I’ve had several concrete projects done at my house and my in-laws over the past 18 months. There is also a lot of construction going on in my neighborhood so I often walk around in the morning (cooler) hours and make usee .. what we used to call being a sidewalk superintendent in the US.
I’ve heard a lot of foreigners complain about the quality of concrete work here in the past, and common theme of "those Filipino workers aren’t putting in enough cement" or words to that effect.
Frankly, there is quite a bit of poor concrete work that gets done here, but, from what I have observed, seldom is it caused by too little cement. Many workers I have watched and talked to don’t really know much about concrete and how it works …so I thought I’d pass on a few tips so you can supervise your own small jobs and have them come out better.
Concrete is one of the world’s oldest and most durable building materials … it was already a material of choice when the Romans built the Colosseum from it,and the rules haven’t changed much. It pays to know a little bit about how it works and what needs to be done to make it last thousands of years as the Romans knew.
Concrete is a blend of three or four major components:
- Cement. Technically called Portland cement because of the process used, this is a mix of limestone and clay, heated in a kiln and then ground to powder. Use a name brand product and keep it dry until use, you should be ok.
- Sand. In the US we specify ‘washed builder’s sand’. here most people take what they get. This is the number one problem I’ve seen. The sand must be free of organic matter and free of silt of fine clay. (here’s a good site about how to test sand for suitability). If the sand is full of fine powder the concrete will not be good, no matter how much cement goes in the mix. I also see workers using dry sand that has been out in the sun for days. Sand should be damp enough to form a lump in your hand when squeezed before you start mixing … if it isn’t, you’re wasting your time in the long run.
- Aggregate. Crushed gravel or stone. You must use aggregate in all but the thinnest layers of concrete. I see whole houses being built with sand and cement only … these will never attain full strength. As with sand, the aggregate must be free of powdery silt, if necessary it must be washed.
- Water. Perhaps the most important and least understood fact is that water for mixing concrete must be clean. You can’t use water from smelly old tanks and drums ..especially if there is any petroleum or food residue. The water you use for your concrete must be as pure as possible … drinking water quality. If it is not, all the rest of the materials and the labor are wasted. In my experience you’ll have trouble with Filipino workers on this issue at times. Be nice, be open to suggestions, but use clean water … it’s your job and you are paying for it.
How much water? This is much more important than how much cement. Both too much or too little can ruin concrete. best to use what is called a ‘slump" test. see a good technical discussion here.
Last thing to remember is .. concrete doesn’t dry, it ‘sets’ which is a simplified term to describe complex chemical reaction (concrete will set very nicely underwater, by the way, where it technically never ‘dries’. I see job sites frequently where fresh concrete is baking in the tropical sun. It will dry before it sets properly and thus never attain it’s proper strength.
Great stuff this concrete,though, when you learn some simple rules and do it right .. it will last many lifetimes and provide a great return on investment.