CivDesignr Journal

A collection of articles and ideas on construction and maintenance. By Alan Howard BSc. Civil (UWI - Trinidad), MEng. Civil (U of Toronto)
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Posts tagged "concrete"

Home Preparation diagram

Source: Miami Herald Hurricane Preparation Guide

Following on from my last post on construction for hurricane protection I have a few more tips but this time for wall construction using concrete blocks (concrete masonry units - cmu). 

This type of construction is very common in the Caribbean and southern Florida due to the higher hurricane resistance of this type of construction.

In order to ensure that your hurricane resistance is maximised first and foremost a qualified structural engineer should work with your contractor to confirm proper practice is adhered to. As a client however there are a few things you can look out for as major red flags.  it is vital that your roof structure is properly connected to your walls. This is usually done using a capping beam and reinforcement in the block holes approximately every 4th block hole in non loadbearing walls and in each hole for walls carrying heavy loads. If this is not the case on your site ask a question of your contractor and/or your engineer. 

Your wall reinforcement should continue all the way to the foundation and tie in correctly there as well. It is also good practice to include stirupps in the corners and wall intersections and to reinforce and pour all block holes in these areas. This will cause your corners to act like columns and bring greater stiffness and strength to your structure. 

Asker Anonymous Asks:
How much concrete is needed for a floor slab that is 10'x 20'
civdesignr civdesignr Said:

To determine the amount of concrete required (or any of the materials that make up the concrete) we start with a calculation for volume.

A typical ground floor slab cast on solid ground (cast “at grade” in engineering terms) is between 4 and 5 inches thick or between 100 and 125 millimetres in metric.

Assuming a 4” thick floor first convert everything to feet and multiply length x width x thickness.

Volume of Concrete = 20’ x 10’ x 0.33’ = 66 cubic feet

You can also Try my Concrete Material Calculator on my calculations page here which can determine how much cement, sand and stone in cubic meters you would require.

Also bear in mind that a 94lb bag of cement holds 1 cubic foot (0.0281 cu. metres)

Here is a sample picture of my calculator below.

                     Click HERE to use calculator shown above

Concrete - What should a client/homeowner know


Concrete is possibly the most common construction product on construction projects. Given how common it is it is essential to understand a few things about its most essential characteristics.
At its core concrete is an attempt to create a moldable, high strength version of natural stone. However, concrete has some significant differences from natural stone.

Firstly, concrete’s main component, cement, is manufactured and therefore has a cost attached to its creation. To offset this cost, sand and stone are added to “stretch” the mix and thereby reduce the overall cost. Concrete is made up of cement paste and natural stones and we can’t change the strength of natural stone so it is the strength of the cement paste that really determines whether the concrete is weak or strong. The cement paste is made from cement dust and water and it is the ratio between these two substances that really determines the strength of cement paste and therefore the strength of any piece of concrete

Secondly, since concrete it is man-made, it has very predictable properties which makes it easy for designers to use this product in their designs. This allows designers to design a part of the structure in concrete without necessarily testing the structure after it is cast to make sure it meets the requirements. Therefore, it is important that the contractor on site make sure that the mix is tested to ensure it meets the requirements of the design. 

When working with concrete it is important to remember these two critical properties. 
  1. The ratio between water and cement is what determines concrete strength.
  2. The designers calculations rely on concrete having certain properties so periodic testing is important.