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.
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 30cm x 30cm x 30cm. 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.
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 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”