Factors Involving Concrete Cutting

When cutting/drilling concrete, several factors influence your choice of diamond blades/bits These include:

• Compressive strength • Type of sand

• Hardness of the aggregate • Steel reinforcing (rebar)

• Size of the aggregate • Green or cured concrete

These guidelines are for general reference only. Your best source for information on the characteristics of the concrete you have to cut is from the original contractor.

Compressive Strength

Concrete slabs may vary greatly in compressive strength, measured in pounds per square inch (PSI). Compressive strength in concrete is a measurement of the load carrying capability of concrete. Most concrete roads are 4-6,000 PSI, while typical patios or sidewalks are about 3,000 PSI.

Concrete Hardness	PSI
Critically Hard	8,000 or more
Hard	6-8,000
Medium	4-6,000
Soft	3,000 or less

MOHs Scale

1 - Talc

2 - Gypsum

3 - Calcite

4 - Fluorite

5 - Apatite

6 - Feldspar

7 - Quartz(SiO₂)

8 - Topaz

9 - Corundum(Al₂O₃)

10 - Diamond

Most aggregates fall into the 2 to 9 range on the MOHs scale. Some commonly used aggregates measure this way on the MOHs scale:

MOHs Range	Description	Aggregates
8-9	Critically Hard	Flint, Chert, Trap Rock, Basalt
6-7	Hard	Some River Rock, Some Granites, Basalt, Quartz, Trap Rock
4-5	Medium Hard	Some Granites, Some River Rock
3-4	Medium	Dense Limestone, Sandstone, Dolomite, Marble
2-3	Medium Soft	Soft Limestone

Size of the Aggregate

The size of aggregate affects diamond blade performance. Large aggregates tend to make a blade cut slower. Smaller aggregates tend to make a blade cut faster. The most common standard sizes of aggregate are:

• Pea Gravel- variable in size, usually 3/8” or less in diameter

• 3/4 inch - sieved size of 3/4” or less

• 1-1/2 inch - sieved size of 1-1/2” or less

Type of Sand

Sand is part of the aggregate mix and determines the abrasiveness of concrete. “Small aggregate” is usually sand. Sand can either be sharp (abrasive) or round (non-abrasive). To determine the sharpness of sand, you need to know where the sand is from. Crushed sand and bank sand are usually sharp; river sand is usually round.

Green concrete is more abrasive than cured concrete. This is because when concrete is not fully cured there has not been enough time to allow the cement to gain strength to hold all the sand in place; sand can easily be scraped off the surface being cut. More loose sand means more abrasiveness.

Steel Reinforcing (Rebar)

Heavy steel reinforcing tends to make a blade(bit) cut(drill) slower. Less reinforcing tends to make a blade(bit) cut(drill) faster. Light to heavy rebar is a very subjective term.

Examples include:

Light

Wire mesh, single mat.

Medium

#4 rebar, every 12” on center each way (OCEW), single mat.

Wire mesh, multi-mats.

Heavy

#4 rebar, 12”（OCEW）, double mat.

#5 rebar, 12”（OCEW）, single mat.

“Heavy” rebar can also result from different grades of steel. Typical rebar is grade 40 steel. Grade 60 steel would make the example of #4 (medium) rebar above into a heavy rebar. Rebar gauges (diameter of the bar) are measured in eighths of an inch - #4 rebar is 1/2” diameter, #5 is 5/8”.

Green or Cured Concrete

The drying or curing time of concrete greatly affects how the material will interact with a diamond blade. Green concrete is freshly poured concrete that has set up but not yet fully cured. It is softer and more abrasive than cured concrete. You need a harder-bonded blade with undercut protectors to cut green concrete. You need a softer-bonded blade to cut the same concrete in a cured state.

As it applies to diamond blade sawing, concrete defined as “green” is less than 24 hours from pour or younger, the actual time can vary widely. Weather, temperature, moisture in the aggregate, time of year, and the amount of water in the mix all influence curing time. Also, much of the concrete poured today

has additives, which can either shorten or extend curing time. Consult your mix design to find the relative curing time for your job. As soon as wet concrete sets up and does not spall or ravel, green cutting should begin.