Precast Concrete

Precast concrete is a sustainable building system that provides slender, efficient structural elements.  Precast plants provide many advantages over concrete cast in the field.  Better control over conditions, materials, and workmanship gives a higher strength, higher quality product in a safer environment.  Repetitive forms can be reused.  The limitations of transportation, however, means widths over 14' can be problematic and expensive to move.


Testing is similar to that of site-cast concrete.

Precast concrete is often pre-stressed - prestressing allows a much more efficient distribution of compressive and tensile forces, thus it allows for smaller, cheaper concrete members.  Tensioned steel squeezes the concrete, putting the entire concrete section under compression, reducing cracking.  The strands can be shaped to more closely match the path of tensile forces through the beam, which tends to be at the top near the ends, but at the bottom near the center of the span.  T








Prestressing comes in two flavors:


PRE-TENSIONED:

Manufactured with steel cables or bars, called “tendons,” are pulled taut prior to the casting of the concrete, which bonds to the tendons and the force is transferred along the surface area of the rebar.  This site goes slightly further in depth.

POST-TENSIONED:
The tendons are not allowed to bond during curing by sheathing them.  After the concrete has cured, the tendons are jacked taut and anchored to the ends of the concrete, pulling the concrete section into compression.  Most commonly used in large building projects such as high-rises and bridges. Post-tensioned concrete slabs-on-grade also can be found where there are unusual soil conditions.

Precast pieces were used to reinforce the stage floor prior to Kirstie Alley's appearance on Dancing With The Stars.

Masonry Mortar

Don't mistake wine for mortar; however, both depend on quality of ingredients

Mortar can be similar to wine in that you should be aware of the labels because each type is different. Similar to wine, Mortar is created by several different ingredients making some mortar stronger than others. There's suitable mortar for colder climates and there's a variety of colors to choose from. I am not encouraging the use of wine on job sites, btw. This is just to help you remember the differences in mortar types.

Mortar is made up of portland cement, hydrated lime, and sand (aggregate) & water.

Lime is added to impart smoothness and workability. Its produced by burning limestone or shells in a kiln to drive off carbon dioxide and leave quicklime (calcium oxide).

Hydrated Lime (slaked lime): released large quantities of heat.

Mortar mix can be obtained in shades ranging from pure white to pure black; makes up 20% of exposed surface area of brick wall, therefore color is important.

Mortar composition standard for unit masonry refer to: ASTM C270

TYPE N: used for most purposes

TYPE M & S: higher strength structural walls (or for severe weather). S is used below grade, as well.

TYPE O: most economical and used for non-bearing walls

For more information on selecting the right mortar for your job refer to Masonry Contractors Association of America

Movie Building Inspector #1

It's time for episode 1 of Movie Building Inspector, where today we are responding to reports of violations at a certain school for wizards and witches... I hear it's one of those liberal ones over in Europe.  Let's just take a look in the main hall and GAAAH!

Hogwart's Dining Room

Holy crap, Hogwart's is a fire hazard if I have ever seen one!

Sure, heavy timber construction is better than light frame, but lets back those 5,000 open flames away from the roof structure, unless you have an anti-litigation spell for all those angry parents of flattened children - class action suiticus dismissum! The more resistant the structure is to heat, the bigger the building can be! Also, should Hogwarts have a proper sprinkler system (which they do, although the fairies-with-tiny-buckets-of-water system is only approved for use in England), the structure could be enlarged and travel distances to exits lengthened. See the chart below for Construction Types.

Click to enlarge

Who isn't a fan of magic stairs that swing from corridor to corridor? Of course, this design would get an immediate stamp of rejection from the DOB...

Stairs to Gryffindor Hall

If you want to bring these up to code, here's a few design rules you should know:

• Handrails at the bottom of stairs shall extend 12" plus extend the slope for a horizontal distance the depth of one more tread. 
• Vertical distance between landings may not exceed 12'-0" with headroom clearance no less than 6'-8".
• Monumental Stairs are stairs 88" in width or wider and require intermediate handrail.
• Avoid single steps; where there are more than three steps, handrails must be provided.
• Handrails must be located 34"-38" above nose of stair or ramp.
• Width of an egress stair is based on occupancy: in most cases, 44" is the minimum but you can go to 36" in a residential occupancy.
• American Disabilities Act requires the below dimensions for handrail design:

You should always refer to your local building code for regulations regarding stair design, as they may have added a few requirements of their own.

Concrete Curing & Testing

Concrete gains about 70% of its strength during first week of curing; it takes 28 Days to fully cure, like Sandra Bullock's heart.

TESTING BEFORE CURING

Slump test.

Slump test: measures consistency of concrete; amount of slump desired depends on how concrete will be used, but typically in the range of 2" to 6".

Even a real Kelly ball looks dirty
on an R. Kelly album cover.

Kelly Ball Test: also "ball penetration" test; hemispheric mass of steel with a calibrated stem is dropped onto a slab of freshly laid concrete. Amount of penetration is measured and compared to 1/2 the values of slump test.

K-slump test.

K-Slump test: uses 3/4" tube that contains a floating scale - the distance the scale floats out is read and measures consistency of concrete.

TESTING AFTER CURING

I will crush you.

Cylinder Test: measures compressive strength; tested in laboratory; During

the initial pour, samples of the concrete are poured into 12 inch by 6 inch

cylinders to be tested. After seven days of curing, the first sample is

given a compression test, and again after 28 days (full strength).

Core cylinder extracted from cured concrete.

Core Cylinder Test: used when a portion of the structure is in place and

cured but needs to be tested; a cylinder is drilled out of the concrete and

tested in lab to determine compressive strength.

Impact hammer: non-destructive field test of concrete strength after it has

hardened; rebound of plunger snapped against surface is measured.

TESTING FOR MOISTURE

Moisture is a critical factor in determining the ultimate strength of concrete.

Calcium chloride test (moisture dome test): common test for moisture in

concrete. Contractors use calcium chloride to test the dryness of concrete

before putting down flooring; tester weighs the container again on the same

scale. The difference in weight represents the water vapor emitted.

Hygrometer test (relative humidity test): moisture emission by measuring the

relative humidity RH of atmosphere confined adjacent to the concrete floor;

test standards recommend that moisture sensitive flooring not be installed

unless RH is 75% or less.

Polyethylene test: presence of visible water indicated concrete is

insufficiently dry for application of finishes. Similar to mat test.

Electrical Impedance test: moisture content in slab is read out directly.

TESTING FOR ALKALINITY

pH level of concrete should be tested -  concrete normally has pH of 12.0-13.3 (7 is neutral).

Alkalinity can screw you in 2 ways; high alkalinity on surface of slab

can damage a tile installation by causing the adhesive to re-emulsify, or

revert to its liquid state. Alkalinity is also responsible for Alkali-Silica

Reaction (ASR), in which cement begins to dissolve sand and rock within concrete.

ASR is reduced by using low lime content, aggregates not susceptible to ASR,

proper curing and not finishing with hard trowel surface.

Titration test: lab test of alkalinity in concrete.