Technical Considerations for Executing Joist-and-Block Ceilings in Building Construction

This ceiling type is a combination of prefabricated joists and filler blocks that, once cast in place with concrete, form a monolithic slab. This system not only provides excellent structural strength but also enables economical and straightforward execution. The joist-and-block ceiling is considered a standard and optimal choice, especially in residential buildings and projects with short to medium spans.

Despite its widespread use, correct installation of this system requires strict adherence to technical details and code requirements, as even minor errors in assembly, shoring, or concreting can impair structural performance or cause serious future damage. Therefore, understanding the technical and practical principles of joist-and-block ceilings is essential for engineers, supervisors, and construction managers.

Main Components of the Joist-and-Block Ceiling and Their Roles

The joist-and-block ceiling consists of a set of prefabricated and in-situ components that work together to deliver the desired structural and architectural performance. A thorough understanding of these elements and their individual functions is essential for proper design and execution.

1. Joist:
The joists act as the main tension members in this system, transferring ceiling loads to the main beams and columns. Joists are commonly produced in either metal (Chromite joists) or prefabricated concrete forms. In concrete joists, the main elements include tension rebars, stirrups, and bottom concrete. The precise placement and spacing of joists directly influence the ceiling's final performance.

2. Block:
Blocks are placed between the joists as fillers; their primary roles are to reduce concrete consumption, lower ceiling weight, and create a uniform surface for concreting. These blocks are usually made from clay, polystyrene, or lightweight concrete. Block selection should be based on criteria such as insulation requirements, weight, cost, and ease of transportation.

3. Top Concrete Layer (In-situ Concrete):
After arranging the joists and blocks, a layer of in-situ concrete—typically grade 25 or higher—is poured over the ceiling. This layer integrates the blocks and joists into a single system, providing composite slab action. It also plays a critical role in providing compressive strength and ceiling rigidity.

4. Shrinkage and Temperature Reinforcement:
At the top of the ceiling, within the concrete layer, a network of shrinkage and temperature rebars is placed. These rebars are vital to prevent cracks caused by concrete shrinkage and temperature fluctuations and help distribute stresses more evenly.

5. Temporary Shoring and Formwork:
Before concreting, temporary supports (shores) are used to stabilize the ceiling during construction. They remain in place until the concrete hardens and achieves sufficient strength. Proper arrangement of shores is essential for controlling deflection and preventing settlements.

Types of Joists and Blocks and Selection Criteria

In joist-and-block systems, the type of joist and block has a direct impact on execution quality, structural strength, speed of construction, and overall project costs. Depending on the project requirements, climate, architectural features, and loading, different types of joists and blocks are used. Here, we introduce the most common types and their selection parameters.

A) Types of Joists

1. Precast or Semi-precast Concrete Joist:
These joists are made of concrete and reinforcement bars, and are generally produced in factories or the project site. The combination of tension rebars, transverse stirrups, and bottom concrete ensures sufficient load-carrying capacity. Due to availability and reasonable cost, concrete joists are widely used in residential buildings.

2. Metal Joist (Chromite Joist):
This type features a bottom flange made from a steel plate or strip and a web formed from diagonal rebars. The top flange is typically created within the poured concrete. The main advantage of the Chromite joist is the elimination of the need for under-ceiling shoring, which speeds up construction and provides more freedom below. This type is suitable for longer spans and projects with tight deadlines.

B) Types of Blocks

1. Clay Blocks:
Clay blocks are among the most common types, produced from fired clay. They provide effective thermal and sound insulation, though they are more fragile and heavier compared to other varieties.

2. Polystyrene (Styrofoam) Blocks:
These blocks are extremely lightweight and easy to transport and install. Their reduced weight helps lower dead loads on the structure and they offer good sound and thermal insulation, though they require special protection against fire.

3. Lightweight Concrete Blocks:
These blocks are made from lightweight concrete (e.g., LECA or perlite) and offer a middle ground between clay and polystyrene blocks in terms of weight and performance. They possess good fire resistance and, given standard production, offer acceptable durability.

Criteria for Selecting Joists and Blocks

• Ceiling span and structural loading
• Execution factors (shoring, installation speed)
• Resistance to fire and environmental conditions
• Sound and thermal insulation
• Availability of materials and workshop resources
• Economic and total cost considerations

Comparison of Joist-and-Block Ceilings with Other Common Ceiling Systems

Ceiling system selection in building projects is influenced by load-carrying capacity, installation speed, cost, dead weight, ease of execution, and available workshop facilities. Thanks to its unique features, the joist-and-block ceiling is one of the most common options in urban construction, particularly in residential buildings. However, comparing it with other popular ceiling systems can help inform better choices in different scenarios.

A) Comparison with One-way and Two-way Concrete Slabs

• Joist-and-block ceilings are lighter than concrete slabs because filler blocks reduce concrete usage. They also don’t require extensive rebar installation across the entire surface, making them simpler to execute.

B) Comparison with Composite (Steel-Concrete) Ceilings

• Composite ceilings generally do not need broad formwork and are constructed using steel beams and galvanized steel sheets, which speeds up the execution and makes them a good option for steel skeletons.

C) Comparison with Chromite Ceilings

· There are similarities to the joist-and-block ceiling, but due to the removal of shoring, Chromite ceilings are quicker to execute and avoid occupying workspace on the lower floor—making them suitable for fast-track projects or those with limited access at lower levels.

Summary Comparison

Given this comparison, the ceiling system should be selected based on specific project needs, execution conditions, time constraints, and budget.

Conclusion

Joist-and-block ceilings, as one of the most widely used ceiling systems in Iran's construction industry, have earned a special place among builders and designers thanks to features like ease of execution, reduced material usage, appropriate weight, and adaptability to various project conditions. However, realizing the full benefits of this system depends on a thorough understanding of its technical and practical requirements.

Choosing the appropriate joist and block types, observing code requirements, careful attention to critical execution stages such as shoring, concreting, and rebar placement, and avoiding common mistakes all directly affect the ceiling’s quality and structural behavior.

In the end, a combination of technical knowledge, hands-on experience, and meticulous adherence to standards is key to successfully implementing joist-and-block ceilings and improving urban construction quality.