Concrete Mix Design

Strength 4500 lbs.: Water-Cement Ratio 0.45.: Cement Content Minimum 5-1/2 Sack (94 lbs. per sack)Maximum 6 Sack.: Aggregate ASTM C 33: Maximum size 1-1/4-inch (Gravel); Maximum size 1-inch (Crushed Limestone).: Slump 3-inch plus or minus 1-inch.:Thickness 4-inch or 6-inch (select one:  6 – inch is recommended for heavier loads).: Entrained Air 6-percent plus or minus 1-percent.: Mid-Range Water Reducer allowed for placement.:  Welded Wire Reinforcement WWR 12 x 12 inch, W1.4 by W1.4

Soils Preparation

Soils under footings for commercial and industrial construction must be tested for load bearing capacity by a professional engineer to certifiy the soils are capable of supporting the structure and recommend the size of footings.  

Compaction

Soils under slab on grade must be compacted to engineering standards to adequately support the slab otherwise it will brak up under pressure.  Reducer allowed for placement.:  

Contraction Jointing

Contraction joints provide a natural pathway for shrinkage cracks to follow that are more aesthetically pleasing than random cracking;  there are projects where it is desirable to allow concrete slabs to random crack rather than cutting contraction joints. 

Contraction joints can be hand tooled or saw cut. Hand tooled joints are better looking, but contractors prefer to saw cut joints because it’s easier.  Also, hand tooled joints will often fill with dirt and cultivate weeds unless caulked on a multi-year basis.  Saw cutting joints should be done in the first   6 – 18-hours or they are ineffective.

Spacing contraction joints is an aesthetic choice but it has practical effects. The smaller the squares, the better the crack control; but more jointing costs extra so there is an economical middle ground to be reached.  SIx foot squares are good for driveways but 10 foot squares is more practical.

Welded WIre Reinforcement (WWR)

Welded wire reinforcement (WWR) holds joints and random cracks tightly together, and combined with mimimally spaced contraction joints can eliminate random cracks altogether. Even concrete with a low water-cement ratio will crack, just less so than concrete with a higher water-cement ratio.

Some concrete people will try to sell you fiber mesh reinforcement as a substitute for WWR, but fiber mesh, in the words of the Portland Cement Association, “should not be expected to replace wire mesh in slab on ground”.

Welded wire reinforcement (WWR) also strengthens the concrete so that it can withstand heavier loads without breaking up.

WWR is placed on chairs to keep it in the middle of the concrete. If WWR is not placed on chairs, the inspector must watch carefully that workers are using a steel hook to pull the reinforcement up into the slab as the concrete is being placed. WWR should be ordered with 12 by 12-inch box squares so the workers will have a place to step without pushing the reinforcement back on the ground. 

Welded Wire Reinforcement WWR 12 x 12 inch, W1.4 by W1.4

Concrete for Slab-On Grade Specifications

Curing Concrete

Concrete hardens by the reaction of water and cement particles in a process called hydration. Most hydration takes place in the first few hours but continues as long as there is free water and particles of cement available. The more complete the hydration process is the better the concrete. After concrete starts to harden, the hydration process slows down, and if the free moisture evaporates too quickly the hydration is halted altogether, leaving the concrete under strength. . 

Curing is the process of prolonging  hydration with methods that keep  water from evaporating. 

Curing methods can be divided into three classes: (1.) Membrane-Forming Curing and Sealing Compounds sprayed on the surface (2.) rewetting techniques including continuous flooding of the concrete for 7 to 30 days and absorptive covers kept continuously wet for the same period of time and (3.) Moisture-Retaining Covers of polyethylene film. There are other methods but these are the most common.