Building our house in the Philippines. Ensuring concrete quality. This essay is about our experiences in the provinces. The easy availability of better material in cities (clean, graded, crushed stone, washed sand and even air-entrained “Ready Mix” concrete is a whole different context and most of our comments don’t apply.
A ponke in action
The above photo shows the workers adding material to the cement mixer using a “ponke”. The ponke is a wooden box with handles. The inside dimensions of the ponke are 40cm x 40cm x 40cm. The ponke is sized to hold one 40 kilo sack of cement. I asked that the ponkes be built and used as a means of controlling the concrete mixture. The use of the ponkes makes it easy to get the mixture right. Ponkes are rarely used in the Philippines now, but formerly were the norm. Now materials are more commonly measured using empty cement sacks refilled with sand or gravel. Never use shovels to measure.
We wanted strong concrete and a strong house. After research, we decided on a very strong mix of one part cement, two parts sand and three parts gravel – a 1-2-3 mix, except for floors where we used 1-2-4. This also led us to buy a gas powered one-bagger cement mixer, a gas powered internal concrete vibrator and to stick with the 1-2-3 concrete mix. “One-bagger” means that the mixer is capable of accepting one 40 kilo (88 pound) sack. As a person with neck problems, it makes me wince to see Filipino workers carrying two 40 kilo sacks on their heads when we get a cement delivery! More on the mixer and vibrator at /building-our-house-getting-started/
Basic Chart of Concrete Mixes (link)
Cement : Sand : Gravel 1 : 2 : 5 for grade C15 (general purpose concrete) = 2175 psi = 152 kg/m2; gen. 2000 psi 1 : 2 : 4 for grade C25 (strong) = 3625 psi = 255 kg/m2; generally 4000 psi 1 : 2 : 3 for grade C30 (very strong) = 4351 psi = 305 kg/m2;generally 4500 psi
We’ve had a number of comments that our 1-2-3 C30 mix is wasteful overkill for routine residential construction — that a C20 or C25 mix would be more than adequate. That would be true if you assumed that the proper materials and conditions to make C30 concrete were feasible; if we had clean, sorted crushed stone aggregate, if we had good clean washed sand, if the mixes were well mixed and contained the right amount of water and if in hot weather the newly poured concrete was kept wet for a few days. If these things were true, then indeed the 1-2-3 mix is overkill. It’s our experience (admittedly limited) that these conditions rarely apply in the provinces. You may aim for C30 and end up with C15 or C20. If you aim for C15 you may end up with who knows what.
We have been amazed to see how easy it is to demolish concrete buildings in the Philippines. Partly it’s the soft hollow block but it’s also the weak concrete.
The concrete in the typical rural house may be 1-2-6 or even weaker. The gravel is not crushed. It’s from the nearest river bed. There might be salt contamination, depending where the gravel is dredged from. The gravel will not be clean. The sand will not be washed. The mix is soupy in the extreme — no slump. Every desirable physical property of concrete that you can measure is adversely effected by adding more water. This mix is used with weak hollow block and as much rebar as the owner or builder cares or can afford to put in. What is the PSI? I shudder to think.
The house you buy an already built house it probably won’t have such strong concrete. 1-3-5 is in common use. I have seen deliveries of substandard reinforcing bar. In fact, when you order rebar you have to specify standard or you may well get substandard automatically. A poorly built house may be built with a “class B” or “class C” concrete mix and not enough rebar. You’ll never know what’s in your house unless you build it yourself. It might never matter, but here’s a photo of the church in nearby Oton, Iloilo which was destroyed in the 1948 Panay Island earthquake.
Magnificent Oton Church, destroyed by 1948 earthquake. About 8KM from our building site.
The plan of our house was designed by a structural engineer. We tried to be quite strict in following the plans. Sometimes good Filipino builders use traditional rules of thumb not based on engineering basics. This can mean too much steel in places which really don’t need it and not enough in places that do.
At least in the provinces, the ambition to have quality concrete for your project can lead to frustration. My crew were hard workers, but accustomed to their ways of concrete construction. The local gravel contains so much sand that the mix probably ends up being 1-3-2 (one cement, three sand and two gravel). The workers like this sandy gravel because it’s easy to work with, flows easily into forms. Using screened, washed, crushed gravel or stone makes stronger concrete. Make sure that the largest aggregate you use will easily flow into forms and around and through the rebar. A large stone can catch in the rebar and cause a void. See our post on columns and beams for more information.
Trying to keep Philippine workers from adding too much water to concrete is a legendary problem. They like to make soupy concrete because it flows easily and does not set-up quickly, it remains “workable” for a much longer time than proper concrete. If it does start to set up, they add more water and re mix. We have read that some on-site Filipino supervising engineers have been so frustrated with this problem that they just disappeared from the project site rather than fight with the workers over this issue. I now understand their problem.
We used a gas powered concrete vibrator
We pushed so hard that it has caused real friction with my crew. I would not allow any concrete to be mixed unless I was there to make sure it was not too wet and that it stayed in the mixer for at least two full minutes. That seems like a short time, but the workers were so anxious that they would dump it early if I was not there with my watch. I would not allow the addition of more water “re-tempering” the concrete. If the mix was too soupy or was setting up and would have them dump the batch. They were truly horrified by that. By the end of the project the crew knew exactly what we expected, but I’m sure if we were not there to supervise, they’ed do it their way.
So, here are our suggestions:
- Buy a good mixer. It will cost about P60,000 but you can sell it when you’re done and recoup most of your money.
- Buy clean washed sand and crushed stone, at least for beams and columns.
- Buy a internal vibrator. Research how to use it (link below) and then teach your crew. It will cost about P20,000 but you can sell it when you’re done. We had no trouble selling our mixer and vibrator.
- Unless you have a trusted foreman, be there whenever concrete work is done.
- Make sure the fresh concrete stays wet 24/7 for a few days. The longer the better. Rice straw makes a good covering.
Download THIS PDF “Concrete Basics” from the Australian Concrete organization. It’s an excellent overview and includes a section on using concrete vibrators that we wish we had seen before rather than after construction of our house.
THIS is a more technical but very useful PDF “Designing and Proportioning Normal Concrete Mixtures”
Happy building!
Related posts:
September 22, 2011 at 1:44 am
A useful and simple method to test the quality of freshly made is the concrete slump test. This basically uses a 300mm conical cone which is filled with concrete, the cone is then removed and the amount of slump in the concrete is measured. There should be an even slump of 50-90mm for normal reinforced concrete placed by a vibrator. An uneven slump or one greater than 90mm should result in the concrete being rejected.
This can also be used to measure the variability between batches. There are video clips on youtube demonstrating the procedure.
August 7, 2011 at 5:36 pm
Hi Bob.
Thanks again for the many and very detailed information’s about building in Philippines.
Until 1986 it was common used to mix salt, (CaCL2) into concrete, as it was a very effective accelerator for the concrete hardening process. Unfortunately, its also very corroding on the reinforcement, and in worst case hygroscopical, so that the concrete is always wet, or moist.
BUT, salt in concrete makes the concrete stronger. !!!
If you want to know if there is salt in the sand or stones, then mix a small amount in water, let it settle and taste the water. small amount of salt is no harm if the concrete is dry and thereby preventing corrosion on the reinforcement.
The whole issue of mix and strength, is a matter of lots of discussions and thoughts. Basically it is about having a certain sorting of gravel, and then secure than all cavities is filled with sand, and that the surfaces of the sand and the stone is covered and bound by the cement. The strength is then according to how much “glue” – cement there is in the mix.
If the gravel contains sand, then measure 1 liter of gravel, wash out the sand, measure that and compensate for the sand in the gravel. But in the end, the perfect concrete contains only the needed amount of sand, but in practice, the amount of sand is so, that there is an overweight of sand, to assure that there is no “stone nest” in the moldings. Or if its a floor, that its possible to make a smooth and not stony surface.
The amount of cement, and thereby the strength, is more complex. Cement is the glue, and must cover all surfaces of the materials in use. so if the stones are small, the surface is bigger, if the sand is very small, again the amount of cement must be higher, that is why the sorting of sand and gravel must be within some defined limits to secure the strength of the concrete.
If the river sand contains high amounts of clay or very fine grained sand, it will need more cement. take a handful of sand, place it in 1 liter of water, and stir it.
Let the sand sink and see what is still swimming in the water. if there is a lot of clay, or even soil or topsoil, then decide to use more cement.
But all that compensating for material quality is all plus minus half an arm, and the end result is not predictable.
Its better to stick to washed materials and then use the proper mix and strength for each building structure, where 25 MP is fully okay for outdoor load and force taking structure, and 20 MP is good for all other structures, such as floors and runways. 15 MP is fine for hollow block filling, as this material in no ways can make the hollow block structure stronger than the blocks.
HOw about the mix.:
1:2:3 is based on weight but is it recalculated to volumen. ??
SO. – 40 kg of cement is 80 kg of sand and 120 kg stone.
But how many liter is this. ?? Cement is packed in many ways, and the volume
varies a lot from manufacturer to manufacturer.
Typical density here would be.
Standard Portland cement. 1 ton = 0,9 CBM
Sand moist 0 – 0,25 mm 1 ton = 0,6 CBM
Gravel 4 – 8 mm 1 ton = 0,7 CBM
Gravel 8 – 16 mm 1 ton = 0,6 CBM
The amount of finished concrete is about 60% of the total of materiel
included in the mix, or in the easy way, the same as the amount of stones.
With regards
John
August 9, 2011 at 5:15 pm
John,
As always, thanks for your excellent comments. Yes, we measured the ingredients for concrete by volume, not by weight. You mentioned that various problems (dirt, clay) can be adjusted for by adding cement. Hopefully, in choosing the 1-2-3 mix we may have addressed the various deficiencies in our sand and gravel and maybe ended up with an adequate concrete. When we had to demolish some work already done to make up for engineering mistakes the crew found the demolition work to be quite difficult, so maybe there’s hope that the mix we used is sufficient.
When we started the project, we knew next to nothing about concrete construction except too much water and too little cement are bad. We have learned a lot and try to share with others through this site.
John, thanks again for you always terrific contributions!
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March 25, 2011 at 3:06 am
Well said my man. I have built a house already in Ph. Now one on the beach. I am thinking of corrigated tin but my Filipina wife shakes her head and thinks I’m crazy. It’s really hard to think out side the box in the Ph. I watched the program called “This Old House”. The guy was flaming a house built in 1778 on a rubble foundation. He yakked about the need to put in a 20 inch wide 48 inch deep foundation on a 24 x 24 footer. I thought to myself, “What a knucklhead, there he goes replacing a foundation that has proven itself already for over two-hundred years for one that is a recent invention by man less than one-hundred years old. like I care if it lasts 50 more years, I’ll be dead and gone and don’t care what its doing then…Daaaaaa