Concrete Mix Attributes, Instructions and Slumps

A concrete mix is like a chef’s good recipe. Concrete is composed of aggregates, Portland cement, water, and any other cementitious material or chemical admixture. Some concrete mixes will have entrapped air by using admixture or air-entraining cement. Concrete mixes could also have chemical components used to accelerate, retard or improve manageability, reducing in certain instances water quantities, increasing its strength or altering concrete properties. Selecting the best concrete mix is a task that has to consider costs and placement requirements while providing a great aesthetic and integral product.

Concrete Mix Key Attributes

A great concrete mix should consider:

1. Workability: The concrete mix workability is the property that determines the mix capacity to be placed and consolidate properly, allowing to finish a product without segregation.
2. Consistency: This property determines the mobility and slump of the concrete mix. This characteristic is measured in terms of slump, the higher slump values, the more manageable, and greater mobility will be obtained.
3. Strength: This is one of the most important characteristics of a concrete mix, and the most known property of concrete, it is measured using it compression resistance, after the concrete has reached 28 days after being poured.
4. Water-Cement Ratio: The W/C ratio on a concrete mix, is defined as the relationship and ratio between the weight of cement, the weight of water being added to the mix, plus added pozzolan. This characteristic has a direct and linear relationship with the mix strength.
5. Durability: A good concrete mix will provide you with concrete that could withstand severe weather conditions and changes without any decay sign. The more durable the concrete is, the more resistant to weather variations such as freezing, wetting, drying, and heating.
6. Density: Concrete mixes could also be specified for certain applications such as counterweights, radiation protection, insulation or endurance and resistance.
7. Heat Release: A concrete mix should also consider heat being released under a chemical reaction that will fade at a reasonable rate without producing cracks or shrinkage.

Concrete Mix Using ACI 211

The concrete mix design provided by the ACI Committee 211.1 is one of the most used methods to design method that will allow you to:

1. Use it on regular or lightweight aggregates.
2. Use it on similar procedures for rounded or angular aggregates.
3. Use it to design air entrained or non-air-entrained concrete mixes.

The ACI 211.1 method could be used to design a concrete mix by following these simple steps:

1. Select the target slump.
2. Select the maximum aggregate size, the larger the aggregate of the concrete mix, the better for minimizing shrinkage and curling.
3. Using ACI table 6.3.3, estimate the water and air content.
4. Select the water-cement ratio for the concrete mix.
5. Calculate the cement content by dividing the water content by the water-cement ratio.
6. Estimate the coarse aggregate content.
7. Estimate the fine aggregate content.
8. Adjust for aggregate moisture; wet aggregate can significantly reduce the amount of water to be added.
9. Make trial batches to determine if the concrete mix meets your design requirements.

Concrete Mix: Recommended Slumps

When preparing a concrete mix, you must consider the type of slump being expected. Follow these recommended slumps:

1. Reinforced foundation walls and footings:
   1. Maximum Slump 75mm
   2. Minimum Slump 25mm
2. Plain footings, caissons, and substructure walls:
   1. Maximum Slump 75mm
   2. Minimum Slump 25mm
3. Beams and reinforced walls:
   1. Maximum Slump 100mm
   2. Minimum Slump 25mm
4. Building columns:
   1. Maximum Slump 100mm
   2. Minimum Slump 25mm
5. Pavements and slabs:
   1. Maximum Slump 75mm
   2. Minimum Slump 25 mm
6. Mass Concrete Mix:
   1. Maximum Slump 75mm
   2. Minimum Slump 25mm