The Good, the Bad and the Concrete block

What is a concrete block?

The concrete block is a special concrete element. It is capable to hold the fresh, plastic concrete, until it luckily gets hard. In general, this is a kind of concrete permanent shuttering, that doesn’t need shuttering plate. It’s a bit like we cut the big shuttering plates into small pieces, so we simple mortal people can build tall and long concrete walls without difficult shuttering. Writing this is quite simple, but in practice, the situation is not that plain: a normal monolithic concrete (poured in place concrete) is always stronger than a concrete made with concrete blocks. This is why industrial builders (not house builders) don’t use concrete blocks. They use only monolithic concrete with Doka or Peri shuttering.

At this point, we get into the picture: we don’t need brutal solid concretes for a house building. But it comes handy, if Johnny is able to build a 30 (12″) cm thick concrete block wall with his boozer head and two dirty hands. So in general: this is a concrete wall building block, with You can build easily, and it gives slightly weaker wall or concrete base course. Lucky for us, that house builders don’t need atomic bomb flash resistant concrete.

Possibility of use

• 

  basement foundations, walls

•  fences
• base course
• retaining walls
• drainage shafts
• insulation protecting walls
• floral blocks, furniture for outside, garden bench
• etc…

What is the concrete block made of?

Concrete, of course. Concrete, that is not made for eternity. I’ve got a feeling, that it’s made of C10 or C12 concrete, because it brakes like sponge fingers. If You knock it with a hammer, it brakes to pieces. The grain size of the sand used to make concrete blocks is quite fine. It looks 0-4 mm for me. With this fine sand, the manufacturer is capable of making accurate, right size products (smaller the grain size of the sand makes accurate the blocks).

With this accurate, but weak concrete combination the manufactures reach easily shape able an low price products. It’s normal to ask: if the blocks are made of weak concrete, will we loose strength on the complete wall? The answer is no, because we don’t calculate with the strength of the concrete blocks (jacket force), only with the reinforced concrete inside it. If the concrete got solid in the blocks, You could peel the shell (don’t do it), the wall should stand still. In general: the concrete block only works until the concrete inside binds. After that, it’s unnecessary like cola in red wine…

Size, shape

For example, a 30 (12″) cm type concrete block is 30 cm wide and 50 (20″) cm long. The height depends on the manufacturer: there are 20 cm, 23 cm and 25 cm types. Unfortunately there are manufacturers, who change the 50 cm length as well.

I will show You the general, 30 cm (12″) wide, 50 cm (20″) long, 23 cm (9,4″) tall concrete block for You. If You order one pallet, there will be 40 pieces of 30 cm concrete blocks on it (sometimes it varies). There are 8 pc. in one row, and there are 5 rows. These rows are held in place by a plastic wire. You can tear these wires by hand, but they hold the pallet very well. The pallets are made of some kind of cheap curved wood, You can’t build furniture of these… The weight of one pallet is about 1 ton, don’t put it on a weak trailer! The size of the pallet is: 1 m wide, 1,2 m long, 1,3 m tall.

If You buy a pallet of 30 x 50x 23 concrete blocks, You will find 3 types of blocks on it (it comes like this without asking)

Normal, 30 cm wide concrete block

There are 30 pieces of this type on one pallet. We use these type of blocks on the straight sections of the wall/fence nicely put in vertical line, and in bonds. You will use this kind in the most of the time.

As You can see it on the picture, it has 4 sides, and slots on the top of the inner walls. The rebars will perfectly fit in these hollows. By bonding, I mean You have to be careful, that the vertical joints are not meeting on top of each other. Just like with the brick bonding. With the concrete blocks, You should have half shift bonding. The blocks can be cut with diamond cutter blade.

The walls of the block are 3,5 cm (1,4″) thick. The walls on the long sides extends over by 4,5 cm (1,8″). It’s unnecessary, but I mention it: You can make 30 cm thick walls with this type of concrete block. From this 30 cm, only the inner concrete works. This way, our supporting structure is not 30 cm, only 23 cm thick. (30 cm – 3,5 cm – 3,5 cm). If you put a 30 cm thick brick on it, the sides will not be supported by the concrete. Luckily, there is a reinforced concrete slab between the brick and the concrete base course. This slab will divide the forces, so there will be no problem.

The weight of one concrete block is 24 kg (52 lb), witch is freaking heavy.

The block has literally no thermal insulation. The U value is 2,71 W/m2K. A brick wall, with the same 30 cm thickness has U=0,83 W/m2K. The brick is 3 times better than the blocks, and You have to insulate even the brick, to reach a normal U value (U= 0,15 W/m2K is acceptable).

Corner or ending block

You’ll find 5 pieces of these on one pallet. We usually use this type for constructing T junctions, and wall endings. It has a double dividing wall in the middle, You should cut it into two pieces at this point. Practically, I never divided them there… Still, if You cut it there, You can start the half bonding with it.

The corner has a little flaw as well: at the corners, You have to put the ending blocks 90 degree rotated to each other, so there will be normal bonding. It’s all nice and good, but the concrete will be totally divided vertically. I believe it’s not too good, so I cut the side walls. This way, the corners will also have continuous concrete inside. The corner concrete blocks have no hollows on the side for the rebars (L shaped ones will be needed here), so You will have to carve these in situ. The walls thickness of the ending blocks are the same as the normal block, 3,5 cm (1,4″), and there is a double divider in the inner hole.

The ending block is absolutely freaking heavy to lift. It weights 32 kg/pc. (70,5 lb). And of course, mostly You will moon about these pieces. There is a good chance that it will tear down your kidneys. The ending block can be used as a normal block, but always check the continuity of the inner concrete!

Cutter ended blocks

You’ll find 5 pc. of these types on one pallet. These blocks looks the same as the normal blocks, but on one end, the walls are weaker, so it can be cut easily. This means, that there are two vertical grooves on the wings on one side. This kind of concrete block can be used as a corner block, if you cut the wings of. As a normal block, it is good as it is. It weights 24 kg/pc (52lb).

Other types of concrete blocks (with prices)

When we say 30’s concrete block, we usually  think about a 30 cm wide type. This 30 cm will give the wall thickness. Of course, there are other types as well (gross prices, with no carriage):

• 10 cm x 50 cm x 25 cm – for barrier walls – 1,2 $/pc
• 12 cm x 50 cm x 25 cm – also for barrier walls, but rare as a white crow – 1,3 $/pc
• 15 cm x 50 cm x 25 cm – 1,17 $/pc
• 20 cm x 50 cm x 25 cm – 1,31 $/pc
• 25 cm x 50 cm x 25 cm – 1,5 $/pc
• 30 cm x 50 cm x 25 cm – 1,7 $/pc – great for base course
• 40 cm x 50 cm x 25 cm – 2,1 $/pc – great for base course

And some other cleverness according to the sizes:

• blocks under 10 cm thickness are not produced, this is the thinnest size. You can barely put concrete in these as well.
• the manufacturers usually make different height blocks as well. There are 20 cm, 23 cm, and 25 cm height type concrete blocks.
• with the different height type concrete blocks we can shoot ourselves in the leg, or fine tune the construction. If we plan right, and choose the correct blocks, we will be able to build our wall or foundation perfectly. But we are dead man if we mix up the sizes when ordering… This is a double sided weapon.

Calculation of the needed materials for the concrete blocks

For a perfectly constructed concrete 30 cm thick block wall, you will need many things: the blocks, some rebars, and concrete. If any of these are missing, you’ll face huge problems.

For 1 m2 wall, you need:

• 8,7 pc of concrete blocks
• d=6 mm steel rebar. 2 strands horizontally for each block story, and also vertically continuously in the holes. You’ll need 12 m rebar for 1 m2 wall.
• 215 l concrete, that is 0,215 m3 of C16 concrete.

So if you need for example 80 m2 concrete wall block (30 cm thick), than you have to calculate like this:

• 80 m2*8,7 pc= 696 pc blocks -> 696 pc/40 pc= 17,4 pallets ->you can order 18 pallets, because you will loose, break, cut, eat some of the blocks
• 80 m2*12 m= 960 m d=6 mm iron rebars -> 960m/6m= 160 strand of rebar. Order 170 strands, to be sure.
• 80 m2*0,215 m3=17,2 m3 C16 concrete -> it can be 18 m3, some of it will be lost, and some will be stuck in the concrete pump.

Concrete block walls are always built on some kind of foundations, mainly strip foundations.

Prices

One 30 cm thick concrete block costs about 1,4-1,8 $. But this is only the price of the block. You’ll have to add packing, craning, transportation, pallet fees. And of course, the bigger the block, the higher the price is.

If you receive the tenders from the traders, keep a look out for these added items, they can set the prices…

I’ll try to detail the prices:

• 1 pc of concrete block costs about 1,4-,1,8 $. I’ll go on with 1,6 $. -> 1 pallet has 40 pc on it. 40 pc * 1,6 $= 64 $/pallet. The same for 1 m2: 8,7 pc * 1,6 $= 14 $/m2
• crane price for 1 pallet: about 6,5 $/pallet -> for this price, the truck uses the crane attached to it, and puts the pallet down somewhere in the construction yard.
• transportation fee: about 1,7 $/km -> calculate with full truck! For one pallet, a big truck is just too expensive. Ask for unique prices for this item.
• packaging price: sometimes the trader calculates with it, sometimes not. The trader decides.

If I have to calculate the price, or I have to compare the prices, I add every item on the list, and divide it by the numbers of the blocks. This way from the starting 1,6 $/block rises up to the stunning 2,5 $/block. This means that the transportation takes away 1/3 of the price. Painful is it?

And we are still not ready:

• we also have to pay for the concrete inside the block: for 1 pc of block (C16 – 67 $/m3) 1,6 $ will be the cost for concrete. For 1 m2 it’s 14 $.
• the price for the steel rebars are: 1 pc of block with two lines of d= 6mm rebar costs 0,5 $. For 1 m2 it’s 4 $.
• and the wages: building the wall with reinforced concrete costs about 35 $/m2. This is only the labor! The cost for the building material is not included! Asking for prices is obligatory!

Let’s calculate the final price for 1 m2 of 30 cm thick concrete block wall!

• 8,7 pc. of concrete block transported, packed, craned to place: 8,7 pc * 2,5 $= 21,8 $
• price of the concrete transported, without the pump, C16 : 14 $
• cost for steel rebars: 2 lines of vertical and horizontal steel: 4 $
• price of the wages: 35 $

All added together: 21,8 $ + 14 $ + 4 $+ 35 $= 74,8 $/m2 , if we mandate a professional stonemason with the work. If we do it ourselves, it costs 40 $./m2

Fence from concrete blocks

This is an interesting way to use the blocks: You can build a perfect fence with these blocks. We usually calculate with a few rows of blocks, and that’s it. Unfortunately things are not this easy. The blocks are heavy as mud. And of course we put the even heavier concrete in it. Because all of this, we have to make a fair strip foundation under it. If we forget to make this foundation, the fence will slowly lean into the garden, or on to the car in the street (hopefully not our car). To avoid this minor problem, you have to build a normal concrete block wall with a deep, reinforced (rebar) strip foundation:

• put a strip foundation under the fence. You’ll have to reinforce the foundation with iron rebars, and fill it with at least C12 concrete. Don’t forget the compression of the concrete (vibrate)!
• the depth of the strip foundation is at least 80 cm, and that is the frost-line.
• usually, we use 3 rows of blocks on the base course. From this base, the columns go higher.
• we build columns every 2-2,5 m. These columns will hold the lath fence (or something for the filling).
• I wouldn’t build a full wall from concrete blocks, but sometimes this is the only way, but it looks brutal without plastering.
• a full concrete block wall needs some kind of waterproof plastering and painting.
• a full, high block wall needs a normal, deep, reinforced strip foundation.
• there are fancy surface fence concrete blocks, but they are fearsome expensive (10 $/pc). The fancy corner blocks can cost 37 $/pc (what the f*ck?)
• don’t forget the coping stones: the whole fence will soak from the top, and the plastering will come off. And of course, it looks nice.

Retaining wall from concrete blocks

Why should You build a retainig wall from concrete blocks? Well, because it’s cheap to build, and doesn’t need much experience.

The good

• cheap to build, and doen’t need much experience
• strong, stable retaining wall can be built with it
• with a rugged surface, it makes the cracks disappear: the cracks will appear at the pointing. If the cracks open a bit, there is no problem. If plastered, the cracks will come to the fore
• the plastered blocks look great
• the retaining wall will hold the smaller movements in the ground. If you fill it with reinforced concrete, it will be very strong

The bad

• You’ll have to build a srip foundation
• the normal, cheap concrete blocks are ugly as hell. After some time, it may get tarnished, but it’s just a concrete wall. It needs plastering and painting, but there will be cracks…
• it’s not enough to fill the blocks with mud. It needs reinforced concrete in it
• the retaining wall lets the water through. This way there will be leaks, and dirty spots
• the rugged surface blocks are expensive as hell

Summary

Building a concrete block wall is just like playing with Lego. It’s easy, safe and clean to build. It gives a relatively nice and clan surface. The price is also good compared to a poured-concrete structure. In return, the wall will be a bit weaker. If we are clever, we can save a lot of money and energy with this technology. If we make the wall with our own hands, we can save the price of the labor. This work will also help with our hernia, it brings it out nicely.

Meet me at the physiotherapy!

The article can be found in beautiful hungarian language as well.

Written by Levi