Decoding Concrete Nuances

Recently, while teaching master’s students from civil engineering and architecture backgrounds, I was struck by a recurring and concerning issue: a lack of fundamental understanding of concrete and its practical applications. Concrete, as we know, is one of the most essential and widely used structural materials—introduced early in undergraduate education and central to construction practices, particularly in India. Its versatility and ubiquity make it indispensable. Yet, surprisingly, even many senior engineers—across both private and government sectors—continue to struggle with its practical nuances and code-based requirements. This is not a new problem. I recall a similar situation nearly two decades ago while working on a medical college project. Despite the involvement of reputed government organizations as consultants and contractors, there was a noticeable gap in their understanding of concrete fundamentals and relevant standards. Sadly, it seems little has changed in the intervening years. This persistent knowledge gap continues to manifest in the compromised quality of structures we see being built today.

Motivated by this concern, I felt compelled to write this piece—not to explore design theory, but to shed light on certain aspects of practical realities of making and using concrete in everyday construction. Topics such as mix proportioning, batching, and acceptance criteria are fundamental to ensuring quality, yet they are often misunderstood or overlooked in practice. Concrete, and the art of building with it, is an expansive field—vast and deep like an ocean. This article aims to navigate some of its essential but frequently neglected and less understood aspects, helping professionals better grasp the nuances that truly influence the durability and performance of concrete structures.

At its core, concrete is a composite material made primarily of cement, coarse sand, stone aggregates, and water. When these components are mixed, they undergo a chemical reaction known as hydration, which gradually hardens the mixture and enables it to gain strength over time. Known for its excellent compressive strength—typically ranging from 20 MPa to 100 MPa—concrete is incredibly versatile. It can be cast in-situ at construction sites, delivered as Ready Mixed Concrete (RMC), or used in precast elements. It offers reasonable resistance to weather and fire, and when combined with steel reinforcement (rebar), its tensile strength improves significantly. Its properties can be tailored by adjusting the proportions of ingredients or by incorporating specialized materials and admixtures to meet specific performance requirements. It is important to note that the quality, testing, and use of concrete materials are governed by BIS standards, with IS 456:2000 serving as the primary code of practice for plain and reinforced concrete.

Table 5 of IS 456:2000 outlines critical parameters for concrete durability, specifying the minimum cement content, maximum water-cement ratio, and minimum concrete grade for various exposure conditions when using normal-weight aggregates of 20 mm nominal size. These requirements are rooted in durability considerations and are non-negotiable. For reinforced concrete under moderate exposure, the code mandates a minimum grade of M20, with at least 300 kg/m³ of cement and a maximum free water-cement ratio of 0.55. The table also extends these specifications to Plain Cement Concrete (PCC), prescribing M15 as the minimum grade for moderate exposure. Notably, it does not specify a minimum grade for PCC under mild exposure conditions. While applying this table, engineers must pay close attention to Clause 6.1.3, which allows the use of concrete grades lower than those listed in Table 5 for certain applications—such as PCC, lean concrete, simple foundations, masonry wall footings, and other temporary or non-critical reinforced concrete works. However, this flexibility should be exercised with caution and sound engineering judgment, ensuring that structural integrity and durability are not compromised.

In most structural applications Design Mix Concrete is used due to its ability to meet specific performance requirements. The proportions in a design mix depend on several critical factors, including the desired concrete grade, type of cement, maximum nominal size and type of aggregates, minimum and maximum cement content, water-cement ratio, workability, exposure conditions, placement temperature, method of placing, level of supervision, type and dosage of admixtures, and sustainability considerations. The mix must be designed to achieve the required workability and a characteristic compressive strength (fck) not less than that specified in Table 2 of IS 456:2000.

To account for variability in materials, site conditions, and supervision, the target mean strength of the concrete should be at least fck plus 1.65 times the standard deviation. This ensures a buffer that helps maintain consistent quality across batches.

Clause 9.2.4 of IS 456 provides guidelines for calculating standard deviation at the site, which requires test results from a minimum of 30 concrete cube samples. This assessment should be carried out as early as possible and repeated whenever there is a change in concrete production. If sufficient data is not available, the assumed standard deviation values listed in Table 8 of the code must be used, ranging from 3.5 to 5 depending on the concrete grade. It is important to understand that a higher standard deviation leads to a higher target mean strength. Ideally standard deviation should be as low as possible. Engineers must approach this process with diligence and precision, as it plays a crucial role in ensuring the quality, durability, and reliability of the concrete used in construction.

In concrete batching, the quantities of cement and aggregates must be determined strictly by mass. Solid admixtures should also be measured by mass, while liquid admixtures may be measured either by mass or volume using calibrated tanks. Whenever feasible, it is advisable to source concrete from Ready Mixed Concrete (RMC) plants or from on-site/off-site batching facilities. For such cases, IS 4926—Ready Mixed Concrete: Code of Practice—should be referred to. Before batching begins, all materials must be tested in accordance with relevant standards, and only approved materials should be used. Among these, water quality is of utmost importance. Water used for mixing and curing must be clean and free from harmful substances such as oils, acids, alkalis, salts, sugars, organic matter, or any other contaminants that could adversely affect concrete or steel reinforcement. Generally, potable water is considered suitable for concrete mixing. Permissible limits for solids in mixing water are specified in Table 1 of IS 456:2000. It is worth noting that contaminated water is one of the leading causes of early deterioration in concrete structures.

Clause 10.2.3 of IS 456 clearly states that all concrete ingredients must be batched by mass. The only exception is outlined in Clause 10.2.4, which permits volume batching only when weigh batching is impractical and accurate bulk densities of the materials to be used have been previously established. In such cases, allowances for bulking must be made in accordance with IS 2386, and the mass-volume relationship should be checked frequently and approved by competent authority. In essence, concrete must be batched by mass—regardless of whether it is a design mix or nominal mix. This requirement is fundamental to ensuring consistency and quality. Whenever possible, concrete should be procured from RMC plants or batching facilities. If site mixing is necessary, weigh batchers must be used to maintain accuracy and reliability.

IS 456:2000 specifies nominal mix proportions (M5 to M20) by weight, as detailed in Table 9, with an emphasis on controlled batching for accuracy. Despite this, volumetric batching using pans or boxes remains common on construction sites due to convenience or lack of awareness, even though it does not comply with the code. While IS 456 permits nominal mixes only up to M20, it strongly advocates weight-based batching for superior quality control. Importantly, a nominal mix does not inherently imply volumetric batching. Concrete produced through weigh batching is significantly more accurate and consistent, as it accounts for moisture content in aggregates, reducing variability in strength and workability. Although volumetric batching persists in small-scale projects, IS 456 clearly states it should be used only in exceptional cases when weigh batching is impractical.

Another significant gap in understanding lies in the sampling, testing, and acceptance criteria for concrete. Clause 15 of IS 456:2000 clearly specifies that samples of fresh concrete must be collected in accordance with IS 1199, and the test cubes should be cast, cured, and tested as per IS 516. While 7-day compressive strength tests may be conducted as supplementary checks, the 28-day compressive strength—outlined in Table 2 of the code—remains the sole criterion for acceptance or rejection of the concrete. The frequency and number of samples required are detailed in Clause 15.2. Each sample must consist of three test specimens. Additional samples may be taken for 7-day testing or when accelerated curing methods are used, as per IS 9103. These provisions are essential for ensuring that the concrete meets the specified strength requirements and performs reliably in service.

The test result of a concrete sample is defined as the average compressive strength of three individual cube specimens. However, if the variation among these individual results exceeds ±15%, the sample is considered invalid. Such discrepancies typically indicate improper sampling, curing, or testing procedures. 

Clause 16 of IS 456:2000 outlines the acceptance criteria for concrete strength. Concrete is deemed to comply with the specified strength requirements only when both the mean strength of any group of four consecutive, non-overlapping samples and each individual test result meet the conditions specified in Columns 2 and 3 of Table 11. For concrete grades M20 and above, the mean strength of the group must be at least equal to either: fck + 0.825 × established standard deviation, or fck + 4 N/mm²,  whichever is greater. Here, one group refers to four consecutive, non-overlapping samples—each sample being the average of three cube specimens. Additionally, every individual test result within the group must meet the minimum strength requirement specified in Column 3 of Table 11. If the concrete fails to meet these criteria, further investigation and remedial measures may be necessary to ensure structural integrity and compliance. I have noticed that many engineers do not have correct understanding of it. Concrete does not forgive mistakes—what you put in is what you get out.

fck refers to the characteristic compressive strength of concrete, defined as the strength below which no more than 5% of test results are expected to fall. This is typically measured using a standard concrete cube of dimensions 15 cm × 15 cm × 15 cm. A concrete grade such as M20 indicates that the concrete is designed to achieve a compressive strength of 20 N/mm² (or 20 MPa) at 28 days, with only 5% of results permitted to fall below this threshold.

I hope this discussion helps clarify many of the persistent doubts and misconceptions among students, project managers, and engineers actively engaged in the design and construction of concrete structures. A sound understanding of concrete—beyond just its theoretical design—is essential for ensuring the durability, safety, and long-term performance of our built environment. It is important to emphasize that the insights shared here are based on IS 456:2000, the current code of practice for plain and reinforced concrete in India. However, this standard is presently under revision, and a new version is expected to be released soon. The upcoming update will introduce significant changes aimed at aligning with modern construction practices, sustainability goals, and advancements in material technology.

Let us continue to build not just structures, but a culture of technical competence and thoughtful execution.

MANOJ MITTAL- OCTOBER 12,2025|NOIDA

© This blog post is the intellectual property of MANOJ MITTAL. Unauthorized use or reproduction is prohibited.