Where Does Concrete Get Its Strength?

Concrete is a construction material, indeed a structural element, produced by combining aggregates—such as gravel and sand—with a binder and water. The binder is typically cement.

Concrete is a widely used building material. It is employed in the construction of water structures such as dams and canals, as well as roads, buildings, bridges and other structures. It serves both as a load-bearing element and as a decorative material. It is preferred for its fire resistance, waterproofing and sound insulation properties. In modern structures, it is also used as protection against nuclear radiation. The cement and aggregates required for concrete are prepared in separate industrial sectors. Subsequently, concrete mortar is prepared by combining these materials in appropriate proportions. The selection of suitable mixing ratios ensures balanced achievement of properties such as economy, workability, strength, durability and appearance. Numerous mixing ratios are used depending on the condition of the aggregate and the type of cement. The ratio of mixing water to cement content is one of the most important factors affecting concrete strength. Another significant factor is the quantity of air in the concrete. This amount is approximately 0.3–3% in normal concrete. These two factors are very important in controlling concrete quality and achieving homogeneous concrete mixtures. The final step in obtaining concrete is the curing and hardening of the cast concrete. Hardening occurs as a result of cement hydration, that is, its chemical reaction with water. Depending on the cement type and temperature, a prolonged curing period may be required to achieve complete hydration. In most cases, seven days is sufficient. Concrete is commonly believed to harden as a result of water evaporation. In reality, this is incorrect. Without water, neither hydration nor hardening can occur.

Water is consumed as a result of cement hydration, and only excess water is allowed to evaporate. The chemical reactions in the concrete formation stages are quite complex.

                         

Admixtures have been developed to address problems arising in produced concrete. These admixtures both increase concrete strength, protect it from water damage, and enable adjustment of fluidity or stiffness depending on the consistency imparted by the admixture.

Concrete durability is its resistance to aggressive elements in the external environment. Concrete can lose its strength over time due to various harmful effects resulting from certain chemical reactions. In such cases, the structure either partially or completely collapses, or becomes unusable, as a consequence of the concrete's inability to withstand the forces to which it is subjected. Concrete must be naturally resistant to chemical damage and not lose its properties as a result of physicochemical external factors. For this reason, it is required to possess sufficient chemical resistance (durability). In addition to cement, aggregates and water, certain admixtures are also mixed into concrete. These are added either before or after water addition. Admixtures can increase the workability, durability and strength of concrete, as well as delay or accelerate its hardening. Furthermore, thermal expansion and permeability can be controlled with concrete admixtures. There also exist admixtures that create millions of microscopic air bubbles in concrete. Water-reducing admixtures that diminish the water requirement of the mixture electrically charge portland cement particles to separate them from each other and produce a more homogeneous mixture, thereby reducing water demand.

Reasons for using chemical admixtures in concrete production:

• Ease of use, • Economy, • Differences in fresh concrete properties, • Creation of differences in hardened concrete properties and hardening mechanism. Concretes whose production is not possible without admixtures: • Very high-strength concrete, • Freeze–thaw resistant concrete, • Highly flowable and simultaneously non-segregating concrete. Chemical admixtures used in concrete production, when classified according to their structure, are divided into two categories: 1- Mineral admixtures; finely ground materials used to improve certain properties of concrete or to impart special characteristics to concrete (TS EN 206 definition). 2- Chemical admixtures; chemical substances added during mixing to alter fresh and/or hardened concrete properties, in quantities not exceeding 5% by weight of the cement content. Chemical admixtures are generally organic or inorganic chemical substances soluble in water. They are used in low quantities relative to cement weight (for example, 0.1–5%). When used in excessive doses, side effects may occur (setting delay or acceleration, strength loss, concrete segregation, air entrainment, etc.). When multiple admixtures are used together, their compatibility must be investigated, and optimum dosage must be determined through trial mixing.

Benefits of using chemical admixtures in concrete production:

• Increasing concrete workability without increasing water content, • Reducing bleeding and segregation, • Adjusting setting time (delaying or accelerating), • Increasing early-age strength gain rate, • Slowing down hydration heat increase, • Increasing resistance to harsh external conditions (freeze–thaw, etc.). Chemical admixtures can be listed according to TS EN 934-2 standard as follows: • Water-reducing/plasticizing admixtures, • High-range water-reducing/superplasticizing admixtures, • Water-retaining admixtures, • Air-entraining admixtures, • Setting accelerating admixtures, • Hardening accelerating admixtures, • Setting retarding admixtures, • Waterproofing admixtures, • Setting retarding/water-reducing/plasticizing admixtures, • Setting retarding/high-range water-reducing/superplasticizing admixtures, • Setting accelerating/water-reducing/plasticizing admixtures. According to their mechanisms of action, chemical admixtures can be grouped into two categories:

1- Surfactants:

By adhering to cement particles, they affect hydration reactions and alter the surface tension of water. Example: Air-entraining and water-reducing admixtures.

2- Setting regulators:

They decompose and ionize, affecting chemical reactions between cement and water within the first few minutes to the first few hours. Example: Setting accelerating or retarding admixtures. Surfactants are long-chain organic molecules with one hydrophilic (water-attracting) end and one hydrophobic (water-repelling) end. The hydrophilic end contains one or more polar groups such as COO or SO3. Surfactants used in concrete production are mostly negatively charged admixtures with polar or nonpolar chains. Nonpolar chain negatively charged admixtures are air-entraining agents, while polar chain admixtures constitute water-reducing/plasticizing admixtures. Air-entraining admixtures are materials that increase the air content of fresh concrete and ensure that air voids are uniformly distributed separately as approximately 0.20 mm diameter voids, maintaining the stability of these voids until concrete setting is complete.

Resin salts, protein-based materials, petroleum acids and synthetic detergents are important air-entraining chemicals.

The purpose of using this admixture is to create small-diameter (? 50µm and ?? 200–250µm) spherical air bubbles (4–6% by volume) in the concrete structure to increase the concrete's freeze–thaw resistance. In addition to the diameter of air bubbles, the average distance between them (spacing factor) should, from the standpoint of admixture effectiveness, be as small as possible (max. 0.2 mm). There are certain advantages and disadvantages to using air-entraining admixtures: • It facilitates the workability and pumpability of fresh concrete. The amount of air entrapped in the concrete increases from 1.5–2.0% to 3.0–6.0%. • They also reduce the risk of segregation and bleeding. • In mass concrete and lightweight concrete, they can be used to achieve the same workability with less cement. • They play an effective role in protecting concrete from freeze–thaw effects. Thus concrete durability is improved. Because air significantly affects compressive strength, the air content of concrete must be frequently checked, and air must not exceed 6%. • The 28-day strength of concrete prepared using air-entraining admixtures must be at least 75% of the strength of control concrete. • When air-entraining admixture is used at high dosages, it retards setting. Air-entraining admixtures are predominantly used in concrete for concrete roads, airfield pavements, airports and water structures. Air-entraining admixtures are used in quantities between 0.03–0.15% of cement weight. For example, 30–150 grams of air-entraining admixture is used per 100 kg of cement.

Cement particles tend to agglomerate by bonding with each other. Plasticizers combine with water to reduce the surface tension and cohesive force of water in concrete. Plasticizers carry a negative electrical charge and tend to move at the water surface.

Due to these effects, by preventing agglomeration and simultaneously facilitating the sliding of particles over one another and providing a lubricating effect, plasticizers reduce the internal friction of concrete and increase its workability. Plasticizers provide a reduction in mixing water content of 5–12% while maintaining the same workability. If used without reducing mixing water, they significantly increase workability; additionally, by reducing segregation, they increase pumpability. Since strength is a function of water/cement ratio, when plasticizing admixture is used, the reduction in mixing water will lower the water/cement ratio, resulting in higher early and ultimate strengths, and producing concrete with fewer voids, better surface appearance and lower permeability. Plasticizers are generally used in ready-mix concrete, mass concrete, pumped concrete, and wherever smooth surface finish is desired, in areas of dense reinforcement, etc.

P 101, from NesChem Additive products, is a powder air-entraining agent based on polycarboxylate containing dimethyl siloxane and silicon reaction products, also possessing anti-agglomeration properties.

Additionally, P 101, while eliminating air bubbles larger than 1000µm that are not desired to be present in concrete, demonstrates excellent performance as an air-entraining agent in all cement-based water-based systems. In concrete tests conducted using P 101 at NesChem Additive Laboratories, spread values were observed to increase from 10% to 30%. Furthermore, through its air-entraining property, it also plays an effective role in protecting concrete from freeze–thaw effects. The quantity of air desired to be present in concrete increases from 1.5–2% to 3–6% (maximum desired air content is 6%). Thus, concrete durability is improved. In today's practice, concrete production without admixtures is virtually nonexistent.

Remember!

• Admixtures are not medicine. They do not correct poorly prepared concrete. • Admixtures do not produce the same results for every type of cement, aggregate and gradation. • Their side effects must be investigated. • The compatibility of two or more admixtures in the same concrete must be verified.   Neslihan Yılmazer Chemical Engineer NesChem Founder and Technical Director