Wednesday, July 12, 2023

Making a concrete ping pong table

I recently completed making a concrete ping pong table. It came out pretty well, and I look forward to playing some ping pong!

Here are the basic steps I took to make and assemble the ping pong table.

First, I made wooden molds. There was a mold made for the table surface, a mold made for the central supporting arches, and four molds for legs which spring from the arches to the corners of the tabletop. Here are the molds, shown upside down.

Here is the arch section being made. There are 4 pieces of #3 rebar (3/8 inch diameter) in the arch form.  I used basaltic FRP rebar (fiber reinforced polymer).  All reinforcement was kindly donated by Nick Gencarelle of Smarter Building Systems. Nick is very knowledgeable and helpful.  We just used a bagged concrete mix, specified as having a strength of 4,000 psi after 28 days of curing.

Here are the four legs being made. Each leg also has 4 pieces of #3 FRP rebar.

Next, we set up a form for the base. The same form was used later for the tabletop. We placed #3 FRP rebar inside the form, at 10 inches on-center.  The arch form and leg forms were placed and cast directly in the concrete of the base.

After the base cured for a few days, we set up the mold for the tabletop. The mold was filled with basaltic FRP mesh reinforcement and also #3 FRP rebar, for tensile reinforcement.  The rebar was located so that it aligned with the legs underneath, for strength. The entire mold was greased with Crisco, used as a mold release agent. A sheet of plastic was placed on top of the wooden form, to help the concrete release from the mold.

The mold was then filled with concrete, with particular attention to place some concrete under the rebar, to help provide proper cover.  The concrete was then screeded (spread evenly with a straight piece of wood, moved back & forth as it is drawn across the form).  This surface was then floated, or smoothed out by hand.  The edges of the form were all vibrated. In this case, we did not have a proper concrete vibrator, so we used a "sawzall" reciprocating saw, which worked pretty well.

Properly floating the surface is important to get a nice, smooth, flat finish.  It is worth spending some time and doing this properly.

The form was then covered and allowed to cure for a full week. It helps to cover the concrete with plastic, so that water remains in the curing concrete to form hydration products.

After one full week, the wooden forms were removed. We also did some landscaping, to create a level playing surface on the ground around the table; this involved a retaining wall being placed also.  This work was simply done with a pick, shovel and rake. It took an afternoon. 

Now, it just needs a net! I will also use a sealant to help protect the concrete from the weather, something like Thompson's Water Seal.  This will also make a great picnic table. I expect it should last a long time. We will also plant some grass on the fresh dirt.

This basic concept could be made much larger, to provide an elevated platform to build homes on. We could use my company's masonry arch system to accomplish this, easily and quite affordably.  This would be appropriate for coastal areas which are prone to storm surges and flooding from hurricanes and severe weather. It is stronger than the wooden posts currently used to elevate homes above a storm-surge plain, and will not rust or rot, like wood. It is also more elegant and looks much better than those wooden posts.

This table cost about $150 in concrete.  The rebar is also inexpensive. If anyone wants a concrete ping pong table and would like to borrow my molds, you are welcome to.  Just let me know.

This thing should be fun, I look forward to using it!

Tuesday, November 15, 2022

A recent video

Recently some dear friends of mine completed a lovely video they made about me, my work and my company.  This work was done by Burton Stein, his daughter Autumn Layne Stein, and Autumn's fiance, Matt Goodwin.  I am very grateful to them for this work. 

Here it is!  

Sunday, September 11, 2022

Experimental prototype one-car garage

 I decided to make a concrete driveway at my property in Alfred, NY. It gets very muddy here, so a concrete driveway will be very helpful. I need the driveway to be strong enough to accomodate heavy trucks. It is 8 inches thick, with #3 (3/8 inch diameter) basaltic fiber reinforced polymer rebar, arranged every foot on center. I'm using 4,500 psi concrete, and doing pattern stamping with a "European fan" pattern, finished with coloring and finally sealed. 

I realized that I had to build a garage at the end of this driveway, beginning at the end. In the past month or so I assembled this prototype, in a simultaneous attempt at several new experiments.  

The garage is 12 ft. wide, 20 ft. deep, and 8 ft. tall at opening. FRP rebar is located every 8 inches, in walls and arch.

Concrete block are anisotropic, which means that they are stronger in one direction than another. They are much stronger in compression in the axis of compaction during their manufacture. The strength increase is around 70% higher along the axis of compaction. Regular CMU walls have this high-strength axis oriented vertically, up-and-down. This means that the weak axis is oriented horizontally. In cases where high strength is desirable in the horizontal direction, such as tornadoes, hurricanes, extreme weather events, and especially waves or driven water, it could be advantageous to have the high-strength axis oriented horizontally.

Anisotropy in manufactured block occurs because aggregate aligns itself preferentially, normal to, or at 90 degrees to, the applied force during violent vibrational compaction of concrete mix in a mold to form the CMU. Here are some optical micrographs taken from a CMU, showing preferential orientation of aggregate.

The walls of this prototype building have the high-strength axis oriented horizontally. There are no hollow cores, it is solid 8 inches of reinforced concrete. Shown below is an improvized 'bond beam' for additional lateral strength; there are 2 rebar emedded in the mortar joint. Such a building should be capable to withstand tornadoes, hurricanes and wildfires.

The roof will have the block oriented radially, toward the outside, so that this high-strength axis feature is maximized. I am getting ready to build the masonry arch roof, which springs from cast reinforced skewback/bond beam at the top of the vertical wall. Inside the building at the top of the vertical walls are horizontal rebar crossing the span, for additional tensile reinforcement.

I will post updates as this project is completed. It's going pretty quickly, and I'm just working alone: moving block, mixing mud and tending myself. One block at a time.

Monday, March 21, 2022

Masonry acoustics, part deux

 I recently finished the second coat of paint on the living room of a building I'm currently completing. Since it is clean and empty, I thought this might be a good time to briefly discuss the room's acoustics.

My dear father left me a pair of Klipschorn speakers, something of a 'classic' pair of loudspeakers, designed and made by Paul Klipsch.  These are a pair of vintage speakers, renowned for their efficiency and warm sound.

Knowing that I wanted to place these speakers in this room, I consulted the Klipsch website to find out what the optimal design and proportions of the room should be. "Klipschorn speakers typically perform best when positioned in the corners on the long wall of a rectangular room. If the room is very narrow and long with corners farther apart than 18 to 20 feet, the stereo image may not be optimal. A room with a length to width ratio of 1.00 to .618 is preferred."  The same ratio is applied to the height of the wall, so that the ratio of the width of the room to its height is 1.00 to .618. These ratios resulted in the room having the dimensions of 30 ft. 8 in. long, by 19 ft. wide, by 11 ft. 8 in. tall.  The arched ceiling goes to a top center height of 16 ft.

The ratio of 1.00 to .618 may sound familiar to any mathematically inclined people.  This is the ratio provided by the Fibonacci sequence.  These ratios are ideally reflected in al three axes: x,y and z.  Thus the proportions of the room are those of a 'golden cube.'  These same ratios are found in the Greek Parthenon.  It is both visually appealing and acoustically beneficial.  Some say that these ratios help prevent constructive or destructive interference of certain frequencies; this idea seems to be disputed by others.

The only other suggestion offered by Klipsch is to include an arched ceiling, for more resonance and a fuller sound.  This room has an arched roof, it's the focus of my work.

The stereo sounds great, it really rocks out: whether it's Beethoven or Jimi Hendrix.

Saturday, February 26, 2022

Friday, January 21, 2022

Where is the most stress?

In designing and assembling a masonry building, the engineering work can provide helpful insight which is simple and powerful.  

For example, if we consider the masonry building I'm currently completing, it's insightful to ask: where is the highest stress in the building? Where is the highest compressive force, squeezing together?  Where is the highest tensile force, pulling apart?

The engineering for this building was done by Cheng-Ning Jong, PE.  He has some familiarity with my company's masonry system, since he helped compose, file and prosecute all of our patents.  We've worked together for several years and have a good rapport, a comfortable back-and-forth as we discuss, develop and fully articulate ideas.  

Mr. Jong's most critical role, in my opinion, is the detailing of the reinforcement and the size of the concrete footer from which the stem wall is laid.  A 'footer' is the base of the building, typically located in an excavated trench.  Here's a picture of the footer, with the first few block being arranged for the stem wall:

A stem wall is the bottom section of all the vertical walls buried below the ground, sitting on the footer.  Here is a completed stem wall for a room:

This building has arched masonry roofs, domes, half-domes, flying buttresses, arches meeting at intersections; there is a lot of structural configuration, rebar, weight, stress and so on within the building structure. Here are some architectural drawings, showing some of this detail.  Our architect for this building was Robert Ferry, AIA, RDP.

So if we consider this entire structure, where is the most stress?  Where is the highest compression?  Where is the highest tension?

The highest compression occurs at the bottom of the stem wall, where the stem wall meets the footer, on the outside of the building.  Why?  The entire weight of the building sits on this point.  In addition, the vertical wall acts as a giant lever, translating any thrusting force from the masonry roof and increasing this force by the length of the lever, or wall height to this location. This location, at the corner of the stem wall and the footer, wants to act as a hinge on which the lever of the vertical wall acts.  The highest tension occurs at the bottom of the stem wall, where the stem wall meets the footer, on the inside of the building.  The same lever action of the wall wants to pull up from the hinging on the outside, a mere 8 inches away: the wall thickness.

It's useful to note these areas of high stress.  It makes one pay closer attention to the detailing of rebar, rebar placement, connections, centering, etc., when you are consciously aware that the building you're making will have these high stress locations.  Build accordingly, get it right.