Draft – Comments Welcome
1. Introduction
In the previous lesson we talked about the need for reinforcement in concrete structures. First of all concrete is not valid in intention. And so we add a supplement where our concrete members could always come into contention. Second, if, when, and where the concrete member does fail, our own backup will fail for a “safe” failure through the “holding member”.
Supply, in the context of this lesson, will refer almost entirely to iron supply. And most of the iron supports will be ‘deformed bars’ of iron; hence the name “rebar” for short. There are other types of supplements, such as fiber supplements, which we will discuss in less time.
2. Steel support
Steel makes concrete a great material to reinforce because it is so strong. It is also ductile, thus adding ductility to other non-ductile (plain) concrete. What this means is that concrete with insufficient support “yields” or “deforms” and relatively large deformations without “rupturing” (crushing). This degradation of behavior past the “seat” point is also described as “plastic” behavior. Therefore, for use in structures, steel adds a lot of strength and even greater safety. If a large earthquake occurs, instead of concrete and ending up in a pile of concrete, a properly reinforced concrete structure can crack and weaken, but not necessarily collapse (break). and this is good news for anyone occupying a building at the time of the earthquake.
Common types of steel supports used in structural concrete are:
1. deformed bars (rebar)
2. Combined Wire Fabric or Mesh (and Deformed Wire Fabric)
3. Deformed wire
4. Pre-tension (Pre-Giant) Steel
5. Post Tension Rods
(Note: we will not discuss fiber reinforcement here, as fiber reinforcement has another purpose.
Deformed bars and deformed wire use deformations on the surface of the bar to transfer the stress (load) to the steel from the surrounding concrete (and the entire lesson is assigned to make a certain contact surface here.). For fabric or mesh joints, the main transfer mechanism is the interlocking of the mesh embedded in the concrete (and wire deformation is used to deform the wire). By pre-laying the steel stress through friction from the Poisson effect transfers (and deformations if present). Post-tensioning steel generally transfers the stress to the concrete through plates or other means.
3. Re-bar
Rebar comes in ‘stars’ usually 20 ft long and diameters ranging from 3/8 in. to 2-1/4 in. diam. Hence the size # is 3/8 of the diameter, and so on. The size of the Rebar can also be designated by the “soft” (approx.) equivalent metric in mm. So, for example, the diameter of the rebar can be specified with a soft metric size #13, referring to 13 mm.
A list of common sizes and equivalent soft metrics follows.
Inch-Pound = Soft Metric
#3 = #10
#4 = #13
#5 = #16
#6 = #19
#7 = #22
#8 = #25
#9 = #29
#10 = #32
#11 = #36
#14 = #43
#18 = #57
“On paper” it might seem that #10, for example, might be indistinguishable as to whether it is an inch-pound size or metric size. Well, stupidly, if you look at the actual rebar, there is a big difference in a bar that is 3/8 inch in diameter versus a bar that is 10/8 inch (1.25) in diameter. In addition, bars 3/8 in. in diameter weigh about 6 ounces per linear foot while 10/8 in. diameter bars will weigh about 4 pounds per foot. If you are looking at the bulk of the construction documents, see the context or Specifications. You probably won’t find, for example, one-inch-diameter bars in a residential footer, but you will find them in a column for a tall commercial structure. Actually, in terms of available sizes, #10 is the only “number” that is available in either system.
If everything ‘still’ doesn’t make sense…then, here: www.crsi.org/rebar/id.cfm
4. A wire joint fabric
A Wire Fabric is made up of wires arranged at right angles to each other at their intersections to form a Grid.
The threads are either smooth (W), or deformed (D). (Wires are ugly like long, mini-rebars).
Voices vary, but usually take the formation of the wires in the grid, the type (W or D), and then the size. But size is different from bars; for the size of the wire “hundreds of square inches” in cross-sectional area (for each wire).
So, for example, WWF 6 x 6 D6/D6 represents “wire-bonded fabric” with wires in 6 in. x 6 in. grid, with each wire having a cross section of 0.06 in.2.
Sizes go down from W31 and D31 to W0.5 and D4 (small).
5. Supplement Level
The grade refers to the “certain yield stress of the steel”. These are the common steps.
Gr. 40 … referring to steel with a minimum specified yield stress of 40 ksi (40,000 psi).
Gr. 60 … 60 ksi (60,000 psi).
Gr. 75 … 75,000 psi.
Supplement levels will appear in one form or another in the Construction Specifications section of the Documents.
The ‘yield’ stress is the force in the steel beyond which the steel ‘yields’ or deforms some permanent amount. Below the yielding force, the steel will bend under stress, but will return to its original shape when the stress is released. In the context of the previous discussion in the lesson we could say that beyond the yield point the steel becomes “plastic”.
Grade 60 is now probably the most common rebar specified for cast-in-place construction. While it can be found next to a large amount of local buildings, it is known to be 50% stronger, and usually a little more than Gr. 40. (Perhaps it is now even less clear).
I generally define a supplement like this: … “Gr. 60 Deformed Bars.”
But perhaps you define something more intense and something like: … to strengthen the bar will be Gr. 60 and meets ASTM A615 (carbon steel)” … where ASTM stands for the American Society for Testing and Materials, and where A615 is the standard of trade for special carbon steel reinforcing bars. ASTM A706 is the standard of trade for low-alloy reinforcing steel.
Bonded Wire Fabric is commonly available in Gr. 60. ASTM A185 covers plain wire fabric and ASTM A497 covers deformed wire fabric.
UPDATE: it appears that ASTM uses the term Wire Reinforcement Joint (WWR) instead of Wire Fabric Joint (WWF).
Soft Metric Equivalents of Grades
Gr. 60 ksi = 420 MPa
Gr. 75 ksi = 520 MPa
6. Distinguishing Rebar
Each piece of rebar has a series of marks that indicate;
Top sign: The letter or symbol of the production mill
2nd symbol: Bar Size
3rd mark: Type Steel
4th sign (of any kind): Degree
See CRSI’s page on identifying rebar… here. (CRSI stands for Concrete Reinforcing Steel Institute).
7. Minimum Reinforcement (Emperor‘s)
The minimum amount of support in a concrete member is generally specified based on the ratio of the cross section in the steel relative to the cross section in the concrete area. We have different minimums, depending on the use of the concrete or the member.
- Minimum supply to tables (temperature / decrease).
… ρ min, t/s = 0.0018 for Grade 60
- Minimum flexural reinforcement of members … based on … effective depth, or area …. ρ min, flex = 0.0050 for Grade 40
… ρ min, flex = 0.0033 for Grade 60
- Minimum reinforcement in walls … is based on full wall thickness (from Chapter 14 of the ACI 318 Code) …. ρ min, walls, H.S. = 0.0020 for # 5 and smaller. rebar, Grade 60 or higher
… ρ min, walls, V.S. = 0.0012 for #5 and smaller rebar, Grade 60 or higher.
(I will say more about the reinforcement in the walls, but we will do that in a later lesson
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The main idea behind minimum amounts of support is to control cracking, safely.
8. Maximum supply (General
In general, the idea behind specifying maximum support weights is that you always want the steel to yield before the concrete begins to crumble. The current approach of the ACI Code to accomplish this is to limit the steel strain. This will be covered in more detail in later lessons, as it is more complicated than simply setting limits. According to the ratio of steel area to effective area, the flexural strength of our members is limited to a few percent (usually less than one or two percent).
The upper columns may bear more weight, but not more than 8 percent.
The main idea behind maximum support weight is that we don’t want to overpower the rest of the concrete.
9. Space and Cover
Spacing and cover requirements are prescriptive (spelled out, “cockbook”).
Spacing limits for retaining barriers are in ACI 318 Section 7.6.
Cover requirements are covered in section 7.7 of ACI 318.
The proper spacing of the bars ensures that the concrete is actually surrounded by bars. The enclosure requirements are adequately protected. Both requirements also affect the bond that can develop between concrete and steel.
10. Notes
Increasing Support Length, Jeff Filler, Content Associate.
ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428.
Concrete Reinforcing Steel Institute (CRSI), 933 North Plum Grove Road, Schaumburg, IL 60173.
Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington Hills, Michigan, 48333.
