Reinforced concrete masonry walls are essential in construction, known for their durability, strength, and resistance to various loads. These walls are built by combining masonry units, such as concrete blocks or bricks, with mortar, reinforcing steel, and sometimes grout. Each material plays a crucial role in the overall performance of the structure, especially in terms of load-bearing capacity, shrinkage control, and resilience against environmental forces. This article explores the materials used in reinforced concrete masonry walls, focusing on their specific properties as defined by industry standards.
Materials Used in Reinforced Concrete Masonry Walls
The main materials used in constructing reinforced concrete masonry walls include:
- Concrete Masonry Units (CMUs): Blocks or bricks made from concrete mixtures, essential for forming the core structure of the wall.
- Mortar: A binding material that holds masonry units together and fills joints.
- Grout: A fluid form of concrete that fills cavities in the masonry wall to embed reinforcing steel.
- Steel Reinforcement: Provides additional strength and controls shrinkage cracks within the wall.
Each of these materials has specific properties that contribute to the wall’s strength, durability, and stability. Below, we discuss these properties in detail.
Properties of Concrete Masonry Units (CMUs)
Concrete masonry units are foundational components in reinforced masonry walls, providing bulk and strength. The properties of CMUs are regulated by ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units), which sets guidelines for both solid and hollow masonry units made from Portland cement, water, and mineral aggregates.
Compressive Strength
The compressive strength of a CMU is critical in determining its load-bearing capacity. According to ASTM C90, the average compressive strength of three tested units must be at least 13.1 MPa, with a minimum of 11.72 MPa for a single unit. For instance, if three units have compressive strengths of 12 MPa, 13 MPa, and 13.85 MPa, the average would be 12.95 MPa, which does not meet the required threshold of 13.1 MPa, and thus would fail to qualify.
Dry Shrinkage and Water Absorption
To prevent cracking, ASTM C90 also sets a maximum dry shrinkage rate of 0.065%. Shrinkage beyond this limit could cause cracks in the wall, affecting both appearance and structural integrity. Water absorption is another vital property, and ASTM C90 specifies varying absorption limits for lightweight, medium, and normal-weight units. Lightweight units have a higher allowable absorption because their aggregates retain more water.
Properties of Mortar
Mortar serves a dual purpose: it separates and bonds CMUs together. Mortar thickness is usually around 9.5 mm, though it can be adjusted slightly to accommodate variations in CMU dimensions.
ASTM C270 Specifications
The ASTM C270 standard regulates mortar used in reinforced masonry walls, specifying three types of cementitious systems:
- Masonry Cement: A mixture of Portland cement, fillers, and plasticizers.
- Cement-Lime Mortar: Composed of cement, hydrated lime, sand, and water.
- Mortar Cement: Preferred in high seismic regions (Category D and higher) due to its superior flexural bond strength.
ASTM C270 further classifies mortar into types M, S, N, and O, in descending order of strength. Types S and N are most commonly used for reinforced masonry walls, while Type S or M is required in seismic regions for added stability.
Joint Types
Mortar is placed in two types of joints:
- Bed Joints: Horizontal joints between masonry units, where mortar is applied on the face shells.
- Head Joints: Vertical joints between units, where mortar fills to the thickness of the face shell on both sides.
These joint types help control the bonding pattern of the wall and contribute to its tensile and flexural strength, which is especially relevant in seismic areas.
Properties of Grout
Grout is a soupy concrete mixture designed to fill the cavities in masonry units, bonding them to the embedded steel reinforcement. ASTM C476 regulates the properties of grout used in masonry walls, specifying that it should contain cement, water, lime, sand, and sometimes coarse aggregate or admixtures.
Purpose and Application
The main purpose of grout is to anchor steel reinforcement within the wall, enhancing load resistance. Due to the absorptive nature of CMUs, the grout must have a high slump (20–28 cm) to ensure it flows smoothly through the masonry units and completely fills the voids. During the pouring process, the grout mixture may lose water to the surrounding masonry, which aids in curing but requires the grout to be initially quite fluid.
Placement Techniques
Proper grout placement is essential for strength and durability. Site engineers observe a change in color (to grey) on the masonry unit’s surface to confirm water absorption and ensure grout has reached the bottom of each cell. Vibration equipment is typically used to compact the grout, ensuring proper density and connection to the reinforcement.
Properties of Steel Reinforcement
Steel reinforcement in masonry walls provides strength, crack control, and resilience. Steel bars used in masonry walls follow the ASTM A951 standard, which requires them to be galvanized to prevent corrosion.
Placement of Steel Reinforcement
Steel bars are usually installed vertically within masonry cells, held in place by spacers to ensure proper positioning. In walls designed for lateral loads, such as wind or soil pressure, steel reinforcement may also be placed horizontally to add strength across the wall. For walls over 30 cm thick, two layers of reinforcement are recommended in seismic zones.
Seismic Considerations
For walls in seismic regions, reinforcement is critical. Vertical bars can be placed on one or both sides of the wall, depending on load requirements. When walls are exposed to lateral or wind loads, designers must ensure reinforcement bars are accurately positioned to withstand these forces.
Conclusion
Constructing a reinforced concrete masonry wall requires careful consideration of each material’s properties to achieve durability, stability, and compliance with safety standards. Standards such as ASTM C90, ASTM C270, ASTM C476, and ASTM A951 provide guidelines for the quality and performance of CMUs, mortar, grout, and reinforcement, respectively. By adhering to these specifications, reinforced concrete masonry walls can be built to endure various stresses, resist cracking, and remain durable over time, making them a reliable choice for modern construction projects.
This article provides an in-depth look at the materials and standards that make reinforced concrete masonry walls a vital component in structural engineering and construction.