good ppt

Date: March 30, 2011

Submitted toProfessor: Dr. Mohamed Ismail

Department of Civil Engineering

Submitted by: Atique Ahmed ID # 989056528

INTERACTION BETWEEN INTERACTION BETWEEN ATMOSPHERE AND CONCRETEATMOSPHERE AND CONCRETE

Introduction of ConcreteProperties of ConcreteTypes of ConcreteAdmixtures of ConcreteHot Weather ConcreteCold Weather ConcretePrecaution Measures in Cold WeatherConclusionCase Studies

Outline

What is Concrete?

Concrete is a composite construction material, composed of cement (commonly Portland cement) and other cementitious materials.

When concrete structure provides required strength, serviceability, weathering resistance, expected service life and maintained engineering properties is call durable concrete.

However, the French Canal du Midi was built using concrete in 1670.

Portland cement was first used in concrete in the early 1840s.

Introduction

Properties of concrete

Following are considered important properties of fresh concrete

1. Setting of Concrete

2. Workability of Concrete

3. Avoid Bleeding and Segregation

4. Hydration Process

5. Air Entrainment

Properties of concrete…..

Setting of concrete

The hardening of concrete before its hydration is known as setting of concrete.

Following factors affect the setting of concrete:1. Water Cement ratio 2. Suitable Temperature 3. Cement content 4. Type of Cement 5. Fineness of Cement 6. Relative Humidity 7. Admixtures 8. Type and amount of

Aggregate


Workability of concrete

Workability is often referred to as the ease with which a concrete can be transported, placed, and consolidated without excessive bleeding or segregation. Factors affecting concrete workability:i. Water-Cement ratioii. Amount and type of Aggregate iii. Amount and type of Cement iv. Atmospheric conditions (Temperature and Wind)


Bleeding and Segregation….. Concrete Bleeding

Bleeding in concrete is sometimes referred as

water gain. Bleeding is predominantly observed

in a highly wet mix, badly proportioned, and

insufficiently mixed concrete. In thin members

like roof slab or road slabs and when concrete is

placed in sunny weather show excessive

bleeding.


Bleeding and Segregation………..

Segregation in ConcreteSegregation can be defined as the separation of the constituent materials of concrete.


Hydration in Concrete Concrete derives its strength by the

hydration of cement particles. The hydration of cement is not a momentary action but a process continuing for long time.

• The desirable conditions are a suitable temperature and ample moisture.

• Concrete, while hydrating, releases high heat of hydration. This heat is harmful from the point of view of volume stability. Heat of hydration of concrete may also shrinkage in concrete, thus producing cracks.


Air Entrainment Air entrainment is intentional creation of tiny air

bubbles in concrete. Air entrainment reduces the density of concrete. Air entrainment is used to produce a number of effects in both the plastic and the hardened concrete. These include:

1. Resistance to freeze–thaw action in hardened concrete.

2. Increased cohesion, reducing the tendency to bleed and segregation in the plastic concrete.

3. Compaction of low workability mixes including semi-dry concrete.

4. Stability of extruded concrete.5. Cohesion and handling properties in bedding mortars

Type of Concretes

Types of concrete Some common and main types of concrete are:

1. Normal Concrete 2. High Strength Concrete

3. High Performance Concrete 4. Air Entrained Concrete (F&T)

5. Light Weight Concrete (reduce DL)

6. Self-Compacting Concrete

7. Shotcrete Concrete 8. Pervious Concrete (Flatwork

9. Roller Compacted Concrete (Rd)

9. and many more

Admixture of Concrete

Admixtures

A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties is called Admixture.

Types of AdmixturesChemical admixtures - Accelerators, Retarders,

Water-reducing agents, Super plasticizers, Air entraining agents etc.

Mineral admixtures - Fly-ash Blast-furnace slag, Silica fume, and Rice husk Ash etc.

Hot Weather of Concrete

ACI - American Concrete Institutes rules out

ACI 305 “Hot Weather Concreting” defines hot weathers as any combination of the following conditions that tends to impair the quality of the freshly mixed or hardened concrete:

High ambient temperatureHigh concrete temperatureLow relative humidityWind speedSolar radiation

Hot Weather of Concrete….

Hot-weather slow the cement hydration reactions within the concrete and to minimize the rate of evaporation of moisture from the freshly mixed concrete.

High temperatures accelerate the hardening of concrete and more water is required to maintain workability. If the water-cement ratio is not maintained by adding additional cement, strength, durability will be reduced and other properties of concrete are adversely affected.

High water contents also mean greater drying

shrinkage. Furthermore, setting is accelerated which shortens the time within which the concrete can be handled and finished.


Potential concrete problems in hot weather are likely to include:

■ Increased water demand

■ Increased rate of slump loss

■ Increased rate of setting

■ Increased tendency for plastic-shrinkage cracking

■ Increased difficulty in controlling entrained air content

■ Decreased 28-day and later strengths

■ Increased tendency for differential thermal cracking


When concrete is to place under hot weather conditions, the following steps must be considered:

•Use cooled concrete, chilled mixing water, ice in the mixture, the use of liquid nitrogen to cool the mixing water or the concrete mixture, or the cooling of the coarse aggregate.•Use concrete consistency allowing rapid placement and effective consolidation at high temperatures•Use day and night placement schedule when the weather conditions are favorable•Minimize the time to transport, place, consolidate, and finish the concrete•Protect the concrete from moisture loss at all times during placement and during its curing period.

Cold Weather of Concrete

Cold Weather Concreting

The cold temperature in your refrigerator speeds up the transition of the liquid mixture into a semi-solid material. With concrete, cold temperature slows this transition, but if this is not the case in your concrete, then it is disastrous for the project. In my opinion, concrete is probably one of most fascinating building materials; however it appears to be a very basic material. High quality concrete doesn't happen by accident. It is a highly sophisticated chemical compound.

Cold Weather of Concrete…What Happens When Concrete Freezes? Pore water in concrete starts to freeze around -1°C (30°F)As some water freezes the ion concentration in the unfrozen water goes up, further depressing the freezing point.At around -3 to -4°C (25 to 27°F), enough of the pore water will freeze so that hydration will completely stop, and depending on the extent of hydration, and thus the strength of the concrete, the forces generated by the expansion of ice (ice occupies ~9% more volume than water) may be detrimental to the long term integrity of the concrete.

Cold Weather of Concrete…Handling Concrete in Freezing Obviously some chemical reactions require heat to complete, while other reactions create heat. The chemical reaction of concrete creates heat; called the heat of hydration. This heat can be trapped and used to fight off the threat of ice. To achieve that goal, cover the fresh concrete with insulated blankets.For every 10°C (18°F) reduction in concrete temperature, the times of setting of the concrete double, thus increasing the amount of time that the concrete is vulnerable to damage due to freezing.Concrete that is protected from freezing until it has attained a compressive strength of at least 3.45 Mpa (500 psi) will not be damaged by exposure to a single freezing cycle.

Cold Weather ConcretePrecautionary Measures for concrete in cold weather

1. Using hot water2. Providing enclosures i.e. covered area3. Using air entraining agents4. Scheduling concreting5. Using admixtures Accelerators6. Type III or high early strength cement7. Using extra proportional amount of cement for early

strength gain in heated mixed environment

Conclusion

Concrete is a strong material because of its chemistry. When water

mixes with the cement powder, it starts an irreversible chemical

reaction. Tiny crystals begin to grow. These crystals attach to one

another with the sand and the gravel in the mixture. When

everything goes right, basically created a compound hard as a rock.

Most concrete chemists and engineers agree that if the concrete

can attain a minimum strength of 500 pounds per square inch (PSI),

it can resist damages.

Case Study In final report includes Case Studies for the following:

Case Study – Bridge Deck

Title : Early age cracking and Corrosion in High Performance Concrete

Location: Vachon Bridge in Laval – Quebec

Built: During 2006

Questions?

Thanks for Your Attention

Documents

good ppt