Concrete is the most common man-made construction material on earth.  Over 55,000 concrete trucks are operating in the United States today and there are countless finishing methods and decisions to be made for your project. How can you be sure you are getting a long-lasting product from your contractor?  Is your project being over-built or under-built?  Replacement of a poor concrete job is expensive!

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Below are common items to consider and industry terms to be familiar with.

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• Concrete vs. Mortar vs. Cement (NOT the same thing)

  • Concrete = cement + ​aggregate (sand and rock) + water.  Concrete is POROUS

  • Mortar = cement + sand

  • Cement is the binding element in concrete and mortar, made of limestone, clay, shells, and silica sand.  It sets and hardens when combined with water

• Strength of Concrete and Curing

  • Typical concrete from a ready-mix truck begins around 3,000 PSI (pounds per square inch)  This can be increased ​to 4,000 PSI or 5,000 PSI depending on your project. 

  • Typical driveways, patios, and walkways use a 3,000 PSI standard outdoor mix.  An 'outdoor' mix entrains air in the mix to increase durability and helps to limit cracking in freeze & thaw cycles.

  • Critical projects typically involve taking sample specimens from each truck, allowing them to cure in a cylinder, then breaking them in a compression testing machine to test the compressive strength of the mix

  • Strength over time: installed concrete gains strength over time - average strengths are 40% in 3 days, 65% in 7 days, 90% in 14 days, and 99% in 28 days. 

  • High humidity slows down curing

  • High temperatures speed up curing

  • Retarders slow down concrete curing and accelerators like Calcium Chloride speed up concrete curing

  • Additional water added to the concrete at the site weakens the concrete and should be limited

• Reinforcement (Rebar, Remesh, Fiber Reinforcement)​​​

  • Concrete is extremely strong in compression (squeeze) but weak in tension (stretch).  Metal (rebar) reinforcement is added into slabs, walls, and footings to increase concrete's tensile strength

  • Rebar is sized in 1/8 increments in the US - #4 rebar means 4/8" or 1/2" rebar​

  • Rebar should be installed suspended in the bottom of footings and slabs.  This is where the concrete is in tension.

  • Rebar should not be left exposed to the ground or air.

  • Welded wire fabric, welded wire mesh, Remesh (WWF or WWR) can be installed in concrete to aid in shrink cracking.  This should ideally be added into the TOP half of the slab on flatwork.  WWF can be purchased in mats and rolls.

  • Fiber can be added into the concrete mix at the plant before delivery and limits microscopic cracking in the concrete.  It is more difficult to get a smooth finished surface with fiber entrained concrete mix.

• Base Considerations (below concrete)

  • Drainage​

    • Ideally, the base below your concrete should be installed to allow water to flow away from the concrete.  Stormwater can erode the base causing pockets and weak spots below the concrete.  Water can also freeze and expand below the concrete, forcing the concrete up and leading to movement and cracking.

    • Aggregate is typically used as a base ​material to increase the drainage and compaction characteristics below the concrete.  57 stone and crusher run are commonly used

    • Vapor barriers are added between the base and the concrete below buildings to limit vapor and moisture from passing from the ground through the concrete

  • Compaction​

    • The more compacted your ​sub-grade and base, the stronger your concrete will be and the less susceptible it will be to cracking from settling

    • A geotechnical engineer can perform a compaction test on the soil below your concrete - typically 95% compaction or better is desired

    • A level base allows a consistent depth of the concrete and therefore a consistent strength throughout​

    • Poor soil under your concrete can be assisted with the use of compacted crushed stone

    • Non-critical projects with a highly-compacted base and good drainage may not require aggregate below the concrete

    • Do NOT allow the use of organic fill (black, woody, leafy) material under any of your concrete.  This will degrade and leave voids under your concrete.

• Order Size & Truck Info

  • Orders are placed in CY (cubic yards)​

  • Most suppliers have a minimum order amount, if you order less concrete than their minimum, there are typically 'small order fees'

  • Most trucks can legally deliver 9 to 10 cubic yards of concrete

• Joints​ (expansion, contraction)

  • Contraction (shrinkage) ​joints are most commonly seen from above slabs and are sawed, tooled, or formed in concrete to allow the concrete to crack in specific, desired locations.  If they are sawed, these should be put in place quickly after the concrete is installed before significant contraction takes place.

  • Expansion joints are separations between adjacent sections to allow movement.  A compressible filler material is used in flat pavement slabs to create expansion joints.

• Finish Options​

  • Surface

    • Broom finish is standard for exterior flat applications where slipping hazards exist​

    • Smooth / Troweled finish is standard for many interior applications

    • Exposed aggregate finish is achieved by washing away the top layer of concrete, exposing the edges of natural stone aggregate. 

    • Stamped concrete can mimic brick, paver, or stone looks 

    • Staining the concrete surface to achieve the desired color.  Stained concrete is typically sealed.

    • Pigmented concrete is a color added into the entire concrete pour - this is typically a lighter color than stained concrete.  Pigmented concrete is typically sealed.

  • Edges

    • Rounded edges are typical to limit edge chipping​ - various radius edges are available

    • Lined edges of stairs, curbs, and sidewalks

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ADDITIONAL CONCRETE TERMS

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