1. DRY CAST CONCRETE FOR BOXES
PART ONE OF THREE PARTS OF CONCRETE FOR BOXES
January 15-18, Murfreesboro, Tennessee
By Tom Umek

2. Rigid Rugged Resilient
Dry Cast Concrete may seem to be the least complicated to make but it is often the most demanding when placing it in a form.
Type of aggregate
Size of the aggregate
Moisture content
Additives
Flowability or slump
Rigid Rugged Resilient

3. Types of the Aggregate
Know your sources!

4. Types of the Aggregate
Both gravel and crushed stone produce quality concrete. Gravel will have a lower water demand compared to crushed stone. Crushed stone will usually exhibit a higher paste-aggregate bond (Photo courtesy of the PCA)

5. Types of the Aggregate
Crushed stone #5 – Sizes are from 1″ down to fine particles. For road and paver base.
Crushed stone #67 – Sizes from 3/4″ down to fine particles. For fill, road and slab base.
Crushed stone #1 – Sizes are from 2″ to 4″. The largest of the crushed stone grades. For larger jobs such a culvert ballast.
Crushed stone #8 – Sizes from 3/8″ to 1/2″. For concrete and asphalt mix.
Crushed stone #3 -Sizes from 1/2″ to 2″. For drainage and railroad projects.
Crushed stone #10 (also called stone dust) – Screenings or dust. For fabrication of concrete blocks and pavers and for riding arenas.
Crushed stone #57 – Sizes of about 3/4″. For concrete and asphalt mix, driveways, landscaping and French drains.
Crushed stone #411 – A mixture of stone dust and #57 stone. For driveways, roads and as a base for retaining walls. It can also be used to patch holes in paved areas. The dust mixes with the larger stone and settles well.

6. Size of the Aggregate

7. Importance of Aggregate Grading
Ideally coarse and fine aggregate should be graded in such a way as to minimize the voids. After compaction the volume of the cement paste must be more than the voids between particles. Underfilling will result in entrapped air and an unworkable mix.
An extreme example of this is a no-fines concrete where the sand fraction is minimized, the course aggregate interlocks, but nothing fills the voids. This will NOT protect the reinforcement from corrosion or provide a weather tight structure.
Similarly achieving a paste volume to ‘just’ fill the voids will result in a mix where the coarse aggregate will interlock but not necessarily in an optimum compacted state making placing difficult and leaving voids. Some overfilling of the void space between the coarse particles by the sand fraction and between the sand particles by a cement paste is necessary for workability, placement and durability of the concrete.
Grading curve envelopes are given in BS 882, Specification for aggregates from natural sources for concrete.
Acknowledgement: The Concrete Society

9. Size of the Aggregate
Other physical and mineralogical properties of aggregate must be known before mixing concrete to obtain a desirable mixture. These properties include shape and texture, size gradation, moisture content, specific gravity, reactivity, soundness and bulk unit weight. These properties along with the water/cementitious material ratio determine the strength, workability, and durability of concrete.

10. The screens are only accurate if they are maintained
The screens are only accurate if they are maintained. Wear or buildup will skew the results!

12. Be careful when comparing materials….
They may look the same but these are from two completely different sources

13. Moisture Content of the Aggregate
The absorption of the aggregate indicates the quantity of water which will be absorbed into the pore structure. Most commonly, aggregates will have a moisture content that is either below or above this absorption limit. When batching, it is of critical importance to account for this water. Obviously, if the aggregates are in a surface wet condition, meaning the moisture content is greater than the absorption, that surface water increases the water-to-cement ratio and affects strength and durability.

14. Moisture Content of the Aggregate
Saturated Surface Dry
Air Dried
Free Water
Bone Dry

15. Additives The primary use is to increase lubricity
To protect from freeze/thaw by adding air entrainment
Used as a retarder or accelerant

16. Potentially equally damaging is a condition when the aggregates are only partially saturated, with the moisture content below their absorption value. While this will potentially increase strengths and even durability if not accounted for, placing these dry aggregates into the wet concrete mixture will allow the aggregates to take up water from the paste, filling their pore spaces but decreasing the amount of water available to allow the particles to slip by one another during pumping, placing, and finishing.