A home foundation of concrete design is not forever, but you won’t know the difference in your time.  For other guy’s, concrete designing for a home foundation – mixing concrete and placing concrete – is their day job

This home designer webpage is intended to highlight key aspects of concrete design in both preparation and placement for your own knowledge and benefit:  you’ll have a clue what the others’ are talking about and doing or not talking about and not doing; and you’ll also have a basis for your own thought, interest, and inquiry

Certain aspects of concrete design standout, among them:   

1. The varied and disciplined applications of water throughout the early life of concrete from mixing through curing

2. This home designer’s preference for gauging aggregate size to specific use

3. Rigorous tamping of all substrates

4. Guidance on amendments

Concrete batch for home building construction shall be delivered to a site as a transit mix in an agitator truck. Each batch shall be shrink mixed.  Each batch shall be of minimum fines and of clean, mixed aggregates and clean sand.  The aggregates shall be to 1/3 diameter relative to slab-on-grade thickness, but not greater than 1 ½ linear inches diameter, shall be well-graded not voided and, preferably fractured.  Sand shall be of a minimum and shall be clean.  Aggregate shall conform to not less than most recent ASTM C-33 (a/k/a American Society for Testing and Materials, “Standard Specification for Concrete Aggregates”). 

Mix, transport, and placement shall conform to not less than ACI 301 (American Concrete Institute, “Specifications for Structural Concrete”, latest edition) and shall conform to not less than most recent ASTM C-94M (a/k/a American Society for Testing and Materials, “Standard Specification for Ready-Mixed Concrete). 

Mix shall be placed over substrate, including, among others, below slab-on-grade and footing trench applications.  Substrates shall be tamped damp, not wet the more compact of not less than 50 beats per square foot and 95% density in conformance with not less than modified proctor most recent ASTM D-1557 (a/k/a American Society for Testing and Materials, “Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort”).  The tamping of substrates shall be tamped without regard to placement as footing, beam, or slab-on-grade. 

Comment:  This home designer knows that tamping substrate to footing trenches is not often applied.  It should be. 

The mix once placed shall cure initially 7 full days, continuously wet, and continuously unloaded.  Curing may be under hose-fed burlene or similar.  Concrete may, after initial wet curing, have applied a chemical curing membrane. 

Comment:  Shared for #1 complaint from the pros in applying concrete foundations and slabs-on-grade learned down all the years by this home designer – right alongside too much water before the placement – is too little water after the placement.  Once a placement is past the plastic phase, you cannot keep the material too wet. 

Comment:  This isn’t about removing the formwork, it’s about wet.  In colder climes, formwork should stay on longer, say, seven days, to warmer climes where 3 days could be sufficient.  Please keep in mind that the sooner the formwork comes off, the more surface area there will be to keep wet for the initial seven-day wet cure. 

Portland cement shall conform to not less than most recent ASTM C-150 (a/k/a American Society for Testing and Materials, “Standard Specifications for Portland Cement); not less than most recent ASTM C-595 (a/k/a American Society for Testing and Materials, “Standard Specification for Blended Hydraulic Cements”).  Furthermore, Portland cement shall be of Type I or Type 2, unless otherwise noted. 

Comment: Shrinkage may be offset with consideration of Type K cement.  See http://www.rapidset.com/TypeK_FAQ.asp#a08 et al. in regard to optimal batching, placement, and finishing, notably including but not limited to water content and slump. 

Water content shall be not greater than .45 in water-to-concrete ratio and no water shall be added to a mix once on a site - not in mix, not in placement, and not in finishing.  

Comment:  It seems that the worst of concrete application offenses among those in the know is that there’s too much water in the mix, often added after the trucks arrive on-site. 

Slump shall not be less than 3.5 and not greater than 4, unless otherwise noted. 

Comment: Slump metrics are another way this home designer corrals water-pushing concrete residential contractors.   

Comment:  Please note, too, that common practice allows dispatchers and batchmasters some plus-and-minus leeway to these limits. Make it so that their tolerance is within your limits or, otherwise, well understood and agreed-to by you the general residential contractor and mixmaster or dispatcher.  Please note that mixes may be modified on-site with a plasticizer by mutual agreement of the general residential contractor and mixmaster or dispatcher, again with your foreknowledge and agreement.   

For example, amendment may be, among others,

1. Fiber (but not steel fiber) at not more than 1 ½ pounds/cubic yard.

2. Plasticizer in lieu of up to 15% water content but within slump and effective water content limits, and added as close to placement as possible and absolutely kept wet not less than 7 days focusing particularly on the early part of that time period due to plasticizers inherent characteristic of high-heat cure.

3. Superplasticizer (a/k/a/superfluidizer, superfluidier, super water reducer, high range water reducer) in lieu of up to 30% of water content, but: within slump; effective equivalent water content limits; added as close to placement as possible; and absolutely kept wet not less than 7 days focusing particularly on the early part of that time period due to superplasticizers inherent characteristic of high heat cure.

4. Pozzolan in conformance to not less than most recent ASTM C-618 (a/k/a American Society for Testing and Materials, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete”).

5. Chemicals in conformance to not less than most recent ASTM C-494M (a/k/a American Society for Testing and Materials, ”Standard Specification for Chemical Admixtures for Concrete”).

Again, mix amendment -each and all of which - must be made only by mutual agreement of the general residential contractor and mixmaster or dispatcher. 

Comment:  This home designer is warming to fiber amendment over most others for host material stability. 

Compressive strength shall be rated generally not less than 3500 pounds per square inch 28-day compressive strength if placed from a batch mix; specifically not less than 4000 pounds per square inch in a garage slab-on-grade and driveway; and not less than 5000 pounds per square inch if from premixed bags. 

Air entrainment shall be at 3%-6% for footings, at 5%-8% for walls 6%-7% and for slab-on-grade in severe exposure (w/ aggregates 3/4 linear inch- 1 linear inch), at 4%-6% for slab-on-grade in moderate exposure.  Air entrainment shall be applied where slabs-on-grade, walls, footings are subject to freezing.  Air entrainment shall conform to not less than most recent ASTM C-260 (a/k/a American Society for Testing and Materials, “Standard Specification for Air-Entraining Admixtures for Concrete”).