What Is a Lean Concrete Mix and How Is It Made?

What is a lean concrete mix? It’s a type of concrete where the cement content is less than about 10% of the total mixture content.

This makes it a low-strength material and it’s used as an alternative to compacted soil. It’s also highly liquid and self-levels.

So you can fairly easily remove lean concrete, if necessary. Whereas rich concrete used in megastructures is near unbreakable.

This perfectly illustrates the versatility of concrete. Because by simply altering concrete’s cement content, you produce a different material.

I’ll further showcase this versatility as I discuss a lean concrete mix in detail. Before we start though, you need to first understand how concrete is made.

How is concrete made?

worker flattening concrete floor

Concrete is a hardened building material. You make it by mixing the following ingredients together:

  • Water
  • Fine aggregate (any natural sand particles)
  • Coarse aggregate (gravel, crushed stone, recycled concrete)
  • Cement
  • Optional: Admixtures are chemicals used to change the properties of a mix

Important Note: Portland cement is the most commonly used cement in concrete. It’s a fine gray powder. 

It’s made by heating limestone and clay minerals in a kiln. This forms clinker. Then you grind the clinker and add 2% to 3% of gypsum. 

When you combine water with Portland cement, an exothermic reaction happens called hydration. In other words, the reaction releases heat. This reaction then leads to the hardening of your mixture.

Now, why add aggregate?

The inclusion of stones and sand increases the strength of concrete. Because for example, the cement bonds to the jagged edges of the stone. This then forms strong bonds and thus increases concrete strength.

Also, you’ll have less shrinkage compared to if you only use cement without aggregate.

Important Note: aggregates either have a smooth surface or a jagged surface. The smoother the surface, the more workable concrete becomes. While rougher surfaces generate stronger concrete. 

To learn more about how concrete is made and the ingredients, read here.

What is a lean concrete mix?

Now that we’ve outlined the ingredients of concrete, we can in detail go over a lean concrete mix.

As already mentioned, the cement content is low with a lean concrete mix.

Also, you use fine aggregate in a lean concrete mix. This is typically made from recycled concrete pavement, discarded sand, or recycled ash. This is the material of choice over heavy and high-density rocks and sand.

As a result, this makes for a cheap and easy-to-make mix.

The following are typical test and performance specs for lean mix concrete:

  • The minimum strength of 2.1 MPa or 300 psi per ASTM C 39 at 28 days after placement.
  • Evaporation of bleed water to not result in shrinkage of more than 10.4 millimeters per meter (1/8 inch per foot) of lean concrete fill.
  • Lean concrete fill to have a unit weight of 1900 to 2300 kg/mg^{3} (115 to 145 lbs/feet^{3}). Measure at the place of concrete pouring after a 60-minute ready-mix truck ride.
  • Mix design to produce a consistency that’ll result in flowable concrete when poured. It shouldn’t require manual labor to move the concrete into place. In return, this provides an easily pourable uniform surface.

The lean concrete mixture ratio

workers construction site

We’ll compare the three most common categorized concrete types together. This will best showcase the unique mixture found in lean concrete.

These concrete types are the following:

  • Lean concrete mix
  • Normal concrete mix
  • Rich concrete mix

Also, what I list for each concrete type are generalizations. But, they’re fairly accurate and they’ll accomplish our goal.

Our goal is to highlight what sets lean concrete mix apart from other concrete types. More specifically, to show how little cement lean concrete mix requires.

To point out, the amount of added water varies from project to project for any given concrete mix. Thus, the application of concrete in a project is important to know. This doesn’t concern us though in our comparison.

Lean concrete mix

It’s used to provide smooth surfaces for structural reinforcements like footings. At the same time, it prevents a structure’s concrete foundation from touching soil.

This is important, as moisture and various chemicals in the soil can damage concrete. For example, sulfate causes concrete to deteriorate making it soften and decay. You don’t want to risk damaging the foundational pillars of your structure.

Also, a lean concrete mix can fill open areas where earth filling isn’t stable. For example, it can strengthen around an excavation or fill an over-excavation. Generally, it’s a good replacement because it’s much sturdier than compacted soil. Plus, it’s flowable to fill all the crevices.

BUT, the concrete isn’t very strong.

Cement content: cement content < 10%.

Mix ratios: 

  • Cement (1 part )
  • Fine aggregate (4 parts)
  • Coarse aggregate (8 parts)

Important Note: for lean concrete mix, the Water/Cement (W/C) ratio is typically high. Typically, the W/C ratio is around 0.4 to 0.5, but I’ve seen as high as 1.1.

This 1.1 W/C ratio helps maintain workability. For example, you can easily get the concrete out of a truck and pour it. 

Also, with lean mixes, you’ll have less shrinkage of concrete. The dry shrinkage increases as the water content increases. Thus, to reduce the shrinkage, you need to keep the water as low as practically possible. 

Normal concrete mix

This is your ordinary concrete. It’s used where there’s not a demand for great tensile strength.

For example, used for roads, residential buildings, or several-story buildings. It’s not used for skyscrapers or any mega structures though.

Cement content: 10% <= cement content <= 15%

Mix ratios: 

  • Cement (1 part )
  • Fine aggregate (2 parts)
  • Coarse aggregate (4 parts)

Rich concrete mix

This form of concrete is widely used for large construction projects. More specifically, when structures are subject to high loading.

For example, it’s used in mega structures such as skyscrapers, dams, and bridges.

Cement content: cement content >= 15%

Mix ratios: 

  • Cement (1 part )
  • Fine aggregate (1.5 parts)
  • Coarse aggregate (3 parts)

Admixtures for lean concrete mixes

Sometimes it’s important to use admixtures with concrete mixes. This includes lean concrete mixes.

The different additives have varying effects that are beneficial for different project types. But it’s always important to check if an additive is permissible for your project.

I’m going to go over the five most common admixtures you can add to your mix.

#1 Air entraining agents: improve the workability of concrete. This is especially important when you work with concrete in freezing weather conditions. Also, when concrete becomes exposed to thawing, sulfates, and seawater.

#2 Accelerators: help accelerate the hardening of concrete in cold weather. The agent used is calcium chloride and non-calcium chloride.

BUT, don’t use calcium chloride accelerators for concrete exposed to seawater. Also, don’t use in reinforced concrete, or concrete that’s in contact with aluminum or any other non-ferrous materials.

#3 Pozzolans: replace or improve the cement in concrete mixes. This way you can use less cement to achieve the specified required concrete strength.

In comparison, the curing time may extend if you use Portland Cement alone in your mix.

#4 Retarders: slow the hardening of concrete. This is useful in hot weather climates.

#5 Water reducers: improve the quality of concrete. Thus, you can get the desired concrete strength at lower water to cement ratios.

Also, you can increase the concrete slump without increasing the water content.

Important Note: slump is the measure of concrete consistency. Also, the fluidity of your concrete mix. This is a critical component in the workability of concrete. 

Conclusion

A lean concrete mix is just one of many types of concrete mixes. It has a certain mix ratio that makes it ideal to use in certain applications.

By learning all the applications of different concrete types, you can better design projects. This versatility is what makes concrete so amazing.

Also with concrete, it’s important to remain consistent from project to project. This way you always know what concrete mixes you’re going to get. Plus, you’ll limit any waste with your raw materials.

Have you used a lean concrete mix before? What are your thoughts on lean concrete mixes?

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