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Understanding Materials: Concrete

Four essentials must be known about the concrete to determine proper diamond-blade selection.

1. Compressive Strength
The hardness of concrete is referenced by its compressive strength measured in Pounds per Square Inch (PSI). Cured concrete slabs vary widely in compressive strength; with moisture, temperature, design of mixture additives, cementitious materials and curing processes often determining their measured level of strength. The higher the compressive strength, the harder the material.

Concrete Hardness PSI Typical Application
Very hard 8,000 or more Nuclear Plants
Hard 6,000 - 8,000 Bridges, Piers
Medium 4,000 - 6,000 Roads
Soft 3,000 or less Sidewalks, Patios, Parking lots

2. Age of the Concrete
The “age,” or length of curing time, greatly affects how the diamond blade interacts with the concrete. Although methods exist to accelerate the curing process, the “state” of concrete from initial pouring to a period of 72 hours and over can be defined in 3 distinct increments, and is influenced by temperature, weather, moisture, aggregate, time of year, admixtures and composition.

State 1 – 0 to 8 hours
The concrete is considered in its “green“ state 0 to 8 hours after the pour, meaning it has set but has not hardened completely. With green concrete, the sand in the mixture has not bonded to the mortar blend firmly and will cause extreme abrasive action once the physics of sawing begins. Further, the slurry generated by green concrete is equally as abrasive and will require special undercutting protection for the steel core of the diamond blade. Typically, sawing control joints of highways, industrial flooring, driveways, runways and similar projects is performed during this state.

State 2 – 8 to 24 hours
The concrete is considered as cured 8 to 24 hours after the pour. The sand is held firmly adhered to the overall mixture. Generally, control joints established in State 1 are widened during this time.

State 3 – 24 to 72
The concrete is considered as cured 24 to 72 hours after the pour. The sand is held firmly in the mortar mixture, and the overall abrasive actions and properties of the concrete are greatly diminished. Now, consideration of the aggregates, compression strength and steel content of the concrete become important factors in determining proper diamond-blade selection.

3. Aggregates and Sand
Aggregates are the granular fillers in cement that can occupy as much as 60 to 75% of the total volume. They influence the way both green and cured concrete perform. Aggregates can be naturally occurring minerals, sand and gravel, crushed stone or manufactured sand. The most desirable aggregates used in concrete are triangular or square in shape, and with hard, dense, well-graded and durable properties. The average size and composition of aggregates greatly influence the cutting characteristics and selection of the diamond blade. Large aggregates tend to cause blades to cut slower; smaller aggregates allow the blades to cut faster.

Difficulty Average Aggregate Size
Harder to Cut (Blade wears slower) 1-1/2" or more
1-1/2" to 3/4"
3/4" to 3/8"
Easier to Cut (Blade wears faster) Pea gravel (less than 3/8")

Aggregate hardness is referenced by the Mohs Scale. This scale assigns arbitrary quantitative units, ranging from 1 through 10, by which the scratch hardness of a mineral is determined. Each unit of hardness is represented by a mineral that can scratch any other mineral having a lower-ranking number. The minerals are ranked from talc or 1 (the softest), upward through diamond or 10 (the hardest). Hard aggregates shorten blade life and reduce cutting speed.

Sand composition is another factor in determining the hardness characteristics of the cement and the abrasive properties of the mortar. Three types of sand are generally used in the mixture:

  • River Sand (round nonabrasive)
  • River Bank Sand (sharp abrasive)
  • Manufactured Sand (sharp abrasive)

River Bank Sand and Manufactured Sand are more abrasive than River Sand. The more abrasive the sand is, the harder the bond-matrix requirements. Sharper, more geometrically defined sands also require harder bonds.

Mohs Hardness Scale

 

Aggregate Classification Map of the United States
     Soft         Medium Soft         Medium         Medium Hard         Hard
Click here for aggregate classifications by state

4. Steel Reinforcement
Further strengthening and structural integrity of concrete is accomplished by introducing concrete reinforcing steel bars (rebar), steel wire strand of wire meshing into the concrete. It costs more to cut concrete that contains reinforcing steel because cutting rates are slower and blade life is reduced. If the cross-sectional area of concrete is 1% steel, the blade life will be about 25% shorter than if no steel were present. Concrete with 3% steel can reduce blade life as much as 75%.

Standard Reinforcing Bars
Metric Size
(mm)
Diameter Imperial Size
(inches)
Diameter
10 9.5 #3 .375
13 12.7 #4 .500
16 15.9 #5 .625
19 19.1 #6 .750
22 22.2 #7 .875
25 25.4 #8 1.000
29 28.7 #9 1.128
32 32.3 #10 1.270
Heavy Rebar: #6 Rebar every 12" on center or 2 Mats of #4 Rebar every 12" on center
Medium Rebar: #4 Rebar every 12" on center
Light Rebar: Wire Mesh, single mat