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Concrete vs. Clay Bricks

Raw materials

Concrete Bricks
Made with a combination of sand, aggregate and cement. The primary aggregate is typically limestone.

Clay Bricks
Made with surface clays, shales, and fire clays.

Manufacturing

In both cases, the raw materials for clay and concrete brick are mined, ground to specifications and mixed with water. Concrete bricks are typically formed in molds using hydraulic pressure while clay bricks are typically extruded through a die. Concrete bricks are cured through a chemical reaction between the water and cement. This process is accelerated through the use of kilns that have a high relative humidity and a temperature of around 150 degrees. Clay bricks are typically cured by a process called vitrification. Vitrification of the clay occurs in a kiln at temperatures reaching upwards of 2,000 degrees. Both concrete and clay bricks are manufactured to meet a series of ASTM specifications that set the minimum design characteristics of each product.

Properties

Concrete Bricks
Concrete bricks will initially shrink after manufacturing due to the loss of moisture. With no voids in the brick, Bilco bricks use about 30% less mortar and sand, resulting in significant savings. A concrete brick typically weighs approximately 1 pound more than a clay brick due to its higher density. The higher density of concrete brick is correlated to fewer air pockets and consequently to a lower water absorption rate. Concrete bricks will typically have a higher compressive strength than clay bricks.

Clay Bricks
Clay bricks will typically expand after manufacturing due to the addition of moisture. Most of this expansion occurs during the first few weeks, but will continue at a much slower rate for several years. This expansion of clay bricks is not reversible. Both concrete and clay bricks experience expansion and contraction due to heat gain and loss. Clay bricks typically have core holes while concrete bricks are usually frogged. Cores and frogs are designed to reduce weight and aid in the drying and firing process.

Heat Flow (R Value)

Although concrete bricks have a higher density than clay bricks, there is very little difference in the R value of clay and concrete.

A typical exterior brick veneer wall is designed to have a void space between the back of the brick and exterior wall sheathing. This void acts as a barrier to keep moisture formed on the back of the brick wall unit from being transferred to the exterior wall sheathing. This void is also an active air space, allowing for moisture to escape through weep holes placed at the bottom of the flashing. Because of this design the exterior brick veneer adds very little additional insulating value to the overall exterior wall system.

Fire Rating

There are several distinct advantages for concrete brick in fire-rated applications:

1. Inherent vs. Assembly-Dependent Ratings

The primary technical differentiator is how the 1-hour rating is achieved.

• Concrete Brick: The 1-hour rating is an inherent property of the material itself. Based on ASTM C140 testing and standardized calculation methods, a single wythe wall of our dry-cast concrete brick provides a fire resistance rating of 1.02 to 1.03 hours. This is determined purely by the equivalent thickness and aggregate types used in manufacturing.
• Clay Brick: In contrast, the fire rating for clay brick is dependent on the entire wall assembly. Test data from the Brick Industry Association (BIA) indicates that clay brick require a specific combination of 1/2″ gypsum board, 2×4 wood studs with batt insulation, and OSB sheathing to qualify for a 1-hour rating.

2. Physical Integrity & Strength

Performance data indicates that concrete brick provides a more robust barrier under thermal stress:

• High Strength: Our concrete units exhibited compressive strengths up to 9,510 psi, far exceeding the 3,500 psi minimum required by ASTM C1634.
• Thermal Limits: During clay brick assembly testing, the units reached their performance limit when flames erupted from cracks in the brickwork after the test panel was removed. Concrete brick rating is based on stable material density (approx. 132-135 lbs/ft?), providing a more predictable performance metric.

Our concrete brick not only meets the 1-hour threshold but does so as a standalone material, offering a significant margin of safety and greater design flexibility by removing the reliance on specific interior wall components for the exterior fire rating.

The effect of concrete brick on slab design

Due to its higher density and strength, a concrete brick will generally weigh more than a similar sized clay brick.

A typical residential foundation is designed for a perimeter wall load of 700 to 1,200 pounds per linear foot. Any additional weight added by concrete brick falls within the normal design parameters and will not have a negative effect on the performance of the foundation.

Strength

One of the more desirable properties of concrete bricks is its strength. Unlike clay bricks, concrete bricks continue to gain strength over time.

The high strength of concrete bricks is important when resisting the forces of nature.

ASTM Requirements

Although the ASTM (American Society for Testing and Materials) specifications for manufacturing clay brick (C216) and concrete brick (C1634) are similar in scope, the requirements for concrete bricks are more stringent in several important areas.

Compared to clay bricks, concrete bricks are required to have a higher compressive strength and a lower water absorption rate and are also required to have tighter dimensional tolerances as well as smaller allowances for chippage.