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What’s the difference in the SRS Carbon Fiber weaves? SRS-600UNI vs SRS-660BI

Updated: Feb 28, 2023

In the two fabrics offered by SRS, the SRS-600UNI and SRS-660BI, the UNI is short for unidirectional and BI is short for bidirectional. Unidirectional carbon fiber means all of the fibers run in the same direction. In the case of the SRS-600UNI the weave consists of and aerial weight of 600 grams of carbon fiber per square meter running the length of the roll.

Bidirectional carbon fiber means that the fibers are running in two different directions. In the case of the SRS-660 BI the weave consists of an aerial weight of 660 grams of carbon fiber per square meter, 330 gsm running the length of the roll of fabric and woven with 330 gsm running perpendicular to the length of the roll, at 0 and 90 degrees.

The raw carbon fiber that is woven together to make a fabric is called a tow and typically comes in 3k, 6k, 12k, or 24k. The SRS fabrics are made up of 12k tow meaning each individual tow contains 12,000 filaments of carbon fiber. These are continuous filaments which is where the strength of the fabric comes from. When the fabric is placed in tension, each of the 12,000 filaments is loaded simultaneously. Most unidirectional fabrics are woven and held together by weft fibers. These are not structural fibers but are simply used to hold the fabric in place. Bidirectional fabrics typically do not contain weft fibers as the two perpendicular directions of tows are woven together. There is however usually a binding material along the edges of the roll used to keep the woven material in place. The SRS-660BI is a twill weave rather than a plain weave which allows it to wrap and conform to substrates more easily.

The 600 and 660 gsm aerial weights that were chosen for the SRS fabrics have the strength capabilities required to tackle virtually any application and have some distinct advantages over lighter weight materials. When compared to a 200 gsm unidirectional for example, the SRS-600UNI contains three times the weight of carbon and therefore, higher strengths.

In order to choose the proper fabric for an application, one has to understand the differences and advantages/disadvantages of each fabric.


The SRS-600UNI is best utilized for strengthening of concrete and masonry components. As we all know, concrete is great in compression but lacks the tensile properties necessary for the demands so is usually poured with steel rebar embedded in it. This rebar provides the necessary tensile requirements so that the concrete can perform for the intended application. If this rebar is missing, damaged, or deteriorated, carbon fiber can be added to an existing structure in order to provide the necessary tensile capacities related to a specific load pattern. When looking at unidirectional fabrics, the carbon fiber layout is usually going to follow the same layout as the rebar for the intend application. The tensile strength added by applying a 6” wide SRS-600UNI strap is 1.5x greater than that of a #6 rebar.

Design Tensile Loads

#6 Rebar – 60,000psi x 0.44 in2 = 26,400 lbs

SRS-600UNI – 169,000 psi x 6” x .039” = 39,546 lbs (1.5 x greater)

As with rebar, the carbon fiber will also require a development length in order to provide sufficient bond capacity for the application. For example, when looking at a 6” wide SRS-600 UNI strap, the development length would be as follows:

Design Tensile Capacity 40,254 lbs = 6” x development length x 400 psi concrete bond

Development Length = 16.78”

The applications below are examples of where the addition of tensile strength in a single direction is needed:

  • Bowing wall stabilization

  • Reinforcement of broken corners

  • Flexural strengthening

  • Shear strengthening

  • Reinforcement of added wall or slab penetrations

  • Confinement of structural elements

SRS-660Bi-directional fabric

The SRS-660BI is best utilized to provide confining strengths within its width. Crack repairs are one of the most common uses of bidirectional carbon fiber. The SRS-660BI has a design tensile capacity of 35,496 lbs per 12”. This strength however is limited by the bond strength of the composite to the concrete substrate. If a 400 psi concrete bond strength is assumed then the strength developed by the epoxy bond on either side of the crack is as follows: 6” x 12” x 400 psi = 28,800 lbs. The ability to add this kind of strength to a crack repair has significant benefits. It can be used to remedy a problem injection or to simply reinforce a crack repair to provide piece of mind that it will not re-open. In addition to the strength properties provided by SRS-660BI, waterproofing capabilities are another benefit to its use.

Another common application for the SRS-660BI is the reinforcement of cracks related to corroded rebar. Once a crack develops and moistures is able to get to the reinforcing steel and corrosion begins. As the corrosion progresses it worsens the condition of the concrete around the reinforcing steel. In order to repair these areas, the deteriorated concrete needs to be removed and the rebar corrosion needs to be addressed. Once the corroded rebar is addressed, the concrete can be repaired. If the corrosion process is not completely stopped, over time, the expansion of the steel can cause the patch to fail. One method to reinforce against these types of failures is to wrap the repaired areas with a bidirectional carbon fiber. This will not only provide confining strength over the repair, but will also help make up for an section loss in the corroded rebar.

The SRS-660BI is commonly used to wrap support columns or other structural components. When a column is wrapped with a bidirectional carbon fiber not only is the confining/load bearing strength of the column increased, but also the flexural strength of the column itself. As the fabric wraps the column it must lap back onto itself in order to achieve the necessary bond capacity to act as a continuous wrap. The joints between vertical wraps of the column should also be lapped in order to maintain the flexural capacity increases of the carbon fiber running in that direction. If the required amount of strength can not be achieved with a single layer of the SRS-660BI, then two layers of the SRS-600UNI can be used. One layer would need to be installed horizontally and would need to be lapped back onto itself. The joints between the horizontal wraps would not need to be lapped and neither would the joints in the vertical layer.

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