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.

Experts agree that controlling or conditioning indoor air is the best way for a business to increase employee efficiency and to reduce costs connected with employee health care.  Fox Blocks ICF walls provide the tight exterior building envelope that is the first step in controlling the indoor air environment.  Fox Blocks buildings have high steady state R-values, low air infiltration rates, and the solid concrete walls provide superior thermal mass.  The healthiest, most energy efficient building is built “tight” with mechanical ventilation to ensure indoor air quality and moisture control. Opening a window and bringing in fresh outside air alone does not give good indoor air quality.  Fresh air enters the building, but that air is not conditioned.  It has moisture, dust pollen and mold particles that need to be controlled and conditioned.

Improving indoor air quality can increase profitability for businesses and building owners by increasing individual productivity, reducing employee or tenant turnover and disputes and reducing HVAC energy consumption and equipment repair.

Health Concerns

The EPA lists indoor air quality as the greatest threat to people’s health.  Good indoor air quality is especially important for people with asthma and other allergies.  The number of asthma patients has doubled in the United States within the last 10 years, but lowering the humidity and unfiltered air in buildings can help reduce the symptoms of asthma.

Dust mites.  90% of the people who have allergies are sensitive to dust mites.  Cleaning duct systems and air filtration alone are not effective methods of treating for dust mites, because dust mites are also found in upholstery, carpets, and bedding.  Since they require 55% humidity to survive, controlling the indoor humidity to a maximum of 50% is the best way of eliminating them.

Mold.  Mold not only creates an unhealthy living environment—it also causes structural damage.  People react to both the mold spores and the toxic gases released by the mold.  Like dust mites, mold also requires higher humidity levels to survive, so controlling humidity can eliminate mold growth.

Create Quality and Healthy Indoor Air

Humidity control is the critical factor for healthy indoor air quality.  The healthiest indoor air environment is 40% to 50% humidity with temperatures ranging from 68º to 70º F.  Also, the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) recommends 0.35 air changes per hour.  The most cost effective way of controlling indoor air quality is with a mechanical ventilation system.  These come in three types:

  • Exhaust Ventilation System – Stale air is drawn to a quiet vent and exhausted outdoors.
  • Balanced Ventilation System – Exhausts stale air out and brings fresh air in from the outside.  These systems are called air-to-air heat exchangers or heat recovery ventilators.  The temperature of the outside air can be conditioned to within 95% of the indoor air temperature.
  • Supply Ventilation System – Pressurizes the building, forcing the stale air out and controlling the humidity all within one system.  It allows for treating the fresh air before introducing it to the indoor air.  The treatment removes pollen, dust and mold spores from the outdoor air.

Conventional HVAC systems are not designed to control the humidity below 50%.  With the removal of the moisture from the indoor air, the air conditioner is able to cool the air more efficiently, making the building more comfortable.  Other ways of controlling indoor air quality include removing pets and pests, and eliminating smoking from the indoor environment.

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