CONCRETE Flashcards
It consists of a hard, chemically inert particulate substance, known as aggregate (usually sand and gravel), that is bonded together by cement and water.
Concrete.
Among the ancient Assyrians and Babylonians, the bonding substance most often used was ____.
Clay.
The _____ developed a substance more closely resembling modern concrete by using lime and gypsum as _____.
Egyptians, binders.
Used in making concrete and should be clear and free of sulfates, acids, alkalies, and humus.
Water.
It is from municipal water systems or wells provides water suitable for use. Water from lakes, ponds, or rivers should be carefully checked for suitability before use.
Potable Water.
_____ containing up to 35,000 ppm (parts per million) of dissolved salts is generally suitable as _____.
Seawater, mixing water for unreinforced concrete.
____ are ingredients added to concrete, other than Portland cement, aggregates, and water. They may be added before or during mixing.
Admixtures.
They change the properties of concrete, so should be used sparingly and only on the advice of a concrete specialist.
Admixtures.
______ should be workable, finishable, strong, durable, watertight, and wear resistant. Whenever possible, these properties are obtained by careful selection of suitable types of aggregate, Portland cement, and the water–cement ratio.
If this is not possible, or special circumstances such as freezing weather exist, admixtures can be of benefit.
Concrete.
They are used to entrain microscopic air bubbles in concrete. Entrainment can be produced by using air-entrained Portland cement or by adding an air-entraining admixture to concrete as it is being mixed.
Air-entraining admixtures.
_____ improves the durability of concrete.
_____ gives improved workability during placement and superior water-tightness. It also improves resistance to sulfate attack from soil water and seawater.
_____ is used in cold climates.
Entrained air bubbles, Air entraining, Air-entrained concrete.
They are used to slow the setting time of cement paste in concrete. They are often employed in hot weather, where hydration is accelerated by excessive heat.
Without this in hot weather, more water is required to achieve the desired slump, producing a lowerstrength concrete.
Retarding admixtures, or retarders.
They tend to reduce the water required, resulting in a better water–cement ratio and ultimately increased concrete strength.
They also help when it is necessary to pour large amounts of concrete or where placement is difficult.
They are sometimes used in concrete mix trucks that have to travel an unusually long distance to a job site.
Retarding admixtures, or retarders.
They lower the amount of water needed to produce concrete of a given consistency. They can also be used to increase the amount of slump without requiring additional water.
This makes for a lower water–cement ratio, resulting in greater concrete strength. Some shorten set time and can cause increased drying shrinkage.
Water-Reducing Admixtures or Water Reducers.
______ speeds up the strength development of concrete. Strength development can also be accelerated by using Type III high-early-strength Portland cement, by increasing the amount of cement to lower the water–cement ratio, or by curing at higher temperatures.
______ are used in cold weather to develop strength faster in order to offset freeze damage.
Accelerator admixture, Accelerators.
It is a frequently used accelerator. It should be added to the concrete mix in solution rather than dry form.
It is not recommended for use in,
(1) pre-stressed concrete, because of the possibility of corrosion;
(2) in concrete containing embedded aluminum, such as electrical conduit;
(3) in concrete subject to alkali–aggregate reaction or soils or water containing sulfates;
(4) in nuclear shielding concrete;
(5) in floor slabs to receive dryshake metallic finishes; and
(6) in hot weather.
Calcium chloride.
It is a siliceous and aluminous material that, when finely ground in the presence of moisture, chemically reacts with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
Pozzolan.
They are sometimes added to concrete to help reduce internal temperatures, helpful when pouring large masses of concrete. Some are used to reduce or eliminate potential concrete expansion from alkali-reactive aggregates, while others improve resistance to sulfate attack.
Pozzolan materials.
When cement and water mix, the wet cement particles can form small clumps that inhibit proper mixing of cement and water.
This reduces workability and inhibits hydration.
They are admixtures that coat the cement particles, causing them to break away from the lumps and disperse in water.
- Reduce water and cement at a constant water–cement ratio, giving a concrete the same strength as a normal mix but reducing the amount of cement used.
- Produce normal concrete at normal water–cement ratios that is so workable it can be placed with little or no vibration or compaction and not have excessive bleed or segregation.
- Produce a concrete of higher strength by reducing the water content required while maintaining the normal cement content.
Superplasticizers.
Dense concrete that has a water–cement ratio of 0.50 by weight and is properly placed and cured will be watertight.
Concretes that have low cement contents, a deficiency in fines, or a high water–cement ratio can have permeability reduced by adding permeability-reducing agents, such as certain soaps, stearates, and petroleum products.
Permeability-Reducing and Damp Proofing Agents Sound.
They are used to reduce moisture that is transferred by capillary action.
This occurs when one side of the concrete is exposed to moisture and the other to air, as in a slab on-grade.
The surface exposed to air tends to dry. _______ occurs as moisture flows to a dry surface.
Damp-proofing admixtures, Capillary action.
When fresh concrete is poured over hardened concrete, the fresh concrete shrinks, breaking its bond to the hardened concrete.
The hardened surface must be prepared so the fresh concrete will firmly bond to the aggregate in the hardened surface.
The condition of the surface is of great importance. It must be dry and clean (free of dirt, dust, grease, paint, etc.), and the proper temperature should be maintained.
Bonding Agents.
