Project Engineers' Guide: Concreting

Fieldwork No. 6

Our goal is to learn the concrete mixes and how to install rebars and form work. Rebars are reinforcing steel bars used to reinforce the cement. Steel is strong in compression and tension, cement strong in compression weak in tension. That is why in the olden times you see huge blocks of stones to compensate against shearing or the stone cuts in half. The old buildings of the University of Santo Tomas which was built in the 1600s has walls one meter thick. Imagine that. However the steel rusts that is why the cement and steel are a perfect combination as the cement coats the steel against rust. 

The cement, sand and gravel mixture are as follows:

1:2:3 = 5,000 psi

1:2:4 = 4,000 psi

1: 2 1/2 : 5 = 3,000 psi

PSI is pounds per square inch which means the compression strength of concrete when hardened. As a contractor the Client will require you to submit and pass compression tests which are shaped in cylinders and blocks. This is usually in large-scale construction when Ready-Mix Concrete (RMC) is supplied. You get 3 samples per batch of RMC and will be tested of strength of 7 days, 14 days and 28 days. Thus concrete is cured in 28 days. See graph. The purpose of testing compression for 7 days is that the graph will indicate its strength and failure to meet the designed strength will be a reason for rework. They also use the Slump Test as allowable water, cement and aggregates ratio.

After one day the concrete has hardened into plastic state at the outside but still wet at the core. These samples are submerged in water as concrete needs water to cure for 28 days. That is why it is critical for columns to remain wet that formwork stays for 7 days. In case you need the form work and you have to remove the formwork you can cover the column with canvass or plastic and water it every now and then.

Slump Test is how much the wet concrete slumps after you pull away the cone cylinder.

However in small-scale construction we get by with experience.

One bag of cement is 40 kg and 50 kg. Nonetheless our 1: 2 1/2 : 5 concrete ratio means 1 bag of cement, 2 1/2 bags of sand and 5 bags of gravel. Okay. However if you are that meticulous in measurements, make a cubit using plywood and timber measuring 1 foot x 1 foot x foot = 1 cubic feet where you can measure your cement, sand and gravel. 

We compact the pits down to the gravel layer, pour the gravel, compact and prepare to pour the lean concrete. We hammer steel bars at the corners of the pit and mark the top level of the lean concrete with tapes. We pour the lean concrete 3,000 psi. Tomorrow we going to transfer the coordinates of the footing to the lean concrete using the plumb bob.

Fieldwork No. 7

Good Morning Class. We have here our revised construction plans. Compare the new plans with the old and adjust the pegs as necessary. Note that we would be using a backhoe so talk to the operator. The estimated allowance on the edge of the footing will increase indefinitely as the machine could be unpredictable. Say 0.30m around the edges. The backhoe has claws 1 foot, 1.50 foot and 2 feet. The 2 feet claw would ideal to dig the 0.40m wall footing that supports the CHB wall. In the Philippines we use CHB blocks mainly but bricks used in the UK is not uncommon. Okay now you will apply what you learned in surveying. 2 meters setback from the edge of the footing. Check the elevation 0.50m of the pegs. Check the squareness. The right angles. Measure the gridlines as provided. We will mobilize the backhoe after we have determined the gridlines. I know we have excavated okay. But we have new plans that supersedes the old plans.

Fieldwork No. 8

Good Morning Class. The backhoe did not come. I hired six workers to dig the trenches. Direct them to the excavation lines as indicated in the plans. Check the elevation of the pegs every now and then. 050m level. Check the gridlines. Offset to the excavation lines using the nylon strings. Make an allowance of 0.10m at the edges of the footing and wall footing. Okay say we have over excavated based on the old plans. Leave them be and proceed with the digging of the new excavation lines. We will backfill the trenches anyway after we have poured the concrete footing, the wall footing and a part of the columns.

Check out my stairway design. The shear wall is provided so that the stairs has three supports - the suspended beam, the slab-on-grade and the shear wall. The shear wall is a reinforced concrete wall designed to carry weight and resist forces. I probably add another beam along the center of the stairs.

We will also fabricate the rebars for the footing. Talk to the Steelman. I have never met a Steelman who cannot measure. Give the worker the cutting and bending list. One helper helps the Steelman. Let the worker do a sample. Minimize errors as possible. Leave the worker when satisfied with the samples. Give the column stirrups cutting and bending list next.

Talk to the mason to mix concrete for wedges. 25mm, 50mm, 75mm square blocks.

The design mix of our footing is 3000 psi as it deals primarily with shear from soil pressure and from the column punching shear. When we make the wedges it should be 3000 psi or higher. Our mixture of 1:2:4 means 1 measure of cement mixes with 6 measures of aggregates. In this case the cement-sand mixture is 1:6. Gravel is 3/4" or 20mm so the bond is not good for 25mm. We can use the 1:2:4 ratio for 50mm, 75mm blocks.

We need 75mm blocks because that is the elevation the footing rebars will sit on. The 50mm and 25mm blocks are for adjustments at indefinite elevations.

Fieldwork No. 9

Good Morning Class. The excavation should be finished today. To review check the 0.50 level you pull nylon strings from peg to peg. Measure offsets from the gridlines. Mark the excavation lives. Monitor the progress of the excavation by checking the elevation with measuring rods. Note that the excavation level for the wall footing is different. While the footing bottom is 0.60m below the 0.00 level, the wall footing bottom is 0.30m below 0.00 NGL level. Our slab-on-grade will be 0.30m above 0.00. That means our house floor level is 0.30m higher than the street level.

Fieldwork No. 10

Good Morning Class. We going to pour lean concrete today after we have compacted the trenches to planned elevation. 0.60 + 0.05 lean concrete + 0.025 gravel bedding = Footing F1 is 0.675m from NGL 0.00. Wall Footing WF is 0.375m. Using the transit level. Pour the gravel. Mix 3000 psi which 1:2:4 ratio for the lean concrete.

Our manpower is as follows:

2 - Carpenters

1 - Mason

1 - Steelman

5 - Helper

The six workers who excavated were led by the two carpenters because you can rely on them with line and grade and as such you will consult them with the history of the pegs, the gridlines and the nylon strings. The Mason and the Steelman are more specialized. The Plumber comes in when installing the preliminary water and sanitary pipes before the slab-on-grade is poured. The Electrician and other specialty trades come in after the hollow blocks are laid.