Could you recommend a mix for us to use based on your experience in Indonesia? We are
currently carrying out soil tests to confirm clay content.
The way I like to develop a mix is to press blocks. I first press blocks with soils that interest me; ones that don't have too much clay. I try different ratios of soil and sand. I evaluate the mix based on the characteristics of the blocks:
After I have a mix I like, I try adding cement. I try about 5 mixes on the same day with ratios from 1:4 to 1:12. I like to press 10 or 15 blocks from each mix. That way I can compression test 5 blocks at 7 days and 28 days and at another age if I like. The other things I'd like to check for in a mix are the block's expansion characteristics.
We have had the soil tests back from the site on Flores, please see them
below. Our contact there is due to carry out further tests on samples
from a 1 metre deep bore hole. It sounds like the soil has a high clay
content. From my research I understand that clay content is a good
thing, however, from your comments I worry that the content here may be
too high? Is there a remedy for this - perhaps more sand added to the
mix to even things out? Will this affect the amount of cement we
I have also been reading a report to suggest lime works well as a
stabilizer within clayey soil - in place of cement. Would you recommend
Soil Test Results:
The Wash Test
When soil was in the hand and water was poured over it, the soil was
easily rolled in to a sausage and the hand felt smooth.
When the hand was washed it was difficult to get the soil off and quite
sticky to the touch, it left colour on the hand - white milky colour
In addition to this Marcell's brother put the rolled soil sample in to a
cut down water bottle and shook it, the rolled sample stayed in its
shape with only some of the colour (white, milk) from the soil
discolouring the water.
The Jar Test
A total of 12 cm was tested, the result showed very little difference in
the sample, through the water bottle (no jars available), so Marcells
brother cut the bottle to try to look at the cross section, when the
bottle was cut the sample divided with 7cm at the bottom and 5 at the
top. Both were white milky colour, no more detail available at the
The wash test
The results were the same as on the Clinic Land, but the sample colour
was very different with a much darker colour, (more a chocolate colour)
The Jar Test
A total of 10 cm was tested, the result showed a change in colour, with
the bottom 4 cm black, dark brown colour and the top 6cm more of a
You are making some progress. Kudos to
A little clay is good; it allows for handling
"green" bricks fresh out of the press. A lot of clay
is bad; it will make the blocks and walls expand
and contract. Yes, you can adjust the clay
content with sand. In your case, if both samples
were difficult to wash off the hand, you have a
high clay content. If that is all that is available
locally, you can try some blocks with high sand
content. Remember that the sedimentation test
should start with dry soil. Putting the rolled cigar
all stuck together in the bottle to start with is not
Yes, you can use lime instead of cement. I
can't remember if it is necessarily better; I think
that you could be correct though. It is more
environmentally friendly. It can also be a bit more
dangerous to work with. You need to have some
knowledge of lime; there are basically 2 types
slaked and unslaked. Check to see if there is a
tradition of using lime on Flores. If there is not,
learn how to use it first, develop a program for
introducing it, then introduce it. I have not used
lime as a stabilizer. In China they use lime a lot
but in the form of lime putty. On Negros,
Philippines, they found a lime soil that worked
I like to follow the mix development that I
described to you: develop the soil/sand mix you
like, then do several ratios of cement (lime), then
do compression test.
You can also do some shrinkage tests.
There are some standard tests you can look up.
For our blocks I am considering writing up a new
test. It will be something like this: Take 5 dry
(not soaked) cured blocks, fill up the large holes
with a concrete mixture (the same one used for
RC with 6 mm aggregate). Let the concrete
harden for 3 days. Soak the blocks. I haven't
tried this test yet but I would hope that the blocks
would not crack. I have had an experience where
blocks cracked in a wall at the place where they
Keep up the good thinking, Geoffrey
We will take your advice and use ratios of soil to cement ranging from 1:4 to 1:12, however, I would be grateful if you could provide some guidance on initial ratios for soil/sand. In a later post you suggest that we use a high sand content - what ratio would you recommend? Is this something that we could also vary during the mix tests? I remember suggesting equal parts of soil/sand to you and you remarking that you would prefer to use a higher soil content - as would we seeing as sand will need to be purchased, whereas soil will come from/around the site. Sophie
Dear Sophie, I would suggest initial soil : sand ratio trials could be: 8 : 1 6 : 1 4 : 1 2 : 1 for a high clay soil the following might be possible to try the following. The soil (clay) must be pulverized very well. If not, lumps of clay will crack the blocks or wall. Some testing of block shrinkage and expansion would be good. 1 :1 1 : 2 1 : 4 1 :6 A high sand content will wear the steel box more rapidly. Still, it can make a strong block if that is what is needed. Geoffrey
I have read the information regarding recommended cement content of the blocks on your forum. You suggest a ratio of 1:8 which I calculate as 11%. I have researched the cement content of a standard concrete block as ranging from 6-20% depending on the required strength. Based on the information provided by the soil tests and the confirmed presence of clay, can you estimate the percentage/ratio of cement that we will require.
I understand the necessity for a robust block, particularly in this earthquake prone area of the world, however, I am also keen that this method, due to the introduction of the press as an improved block production method, uses less cement than the current method of construction. This is really down to two reasons - you will agree cement is an unsustainable resource that we should use sparingly, and the project is a charity project - therefore we must be committed to spending funds carefully, cement is an expensive commodity.
Dear Sophie, Well, I'd still approach the mix design as I outlined before: - try different ratios of soil/sand to get the 5 characteristics to be acceptable to me: mixability, compressability, ejectability, color and texture - then do 3 or 4 trials of different cement/aggregate (soil + sand) ratios. Cure and then test the blocks. If you need to do a cost estimate; do it as a high / low estimate.
It is environmentally friendly to use less cement. The embodied energy of cement base products is high. Cement has a high contribution to global warming. You can use less cement if you make the walls thicker. You can use close to no cement if you white wash the walls. To find out for sure you need to check with a civil or structural engineer.
Bamboo reinforcement for seismic resistance:
I am keen to explore the possibility of using bamboo to provide the structure with seismic resistance. Is this also your reasoning behind using split bamboo as rebar (rebam) on your projects in Thailand or is there another reason you suggest this technology? I understand that bamboo must be kept away from wet conditions to prevent its strength deteriorating. Is this a problem you have encountered by using bamboo alongside concrete?
We used bamboo reinforcement here in Thailand because we don't have earthquakes. In Indonesia we have not yet used it. The US Navy did a study on it in 1966 (http://dspace.uta.edu/bitstream/handle/10106/210/umi-uta-1098.pdf;jsess…). Moisture is probably not so much a problem. The thing that concerns most people is the fear that the alkali in the cement will attack the plant fibres. We haven't see a problem with this. I like rebam. It is nice to have your interest. The really big advantage to rebam on the island of Flores is that the bamboo doesn't rust. Rust is a big problem with rebar in wet and salty regions. If the concrete is to porus or the cover to thin then the rebar will soon start to rust. Iron oxide takes up more space than just Iron molecules. So, the rust cracks the concrete. I see this a lot in Asia. If you want a picture let me know.
With regard to the split bamboo you use as rebam do you treat this in any way against the alkali in the cement?
Dear Sophie, We have not treated the bamboo in any way. We have used coconut husk fibres in concrete roof tiles. If alkalai attack is a problem with bamboo then we should have seen something similar with the coconut husk fibers. We did a test on them. After 20 years we have not seen them deteriorate. I just dug some out of our store room. The fibres are intact. I don't think that there is any danger of bamboo being attacked by alkalai in cement. Now that I think of it, you can always specify the Pozolan cement that is common in Indonesia - it should be non-alkalai.