※Concrete☆World※

Introduction

        

          Concrete consists of a hard, chemically inert particulate substance, known as an aggregate, which is bonded together by cement and water. The word concrete comes from the Latin word “concretes “,which meaning of compact or condensed. Roman concrete was made from quicklime and an aggregate of pumice during the Roman Empire. Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs in arms of both structural complexity and dimension.

          Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand- layering together with the placement of aggregate, which in Roman practise, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.

          Its history can be traced to ancient Egypt and Rome, it is often falsely perceived as a simple material. Actually, the micro structure of concrete tends to be highly complex. Furthermore, the structure and the properties of this composite material can charge over time. Most modern concrete structures are reinforced with steel, since concrete itself displays relatively low strength when loaded in tension. While steel reinforced concrete is obviously a widely used, cost-effective construction material, degradation of such structures has become a major problem in many parts of the world and not gone unnoticed. The basic constituents of concrete are cement, water and aggregate.

Concrete Composition

         There are many types of concrete available in market. There created by varying the proportions of the main ingredients: water, cement, and aggregates. The ratio of the ingredients changes the properties of the final product, which allows the engineer to design concrete that meets their specific needs. Admixture are added to adjust the concrete mixture for specific performance criteria. Besides that, reinforcement are added to strengthen the concrete. Now lets us has a detail look on those composition.

• Water

        The water in the concrete mix should be clean and free of impurities. This is because impure water can cause problems when setting or in causing premature failure f the structure. The amount of water relative to the amount of cement changes how easily the concrete flows, but also affect the final strength of the concrete. More water make for easier flowing concrete, but also makes for lower strength (high slump) concrete upon curing.

• Cement

          Cement hardens when mixed with water, which binds all of the ungredients together. Portland cement is the most common cement used and is composed of alumina, silica, lime, iron, and gypsum. Small amounts of others ingredients are also include.

• Aggregates

          Sand, gravel, and crushed stone are used as the aggregates in concrete. Aggregates make up the majority of a concrete mixture.Besides that, recycled aggregates are increasingly used as partial replacements of natural aggregate. Those aggrates including air-cooled blast furnace slag and bottom ash are also permitted.

• Admixtures

          Admixtures accomplish a variety of goal. This can be as simple as adding a pigmen to colour the concrete. Other admixtures are used for faster curing times in cold weather, creating extremely high-strength concrete, of for increasing the flowable nature of concrete without compromising the strength. Unfortunately, admixtures can generate unwanted result such as poor adhesion of finish-flooring. For this reason, many structural engineers and architects are hesitant to use admixture.

• Reinforment 

          Since concrete is a material that performs well under compressive loads due to the aggregates efficently carries the compression load, but performs poorly under tensile loading, so steel is added to the concrete. The steel, which performs well under tensile loading, so it addresses this concern. Steel reinforcing is also used to tie different concrete elements together; for instance, to tie a concrete beam to concrete column.

 

Types of concrete

Some common and main types of concrete are:

1. Normal concrete
2. High Strength Concrete
3. High Performance Concrete
4. Light Weight Concrete

Lets us has a look on those concrete

1. Normal Concrete

• Composition : aggregate, water ,cement

• Density : 2240 – 2400 kg/m3 (140 – 150 Ib/ft3)

• Compressive strength : 20 – 40 MPa (3000 – 6000 psi)

• Flexural strength : 3 – 5 MPa (400 – 700 psi)

• Modulus of elasticity : 14000 – 41000 MPa (2 – 6 x 106 psi)

• Permeability : 1 x 10 -10 cm/sec

• Coefficient of thermal expansion : 10-5 oC-1 (5.5 x 10-6 oF-1)

• Drying shrinkage ; 4 – 8 x 10-4

• Drying shrinkage of reinforced concrete : 2 – 3 x !0-4

• Poisson’s ration : 0.20 – 0.21

• Shear stress : 6000 – 17000 MPa (1 – 3 x 106 psi)

• Specific heat capacity : 0.75 kJ/kg K (0.18 Btu/Ibm oF (kcal/kgoC)

• Yield strength is from 10 MPa (1450 psi) to 40 MPa (5800 psi)

• Its slump varies from 1 – 4 inches.

• Air content 1 – 2 %

• Not durable against severe conditions e.g. freezing and thawing.

2. High Strength Concrete

• It made by lowering the water cement (W/C) ratio to 0.35 or lower. 

• Normally, silica fume is added. (It can prevent the formation of free calcium hydroxide crystal in the cement which may reduce the strength at the cement aggregate bond.) 

• Low w/c ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. 

• Superplasticizers is added to compensate for the reduced workability in the high strength concrete mix. 

• Aggregate must be selected carefully for high strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate.

Figure above shows the high strength concrete was used

in the U-beams of the North Boulevard Bridge.

3.High Performance Concrete

Visualisation of High Performance Concrete

A high performance concrete range designed to overcome site

issues relating to time, quality and cost.

This video demonstrates the manufacturing process of High Performance Insulated Precast Wall Panels. Phase 1 shows the placement of the first wythe of concrete followed by a layer of insulating foam. The last steps show the placement of the 2nd layer of strand and concrete creating a thermally efficient wall system with no exfiltration or infiltration of air and no cavities for moisture to collect. High performance concrete is very useful in construction. 

• Has a high strength of high performance concrete ranges from 10000 psi - 15000 psi 

• High workability.is achieved by super plasticizers

● Water cement ratio can be reduced to 0.25

● Fly ash causes ball bearing effect increasing workability

• High durability

● Due to fly ash and silica fume which modify the mineralogy of cement, it can enhance the compatibility of ingredients in concrete mass and reduces the CH amount 

• Ease of placement.

• Compaction without segregation.

• Early age strength.

• Long-term mechanical properties.

• Permeability.

• Density.

• Heat of hydration.

• Toughness.

• Volume stability.

• Long life in severe environments.

Concrete Production

Lets us look at the manufacture of concrete^.^...

• First we need to prepare:

• cement (Portland cement),

• aggregate (such as sand or gravel),

• admixtures (chemical additive),

• fibers,

• water.

• Then mix them together to form concrete. 

• After mixed together, Portland cement and water form a gel. It will continue to react over time to improve workability. 

• In short, the concrete will conveying to the work site and placed, compacted and cured.

Concrete plan facility and delivery truck.

• Mixing Concrete

          Mixing is essential for the concrete production to produce a high quality of concrete. Equipment and method should be able to mix the concrete effectively and produce uniform mixtures for the work.

Steps to mixing concrete
The cement mixer is a device to combine the Portland cement, water, and aggregate such as sand and gravel to form concrete.

1. First of all, 1 gallon of water should pour into the mixer. Ensure that the amount of the water is accurately according to the specification.
2. Secondly, pour the cement into the mixer and switch on the concrete mixer to begin mixing the powder cement mix and the water together. Allow the mixer to continue mixing until the powder and water are completely mixed.
3. Then, turn off the mixer before pouring.
4. Lastly, pour the mixed cement into the wheelbarrow to convey to the construction site.

Cement mixer and way to put inside gravel and sand.

Besides that, if it is small amount that we need, we can do it by manually. The step is just almost same as the mechanical step, just we mix those material on the floor.

Pouring concrete.

Mixing concrete on the ground.

• Workability

          Workabilityis the property of fresh mixed concrete which can be mixed, transported, placed, compacted and finished. Higher workability can be achieve by using well graded aggregate, add chemical admixtures such as superplasticisers, and higher water content but it will cause side-effect. For example, higher water ratio can achieve higher workability but there is the risk of segrefation of aggregate which will make the strength lower.

           Workability can be measured by the slump test, compacting factor test, vebe test, and flow table test. Slump test is the common method to measure the workability of fresh concrete. It is use to measure the consistent of the concrete in specific batch. Besides, it is also used to ensure uniformity for different batches of similar concrete under field conditions.

The flow table test is a method to determine the consistence of fresh concrete. This method will be carrying out when the concrete is delivered by a truck to check its consistence before using it or pouring it to the formwork.

The other 2 tests have a similar function as the slump test.

Slump test.

  

The compacting factor test

The flow table test 

• Curing

           The concrete must take a properly cure after it is placed and compacted to ensure that the concrete is does not dry too quickly and achieve best strength and hardeners. Besides, it is to reduce plastic shrinkage cracking and ensure adequate supply of water for continue hydration and strength gain. Moisture such as concrete shrink will influenced the concrete’s strength during the hardening process. Concrete’s strength can be increase by keeping it damp.

Concrete curing can classify into four categories 

1. Watering the surface of the concrete
2. Surface of the concrete should be protected
3. Place over the wet hessian on the surface
4. Membrane curing- spray applied resin seal over the exposed surface.

Proper curing can concrete can increase strength, avoid cracking and lower permeability. Conversely, improper curing may cause scaling, cracking and poor abrasion resistance.

Polyethylene sheets as a moisture barriers for moist-curing concrete.

Keep saturated with water for curing concrete.

Building with concrete

          Concrete is one of the most durable building materials. It used as heat insulation, fire resistance and as an envelope of a building. It has a higher strength compare with other materials. Concrete is a material which common used in construction world. 

• Energy efficiency

          Concrete offers significant energy efficiency over the lifetime of a building. Thermal mass is a property that enables building materials to absorb, store and release the amount of heat. Concrete has an energy-saving advantage due to its inherent thermal mass. It absorbs energy slowly and keeps it for a longer period. This delays and reduces heat transfer through a thermal mass building component. It can reduce temperature inside the concrete and minimizing heating and cooling cost by store and release the energy.

          Therefore, a massive building uses less energy than a similar low mass building due to the reduced heat transfer through the massive elements.

• Previous Concrete

          Previous concrete is a type of concrete which has a high porosity that can allows the water and other sources pass through directly. It is mix by cement, water, coarse aggregate and little or no fine aggregate. It is using sufficient paste to coat and mix with the aggregate particles together. In addition, it will create a system of highly permeable and interconnected air void which drain out quickly. Previous concrete commonly used in parking area, walkways, streets and greenhouses.

Previous concrete

• Fire safety

          Concrete does not burn easily when fire happen. It has a higher degree of fire resistance compare with other materials such as wood and steel. When fire happen, it can act as a fire shield to block the fire and can protect itself from damage. The risk of structural collapse will be reducing and occupants can gain enough time to escape. By the way, concrete does not require any fire protection because it is a non-combustible material. 

          Besides that, concrete also provide high winds resistance due to its lateral stiffness.

Deismic retrofitting

• Earthquake Safety

           From our knowledge we know that concrete strong with compression, but weak on tension. When earthquake occur, it will produce a large shear loads on structure which is combination of tension and compression loads. During earthquake shaking, concrete without reinforcement will fail.

          Therefore, Seismic retrofitting is the way to minimize the risk of earthquake. It can making the building safer and reduces the risk of structure damage.