Spring 2020
Jonathan Ochshorn
Also see these Milstein Hall construction videos: Part 2: Substructure and Part 3: Slabs.
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:
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:
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 | Formerly used in hot weather, or where slow curing is desired (e.g., large dams); not used much anymore -- instead, Type 2 (with a moderate heat of hydration) has been found adequate for such applications. |
Type V | High resistance to sulfate attack |
Alternatives to traditional Portland Cement:
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.
Belitic calcium sulfoaluminate (BCSA) cement:
BCSA cement "offers design and construction professionals a chance to work with a concrete mixture that does not shrink and sets quickly, reaching 31.026 MPa (4500 psi) compressive strength in about one hour. This is a completely different binder than Portland cement, but one with a long history of successful use in a variety of applications." (source). Problems: sets rather quickly and is 3 times the cost.
Admixtures:
These extra ingredients (sometimes pre-mixed with cement) modify concrete properties in various ways:
Advanced chemical admixtures:
Mixing of concrete:
Quality control:
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:
Factors affecting resistance to corrosion:
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 placed rather 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):
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)
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.
Image source
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.
Welded-wire mesh reinforces a sidewalk at Cornell (top); Monroe and Russell leave their mark in Hollywood (left); typical sidewalk details (right): Photos by J. Ochshorn.
Sidewalk failure at expansion joint due to uneven soil settlement or expansion, photo by J. Ochshorn, State St., Ithaca, NY, April 2019.
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: April 10, 2019
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2007–2019 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.