I do have another question that is on my mind.
The compression test is done after five days of curing, and two days
soaking. Obviously the blocks would cure more over a longer period of
time. As we move toward a mostly sand/cement block, I am concerned
that the compression test designed for earth blocks is not really
appropriate. I think the sand/cement block will harden more with time
because of the higher cement content, than the standard earth block.
A couple of months ago Vaughn and I tested a couple of our blocks that
were almost a year old. We soaked them in water for two days and they
tested out around 45kg/cm2. My guess is that our sand/cement blocks
will also gain significant strength with time.
Have you noticed a difference in block strength depending on heat
during the curing process? I am wondering if heat would hurt the
cement curing process, or help it? I read somewhere that black
plastic cover in the sun significantly raises the temp, and helps
strengthen the blocks. Have you seen a difference with your blocks?
Good Observation Andy. The standard civil engineering strength test for concrete is done at 28 days. We test our blocks at 7 days because: 1. we don't want to produce for 4 weeks before we know we have a problem, and 2. We want to be able to sell the blocks after 7 days. I don't think testing after 28 days is needed.
Your second observation has a lot behind it. First, the only international standard that we have right now has 3 strength categories: 1MPa, 2MPa, and 3MPa (or roughly 10ksc, 20ksc and 30 kilograms per square centimeter); generally appropriate for buildings of 1,2 and 3 stories. These are appropriate strengths for blocks used in load bearing walls. In my opinion, making blocks stronger than this is not necessary and a waste of cement. Testing at 7 days and 28 days to get 20ksc and 30ksc strength respectively would be appropriate. Reducing cement is important for taking care of the earth. Cement is very energy intensive and contributes greatly to CO2 emmisions and the green house effect. Using too much cement is wasteful of resources and degrading to the environment. It will also make our building material more costly and less competitive. Have we tested sand/soil/cement blocks with a leaner cement content?
About curing: Please read this article:
Solar High Humidity Curing Tarp
Regarding BBG curing, how much water is optimal for curing? If the tarp
has water droplets on the inside is that indication that the humidity is
high and enough for curing? My guys turn the water on and let it run
until it's running out the bottom.
Good question. First of all you are on the right track: don't let the concrete dry out; it needs water to get strong. Using the high-humidity curing technology is good. Yes, seeing condensation on the tarp is a good indicator that you have enough humidity. I can't comment on your last water-filling observation; are you using a field chamber or a purpose-specific curing chamber with a sealed trough below to keep the water from leaking?
Regarding stacking the blocks for curing, what is the optimal height for
stacking these nice, newly pressed blocks?
That's an interesting question. Standard production and curing procedure is to press blocks and pour water on them after lunch and before leaving at the end of the day. The blocks are all on individual plates. We want the blocks to take an initial set so they don't erode when we pour water on them. The following morning we move them to a solar high-humidity curing chamber. At the CVBT we stack the blocks 6 layers high because that is the height that our 3.6 meter wide tarp will accomodate. We get no breakage of the bottom blocks. Occasionaly we have a bottom layer block that breaks in our stock area because of an uneveness in the floor. In the stock areas, we stack them as high as is comfortable for the workers; about 1.5 meters.