this.

Properties of concrete

1) Strength:

• Concrete should be strong in compression. For structural concrete the minimum compressive strength should be more than 2500 psi (17MPa (N/mm2), 1 MPa= 145 psi).

• Concrete is week in tension. Usually its value is 1/10th of compressive strength.

Effect of Age on concrete strength

2) Elastic:

• The modulus of elasticity of concrete is an important property because of its bearing of high compression load in a very low deformation.

• The American Concrete Institute (ACI) allows the modulus of elasticity to be calculated using the following equation:

For example, for a concrete of unit weight, w =150 lb/cft and compressive stress, f=3000 psi,

Fig 1: Concrete stress-strain curve

Fig 2: Concrete and Steel stress-strain curves

3) Fatigue of concrete:

In flyovers, beams, slabs where load gets repeated, for a very large number of cycles, concrete undergoes stress concentration, exhibits excessive cracking and may eventually lead to failure after a sufficient number of load repetitions, even if the maximum stress is less than the static strength of a similar specimen.

Such members under– go a process of progressive, permanent internal structural (micro-cracking) change. A material subjected to such fluctuating stresses/ strains, conventionally termed as "fatigue."

Source: http://www.nbmcw.com/articles/concrete/18265-fatigue-in-concrete-concerns-security-and-stability-of-flyovers.html

Figure : Load gets repeated

Figure : Stress and strain in repeated loading

Zone A: stable micro-cracking; stress 0+0.55fcu. The approximate proportion 0.55 is not sensitive to the value of fcu. Strain is partly elastic and partly caused by non-reversible rupture of crystalline bonds, known as micro-cracking, hence the bend of the curve.

Zone B: unstable micro cracking; stress 0.55 fcu – 0.80 fcu. Micro-cracking is stable under a single application of load but increases with repeated applications until the micro-cracks link together to form macro-cracks which in turn progress in extent until rupture ensues.

Zone C: unstable Under macro-cracking : stress 0.8 fcu – 1.0 fcu–Under a single application of load in this zone, micro-cracks get formed as the load increases, which then link together to form macro-cracks. These macro-cracks extend and lead to rupture with either the repetition or the maintenance of the load.

Zone D: post-rupture stress - with a load possessing sufficient energy-potential (piled-up weights), strain (an extensible test rig) or dynamic (an impact)-as soon as the concrete reaches the maximum stress which it can support, it collapses.

Figure 1: Typical stress-strain curve for concrete in compression

4) Durability of concrete

The durability is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration.

Durable concrete will retain its original form, quality, and serviceability when exposed to environment.

5) Impermeability of concrete

Impermeability of concrete is an ability of concrete to resist pressurized water penetration. Penetration of liquids into the concrete has considerable effect on its durability.

In concrete, penetration of moisture and air causes corrosion of steel reinforcement, resulting in volume gain, cracking and peeling of the protective concrete layer.

6) Workability of concrete:

A concrete is said to be workable if it is easily transported, placed, compacted and finished without any segregation. Workability is a property of freshly mixed concrete, and a concrete is a mixture of cement, aggregate, water & admixture.

Factors affecting workability of concrete

• Water Content: Workability of concrete increases with increase in water content.

• Aggregate/Cement Ratio: The higher the aggregate/cement ratio, the leaner is the concrete (that is slope down or settle down)

• Size of Aggregate: For a given quantity of water and paste, bigger size of aggregates will give higher workability.

• Shape of Aggregate: Better workability is ensured to rounded aggregate than Angular, elongated or flaky aggregate

Factors affecting workability of concrete (continues)

• Grading of Aggregate: This is one of the factors which will have maximum influence on workability. A well graded aggregate can lead to good workability.

• Surface Texture of Aggregate: rough textured aggregate will show poor workability and smooth or glassy textured aggregate will give better workability.

• Use of Admixture: The right way of improving workability is to use chemical admixtures such as plasticizers, super plasticizers, air entraining agents etc

Measurement of workability of concrete

Concrete slump test:

• The concrete slump test is an empirical test that measures workability of fresh concrete.

Collapse Shear True

In a collapse slump the concrete collapses completely.

In a shear slump the top portion of the concrete shears off and slips sideways.

In a true slump the concrete simply subsides, keeping more or less to shape.

Factors affecting properties of concrete

1. Quality of Raw Materials:

• Cement: Provided the cement conforms with the appropriate standard and it has been stored correctly (i.e. in dry conditions), it should be suitable for use in concrete.

• Aggregates: Quality of aggregates, its size, shape, texture, strength etc determines the strength of concrete. The presence of salts (chlorides and sulphates), silt and clay also reduces the strength of concrete.

• Water: frequently the quality of the water is covered by a clause stating “..the water should be fit for drinking..”. This criterion though is not absolute and reference should be made to respective codes for testing of water construction purpose.

2. Water / Cement Ratio:

• Up to a water/cement ratio concrete strength is increased. But after that it falls.

• W/c ratio increases concrete workability, compaction.

3. Age of concrete:

4. Compaction of concrete:

• Any entrapped air resulting from inadequate compaction of the plastic concrete will lead to a reduction in strength. If there was 10% trapped air in the concrete, the strength will fall down in the range of 30 to 40%.

5. Size of Coarse aggregate

6. Mixing time

7. Curing time

7. Temperature:

Creep of concrete

Age (day) Minimum compressive strength, (Mpa)

3 12.4

7 19.3

28 27.6

Table: Compressive strength of mortar as per ASTM standard

ACI Mix Design Example

• Given information-

Estimation of mixing water and air content

Given information-Water/cement ratio for different compressive strength

Estimation of coarse aggregate content

Question

Answer

Estimation of fine aggregate content by the absolute volume method.

Adjust the amount of water based on moisture content