Reinforced concrete properties and systems

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. 

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)

Concrete construction systems

Reinforcement: consists of "deformed" bars sized in nominal increments of 1/8" diameter from No.2 (2/8" diameter) up to No.18 (18/8" diameter). Note that No.2 bars are not deformed. Also, current bar markings are in millimeters, rather than No.3, No. 4, etc. (However, US concrete codes still use the older designations.) Grade 60 is commonly used (60,000 psi). Other types of reinforcing steel is used in sidewalks and slabs-on-grade (welded wire mesh), or in pre-stressed or post-tensioned concrete. Spiral wires are used in circular columns, whereas ordinary bent "rebars" are used in rectangular tied columns. Slabs are reinforced in the direction of span: either 1-way (spanning between parallel walls or beams), or 2-way (spanning in both directions simultaneously where the slab is supported on all 4 sides, and is in a more-or-less square proportion. Note that even 1-way slabs are reinforced perpendicular to the direction of span to control shrinkage and temperature-induced cracking.

Conveying, placing concrete: Concrete is moved from the mixer to the formwork by various means, including wheel barrows, buckets, pumping. A danger in such movement is segregation, where heavier aggregate settles and water rises. Concrete is placedrather than poured, although the latter term has insinuated itself into the construction-place vocabulary, and cannot be avoided. So, "cast-in-place" is better than "poured-in-place." To make sure that concrete has reached all parts of the formwork, it is often "vibrated" with special tools (vibrators). This prevents "honeycombing" (where voids appear after the formwork is removed). Concrete should be protected from moisture loss (evaporation) for at least 7 days, by sprinkling water on its surface, or by covering it with sheets such as polyethylene.

Formwork for concrete: Lumber was the primary material used to create forms into which concrete is placed, or cast. Now, other materials are also used, especially metal (reusable) forms, and plywood (rather than boards). The formwork must be structurally able to withstand the lateral pressure of the "wet" concrete before it cures (hardens). Metal formwork ties are often used for this purpose, leaving small circular marks in the surface of the concrete that are often used for aesthetic purposes in so-called "architectural" concrete (i.e., concrete where the architect/client cares about the surface qualities).

For economy, it is prudent to reuse formwork where possible, either within a single job or on multiple jobs. 35-60% of the concrete cost is associated with the need to build forms. Forms also impart a texture to the surface of the concrete; this fact has been exploited by many architects, either with a cabinet-maker's sensibility or with a rough (brutalist) aesthetic in mind. Examples from Corbusier, Kahn, Ando, Rudolph, Pei, and Moneo were shown. For a discussion of the "fake" aggregate created using form liners in Milstein Hall (OMA), see this video. Forms can also be made with rigid insulation, which stays in place after the concrete cures, as shown in the advertisement below (left) and the College Avenue construction example (right):

"PolySteel" and "PolyPro" advertisement (left images); College Avenue apartment construction, Ithaca, NY (right; photo by J. Ochshorn)

In multi-story construction, one floor is typically cast at a time; the horizontal joint in a wall that results (the construction joint) can be hidden within a "reveal" so that the inevitable imperfections of the joined condition are not as obvious.

Of course, this is a purely aesthetic bias; the opposite approach, i.e., exploiting the imperfections of the joint, is equally possible.

Formwork ties: Formwork for walls is often held together with metal ties that actually penetrate the concrete in order to keep the formwork surfaces from spreading apart due to the pressure exerted by the "wet" concrete. Some examples of commercial products, and a diagrammatic sketch, are shown below:

Jose Rafael Moneo: Cathedral of Our Lady of the Angels, Los Angeles, 2002 (with pattern of formwork tie holes articulated in the concrete surface — photo by J. Ochshorn)

Slab systems: The most common ways of framing reinforced concrete slabs are as follows:

• flat plate systems subsume the girders that normally would be articulated between columns within the depth of the slab itself; therefore the appearance is of a horizontal slab and columns only, without any articulated girders or beams. This system was most famously propounded by Le Corbusier as the "Dom-Ino" system; shown below:
  
  Flat plate systems are often used in residential construction to minimize floor-to-floor heights. They are 2-way systems.
• flat slab systems are similar to flat plates (also 2-way systems), except they employ capitals and/or dropped panels at each column-slab intersection to order to provide more surface area at this point of greatest shear.
  
• Variations on flat plate and flat slab systems can incorporate reusable metal or plastic "pans" in a grid module. This eliminates unnecessary concrete in the "tension" zone, while providing an economical formwork system.
  
• 1-way systems employing a hierarchy of girders, beams, or joists can be framed in concrete, and are similar in principle to wood- and steel-framed floor systems.
• precast (and prestressed) concrete planks can be used as floor systems, whether supported on walls (brick, concrete, precast concrete, or concrete masonry) or beams (concrete or steel). The photo below shows Cornell's West Campus dorms designed by Kieran-Timberlake Architects.
  
  

Finally, concrete sidewalks are reinforced with welded wire mesh, and contain control joints to control cracking due to the shrinkage of the concrete, as well as expansion joints to deal with differential movement and settlement at a larger scale.