In a bid to understand how to determine concrete mix ratio in relation to the number of head pans of sand and gravel actually needed for a bag of cement, it is important to understand the relationship between these materials.

Concrete mix ratio is widely used by civil engineers to define the relationship between cement, sand, and gravel.

Over the years, concrete mix ratio has been used by many civil engineering authors to describe the quantity of cement in relation to the quantity of sand and gravel required to achieve a particular concrete strength, usually measured after 28 days.

Having this in mind, care must be taken when deciding the concrete mix ratio to use at a construction site and how to convert it into bags of cement and head pans of sand and gravel.

To understand the concept of concrete mix ratio, the concept of concrete mix design cannot be underestimated. If you would like to know more about concrete mix design in detail, please check the Nigerian manual on Concrete Mix Design, described as the first of its kind.

For the purpose of this article, I will explain some terms that will help us better understand the concept of concrete mix ratio.

Concrete Mix Design

This can be explained as a step-by-step method of determining the right materials needed to produce a specific strength of concrete. For the purpose of our discussion, I will focus on how you can determine concrete mix ratio at a construction site using a specified concrete grade.

Concrete Mix Ratio

As explained earlier, concrete mix ratio is the relationship between cement, water, sand, and aggregates that results in a particular concrete grade.

Concrete Grade (C)

Concrete grade can be explained as the minimum compressive strength of concrete at a specific period, usually 28 days, under proper quality control conditions.

Characteristic Strength of Concrete (Fcu)

This can be explained as the compressive strength that concrete must develop after 28 days of curing before it fails under crushing.

Concrete Curing

Concrete curing is the process of ensuring that concrete receives adequate moisture and temperature to achieve the required strength and durability over a specified period.

Now that we have a clear understanding of the terms above, let us look at different concrete grades in relation to their mix ratios and strengths after a specified period.

The BS code recommendations indicate that different concrete grades should be used for different purposes in construction work. These concrete grades need to be converted into their respective mix ratios, which can then be used at construction sites.

There are different types of concrete mixes: nominal mix and design mix. Nominal mixes are generally specified by volume and are widely used in Nigeria.

Design mix, on the other hand, is a more economical approach in which laboratory testing uses the weights of different constituent materials to determine the strength or grade of concrete required at a construction site.

The confusing part of concrete mix ratio is that many young engineers, builders, and masons do not know how to relate the mix ratio to bags of cement and head pans of sand and gravel. For the purpose of this article, we will look at some specified concrete grades and their mix ratios.

Note: The cement grades of 32.5 and 42.5, which are popularly used in Nigeria, are not taken into account in this analysis.

The Concrete Grades Are:

Grade C-10 — Concrete Mix Ratio of 1:4:6

Grade C-15 — Concrete Mix Ratio of 1:3:5

Grade C-20 — Concrete Mix Ratio of 1:2:4

Grade C-25 — Concrete Mix Ratio of 1:1.5:3

Now that we understand the relationship between concrete grades and mix ratios, it means that to produce a concrete grade such as C-20, you would need a mix ratio of:

1 head pan of cement : 2 head pans of sand : 4 head pans of gravel.

This is the basic principle. However, the confusion is that cement is usually measured in bags at construction sites rather than in head pans.

The question, therefore, is: how do we determine the number of head pans of sand and gravel actually needed for one bag of cement when producing concrete grade C-20 with a mix ratio of 1:2:4?

With reference to Building Contractor Secret, one bag of cement should be able to fill two head pans of cement. If this is correct, it means that for every:

1 head pan of cement, we need 2 head pans of sand and 4 head pans of gravel to produce the required concrete mix.

Mathematically:

1 pan of cement = 2 pans of sand = 4 pans of gravel.

If we double the quantities, we get:

2 pans of cement = 4 pans of sand = 8 pans of gravel.

Remember that:

2 pans of cement = 1 bag of cement.

Therefore:

1 Bag of Cement = 4 Head Pans of Sand = 8 Head Pans of Gravel

This means that for every 1 bag of cement, you will need 4 head pans of sand and 8 head pans of gravel to produce concrete intended to achieve grade C-20, corresponding to a characteristic strength of 20 N/mm², according to the calculation presented in this article.

If you convert this to wheelbarrows, it means that for every 1 bag of cement, you will need approximately 1 wheelbarrow of sand and 2 wheelbarrows of gravel to achieve the concrete mix described above. The article presents this as the minimum standard that should be used at construction sites.

You can watch the video below for more explanation.

Watch the concrete mix ratio explanation

In Nigeria, many construction sites use less than the minimum standard by adopting concrete mix ratios that can reduce the resulting concrete strength below grade C-10. For instance:

A concrete mix of 1 bag of cement with 10 head pans of sand and 12 head pans of gravel gives a mix ratio of approximately 1:5:6, which falls below the C-10 mix ratio listed above.

A concrete mix of 1 bag of cement with 8 head pans of sand and 10 head pans of gravel results in a mix ratio of approximately 1:4:5, which differs from the C-10 and C-15 nominal ratios listed above.

These two concrete mixes are described as being commonly used at construction sites in Nigeria. This raises concerns about the use of concrete mixes that may not meet the required minimum strength for reinforced concrete, as discussed in the Nigerian design mix manual and British code of practice.

The use of concrete below the required minimum standard should not be encouraged. This may result from the high cost of cement or an inability to determine the correct volume of materials needed for a bag of cement.

Finally, proper steps should be taken by the relevant authorities, including the Council for the Regulation of Engineering in Nigeria (COREN) and the Nigerian Society of Engineers (NSE), to educate Nigerian artisans, craftsmen, masons, and newly graduated engineers about the dangers of using substandard concrete mix ratios. They should also be taught how to achieve the appropriate concrete mix ratio for a bag of cement.

I hope this article will help young engineers, builders, and masons make informed decisions about the number of head pans of sand and gravel needed for one bag of cement when aiming to produce a concrete grade of C-20.

If you have any comments or suggestions that could improve this article, please feel free to share them and help someone today, or contact us.

Thank you for spending time reading this article.

Technical note: The quantities above preserve the original article's calculation and explanations. Actual concrete strength depends on factors such as material properties, water-cement ratio, batching accuracy, compaction, and curing. For structural work, use an appropriate approved mix design rather than relying on nominal volume ratios alone.