Chapter 7 Concrete: Review Questions

What are the basic ingredients in concrete?

Portland cement, coarse and fine aggregates (like gravel (or stone) and sand), and water (with possible admixtures)

How does concrete cure and harden?

hydration

What ingredients normally make up Portland cement?

lime, iron, silica, and alumina

What are the designations for the different types of Portland cement (6 + 3)?

type I: normal

type II: moderate

type III: high early strength

type IV: low heat of hydration

type V: sulfate resisting

white Portland cement

type IA, IIA, and IIIA: same as type I, II, and III just air-entrained (A= air-entrained)

What is meant by air-entraining?

air-entraining encapsulates millions of microscopic air bubbles into the concrete mix.

How does the addition of air-entraining materials to Portland cement improve the concrete (2 winter-related, 2 regular)?

- greatly improves the durability of concrete that is exposed to moisture and freeze and thaw cycles during winter

- increases the concrete's resistance to surface scaling, which is caused by salts used to remove winter ice

- improves the workability of concrete

- reduces segregation of the aggregate, bleeding, and the amount of water needed.

What ingredients may be used in masonry cements?

Portland cement, slag cement, and hydraulic lime. They may also contain hydrated lime, limestone, chalk, calcareous shell, talc, slag, or clay

How does the fineness of the cement affect the concrete?

the rate of hydration increases which accelerates the strength development of the concrete.

What natural aggregates are used in making concrete?

crushed stone, gravel, sand

Explain how Portland cement should be stored.

It is moisture sensitive, so it must be protected from dampness.

- watertight

- doesn't touch floor or exterior walls

- Sacked material must be stored on pallets

- Bags should be packed closely

- if it is stored a long time, roll the bags on the floor or ground before use.

- Bulk cement is stored in watertight bins or silos.

What aggregates should be avoided when making concrete?

They must be clean, strong, free of absorbed chemicals, and devoid of coatings of clay, humus, and other fine materials. Aggregates containing some shale, shaly rocks, soft or porous rocks, and some types of chert are not suitable

What aggregates are used in insulating concrete?

Perlite, diatomite, vermiculite, pumice, and scoria

What are the commonly used admixtures (11)?

air-entraining, water-reducer, retarder, accelerator, water reducer and retarder, water reducer and accelerator, pozzolan, gas former, damp proofing and waterproofing, pumping aids, air detrainer, superplasticizers

What is the recommended water-cement ratio?

0.40 to 0.60 (The lower the water-cement ratio, the stronger the concrete).

What five factors influence the quality of finished concrete exposed to the elements?

water-cement ratio, minimum cement content, aggregates, entrained air and slump.

What can be done in hot weather to help retard rapid setting times?

retarding admixtures

What are three concrete standards and quality organizations?

American Society for Testing and Materials (ASTM), Portland Cement Association (PCA), and the American Concrete Institute (ACI)

What is the binding agent in the concrete mix?

cement and water

What is the relationship between the compressive strength and workability of the concrete in regards to the water/cement ratio?

the higher the water cement ratio: the more workable but the lower the strength

the lower the water cement ratio: the greater the strength but the lower the workability

What is an ideal curing temperature?

68 degrees Farenheit

What is the percent of concrete that is admixtures typically?

80%

Water-reducing

allows a reduction in the amount of mixing water while retaining the same workability

Super-plasticizers

organic compounds that transform a low-slump mix into one that flows into the forms

Accelerators

causes concrete to cure more quickly

Retarding Agents

slows the curing time

Workability Agents

improves plasticity

Shrinkage-reducing agents

helps prevent cracking

Corrosion inhibitors

reduces rusting of rebar

In a regular reinforced concrete beam, where is the best place for rebar?

bottom of the beam (bc that is where the tension forces are)

What is a concrete cover?

the amount of concrete on the outside of the reinforcement placement

What weight concrete is the most common (lightweight, normal weight, or heavyweight)

normal weight

What is heavy weight concrete used for?

radiation shielding in medical or nuclear applications

What is self consolidating concrete?

concrete that flows easily without segregating

What is green concrete?

uses less in energy in its production and produces less carbon dioxide than normal concrete.

What is bio-concrete?

a self-healing form of concrete designed to repair its own cracks. It was developed by Dutch researcher and microbiologist Hendrik Jonkers using an extra ingredient acting as a healing agent and requires no human intervention to be repaired once placed.