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👉HOLLOW CONCRETE BLOCK (HCB) – CLASS B LABORATORY TESTS

💫Class B Hollow Concrete Blocks are used for non-load bearing walls or medium-strength construction.

🚧To certify quality, the following laboratory tests are performed.

✅ 1. Compressive Strength Test

⏺Standard: ASTM C140
Purpose: To measure block strength.

❇️Typical Requirement for Class B HCB:

Minimum compressive strength: 3.5 MPa to 5.0 MPa (varies with project spec)

Test 3 blocks (average value)

✅ 2. Dimension & Shape Tolerance

Standard: ASTM C140 / ES Standard
Purpose: Check if the block meets size and shape requirements.

Requirements (typical):

Length, width, height within ±3 mm tolerance

Faces should be flat and perpendicular

No large cracks or visible defects

✅ 3. Density Test (Dry Density)

Purpose: Confirm block classification (lightweight, medium, or normal).

Typical values:

1400 – 1700 kg/m³ for Class B HCB

✅ 4. Water Absorption

Standard: ASTM C140
Requirement:

Water absorption ≤ 12–15%

High absorption indicates weak block quality.

✅ 5. Block Weight Measurement

Purpose: Ensure consistency and structural performance.

Values depend on block size (e.g., 150 mm, 200 mm thickness).

✅Optional Tests (if required)

Ultrasound pulse velocity test – for uniformity

Efflorescence test – check salt content

Moisture content

📌 Summary Table for Easy Use

Test Purpose Typical Requirement (Class B)

Compressive Strength Strength ≥ 3.5 – 5.0 MPa
Dimensions Size accuracy ± 3 mm tolerance
Density Quality classification 1400–1700 kg/m³
Water Absorption Durability ≤ 12–15%
Weight Consistency As per design block type

@etconp

👉THE CHEMISTRY OF A CONCRETE MIX

💫When you combine Cement + Water + Aggregates + (Admixtures) a set of chemical reactions begin automatically.

🚧The most important chemical process is hydration — the reaction between cement and water.

🏷1. Chemistry of Cement

Cement is mainly made of four compounds:

*Tricalcium Silicate , C₃S for Early strength + heat
*Dicalcium Silicate, C₂S for Later strength
*Tricalcium Aluminate, C₃A to Controls setting (fast reaction)
*Tetracalcium Aluminoferrite,  C₄AF for Minor strength

Gypsum (CaSO₄·2H₂O) is added to control setting time.

🏷2. When water is added → Hydration begins

Water reacts with cement compounds to form new materials that make the concrete hard and strong.

🧬 A. Main Reaction: Formation of C–S–H Gel

C₃S + Water → C–S–H (Calcium Silicate Hydrate) + Ca(OH)₂

C–S–H gel is the glue that binds everything

Gives strength + durability

Forms 50–60% of the final hardened concrete

C₂S reacts slower:

C₂S + Water → C–S–H + Less Ca(OH)₂

This provides long-term strength.

🧬 B. Formation of Ettringite (Setting Control)

C₃A reacts extremely fast, so gypsum controls the reaction:

C₃A + Gypsum + Water → Ettringite

Ettringite crystals cause initial setting (stiffening).

If gypsum is finished, ettringite converts to monosulfate:

Ettringite → Monosulfate

🧬 C. Reaction of C₄AF

Produces ferrite hydrates that help setting but add little strength.

🏷3. Chemistry of Aggregates

Aggregates do not react much chemically.
Their main role is:

*Fill space
*Provide bulk
*Reduce cement content
*Improve strength by interlocking

But their surface bonds chemically with the C–S–H gel.

🏷4. Water–Cement Ratio (w/c) Chemistry

Lower water = denser hydration products.

Low w/c → more C–S–H, higher strength

High w/c → more pores, weaker concrete


🏷5. Chemistry of Admixtures
Admixtures modify hydration:

*Plasticizers / Superplasticizers

Disperse cement particles

Reduce water demand

Delay or accelerate hydration based on type

*Retarders
Slow down C₃A + C₃S reaction

*Accelerators
Speed up early hydration

🏷6. Chemistry Timeline of Concrete Hydration

*0–2 Hours: Initial setting

-C₃A + Gypsum → Ettringite
-Early C₃S reaction begins

*2–10 Hours: Final setting

-Needle-like crystals lock particles
-Concrete becomes solid

*1–7 Days: Early Strength

-Rapid C–S–H growth from C₃S

*7–28 Days: Major Strength Gain

-Slow hydration of C₂S
-Pore structure becomes tighter

*Months–Years: Long-term Hydration

-Continuous formation of C–S–H gel
-Increase in durability

📌 In simple words:

⏺Concrete gets hard because cement reacts with water and forms glue (C–S–H gel) and crystals that bind sand and stone together.

@etconp

👉Claim on Additional floor Construction/Addition Works/:

💫A contractor can generally claim for an additional floor construction requested by the client, but it depends on the contract agreement and the change management procedures in place.

🏷How it usually works?

🚧1. Check the Original Contract

If the signed contract was for G+8, and now the client wants to extend to G+10, this is outside the original scope.

The contractor is not obligated to do it without a variation order (VO) or new contract agreement.

🚧2. Variation / Change Order

Most contracts (FIDIC, PPA Ethiopia 2011, etc.) have clauses for variations.

The client (or Engineer/Consultant) issues a variation instruction in writing.

The contractor then submits a claim for:

Additional costs (materials, labor, equipment, subcontractors).

Time extension (because adding two floors increases duration).

🚧3. Contractor’s Right to Claim

The contractor can claim if:

The client formally instructs the change.

The change affects cost and time.

Contractor must submit the claim within the time frame stated in the contract (often 28 days in FIDIC).

🚧4. Evaluation and Approval

The Engineer/Consultant or Client evaluates the claim.

Once approved, the contract price and completion date are adjusted.

🚧5. Risks if No Formal Agreement

If the contractor builds extra floors without written approval, payment disputes may arise.

The client may later refuse to pay, claiming it was not properly instructed.

📜In summary:

⏺The contractor has the right to claim for additional floor construction requested by the client, but it must go through the formal variation/change order process, with clear documentation of cost and time impact.

@etconp