A home foundation design of concrete is not forever, but you won’t know the difference in your time
For some, concrete designing details for a home foundation – mixing concrete and placing concrete – is their day job
This article is intended to highlight key aspects of concrete design detail in both preparation and placement for your own knowledge and benefit
What's it to you as owner?
You’ll have a clue what the pros are
Talking about and doing or
Not talking about and not doing
You have a basis for your own thought, interest, and inquiry
What's it to you as home designer?
Specifying guidelines and latitudes on salient matters of materials and methods
Certain aspects of concrete design standout, among them,
The varied and disciplined applications of water throughout the early life of concrete from mixing through curing
This designer’s preference for gauging aggregate size to specific use
Rigorous tamping of all substrates, and
Guidance on amendments
SOME DETAILS OF CONCRETE DESIGN
Concrete
Shall be delivered to a site
As a transit mix
In an agitator truck
Shrink mixed
Shall be of
Minimum fines
Aggregates
Mixed
Clean
Which aggregates shall be to 1/3 diameter relative to slab-on-grade thickness but not greater than 1 ½ linear inches diameter
Which aggregates shall be well-graded not voided
Which aggregates shall be preferably fractured
Sand
Minimum
Clean
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")
Shall be placed over substrate, including, among others, below slab-on-grade and footing trench applications
All disturbed or amended substrates shall be tamped damp, not wet
Shall be tamped 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, “Substrate Tests for Moisture-Density Relations of Soils and Soil Aggregate Mixtures Using 10-Pound Rammer and 18-inch Drop")
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.
Comment: Substrate preparation is addressed elsewhere on this website at http://searchwarp.com/swa210956.htm comlete with pic of substrate layering.
Shall cure initially
7 full days
Continuously wet
Continuously unloaded
May cure under hose-fed burlene or similar
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 – 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
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")
Shall conform to not less than most recent ASTM C-595 (a/k/a American Society for Testing and Materials, “Standard Specification for Blended Hydraulic Cements")
Shall be of Type I or Type 2, unless otherwise noted,
Shall have aggregate conforming to not less than most recent ASTM C-33 (a/k/a American Society for Testing and Materials, “Standard Specification for Concrete Aggregates")
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 of water content 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
Not in finishing
Comment: It seems that the other #1 complaint about applying concrete 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 guidelines are another way the AG corrals water-pushing concrete contractors. 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.
Unless by mutual agreement of the general contractor and mixmaster or dispatcher
May be modified on-site with a plasticizer by mutual agreement of the general contractor and mixmaster or dispatcher
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 and
May be amended
With fiber (but not steel fiber) at not more than 1 ½ pounds/cubic yard
Comment: This home designer’s warming to fiber amendment over most others for host material stability.
Air entrainment
Shall be at
3%-6% for footings
5%-8% for walls
6%-7% for slab-on-grade in severe exposure (w/ aggregates 3/4 linear inch- 1 linear inch)
4%-6% for slab-on-grade in moderate exposure
And shall be applied where slabs-on-grade, walls, footings are subject to freezing
And 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")
With a 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
With a 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 and effective equivalent 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 superplasticizers inherent characteristic of high heat cure and
With 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")
With 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")
May be amended otherwise, each and all of which must be only by mutual agreement of the general contractor and mixmaster or dispatcher
Shall be repaired in conformance to not less than most recent ASTM C-10 (a/k/a American Concrete Institute, “Repair and Rehabilitation of Concrete Structures")
Concrete applied in remediation shall be matched closely in 28-day compressive strength to existing
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