Chapter 9: Compressive Strength of Concrete Hollow Blocks

CMT

Masonry

A construction method using individual units (such as bricks or blocks) bound together by mortar.

Unfired clay bricks, fired clay bricks, concrete bricks, and hollow concrete blocks.

Common Masonry Materials

Unfired Clay Bricks

Bricks made from clay that are dried but not kiln-fired; typically less durable and more susceptible to water damage.

Fired Clay Bricks

Bricks made by molding clay and then hardening them by high-temperature firing in a kiln.

Concrete Bricks

Bricks made from a mixture of cement and aggregates, typically solid and denser than hollow blocks.

Hollow Concrete Blocks

Modern masonry units with hollow centers, made of cement, sand, and aggregates; widely used due to performance and environmental benefits.

Environmental concerns, the need for sustainability, and technological advancements in block manufacturing.

Reasons for Abandoning Clay Bricks

They reduce raw material usage, environmental impact, and offer better energy efficiency.

Environmental Benefits of Hollow Concrete Blocks

Thermal Resistance

The ability of a material to resist heat flow, contributing to better energy efficiency in buildings.

Sound Resistance

The capacity of a material to reduce sound transmission through walls, contributing to acoustic comfort.

Fire Resistance

The ability of hollow concrete blocks to resist fire, increasing safety in buildings.

20

Hollow concrete blocks can have a lifespan of more than ______ years.

Length: 40 cm, Height: 20 cm, Depth: 10 cm (4”) or 15 cm (6”)

Standard CHB Size in the Philippines

Crushed coarse sand or gravel; river or sea sand should be avoided.

Optimal Material for CHBs

Interlocking Compressed Earth Blocks (ICEB)

Environmentally Friendly Alternative to CHBs

Suppliers should be certified by DTI (Department of Trade and Industry) and DPWH (Department of Public Works and Highways).

CHB Cement Supplier Certification

PS (Philippine Standard) and ICC (Import Commodity Clearance) markings.

Certification Marks to Look For

National Building Code of the Philippines and National Structural Code of the Philippines (NSCP)

Governing Standards for CHBs in the Philippines

6” CHBs

CHB Type for Load-Bearing Walls

Type I Class A or B unit, conforming with ASTM C-90-70

CHB Type Standard for Load-Bearing Walls

5.41 MPa (800 psi)

Minimum Compressive Strength of Individual CHB

6.89 MPa (1000 psi)

Minimum Average Compressive Strength (5 Units) CHB

4” CHBs

CHB Size for Non-Load-Bearing Walls

1 part Portland cement to 3 parts sand (1:3)

Steel Bar Mortar Mix Ratio

Type 1, 2, 3, or 4 confirming to ASTM C-150

Portland Cement Types Allowed

Portland cement and sand

Steel bars must be embedded in a ______ mortar mix (1:3 ratio).

C-150

Portland cement for mortar or grout must conform to ASTM ______.

By weight or by volume (volume is most commonly used, e.g., with buckets).

Two Common Methods of Proportioning Concrete

1 part cement : 7 parts sand (1:7), as per structural engineer’s specification.

Typical Mix Proportion for CHBs

Clean water; shall not exceed 28 liters per 40 kg bag of cement.

Water Requirement for CHB Concrete

10

The slump should not exceed ______ cm as per ASTM C-143.

28

The water amount should not exceed ______ liters per 40 kg bag of cement.

½, 1, 7

A common CHB mix includes ______ bucket(s) of water, ______ bucket(s) of cement, and ______ buckets of sand.

1, 1, 3

A common mortar mix includes ______ bucket(s) of water, ______ bucket(s) of cement, and ______ buckets of sand.

14, 20

When using a batch mixer, revolutions per minute should range from ______ to ______.

7

CHBs should be cured for a minimum of ______ days.

Damp, shaded

During curing, CHBs should be kept ______ and ______.

Water

One way to cure CHBs is to keep them under ______ in curing tanks.

Beach

______ sand should be avoided because it contains salt which weakens concrete.

Flow

A good concrete mix should stand up, not ______ away due to too much water.

Mixing board

Concrete should be mixed on a ______ to prevent contamination from dirt and grass.

Weak

Air in concrete acts as ______ points and should be removed through compaction.

14

CHBs should be stored for at least ______ days before being used in construction.

Air

CHBs should be stored in a way that allows ______ to circulate around and between them.

Not

A CHB that breaks when dropped from head height should ______ be used.

Dampened

Before laying, CHBs should be ______ to prevent them from absorbing moisture from mortar.

Vertical, horizontal

To strengthen buildings against lateral forces, CHBs should be connected to reinforced concrete columns and beams using ______ and ______ reinforcing bars.

Water

Weak concrete results from adding too much ______ to the mixture.

Uniformity

Consistent batch mixing ensures ______ in CHB quality.

Filled

Vertical joints in CHB walls should always be ______.

Mortar

CHBs used in construction should be laid on a full bed of ______.

Compressive Strength of Concrete Blocks

The ability of a concrete block to resist a force that attempts to crush it; important for determining construction suitability.

cement, aggregate, and water

Concrete masonry units are usually made of ___,___,___.

100

The standard drying temperature for block density testing is ______ °C.

Caliper

The tool used to measure the face shell and web thickness of a hollow block is a ______ ruler.

Mass divided by volume.

The formula to calculate the density of a block is

1500

The density value for Grade A(5.5) hollow unit should be greater than or equal to ______ kg/m³.

1100, 1500

For Grade B(3.5) hollow units, the acceptable density range is from ______ to ______ kg/m³.

1800

A solid concrete unit of Grade C(5.0) must have a density of at least ______ kg/m³.

400 mm, 500 mm, or 600 mm.

Common Lengths of Concrete Masonry Blocks

200 mm or 100 mm.

Common Heights of Concrete Masonry Blocks

50 mm, 75 mm, 100 mm, 150 mm, 200 mm, 250 mm, or 300 mm.

Common Widths of Concrete Masonry Blocks

Dimension Measurement Test

Test where block length, width, and height are measured; includes face shell and web thickness for hollow blocks.

8

A total of ______ blocks are tested to determine average compressive strength.

3

Blocks must be tested within ______ days after being collected in the lab.

28

The required age of each block before testing is ______ days.

Steel plates

If the block is larger than the steel bearing blocks, ______ are used to ensure proper loading.

Gypsum

Capping materials used in testing include sulphur, granular materials, or ______ plaster.

2

Half of the expected load is applied first, and the rest over at least ______ minutes.

Compressive Strength = Maximum Load ÷ Gross Sectional Area (N/mm²)

Formula for Compressive Strength

5.6

Grade A(7.0) hollow block must meet a minimum compressive strength of ______ N/mm².

4.0

Grade B(5.0) hollow block must meet a strength of ______ N/mm².

3.2

Solid unit of Grade C(4.0) requires a minimum strength of ______ N/mm².

4.0

Solid unit of Grade C(5.0) requires a minimum strength of ______ N/mm².