7 myths and half-truths about concrete The more cement added to concrete, the stronger it will beThis is a common belief, and while it’s partially true, it’s often attributed with almost miraculous properties. It’s important to understand that the amount of cement affects strength only up to a certain point, beyond which increasing the cement content has only a minimal impact on the final strength of the concrete. For typical concrete strengths used in most smaller constructions (20-30 MPa), approximately 250–300 kg of cement per cubic meter is sufficient. Increasing the cement content slightly enhances strength, but not proportionally. Doses above 500 kg no longer improve strength and may even reduce it due to microcracks caused by increased shrinkage (drying).The strength of concrete is far more significantly influenced by the so-called water-cement ratio (the ratio of the weight of water to cement). To achieve higher strengths, the water content should be kept as low as possible (while ensuring the concrete remains workable, otherwise it will have the opposite effect). For high-strength concretes (60–90 MPa), the water-cement ratio plays a key role and is around 0.30, compared to the typical 0.6. However, concrete with such a low water-cement ratio would be unworkable without chemical additives (plasticizers), making such strength practically unattainable in domestic conditions.Excessive cement doses are therefore ineffective. Moreover, higher concrete strength doesn’t always mean greater load-bearing capacity... (see point 3).The older the concrete, the stronger it getsThis statement is partially true, but it’s often exaggerated to almost magical proportions. It’s true that concrete strength increases over time, but not as significantly as some believe. Concrete strength is typically specified at 28 days, with about 80% of this strength achieved in normal conditions after just 8 days. Strength continues to increase over time, but the strength after 50 years is only about 30% higher than at 28 days. Older concretes (from 60+ years ago) had a more pronounced strength increase due to coarsely ground cement, but even then, the increase was no more than about 60% of the 28-day strength. Thus, claims like “it’s old, well-cured, strong concrete” should be taken with skepticism.The higher the concrete strength, the greater the load-bearing capacity of the concrete elementThis is perhaps the most persistent and hardest-to-debunk myth, or half-truth. For typical structural elements (slabs, beams, and lintels), the concrete’s strength plays only a minimal role in bending load-bearing capacity. For example, if a slab has the correct thickness for a given span, doubling the concrete strength increases its load-bearing capacity by only a few percent (1-3%). This means that standard concrete strength (around 25 MPa) is entirely sufficient for bending and shear capacity. Significant increases in load-bearing capacity are only achievable by adding reinforcement (placed correctly). Therefore, for reinforced elements, proper dimensions (thickness, height, etc.) and the amount of reinforcement are more important than concrete strength, which is often cited by laypeople. Concrete strength is significant for columns, but for most typical constructions (e.g., family homes) with low loads and commonly used column sizes, it’s less critical. The conclusion is that for typical constructions, average concrete strength (around 25 MPa) is sufficient, and to increase the load-bearing capacity of reinforced elements, it’s better to increase the amount of reinforcement in the right places or enlarge the element’s dimensions. The myth that “we put a lot of cement in the concrete, so it will be stronger and this slab will hold anything” is simply not true.The most reinforcement should be in the middle because that’s where the greatest stress isWhen someone without sufficient knowledge starts talking about where an element is most stressed, it makes my hair stand on end. It’s true that for some simple structures, this is obvious, but it’s not always the case. There are different types of “stresses” on elements, such as bending or shear. When you ask such an “expert” what type of stress they mean, they quickly change the subject. I’ve often seen stirrups placed in the middle of a simple beam “where the stress is greatest,” even though the design specified them at the ends because “the designer doesn’t know what they’re doing.” The truth is that shear forces, which stirrups are meant to resist, are usually highest near the supports and nearly zero in the middle. I’m not saying designers can’t make mistakes, but it’s better to consult them than to act independently. Such errors can cost lives. I once encountered a case where reinforcement for a balcony was placed at the bottom surface by “experts” to “support the balcony,” claiming they’ve done it this way for years. I dread to think about those balconies and the people on or under them...Concrete lasts foreverThis myth stems from ancient Roman concrete structures that have survived over 2,000 years. However, those concretes were not reinforced with steel, which only began being used about 120 years ago. The trade-off was the need for massive structures; for example, the Pantheon’s dome used 100 times more concrete than a similarly sized dome built in the mid-20th century. Reinforced concrete structures are generally much cheaper than unreinforced ones (if the latter could even be built). The problem is the corrosion of reinforcement, which is the most common cause of reduced lifespan in reinforced concrete structures. Corrosion can be prevented with sufficient concrete cover, but this is often neglected, especially in small constructions, where reinforcement is practically laid directly on the formwork. Additionally, if the concrete isn’t properly compacted and remains porous, it’s no surprise that such structures don’t last even a fraction of the presumed “eternity.” The need for concrete cover conflicts with the structural requirement to place reinforcement as close to the surface as possible for maximum effectiveness. Thus, an optimal position is sought to ensure both sufficient load-bearing capacity and long lifespan. For typical constructions, a cover of about 3 cm is usually sufficient to ensure a reinforced concrete structure lasts at least 50–100 years. Halving the cover reduces the structure’s lifespan by about 75%.Reinforced concrete doesn’t crackThe paradox is that reinforced concrete is only fully effective when cracks form—only then does the reinforcement fully engage. However, it’s important to distinguish where and what kind of cracks appear. Typical bending cracks in reinforced concrete elements are usually only up to 0.3 mm wide. Larger cracks may or may not indicate overloading or reduced load-bearing capacity, depending on where and under what conditions they appear, and must be evaluated by a structural engineer. A well-designed reinforced concrete structure is engineered to warn of collapse through increased deflection and visible wide cracks (except in a few special cases where this is physically impossible). Thus, not all cracks are equal.Concrete has been used since Roman times, over 2,000 years agoWhile it’s true that concrete was used over 2,000 years ago, it wasn’t quite the same as the concrete we know today. Moreover, the technology was largely forgotten after the fall of the Roman Empire and had to be rediscovered about 250 years ago.
