The Rebar for an Irregular Shaped Slab calculator estimates the total length and weight of reinforcement bars (rebar) needed for an irregular shaped concrete slab. NOTE: this calculator was updated on 16 Feb 2022 to accommodate the inset of rebar placement from the edge of the slab. The net result is a more accurate and less conservative estimate.
INSTRUCTIONS: Choose units and enter the following:
Irregular Slab Rebar and Concrete (IR): The calculator returns:
However, these can be automatically converted to compatible units via the pull-down menu.
The rebar weight algorithm calculates the length and weight of reinforcement steel bars in an irregular shaped concrete slab. The algorithm uses the Rough Area, Perimeter and the onCenter spacing to estimate the total length of rebar in one mat. The total length of rebar is the length in one mat times the number of mats.
Once the algorithm calculates the length of rebar, the length is used with the user specified rebar size, and density constants from the vCalc library to calculate the total weight of the reinforcement steel bars. The area and depth are used to compute the volume of concrete. The formula the length of rebar in one mat of an irregular shaped slab is:
L = 4*oC*(A/(2*oC*oC)) + rP/2
To accommodate an inset from the edge of the slab for the placemen to the rebar, the area (A) and perimeter (rP) have to be reduced BEFORE the formula above. The method used is to treat the slab as a circular slab and reduce the area and perimeter accordingly as follows:
The calculation determines the need for an extra rebar using an internal tolerance for for a spacing fraction that exceeds the tolerance with a default of 1". It also assumes that the uncut rebar is in 40' lengths and that lapping is needed for dimensions in excess of 40`. Furthermore, the lapping factor, when lapping is needed, is 40. For the same calculations with the ability to add a different "uncut" length and/or a different lapping factor CLICK HERE.
A major challenge with rebar in an irregular slab is minimizing wasted rebar. The general guidance is this. When possible, start with the longest cross-section of of your slab and work outwards from there. These will be the longest stretches of rebar in your slab. When you cut off the ends, line them up in a row from shortest to longest. When you get to shorter runs, first see if there is a cut-off section of rebar from your row of cut-offs. This is only a little extra work and will save in both materials and labor removing cut-offs at the end of the project.
Rebar is short for reinforcing bar. Rebar is a roughly circular steel bar with ribs used to provide added tensile strength to concrete structures. Rebar is put in place before concrete is poured. When the concrete has hardened, the concrete around the rebar ribs keep the rebar in place. Rebar and concrete expand similarly with temperature variations. This all has the net result of substantially added tensile strength when rebar is part of the concrete form. Carbon steel is the most commonly used material for rebar, which may also be coated with zinc or epoxy resin.
Rebar is laid out in grids, crisscrossed patterns of rebar, tied at the intersections where runs of rebar touch. The grids have spacing between the rebar rows, and they are placed within the concrete form by a specified inset from the edge of the concrete. Multiple parallel grids, at uniform space intervals, are referred to as rebar mats.
In the United States, rebar sizes are in increments of 1/8th inches in diameter. Therefore, size 4 is 4/8th of an inch, which is 1/2", and size 8 is a full inch in diameter. Based on this and the density of steel used in rebar, the Rebar Size Table contains reasonably accurate specifications of rebar linear weight and lateral (face) area based on rebar size.
The most common lengths of pre-cut rebar in the United States are 20`, 40' and 60'. These are known as rebar sticks. When the dimensions of a slab, wall or other form exceed the length of a single stick of rebar, it is required to rebar lapping length. The length of the lap is specified by a "Lapping Factor (LF)" which is often 40 or 60 times the diameter of the rebar. Engineering specifications of a lapping factor should always be applied.pieces to create the added length. This process is called lapping, and the length of the overlapping rebar is the
The following are price estimates of rebar products based on the current market price of steel and a reasonable markup from raw steel to retail rebar product. NOTE: Always use local pricing! The following prices are for calculating convenience. vCalc makes no warranty on the pricing, and does not guarantee material availability at this price.
|Current Rebar Price Estimate (2024-02-22)
|#3 - 3/8"
|#4 - 1/2"
|#5 - 5/8"
|#6 - 3/4"
|#8 - 1"
The above price estimates are derived from the current market price of steel and an observed markup from raw steel to rebar products.