Cement (Leaving Cert Construction Studies): Revision Notes

Cement

What is cement?

Cement is a construction material that forms the basis of many building products like mortar, plaster and concrete. In Ireland, Portland cement is the most commonly used type. This fine, grey powder undergoes a chemical reaction called hydration when mixed with water.

The hydration process occurs in two stages. First, the cement mixture begins to stiffen. Then it gradually gains strength over time. This chemical reaction continues as long as cement and water remain in contact.

Portland cement manufacturing

Portland cement production involves heating limestone (calcium) and clay together to create clinker. The manufacturing process follows these key stages:

1. Raw material extraction: Limestone is quarried and clay is sourced
2. Crushing and grinding: Materials are broken down into smaller pieces
3. Proportioning: Raw materials are mixed in correct ratios
4. Heating: The mixture is heated in a kiln to form clinker
5. Final grinding: Clinker is ground with gypsum to create fine cement powder
6. Storage and shipping: Finished cement is stored and distributed

Understanding concrete

Concrete forms when cement combines with water and aggregates. Aggregates are granular materials including sand, gravel and stone. These materials are graded by size:

• Coarse aggregates: Natural gravel, crushed rock and stones larger than 5mm diameter
• Fine aggregates: Materials that pass through a 5mm sieve

The resulting concrete can be used for foundations, floor slabs, walls and various pre-cast products.

Batching and mixing concrete

Batching refers to measuring materials before mixing concrete. This can be done by weight or volume:

• Weight batching: Materials are individually weighed - most accurate method used in ready-mix systems
• Volume batching: More common for smaller amounts or domestic mixers

During mixing, water and cement react chemically to form a paste. This paste coats the aggregates and binds everything together as the mixture sets and hardens.

Concrete mix proportions

A standard concrete mix ratio is 1:2:3 (cement : sand : gravel). This mix produces strong, durable concrete suitable for most construction work. However, the amount of Portland cement must be correctly balanced to avoid problems.

Workability testing

Workability describes concrete's ability to be shaped into moulds and compacted properly. Good workability means concrete can be formed, compacted and screeded without trouble. The amount of water and aggregates determines workability levels.

The slump test

This simple test measures concrete workability by examining how concrete behaves when support is removed. The test requires:

• Three concrete samples from different batch sections
• Mixing tray, truncated cone, steel tamping rod
• Metre stick and smooth surface

Placing and compacting concrete

Fresh concrete requires proper containment in formwork - boarding or sheeting that holds wet concrete as it sets. Formwork is typically made from timber and can be reused. During placement, concrete is tamped and vibrated to remove air bubbles.

Curing concrete

Curing achieves optimal strength and hardness in concrete. The process involves spraying water directly onto concrete or onto mats placed over it. This water protects concrete from extreme temperatures and dry conditions that harm the curing process.

Strength testing

Concrete strength varies between batches. The concrete cube test determines whether concrete meets British Standards requirements.

Cube test procedure

Equipment needed:

• Cast iron cube moulds, concrete samples, spanners, oil
• Tamping bar, float, damp cloths, plastic sheets

Results indicate compressive strength and British Standard compliance.

Reinforced concrete

Concrete performs excellently under compression but poorly under tension. Steel reinforcement addresses this weakness. Steel, being stronger in tension than compression, makes ideal reinforcement material.

Steel reinforcement, known as rebar (reinforcing bar), can be:

• Placed in concrete as individual lengths
• Formed into mesh or cage structures

In foundations, cages are positioned in excavated areas with rebar chairs for support, then concrete is poured around them.

Pre-stressing concrete

Pre-tensioning and post-tensioning are advanced reinforcement techniques that provide several advantages:

Pre-tensioning process

Steel wire tendons are stretched between anchors before concrete pouring. After concrete hardens sufficiently, wires are released from anchors. This creates upward buckling tendency in the middle, but concrete straightens under load. Lintels and suspended concrete floors commonly use pre-tensioning.

Post-tensioning process

Here, concrete hardens before steel tensioning. Flexible ducts are positioned before concrete pouring. After setting, steel tendons feed through ducts and anchor at one end. Stressing jacks apply required tension, with grouting filling spaces between tendons and ducting to prevent moisture contact with steel. This method suits large-scale projects like bridges and roads.

Concrete blocks

Concrete blocks are pre-cast products sold in bulk. They come in solid or hollow varieties:

Solid blocks: Used for cavity wall construction and domestic purposes

Hollow blocks: Common in uninsulated buildings like garages and large sheds. In dwellings, they must be south-facing and cannot bear loads. Internal cavities reduce heat gain.

Additional block types:

• Fine texture blocks: Smooth finish blocks found in schools and offices, can be painted directly
• High-strength blocks: Higher cement content than standard blocks
• Aerated blocks: Lightweight products for general construction