Particle size distribution of Crickcrete

If we consider concrete, the main ingredient is aggregate:  rocks, stones and sand.  I briefly discussed this here on this blog.

Of real importance is the particle size distribution in the aggregate mix.  The goal in good concrete is to get a complete space filling by using different sized particles.

Aggregate (stones, rocks and sand) is generally not spherical, but has a longer dimension and a shorter dimension.  This results in a “tip” which is located at the ends of the longer dimension, and a “face” which is located at the end of the shorter dimension.  One of the keys to good concrete is tip-to-face contact between larger aggregate.

One of the other keys to good concrete is that the gaps between large aggregate are filled with smaller aggregate, so that there are not empty spaces, or gaps, or interstitial sites between aggregate.  This is what is meant by “space filling.”

There is a field of science which concerns itself with space filling between particles.  My own exposure to this science came in studying ceramics, wherein scientists are typically looking at very small particles.  One of  the insights into space-filling came about in 1930, and was proposed by two scientists (A.E.R. Westman and H.R. Hugill) who worked together to develop a diagram which represented space filling as a percentage of volume based on different sized particles, and is known as a Westman-Hugill diagram.

Here is a quote from an abstract of their paper “The Packing of Particles” published by the Journal of the American Ceramic Society, June 12, 1930: “It is axiomatic that the mode of packing of very large volumes of particles of uniform shape and size is independent of the size of the particles, provided they are large enough for the effect of electrostatic forces, air films, etc., to be negligible. An apparatus is described, in which equal true volumes of approximately spherical particles, ranging in diameter from 0.2 to 0.0035 inch, pack practically to the same apparent volume. This apparatus was used in studying the packing of mixtures of two and three sues of particles. By plotting the data so obtained in diagrams of a particularly convenient character, it is shown that the apparent volumes of mixtures containing unit real volume of solid fall between limiting values which can be calculated from simple assumptions, and that their deviation from these limits depends in a definite manner upon the diameter ratios of the component particles. The conditions governing the application of the results of the study to ceramic technology are pointed out.”

While Westman and Hugill were considering spherical particles for their model, the basic ideas hold for irregular shapes, which is what one encounters in concrete mix.

Here is what I find interesting about this whole concept.  If you go outside and scoop up a shovel full of rocky, sandy mix (not soil, but aggregate, such as one finds in a stream or creek bed) then the mix is very close to the ideal particle size distribution one would design if starting from “scratch.”

I find this incredible!  Nature has provided us with a close to ideal particle size distribution for very good concrete.  Almost everyone fails to appreciate this fact.  Everything we make from concrete would be much more difficult to make if this were not the case.  If we lived in a world of only tiny sand, we would be making large rocks to provide large aggregate.  If we lived in a world of only large rocks, we would be making sand (at a huge cost of time and energy).  As it is, nature has provided us with a very close to ideal concrete mix in terms of aggregate particle size distribution.

There is a commercial brand of concrete known as “quikcrete” which is sold in dry bags.  Friends of mine who are aware that a creek bed provides an ideal mix of aggregate particle size also live in the country, where a creek is known as a “crick”.  They call their homemade concrete “crickcrete” and chuckle and guffaw like country bumpkins.

So grab a shovel, head to the creek and make some of nature’s own crickcrete.

Water storage tanks

I am completing a unique small masonry project.  This is to be a sub-surface water storage tank, made from triangular manufactured concrete block.  I will be using it as a “plunging tank” to cool off after taking a sauna.

This simple prototype uses an early version of the triangular block system described on this blog (here, here and here).  The blocks used for this tank do not utilize the interlocking key and keyway system.  I was just using up some of my older block.

This tank assembled quite easily.  It is incredibly strong, and is expected to last a very long time.  It will be filled with rainwater, as a proof-of-concept for a rainwater harvesting system.  Rain will be collected from the roof of the sauna (not yet built).  Water will be refreshed and kept from stagnating by simple replenishment from fresh precipitation.

Mortar was used in this model, although this system can also be assembled as a dry-stacked sphere.  Dry-stacking is especially easy with the key and keyway system, as described here.  A dry-stacked tank would require a bladder, or parge coat and sealant.

The use of mortar allows for the shape of the tank to be modified, if so desired.  Mortar can be applied in thicker or thinner amounts to vary the shape of the tank.  In this case, I made the sphere deeper, or elongated it almost like an egg.  This was to provide a deeper tank, for easy submersion.  It is around 8 feet deep.

I cast a pipe under the tank, so that I can run electrical wires, leading to a waterproof underwater light, at the bottom center of the tank.  This will be powered by a solar panel on the small roof of the tank.  I’m hoping it will provide an illuminated fishbowl effect.

I plan to coat the inside of the tank with inexpensive pool paint, since people will be “swimming” in it. 

The top of the tank will have a “wishing well” type roof on it.  It is important to shield the tank from sunlight, since this will encourage algae to grow.  There will also be a hinged lid on the tank to keep insects out.

I am hoping to be able to use this tank without chemicals such as chlorine or bromine.  I’ve installed a pump for circulation, as a back-up in case rain is infrequent.  This will help keep the water from stagnating, and will keep it somewhat oxygenated.

I plan to build another sphere soon, much like the water storage tank.  This other tank will be used as part of a septic system.  These tanks are inexpensive, very strong, high volume (around 1,500 gallons) and easy to install.  I will post my efforts on this blog as things move forward.

To see the tank completed, please look here.

The straightest wall

I’ve been told by a fistful of masons that there is one client that consistently requires –demands, even- the most stringent specifications for concrete block walls far above and beyond any other client they ever dealt with.

Who could this client be?

Some federal agency?  Nuclear facilities?  Chemical companies? Bio-containment?  The pentagon?  Prisons?

No, it’s Wal-Mart.

Next time you’re at Wal-Mart (we all do it) look at the concrete block walls.  If you find a flaw, go find management and complain.  Tell ‘em I sent ya.

Rubble and masonry

Rubble is broken stone of irregular size, shape and texture.   ‘Rubble’ derives from rubbish.  Masons make good use of this garbage.

Rubble masonry is rough, irregular unhewn building stone not laid in regular courses.  It may appear as the outer surface of a wall, or may be used to fill a wall. 

Many thick walls which appear as solid stone are commonly filled with rubble.  It is almost counter-intuitive; that a massive wall -the very symbol of solidity- is often filled with rubble.  There is much more rubble in many more exquisite masonry buildings, from The Pantheon, to cathedrals to the Taj Mahal, than most people realize.

The Great Wall of China is made mostly from rubble, which was used to fill the space between the outer bricks and below the road’s surface.

Dry stacked stone walls rely on strategically placed and compacted rubble to help bind the masonry elements together into a consolidated mass.  Good rubble will have a size distribution (big and small chunks) for space filling.  Proper use of rubble in dry stacked stone walls is an art.

Here’s to rubble.