18 Identify at least 5 and explain their meanings of reducin

18. Identify (at least 5) and explain their meanings of reducing bleeding in concrete? 19. Identify and explain the early year of Portland cement. When was it first produced in the US?

20. Aggregates are usually divided into how many groups?

21. Name and describe three chemical admixtures in concrete?

22. What are the major reasons for using chemical admixtures in concrete mixtures?

23. Why is concrete so widely used?

24. How can we produce air-entraining concrete?

25. What are concrete components?

26. What are footings? Name two types and show how they relate to the building?

27. What are two types of Concrete Buildings Systems?

28. What are the six categories of concrete wall systems in current use?

18) Bleeding in fresh concrete refers to the process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic. The way you work that bleed water can affect the durability of your products.

The biggest factor in bleed water rates is the water-to-cement ratio. A higher ratio can lead to excessive bleeding. The cement type and fine aggregates can play a role in determining the bleed rate. The fewer fines you have in your mix, the more bleeding will occur. Factors also include concrete height and pressure. The relationship between bleed water and concrete heights starts off as linear, but eventually becomes nonlinear at increased heights.

Ways to reduce bleeding in concrete include:

19) Portland cement is a basic cement mixture and a fundamental ingredient for many common cement applications. Because Portland cement is very common, it is also often called “Ordinary Portland Cement” or simply “OPC.” Since the early 20th century, Portland cement has been used internationally for a wide variety of applications including concrete projects, mortar pastes, stucco decorations, and grout fillings

Roman cement was the predecessor to ordinary Portland cement. A crude form of Roman cement was rediscovered by English engineer John Smeaton in 1756. The formula for Roman cement was refined and patented in 1796 by James Parker.

Around 1811, James Frost developed British cement: a new way of processing cement that became instrumental in making Portland cement. In 1824, the first form of Portland cement was patented by English cement manufacturer Joseph Aspdin. Aspdin’s version of Portland cement was further refined by his son, William, in 1843, and it closely resembled the ordinary Portland cement we know today.

The United States imported Portland cement from Germany and England as early as 1868. Between the 1970s and 1980s, the United States began manufacturing their Portland cement in Michigan and Pennsylvania. Within a few years, most of the Portland cement used in the United States was produced locally.

20) For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.

• Aggregates are divided into either ‘coarse’ or ‘fine’ categories.

- Coarse aggregates are particulates that are greater than 4.75mm. The usual range employed is between 9.5mm and 37.5mm in diameter.

- Fine aggregates are usually sand or crushed stone that are less than 9.55mm in diameter.

• Typically the most common size of aggregate used in construction is 20mm. A larger size, 40mm, is more common in mass concrete.

• Larger aggregate diameters reduce the quantity of cement and water needed.

21)Chemical admixtures are the ingredients in concrete other than portland cement, water, and aggregate that are added to the mix immediately before or during mixing. Producers use admixtures primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, and curing; and to overcome certain emergencies during concrete operations.

Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers). All other varieties of admixtures fall into the specialty category whose functions include corrosion inhibition, shrinkage reduction, alkali-silica reactivity reduction, workability enhancement, bonding, damp proofing, and coloring.

Water-reducing admixtures usually reduce the required water content for a concrete mixture by about 5 to 10 percent. Consequently, concrete containing a water-reducing admixture needs less water to reach a required slump than untreated concrete. The treated concrete can have a lower water-cement ratio. This usually indicates that a higher strength concrete can be produced without increasing the amount of cement. Recent advancements in admixture technology have led to the development of mid-range water reducers. These admixtures reduce water content by at least 8 percent and tend to be more stable over a wider range of temperatures. Mid-range water reducers provide more consistent setting times than standard water reducers.

Retarding admixtures, which slow the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting. High temperatures often cause an increased rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable during placement and delay the initial set of concrete. Most retarders also function as water reducers and may entrain some air in concrete.

Accelerating admixtures increase the rate of early strength development, reduce the time required for proper curing and protection, and speed up the start of finishing operations. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.

Superplasticizers, also known as plasticizers or high-range water reducers (HRWR), reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction. The effect of superplasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the jobsite.

Corrosion-inhibiting admixtures fall into the specialty admixture category and are used to slow corrosion of reinforcing steel in concrete. Corrosion inhibitors can be used as a defensive strategy for concrete structures, such as marine facilities, highway bridges, and parking garages, that will be exposed to high concentrations of chloride. Other specialty admixtures include shrinkage-reducing admixtures and alkali-silica reactivity inhibitors. The shrinkage reducers are used to control drying shrinkage and minimize cracking, while ASR inhibitors control durability problems associated with alkali-silica reactivity.

22)The major reasons for using admixtures are: 1. To reduce the cost of concrete construction 2. To achieve certain properties in concrete more effectively than by other means 3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions 4. To overcome certain emergencies during concreting operations

23)Concrete still remains the building material of choice for most structures and is the single most widely used material throughout the world. Its properties are such that it resists weathering, erosion and other natural disasters, and requires very little maintenance. Dating back to early Egyptian times, humans have mixed together the basic ingredients of concrete from sand and gravel, a cement-like binder, and water for thousands of years.

Concrete’s continued popularity comes from the fact that it is not only economical but it is very strong and durable. When compared to wood, asphalt or other building materials, concrete outlasts them by decades and even gains strength over time. Concrete is an extremely versatile material that is used to construct buildings, bridges, dams, tunnels, pavements, runways and roads. Additionally concrete is extremely fire resistance so it is a safe material to use.

The raw material used to make cement, which is a part of concrete, is limestone, and is the most abundant mineral on earth. Also concrete can be made from waste byproducts that come from power plants, steel mills and other manufacturing facilities such as fly ash, slag cement, and silica fume.

24)Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes. Air-entrained concrete is produced using air-entraining portland cement, or by the introduction of air-entraining agents, under careful engineering supervision, as the concrete is mixed on the job. The amount of entrained air is usually between four and seven percent of the volume of the concrete, but may be varied as required by special conditions

25)

There are three basic ingredients in the concrete mix:

Portland Cement - The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.

Water- Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio, the stronger the concrete. (higher strength, less permeability)

Aggregates- Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes

26)Footing is one of the most important parts of a structure which transfers loads of a structure to the underlying soil. The selection of suitable type of footing generally depends on the following factors:

1. The depth of the soil at which safe bearing strata exists.

2. The type and condition of soil.

3. The type of the superstructure.

TYPES OF FOOTINGS:

The different types of footings used for building construction are described below:

1. Wall footing/Strip footing.

2. Spread Footings