CEB Strength Testing: On-site
For those of you who have been following this blog for information on the process and progress of our CEB project, you know that quality control is paramount for safety in construction. Not only do we want the block machine to make very uniform blocks, they must also be strong.
The importance of being able to test the strength of the blocks before putting them in the wall is obvious. There are several different technologies for testing compression and the modulus of rupture. These two tests must be conducted on a sample from every hundred blocks. We have determined that we will randomly sample three blocks from every hundred, and two of them must pass both quality tests-- compression and modulus of rupture.
This blog discusses how we have determined compression testing on-site. You can see the standard shop H frame jack or hydraulic cylinder in the picture below. We fabricated a strong 5/8 inch steel plate and inserted the tip of the hydraulic jack into it. Beneath the block is another heavy steel plate so that there is no bending or distortion that can cause failure before the block actually collapses under uniform pressure.
The complete 10 ton, hand operated hydraulic press with a reinforced press head
Once the block is inserted, then a hand pump allows very controlled l pressure to be applied to the block while watching the gauge which is calibrated in tons of pressure. The question arises as to how to translate the pounds per square inch on the block ( compression) from the pressure gauge. The following is a brief summary of the formula for converting tons of pressure on the cylinder tube to pounds per square inch(psi) on the block.
1. The formula is: pi x r² x psi (assuming r is the radius of the piston in inches). This gives total force applied hydraulically.
2. This force, divided by the surface area of the plate pushing on the block, gives the psi on the block face (assuming the plate and the block face are the same dimensions). Eg. 2" diameter cylinder at 2,000 psi and 63 in² plate: 3.14 x 1 x 2000 / 63 = 6280/63 = 99.6 rounded to 100 psi on the block.
3. Using 100 psi per ton of piston pressure, it is easy to extrapolate to any higher pressure.
For example: in the lower picture you can see a failed block with pressure reading on the gauge of 5 tons. The block failed to withstand 500 pounds per square inch of pressure. This block was completely saturated with water, and New Mexico and California standards require that CEB blocks must achieve 300psi under those conditions. This block had only 20% clay and 70% silt and 10% cement, but it still passed code requirements. We would not use it, however, since the next one off the press beside it failed to reach 1000 psi dry.
This block failed at five tons or 500 psi.
We have 10 stacks of 100 blocks with 10 different formulas to test before we select the one. Optimum formula makes uniform blocks that consistently crush at 1000 PSI when they are cured for 30 days.
This same block testing H frame unit can also be fitted to test modulus of rupture, and when we do that after we have chosen the ideal mixture of clay sand cement and silt, we will report on that procedure.
It is exciting and fun to make blocks. However, without a standard for testing uniform compression, all that work and expense may result only in tragedy, if a wall fails. Why take a chance? We won't. This technology is inexpensive and reliable for single family projects of a few thousand blocks.