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ARCH 2602/5602 Lecture notes

Jonathan Ochshorn

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Week 5a, Monday lecture:
Concrete properties

Early concrete:

Example of Pantheon and early Roman walls.

Ground volcanic rock from Pozzuoli (near Naples) was found to be hydraulic (hardened when mixed with water) when blended with lime and sand. Early Roman concrete tended to use large aggregate.

Use decreased until revival of interest in 18th century:

1756 John Smeaton researched possibilities of hydraulic products in order to rebuild the Eddystone Lighthouse.

Portland Cement:

Patented in 1824 by British stone mason, Joseph Aspdin. His mix (literally mixed in his kitchen) contained finely ground limestone and clay first heated and then ground into a powder. The stuff hardened when mixed with water, i.e., was hydraulic, and got its name from a resemblance to stone found on the Isle of Portland.

Early use of concrete was "non-architectural," and included foundations or "fireproof" floors (with I-beams). Reinforcement came later, including 1854 example of reinforcement system by W. Wilkinson of Newcastle. See concrete history.

Modern Portland Cement contains:

calcium, silicon, aluminum, and iron, found in these common raw materials:

• limestone
• shells or chalk
• shale, clay, sand or iron ore.

Dry or wet process: proper proportions of the raw materials are ground, blended, and heated in a kiln, either dry or in a wet slurry. A type of fusion takes place at 2700 degrees F to create what is known as cement clinker; cooled, it is blended with gypsum and ground again into a fine powder: portland cement.

Concrete components:

• Aggregate (course/gravel and fine/sand)
• Cement (portland cement)
• Water
• Admixtures (optional)

Types of Portland Cement:

Type I	Normal	normal use
Type IA	Normal, air-entraining	normal use where subjected to freeze-thaw cycles
Type II	Moderate resistance to sulfate attack	especially from atmospheric pollution
Type IIA	Moderate resistance to sulfate attack, air entraining	pollution, plus freeze-thaw
Type III	High early strength	Use in cold weather, or where early strength is desired
Type IIIA	High early strength	Use in cold weather, air entraining
Type IV	Low heat of hydration	Use in hot weather, or where slow curing is desired (e.g., large dams)
Type V	High resistance to sulfate attack

Aggregate:

• approx. 70% concrete volume
• sand (fine aggregate) passes #4 sieve; use typically 3 grades of sand in concrete mix
• gravel (course aggregate); use typically several grades of course aggregate in concrete mix
• grading charts used
• maximum aggregate size determined by:
  • must fit in forms (1/5 narrowest form dimension)
  • must pass between reinforcing bars (3/4 distance between rebars)
  • 1/3 slab depth maximum
• weight:
  • normal is 140-152 pcf (so 145 pcf can be taken as normal concrete weight)
  • lightweight: pumice, cinders used for low-density insulation, or moderate-strength (60-85 pcf) for non-structural fill; also can be structural concrete with 90-120 pcf weight.
  • heavyweight: protective concrete used in reactors, counterweights.

Admixtures:

These extra ingredients (sometimes pre-mixed with cement) modify concrete properties in various ways:

• air-entraining agents: increase resistance to freeze-thaw deterioration. May work by creating very small pores in concrete that are hydrophobic, but provide expansion room for freezing water.
• water reducers: allow workable concrete at lower water-cement ratios, decreasing permeability and resulting in greater durability and strength.
• set-controlling admixtures: allow concrete to set properly in high or low temperatures (keep concrete workable longer in high temperature; hasten setting time in low temperature; or produce faster strength gain.

Advanced chemical admixtures:

• mid-range water reducers: provide a bit more water reduction than the conventional product; aid in finishing the concrete surface by reducing "stickiness" associated with high-cement mixes.
• high-range water reducers [HRWR] also known as plasticizers or superplasticizers can reduce the amount of water used up to 30%. Concrete produced is highly "flowable" (almost self-leveling), and can be pumped. Advantages include strength gain, reduced shrinkage, cracking, and permeability.
• viscosity modifying admixtures: allows complex formwork to be completely filled without vibration or risk of segregation of aggregate.
• corrosion inhibitors: protect rebars from chlorides (de-icing compounds, marine environments, or nasty aggregate); an alternative would be to use epoxy-coated rebars.
• pozzolanic admixtures: reduce portland cement up to 40%, thereby saving money, but also some benefits: can react with damaging calcium hydroxide (which is a byproduct of the hydration of portland cement) resulting in greater strength and reduced permeability. Most famous and widely used pozzolan is fly ash. See sustainability lecture.

Blended hydraulic cements:

One can combine Portland Cement with other hydraulic products, including granulated and ground blast-furnace slab, fly ash, natural pozzolans, and silica fume.

Mixing of concrete:

• mix is typically designed by lab to obtain specified strength (e.g., 3000 psi)
• "ACI method" commonly used; slump measures workability (typically 3-4" on 12" high cone).
  
• proportioning: since cement is expensive, aggregate is graded to minimize voids.
• water-cement ratio is key parameter. Too much or too little water reduces strength; while not enough water inhibits workability and may lead to honeycombing (large voids revealed only after formwork is removed)

Quality control:

• site inspection and testing needed for anything other than small residential construction. create 6"x12" test cylinders whose "cylinder" strength is measured after 28 days.
• ACI code requires 2 cylinders per 150 cubic yards or 5,000 sq.ft. (for slabs) and stipulates that:
  • either no test is more than 500 psi below specified strength; or
  • the average of any 3 consecutive tests is no less than the cylinder strength, f'_c.

Concrete reaches its "design strength" in 28 days.

Typical concrete strengths range from 2500 psi to 5000 psi, but higher strengths are certainly possible, especially for high-rise concrete structures.

Concrete is reinforced where tension is expected. The reason is that concrete itself cannot resist tension very well. In a "simply-supported" beam, for example, reinforcement would be placed at the bottom:

In reality, most concrete beams and slabs are continuous, rather than simply-supported. In these situations, the tensile reinforcement is alternatively at the top and bottom of the beam. For convenience, and to provide reinforcement for diagonal tension (shear), longitudinal rebars and vertical stirrups are joined together to form a "cage" of reinforcement that is inserted into the formwork.

Potential problems with concrete:

1. Carbonation: chemical reaction between cement and acidified rainwater (specifically calcium oxide or alkali free lime already in portland cement reacts with rain that has become acidified as it absorbs carbon dioxide). The result is a reduction in alkalinity of the concrete, so that the rebars have less protection against corrosion. Only affects exterior concrete; severity of problem depends on how permeable the concrete is.
2. Chloride attack: causes corrosion (rusting) of rebars. Some older concrete from the 1970s is affected due to use of calcium chloride as an "accelerating admixture." Also can result from use of hydrochloric acid as "etching" medium for certain surface treatments, or from de-icing salts.
3. Sulfate (sulphate in Britain) attack: results from contact with sulphate-based materials, such as sulphated groundwater; also possibly from the use of residual oil shale, pulverized fuel ash, and blast furnace slag used in concrete mix. Two problems: (1) the byproducct of chemical reactions with sulphates occupies a larger volume than the original cement; and (2) alkalinity is reduced, promoting corrosion (rust) of rebars.
4. Alkali-silica reaction (ASR): occurs when alkalis in portland cement (or from other sources) react with certain aggregate in the concrete mix, forming an alkali-silica gel that expands -- leading to cracking.

Factors affecting resistance to corrosion:

• surface treatments or coatings
• quality of concrete, including materials, proportioning, compaction, curing
• steel coatings (e.g., epoxy) or corrosion inhibitors (admixtures)

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Disclaimer: Students are responsible for material presented in class, and required material described on course outline. These notes are provided as a tentative outline of material intended to be presented in lectures only; they may not cover all material, and they may contain information not actually presented. Notes may be updated each year, and may or may not not apply to non-current versions of course.

first posted Aug. 24, 2007 | last updated: Sept. 5, 2008

Copyright 2007-2008 J. Ochshorn. All rights reserved. Republishing material on this web site, whether in print or on another web site, in whole or in part, is not permitted without advance permission of the author.