Concrete

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Concrete

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Concrete is a more mixture of fine aggregates (sand), coarse aggregates (gravel/crushed rock), Portland cement, and water

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Portland cement

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Portland cement is manufactured from lime, silica, iron ore, and alumina. They are properly proportioned, ground, and burned to form clinkers, which are pulverized to form cement.

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Types of Portland cement

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Type I - standard: for General all-purpose use

Type II - modified: for slow setting and less heat

Type III - high early strength: quick setting and early strength

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Type IV - low heat: for very slow setting (little heat)

Type V - sulfate Resisting: for alkaline water and soils

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Aggregates

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They after chemically inert ingredients that are combined with cement and water to make concrete. They effect quality of concrete, reduce shrinkage, and serve as a filler for economy. Classified by size - Fine if usually sand and material is ≤ 1/4" in diameter. Coarse is usually gravel or crushed rock which vary in size from 1/4" to 1 1/2" in diameter. The most important aspect is the grading (distribution of particle sizes) is critical to a proper mix. Large coarse aggregate with finer particles fitted in between. Cement paste (water and Portland cement mix) is then added to graded aggregate. Max aggregate size should be 1/3 thickness of concrete slabs or 3/4 the minimum space between rebar.

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Admixtures

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Sometimes other materials are added to concrete mix to alter certain characteristics or achieve special qualities.

Accelerators: calcium chloride - speeds up setting time.

Air-entraining agents: resins, fats, and oils - resists freezing action.

Retarders: starches, sugars, and acids - slows down setting time.

Waterproofing: Stearate compounds - decreases permeability.

Water-reducing: Organic compounds - reduce water content.

Workability agents: powdered silica's and lime.

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Water-cement ratio

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Most important factor to creating ultimate strength of concrete as well as workability. The least amount of water creates the maximum amount of strength, yet is unworkable. More water increases workability but decreases strength. The optimim water-cement ratio provides the minimum amount of concrete pastento cover each aggregate and fill voids while providing the required strength with adequate workability. Excess water also produces laitance which would need to be removed prior to additional concrete pours.

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Proportioning

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The proportioning of the materials is called the mix. It involves determining the optimim combination of ingredients to produce concrete that is workable, economical, and has the required strength and other properties when hardened. Several trial mixes may be needed to achieve optimim outcome. Ordinary concrete ways 150#/ft³. Standard sack of cement contains one ft³ and is 94#. Aggregates are about 100#/ft³ and water is measured per gallon about 8#. For small relatively uncontrolled mix is usually volume. Ex: 1:3:5 mix consists of 1 part cement, three fine aggregate, and 5 parts coarse aggregate.

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Strength

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The compressive strength of concrete refers to it's strength after 28 days. Depends primarily on water-cement ratio. Typically ranges from 3000-6000 psi with 4000 being most common. High early strength can achieve this in 7-14 days.

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Structural Light Weight Concrete

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Made from lightweight aggregates such as expanded shale or clay osntead of natural aggregates can produce concrete 90-115 #/ft³. Has strength, workability, and appearance comparable to normal weight concrete. Max size for coarse lightweight aggregate is 3/4", air-entrainment is almost always used. Handling and placing is easier, has higher deflection, drying shrinkage is slightly larger, thermal properties are better, and the cost is almost always higher.

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Reinforced concrete

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Steel rebars or welded wire fabric is added to concrete to increase tensile strength. Most common rebar is ASTM 615 grade 60. WWF is designated by spacing and size of wires (WWF 6x6 - W2.9x2.9, spaced 6x6 with a cross section of .029in). Reinforcing bars are spliced by lapping a specified number of bar diameters. Can be pre-assembled, and need to be held rigidly in place by ties and supports, and need to have adequate concrete coverage to protect from corrosion. For example, footings need at least 3in, retaining walls need 2in, beams and columns need 1.5in and slabs need 3/4in.

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Curing

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Curing contract consists of maintaining the proper humidity and temperature for some period of time after placement. Excessive water loss can cause reduced strength of concrete, shrinkage, and surface cracks. Can be cured by:

1- supplying additional moisture by ponding and sprinkling.

2- using wet covering like moist sand, burlap, or straw.

3- covering with membrane that prevents evaporation.

4- leaving wood forms in place and keeping them moist.

Curing time lasts between 3-14 days and the longer the better. Hot and cold weather effects curing and needs special care.

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Construction joints

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The horizontal and vertical joints between two successive concrete pours called construction joints. Exist where concrete pours are interrupted and when new concrete placed against old. To ensure maximum bond, old surface should be roughened, cleaned, and wetted before placing new. CJ are always planes of weakness and should be located at sections of minimum shear. Often keyed to provide some shear strength.

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Expansion joint

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Expansion joints are designed to allow free movement of adjacent parts due to expansion or contraction in concrete which could be caused by change in temps. Provide complete separation through structure from top of the footing to the roof. They are waterproof, weather tight, and typically filled with elastic joint filler. Placement is function of the size of the building and the maximum expected temp. change. Also required on buildings more than 200ft long, at joints of building wings, and at additions to existing buildings.

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Control joints

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Control joints are tooled, sawed, or premolded joints to allow for shrinkage of large concrete areas. Control joints create a weakened section that induces cracking to occur along the joint, rather than in a random fashion.

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Isolation joint

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Provide a separation between a slab on grade and columns or walls, so that each can move independently.