Precast cement concrete blocks are concrete masonry units that are molded and set before being transported to the construction site. They are generally produced in a factory under controlled conditions which contribute to uniform dimensions, strength and quality of the finished product.
Precast concrete blocks are primarily composed of cement, sand, aggregate and water. They are manufactured in different sizes and shapes by manual, semi-mechanized and fully mechanized processes.
Precast cement concrete blocks can be solid or hollow. The use of hollow blocks is more common as they less the dead load of the structure and may improve the thermal insulation as well. Typically, a concrete block weighs between 17.2 and 19.5 kg.
The standard size of a block is 400, 500 or 600 mm in length, 100 or 200 mm in height and 50, 75, 100, 200, 250 or 300 mm in width. “During production, always keep in mind the tolerances required by the applicable standards.”
These units are widely used in masonry construction due to their durability and resistance to fire, rain, abrasion and harsh environmental conditions. But block quality alone isn’t enough. Good workmanship and proper construction practices are also necessary to construct a strong and durable masonry wall.
Production of Precast Cement Concrete Blocks
The production process generally includes batching, mixing, moulding, demoulding, covering, and curing.
1. Batching and Mixing
The required quantities of cement, sand, aggregate, and other materials are measured accurately and mixed together.
Proper batching helps maintain consistent block strength and quality.
2. Adding Water to the Mixture
Water is added gradually until the concrete mixture reaches the required consistency.
The mixture should contain enough water for compaction and cement hydration, but excessive water should be avoided because it may reduce strength and increase shrinkage.
3. Moulding
The prepared concrete mixture is placed into moulds and compacted.
Moulding may be carried out using:
- Manual machines
- Semi-mechanized machines
- Fully mechanized production systems
Good compaction is important because it reduces voids and produces stronger, more uniform blocks.
4. Demoulding
After compaction, the blocks are carefully removed from the moulds.
The blocks should be handled gently during this stage to prevent cracking, edge damage, or deformation.
5. Initial Covering
After demoulding, the blocks should be covered for about 24 hours.
This protects them from rapid moisture loss during the early stage of strength development.
6. Curing
The blocks should normally be cured for at least seven days.
Proper curing allows cement hydration to continue and improves:
- Compressive strength
- Surface hardness
- Durability
- Resistance to cracking
- Overall block quality
Dimensions of Precast Concrete Blocks
Concrete masonry units are produced in different sizes and shapes to suit various construction requirements.
Common block types include:
- Stretcher blocks
- Corner blocks
- Double-corner or pier blocks
- Jamb blocks
- Header blocks
- Bullnose blocks
- Partition blocks
- Concrete floor units
Concrete blocks may be hollow, open-ended, closed-ended, or solid. They are usually identified by their nominal dimensions.
Typical nominal dimensions are as follows:
Length
- 400 mm
- 500 mm
- 600 mm
Height
- 100 mm
- 200 mm
Width
- 50 mm
- 75 mm
- 100 mm
- 200 mm
- 250 mm
- 300 mm
Half-length blocks may also be manufactured in lengths of:
- 200 mm
- 250 mm
- 300 mm
These half blocks should correspond to the dimensions of the full-size units.
Dimensional Tolerances
Precast cement concrete blocks should be produced within the tolerances required by the applicable standard.
Typical maximum variations are:
- Length: not more than ±5 mm
- Height: not more than ±3 mm
- Width: not more than ±3 mm
Accurate dimensions help produce uniform mortar joints, straight walls, and better overall masonry quality.
Precast Block Masonry Construction
The construction method depends on the building height, structural requirements, block type, and design drawings.
For single-storey buildings, the hollow portions of blocks used in foundation and basement masonry may be filled with sand. However, the top foundation course should normally consist of solid blocks.
For buildings of two or more storeys, solid concrete blocks should generally be used in foundation courses, plinths, and basement walls unless the structural drawings indicate otherwise.
Where hollow blocks are used in these areas, their voids may be filled with 1:3:6 cement concrete using aggregate with a nominal maximum size of 12.5 mm.
1. Wetting of Blocks
Concrete blocks normally do not need to be soaked before laying.
However, in hot and dry weather, the top and sides of the blocks may be lightly moistened. This helps prevent the blocks from absorbing too much water from the mortar.
The blocks should not be saturated. Excessive wetting may affect mortar bonding and wall construction.
2. Laying of Precast Concrete Blocks
Blocks should be laid in the specified mortar and properly bedded.
The mortar should cover the full top surface of the previous course and should normally have a uniform thickness of:
- Not less than 10 mm
- Not more than 12 mm
All courses should be laid horizontally, while vertical joints should remain plumb and properly aligned.
Blocks in one course should overlap the joints in the courses above and below by at least one-quarter of the block length.
Precast half-length blocks should be used wherever possible. Cutting full-size blocks on site should be avoided unless necessary and approved.
For battered walls, bedding should normally remain at right angles to the face unless the drawings or specifications state otherwise.
Care should also be taken during handling and laying to avoid damaging block edges and corners.
3. Provision for Door and Window Openings
A course of solid concrete block masonry should be provided below door and window openings.
Alternatively, a 100 mm thick precast concrete sill block may be installed below windows.
The solid block or sill should extend at least 200 mm beyond the opening on both sides.
For the jambs of large doors and windows, either solid blocks should be used or the hollow portions should be filled with 1:3:6 concrete containing 12.5 mm nominal-size aggregate.
These details help strengthen the areas around openings, where stress concentrations and cracking may occur.
4. Intersecting Walls
Where two walls meet and are constructed at the same time, a proper masonry bond should be formed.
At least 50% of the masonry units at the intersection should be properly bonded between the two walls.
When intersecting walls are built separately, pockets should be left in the first wall. The corresponding courses of the second wall should then be built into these pockets.
The vertical spacing of the pockets should not exceed 200 mm.
This arrangement improves connection and load transfer between the walls.
5. Provisions Below Roof Slabs
The course immediately below the roof slab should be constructed using solid concrete blocks.
The top of this course should be finished smoothly with a 10 mm thick layer of cement and coarse sand mortar in a 1:3 proportion.
The surface may then receive a thick coat of whitewash or crude oil, where specified, to reduce bond and allow limited movement between the roof slab and the masonry wall.
This detail helps reduce cracking caused by slab movement, temperature changes, or differential deformation.
Advantages of Precast Cement Concrete Blocks
Precast cement concrete blocks offer several benefits in masonry construction.
High Compressive Strength
Good-quality concrete blocks can safely carry masonry loads when they are properly designed and constructed.
Good Fire Resistance
Concrete does not burn and can provide effective fire resistance when the wall thickness and construction details are suitable.
Good Abrasion Resistance
The hard concrete surface can resist wear and minor impact better than many lightweight wall materials.
Good Stability
Properly constructed concrete block walls provide stable and durable partitions or load-bearing walls.
Faster Construction
The large and uniform units allow masons to build walls more quickly than with smaller masonry units.
Lower Self-Weight for Hollow Blocks
Hollow units reduce the weight of the wall and the dead load transferred to the structure and foundations.
Ability to Receive Reinforcement
Vertical and horizontal reinforcement can pass through or between hollow blocks.
The hollow cells can also be filled with grout or concrete to improve:
- Strength
- Stability
- Earthquake resistance
- Resistance to lateral loads
Improved Thermal Insulation
The air spaces inside hollow blocks reduce heat transfer and may improve the thermal performance of the wall.
Consistent Dimensions
Factory production can provide more uniform block dimensions, which improves wall alignment and helps maintain consistent mortar joints.
Important Construction Considerations
The performance of precast concrete block masonry depends on both material quality and workmanship.
The site team should pay attention to:
- Correct block dimensions
- Required compressive strength
- Proper mortar proportions
- Uniform joint thickness
- Wall alignment
- Bond between intersecting walls
- Reinforcement details
- Concrete or grout filling
- Protection of newly completed masonry
- Compliance with project drawings and standards
Poor workmanship can result in cracking, uneven joints, weak connections, and reduced durability, even when high-quality blocks are used.
Final Thoughts
Precast cement concrete blocks are widely used because they provide strength, durability, fire resistance, and fast construction.
They are available in solid and hollow forms and can be produced in many sizes and shapes to suit walls, foundations, openings, corners, and partitions.
However, successful precast block masonry depends on more than the blocks themselves. Correct production, adequate curing, accurate dimensions, suitable mortar, proper bonding, and careful workmanship are all necessary.
When these requirements are followed, precast cement concrete blocks can provide a reliable and long-lasting masonry system for residential, commercial, and industrial construction.
Frequently Asked Questions
What are precast cement concrete blocks?
Precast cement concrete blocks are masonry units manufactured and cured before they are delivered to the construction site.
What materials are used to make concrete blocks?
The main materials are cement, sand, coarse or fine aggregate, and water.
Are hollow blocks better than solid blocks?
Hollow blocks are lighter and may provide better thermal insulation. Solid blocks are often preferred where higher strength, impact resistance, or load capacity is required.
Should concrete blocks be soaked before laying?
Concrete blocks normally should not be soaked. In hot weather, they may be lightly moistened to prevent excessive water absorption from the mortar.
What mortar joint thickness is used for concrete blocks?
A mortar joint thickness of about 10 to 12 mm is commonly used, depending on the project specifications.
How long should precast concrete blocks be cured?
The blocks should generally be cured for at least seven days, although the required period may vary according to the cement type, production method, and applicable standard.
Can hollow concrete blocks be reinforced?
Yes. Reinforcement can be placed through hollow cells, which may then be filled with grout or concrete to improve structural resistance.
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