Compressive Strength of Concrete Block: Everything You Should Know

A concrete block is a type of building block composed completely of concrete that is linked together with mortar for the construction of walls. Instead of bricks as a construction material, concrete blocks can be used as masonry units. For this reason, it’s also known as the concrete masonry unit (CMU). The traditional brickwork for the construction of buildings is rapidly being replaced by concrete masonry units over the years for their unique advantages over bricks. There are several reasons why it has become a popular building material in recent years. It is strong and durable, meaning it can withstand a lot of wear and tear. It is also easy to work with, making it a popular choice for building structures that can last for a lifetime. Additionally, the concrete block is fire-resistant and insect-resistant, making it a low-maintenance option for both homes and other structures. Plus, concrete block is a sustainable building material, as it can be made from recycled materials. But most importantly, one of the key factors in its success is its compressive strength. 

What is Compressive Strength?

Concrete block or CMU is one of the most versatile building materials available. It can be used to create foundations, walls, floors, and more. Because of its tremendous compressive strength, it is often considered to be the most durable building material on the market. Compressive strength refers to the amount of pressure that the material can withstand before it breaks, or, in other words, the maximum crushing stress that a concrete block can take. The ability of a concrete block to endure compression stresses applied to its surface is a highly essential physical attribute of this popular construction material. The formula for calculating compressive strength is as follows: Compressive Strength = Applied Maximum Load / Surface Area of the Specimen. For concrete blocks, the compressive strength is typically around 17.5 N/mm2 or 2500 psi. This means that the blocks can support a load of up to 17.5 N per square millimeter before they start to crack. Compressive strength is tested to determine how much compression a concrete block structure can endure. Concrete block compressive strength is hence an essential element of structural designing.

However, there are different types of concrete blocks or CMU used in construction, notably solid or hollow blocks of load-bearing or non-load-bearing categories. Moreover, these hollow or solid blocks can be further categorized into several types according to their density, such as ultralow, light, normal, or high density. Interestingly, depending on their nature, the standard compressive strength can vary from type to type. In addition, as we’ll see in the following section, the standard compressive strength of each kind might vary widely depending on a variety of conditions. As a result, regulatory agencies in each country usually set a standard requirement for compressive strength to guarantee that the concrete blocks meet a specific degree of strength. Because of this, the compressive strength of used concrete blocks is routinely tested at various stages of manufacturing and construction. 

Standard Range of Compressive Strength 

As mentioned in the previous section, the required standard of compressive strength in construction work varies for different types of concrete blocks. Usually, the average compressive strength of concrete blocks ranges from 3.5 N/mm2 to 17.5 N/mm2. The compressive strength of individual types is shown in the table below.

How is Compressive Strength Determined

Compressive strength refers to a CMU’s capacity to withstand the stresses on its surface without cracking or deflection. It is important to know if these can be used in construction work for different purposes. It is first checked during mix design to make sure that the quality of concrete that was taken into account is of standard quality. The average compressive strength of a concrete masonry block is typically measured using eight blocks. A compressive testing machine, or CTM machine, is used to test the strength of 8 full-size concrete blocks to see how well they hold together. After being collected at the lab, the blocks should be analyzed within three days. Each block will be allowed to cure for 28 days. Compressive strength test equipment has two steel bearing blocks, one fixed and the other moving, which are used to provide the load to the masonry unit when it is put on the fixed block. Separate steel plates are utilized if the bearing area of the masonry unit exceeds the bearing area of steel blocks. The masonry unit centroid and block center of the thrust are aligned with the plates set on steel blocks.

Concrete masonry units are topped with sulfur and granular materials or gypsum plaster. Half of the predicted maximum load is applied continuously once the unit is placed in the testing machine; the remaining load is applied in no less than two minutes. Concrete block compressive strength is determined by dividing the maximum load for a given masonry unit by the unit’s total cross-sectional area. In the same way, test the remaining 7 blocks. The final compressive strength of a  concrete masonry unit, or CMU, is the average of the strengths of all 8 blocks. 

Factors Affecting Compressive Strength of Concrete Blocks

Compressive strength of concrete block depends on various factors such as the mix proportion of concrete, properties of ingredients of concrete, curing time, its physical dimension and how it is loaded (flat or on edge), solid or hollow, wall thickness and height (slenderness ratio), age, temperature, grove cuttings (due to electric and plumbing works), type of mortar and masonry bond type.

Raw material quality 

The compressive strength of concrete blocks vastly depends on the raw materials from which they are constructed. Generally, concrete masonry units are built with three basic compounds: Cement, Aggregates, and Water. The quality of these components will determine how solid the masonry units will be. 

• Cement: The strength of concrete blocks will be equally proportional to the quality of cement. By creating bonds with aggregates in the presence of water, cement forms the concrete particles. While assessing the quality of cement, some key factors must be thoroughly checked. They are:

1. Method of storage
2. Manufacturing and expiration date
3. Strength and shape tests
4. Presence of lumps
5. Consistency testing

• Aggregates: Covering 80% of the volume of the concrete block structure, aggregates are one of the key components that determine the compressive strength of concrete masonry units. Basically, two types of aggregates: fine and coarse aggregates, are present in the making of concrete blocks. The standard of aggregates is determined on the basis of some crucial factors:

1. Particle size and weight
2. Shape and texture
3. Absence of chlorides and sulfates
4. Water content
5. Consistency 

• Water: The quality of water should be maintained according to respective standards for construction purposes. Water catalyzes the bonding of cement particles with aggregates, so the standard quality of water must be maintained for the durability and strength of concrete blocks.

Water/Cement ratio

The compressive strength of concrete blocks is inversely proportional to the water/cement ratio. The higher the ratio is, the lower its strength will be. This relationship can be depicted in the below graph:

The water/cement ratio is an indication of the durability of concrete. A higher water/cement ratio indicates that the initial spacing between cement particles is greater and residual voids are not filled with hydration products in greater volumes. That is the reason behind the lower workability of concrete blocks. The compressive strength of concrete blocks largely depends on keeping this ratio perfect in mixed designs. A few parameters, such as environmental conditions, construction materials, etc., have to be assessed to identify the expected ratio for a particular project to achieve the required strength. 

The quantitative relation between aggregates 

As mentioned earlier, two types of aggregates are present in the concrete mixture: coarse and fine aggregates. The ratio of coarse aggregates to fine aggregates is an important parameter of the compressive strength of concrete blocks. The coarse/fine aggregate ratio indirectly affects the water demand in the mixture, which in turn determines the water/cement ratio, which finally influences the strength of concrete masonry units. If the amount of fine aggregates in contrast to coarse aggregates decreases, the overall aggregate surface area will decrease, and as a result, the water demand will decrease too. More compressive strength may be achieved with less water because of a lower water/cement ratio. So, the coarse/fine aggregate ratio is a significant indicator to be considered while making the mix designs.

Aggregate to cement ratio

Aggregate particles bond with cement catalyzed by water, and this is how concrete blocks are manufactured. For the constant workability and strength of concrete blocks, the value of aggregate: cement is an important measurement. 

While keeping a specific volume, if the ratio of cement is increased in relation to that of aggregates, it will lead to the increase of solid surface area. But for constant durability, the water demand will remain unchanged, leading to a decrease in the water/cement ratio. The reduction of this ratio indicates the higher compressive strength of these masonry units.

Entrapped air 

The amount of entrapped air improves the workability of the concrete block but reduces its compressive strength. This contradictory feature of concrete blocks should be kept in mind as it controls the expected strength. Air voids inside blocks reduce the compressive strength by 5%. Even though air voids resist the damage from freezing, the portion must be kept controlled for expected strength for particular construction purposes.

Concrete compaction

Concrete densification is the process of compacting a concrete block after it has been manufactured. There is a direct correlation between inadequate compaction and weaker concrete masonry units. Compressive strength is directly related to the degree of compactness. Strength decreases due to air pockets that are formed when the material is not compacted properly. For increased strength, concrete blocks must be properly compacted.

Age of concrete

The older the concrete blocks, the stronger they are. Aging has a huge contribution to increasing the compressive strength of concrete. However, after a certain period of time, the increase in strength is negligible; but concrete particles gain strength in an indefinitely constant manner.

Curing time

The curing time of concrete directly controls the increase in compressive strength in CMU. In general, the majority of strength is achieved during the 28th day of its curing period. Nevertheless, curing concrete blocks is an infinite process, and the achieved strength after 28 days is very minimal.

Temperature & relative humidity 

The process of achieving the strength of concrete blocks is vastly affected by temperature and relative humidity. The hydration rate of the concrete block is a good indication of its compressive strength. High temperatures and dry environments speed up the hydration process, resulting in the quick hardening of concrete blocks. When the concrete blocks do not receive adequate time for hardening, the resulting concrete becomes less well-structured and more porous, which is a reason for the decreased compressive strength of CMU. Sufficient moisture content and proper temperature are required to maintain while storing concrete blocks to achieve expected strength.

Summary

Concrete blocks are a smart solution for a wide range of construction projects. Because of the prevalence of high-rise buildings, construction projects now necessitate the use of heavy-duty load-bearing components. Such situations need the use of concrete blocks. Because of its highly practical and useful range of compressive strength, it has been able to outperform red bricks in these kinds of structural constructions. For all of these reasons, it is no wonder that concrete blocks are such a popular choice for both new construction and renovation projects.

F.A.Q.

What is the most important determinant of the compressive strength of concrete blocks?

Ans: The water/cement ratio is the most significant factor in determining the compressive strength of concrete blocks.

What is the compressive strength of concrete blocks?

Ans: A concrete block’s compressive strength is the amount of force it can withstand before it breaks.

How long do concrete blocks take to cure?

Ans: Concrete blocks cure for an infinite period of time. There is no limit to how long concrete blocks may cure. In the first 28 days of curing, the majority of the curing is completed; after that, just a little amount of curing takes place.

Why are concrete blocks prone to water leaks?

Ans: Due to their porous nature, concrete blocks are susceptible to water leaks.

Where can I get concrete blocks in Bangladesh?

Ans: A wide variety of high-quality concrete blocks are made by a number of Bangladeshi producers, including Mir Concrete Block.