Does Concrete Mix Really Matter?

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The answer is yes. In fact, one of the most important aspects of concrete placement is the proper concrete mix design. Proper concrete mix saves time on labor, facilitates concrete consolidation and ensures a successful Fox Blocks wall pour. Cutting corners on the concrete mix can add cost and problems when you place the concrete into the Fox walls.

Concrete mix can vary from region to region throughout the country because the raw materials differ from region to region. Therefore, you must work closely with your local concrete supplier and explain that you will be placing the concrete in ICFs. Although Fox Blocks can provide concrete mix guidelines, we cannot provide the specific mix design for your area.

Concrete Mix Guidelines

The following concrete mix guidelines must be conveyed to the concrete supplier. The concrete supplier has the most knowledge about the materials in the area and will design the best mix for your Fox Blocks ICF project.

Concrete Compressive Strength - The 28-day compressive strength of the concrete must be specified to meet the strength used in the design of the Fox walls. The minimum strength that should be specified is 2,500 psi (17 MPa). A slightly higher compressive strength of 3,000 to 4,000 psi (20 to 30 MPa) can help the flowability and pumpability of the concrete and provide additional strength to the walls with minimal or no additional cost.

Slump - The proper slump allows the concrete to flow and be pumped at an efficient rate. Inadequate slump can create voids and honeycombs in the walls or put excessive pressure on the forms during concrete placement. Fox Blocks recommends a concrete slump as shown in the table below. The slump specified must be the slump that arrives in the cement truck at the job site. Water should not be added to the concrete at the job site. Additional water will reduce the compressive strength of the concrete and affect the water to cement ratio.

Form Size


Maximum Aggregate Size

9 1/4" (228 mm)

6.5”-7.0” (165 – 178 mm)

3/8" (9.5 mm)

11 1/4" & 13 1/4" (279 & 330 mm)

5.5”-6.5” (140 to 165 mm)

3/8" to ½" (9.5 to 12.7 mm)

15 1/4" & 17 1/4" (381 & 432 mm)

4.5”-5.5” (114 to 140 mm)

¾" (19 mm)

Aggregate Size - The aggregate size is also important in providing concrete with the proper strength that will flow well through the forms. A maximum aggregate size, as shown in the preceding table, is recommended for the best results. If the aggregate in the forms is too large, it could create damming of the concrete within the forms resulting in a void.

Admixtures - Admixtures are ingredients in the concrete other than Portland cement, water, and aggregates. They improve the pumpability, durability and workability of concrete and prevent weathering. There are many different types of admixtures available in different parts of the country. Some types of admixtures include air-entraining, water-reducing, super plasticizers, retarding and accelerating. Fly ash, air, plasticizers and super plasticizers are common admixtures used in concrete in the ICF industry. The concrete supplier should address admixtures at the concrete mix design stage. The goal should be a good flowable concrete mix.