领域：学术论文 · 自然科学

Concrete is a composite construction material composed of a cementitious binder (usually Portland cement), water, and aggregates (sand and gravel or crushed stone), along with various admixtures that may be added to modify its properties. When these ingredients are first mixed together, the fresh concrete is a viscoplastic, moldable mixture that can be placed into forms. A series of chemical reactions called cement hydration begins as soon as water contacts the cement particles. Through hydration, the cement paste gradually hardens and gains strength, binding the aggregates together into a solid mass. This transformation from a fluid plastic state to a rigid, load-bearing solid typically occurs within a few hours after mixing (initial setting often happens after around 2 hours for a normal mix). Strength development then continues over time as curing progresses – concrete keeps gaining strength for months or even years, although the rate of strength gain diminishes with time. In practice, the compressive strength measured at 28 days (the 28-day strength) is commonly used as a benchmark of concrete quality (denoted as f’c in many specifications). 
About 90% of the long-term strength is usually achieved in the first month under proper curing conditions. Proper curing during this period is critical to concrete quality: the concrete must be prevented from drying out or freezing at early ages. Newly placed concrete is typically kept moist (for example by frequent water spraying or by covering with wet burlap and plastic sheets) and maintained at a suitable temperature. Such curing ensures that hydration can progress and that the concrete’s surface does not crack or lose strength prematurely. If concrete is allowed to dry too soon, it will stop gaining strength and may develop surface shrinkage cracks; if it freezes before sufficient hardening, its internal structure can be disrupted. Therefore, curing measures are often applied for at least 7–14 days to promote good strength and durability development. The strength of hardened concrete depends on many factors, including the mix proportions, the properties of the constituent materials, the curing regime, and the presence of any admixtures or additives. One of the most important factors is the water-to-cement ratio (w/c ratio). This is the ratio of the mass of water to the mass of cement in the mix. A lower w/c ratio generally leads to higher strength and density (since less excess water leaves behind fewer pores in the hardened concrete), but if the w/c is too low, the mix may become unworkable (too stiff and difficult to place). For complete hydration of cement, a minimum w/c of around 0.22–0.25 by weight is needed, but typical structural concrete mixes use a w/c in the range of 0.4 to 0.6 to ensure adequate workability. Workability refers to the ease with which fresh concrete can be mixed, placed, compacted, and finished without segregation. To improve workability without adding excess water (which would reduce strength and increase permeability), chemical admixtures such as plasticizers (water-reducing agents) are often used. For example, a high-range water reducer (superplasticizer) can produce a highly flowable concrete (even self-leveling) at a w/c as low as ~0.30, enabling very high strengths while still allowing the mix to be placed easily. 
A highly workable mix can flow and fill formwork and surround dense reinforcement with relative ease, but if too much water or improper proportions are used, the mix may segregate or bleed. Segregation is the separation of coarse aggregate from the mortar (cement paste and fines) due to settling, resulting in a non-uniform composition. Bleeding is a related phenomenon where excess water rises to the top surface of the concrete before it sets. Both segregation and bleeding indicate an unstable mix; they can cause weak, porous regions and lead to finishing problems or reduced durability in the hardened concrete. Proper mix proportioning and the use of fines and viscosity-modifying admixtures can minimize these issues. The workability of fresh concrete is commonly measured by the slump test, which provides an empirical indication of concrete consistency. In a standard slump test, fresh concrete is placed into a frustum-shaped cone mold (12 inches high) in three layers, and each layer is rodded (tamped) a specified number of times. The mold is then carefully lifted off, and the unsupported concrete slumps under its own weight. The vertical drop of the concrete’s top (measured relative to the mold height) is called the slump. A higher slump (e.g. 7 or 8 inches) indicates a very wet or fluid mix, while a low slump (e.g. 1 inch) indicates a dry, stiff mix. Typical concretes for general construction have slumps in the range of about 2 to 4 inches (50–100 mm) for a good balance of workability and strength. However, slump does not capture all aspects of workability; different mixes can have the same slump yet handle very differently (for instance, one may segregate more easily than another). Other tests and measures (such as flow table tests for mortar, the compaction factor test, or more advanced rheometer tests) can characterize workability, especially for specialized concretes. Still, the slump test remains the simplest and most widely used field test to verify batch-to-batch consistency of concrete and ensure that the fresh mix has not changed. In recent years, self-consolidating concrete (SCC) has been developed as an extremely high-workability mix that can flow and compact under its own weight without vibration. SCC is achieved by using advanced admixtures (superplasticizers) and viscosity-modifying agents to produce a fluid yet stable mix. It demonstrates that with appropriate mix design, one can obtain both high workability and high strength (through a low w/c ratio), which is valuable for placing concrete in structures with congested reinforcement or complex formwork. As the fresh concrete gradually sets and hardens, its properties transition to those of the hardened material. The density of normal-weight concrete is around 2400 kg/m³ (about 150 lb/ft³), and its compressive strength can range widely. Ordinary structural concrete typically achieves a 28-day compressive strength on the order of 20–40 MPa (3000–6000 psi), whereas high-strength concrete used in high-rise buildings or other special applications can reach strengths of 70 MPa (10,000 psi) or more. Concrete’s tensile strength is much lower than its compressive strength (on the order of only 10% of the compressive strength). For this reason, concrete is usually reinforced with steel bars (reinforcing steel, or “rebar”) to carry tension. The stress–strain curve of concrete in compression is nonlinear – unlike steel, concrete does not have a long elastic range or a well-defined yield point. It initially deforms approximately elastically at very low stress, but as stress increases, the curve gradually bends (due to microcracking and increasing inelasticity) and eventually reaches a peak, followed by a drop-off as the concrete fails by crushing. Because there is no sharply defined elastic limit, the modulus of elasticity of concrete is usually defined either as an initial tangent modulus or a secant modulus to a point on the curve (for example, to 50% of ultimate strength). For typical normal-strength concrete, a common empirical approximation is that the Young’s modulus is about 20–30 GPa (gigapascals), which corresponds to around 3 to 5 million psi. 
Higher-strength concretes have higher stiffness, though not in direct proportion to their strength. Volume stability is another important aspect of concrete performance. As concrete cures and dries, it undergoes shrinkage – a reduction in volume. Drying shrinkage occurs as water that is not chemically bound in hydration products gradually evaporates from the hardened concrete. If a large element (like a long slab or wall) is restrained while it is shrinking (for example, by the subgrade or by adjacent structural elements), tensile stresses can develop, potentially leading to cracking. To control crack formation, engineers often incorporate contraction joints (planned grooves or weak planes where shrinkage cracks can form in a controlled way) and provide sufficient distributed reinforcement (sometimes called temperature and shrinkage steel) to hold any cracks tightly closed. Thermal changes can also cause concrete to expand or contract. For instance, the heat released during cement hydration can raise the temperature of a large concrete mass in early-age, and subsequent cooling can induce tensile stresses. In hot weather, rapid surface drying and cooling of freshly placed concrete can cause a surface contraction while the interior is still warm, sometimes leading to a pattern of shallow cracks called thermal cracking. Measures such as gradual cooling of massive pours, insulating blankets, or using low-heat cements help mitigate these thermal stresses. In addition to shrinkage and thermal effects, concrete exhibits creep, which is the long-term gradual deformation under sustained load. Under a constant compressive stress (such as a heavy dead load on a column or beam), concrete will slowly deform over months and years beyond the initial elastic strain – this time-dependent strain is creep. Creep strain can be significant, sometimes on the same order as the initial elastic strain. It is influenced by factors like the concrete’s compressive strength, mix composition, age at loading, and the magnitude of the sustained stress (relative to strength). In structural design, creep is taken into account because it can increase deflections of beams over time and can cause redistribution of internal forces in statically indeterminate structures. However, keeping sustained stresses low (e.g. by using a higher concrete strength or adding extra reinforcement) can limit the magnitude of creep deformation. Durability is a critical aspect of civil materials engineering, and concrete must be designed to withstand the environmental conditions to which it will be exposed over decades. A key durability concern in cold climates is resistance to freeze–thaw cycles. When porous, saturated concrete freezes, the water in its pores expands about 9% in volume, which can create internal stresses that crack the concrete. Repeated freezing and thawing can cause progressive damage such as surface scaling and loss of strength. To improve freeze–thaw durability, modern concrete often includes air-entraining admixtures that create a network of microscopic air bubbles in the hardened concrete. 
These tiny air voids (~0.1 mm in size, with a proper spacing factor) act as pressure relief chambers for freezing water, thereby reducing internal stress. Air-entrained concrete can survive hundreds of freeze–thaw cycles without significant deterioration, whereas non-entrained concrete (especially if it has a high w/c ratio and hence high saturation) may suffer severe damage after just a few winters. Another long-term durability issue is chemical attack. For instance, sulfate attack occurs when sulfate ions (from sulfate-rich soils, groundwater, or seawater) penetrate the concrete and react with components of the cement paste (like calcium aluminate hydrates), forming expansive products such as ettringite and gypsum. These expansions can disrupt the concrete’s matrix, causing cracking and loss of cohesion over time. To combat sulfate attack, engineers can use sulfate-resistant cement (with low tricalcium aluminate content), incorporate pozzolanic materials, and ensure the concrete has a low permeability (achieved by a low w/c ratio and adequate curing). Similarly, a deleterious reaction called alkali–silica reaction (ASR) can occur between alkali hydroxides in cement paste and certain reactive silica minerals in some aggregates. ASR produces an expansive gel that swells as it absorbs water, leading to internal pressures and a pattern of map cracking in the concrete over years. To prevent ASR, engineers may use low-alkali cement and incorporate supplementary cementitious materials (like fly ash or slag) to tie up alkalis and dilute the reactive constituents, and they avoid using known reactive aggregates when possible. Moreover, many modern concrete mixtures include supplementary cementitious materials (SCMs) such as fly ash (a by-product from coal power plants), ground granulated blast-furnace slag, and silica fume. These SCMs participate in secondary pozzolanic reactions with the cement hydration products, refining the pore structure of the concrete and often improving its long-term performance. Concretes with SCMs tend to have lower permeability and higher later-age strength, which enhances durability by reducing the ingress of harmful agents. For example, adding fly ash or slag can significantly mitigate sulfate attack and alkali–silica reaction, while silica fume is well known for increasing strength and making the concrete matrix denser. The inclusion of SCMs also has sustainability benefits by recycling industrial by-products and reducing the required amount of Portland cement. Furthermore, reinforcement corrosion is a major threat to reinforced concrete structures, especially in harsh environments. Steel rebar embedded in concrete is normally protected by the high alkalinity of cement paste (which forms a thin passive oxide layer on the steel surface). 
However, if chloride ions (for example, from deicing salts or marine spray) penetrate through concrete and reach the steel, they can break down this passive layer and cause corrosion of the rebar. The corrosion products (iron oxides and hydroxides) occupy a greater volume than the original steel, exerting expansive pressure inside the concrete. This leads to cracking and spalling of the concrete cover, further accelerating deterioration. To improve durability against reinforcement corrosion, concrete can be made with low permeability (to slow chloride ingress) by using a low w/c ratio and SCMs; sufficient concrete cover thickness over the steel is provided to lengthen the travel path for chlorides; and sometimes corrosion-inhibiting admixtures or epoxy-coated (or galvanized) rebars are used. Good construction practices, such as adequate curing and avoiding any premature drying or thermal shock, also help produce a dense, impermeable concrete less vulnerable to these forms of attack. In summary, civil engineering materials like concrete must be designed not only for initial strength and workability but also for long-term durability. A well-proportioned concrete mix with an appropriate water–cement ratio, good-quality aggregates, and suitable admixtures can achieve the required strength while remaining sufficiently workable for placement. Through proper curing and quality control, the concrete will develop the desired mechanical properties (high compressive strength, adequate modulus, etc.) and a dense microstructure that resists deleterious agents. Attention to potential deterioration mechanisms – such as freeze–thaw damage, sulfate or chemical attack, ASR, and steel corrosion – is essential to ensure that structures made of concrete remain safe and serviceable throughout their intended life span. Ongoing advancements in materials engineering continue to improve concrete technology, yielding new types such as high-performance concrete (HPC) and fiber-reinforced concrete, which offer enhanced workability, strength, and durability for the infrastructure of the future.

翻译考点：
1.        “workability” 推荐翻译为 “和易性”，不应该译为“工作性”
2.       “slump test” 推荐翻译为 “坍落度试验”
3.       “water-cement ratio (w/c)” 推荐翻译为 “水灰比”
4.       “curing” 推荐翻译为 “养护”
5.       “hydration” 推荐翻译为 “水化作用”
6.       “creep” 推荐翻译为 “徐变”
7.       “shrinkage” 推荐翻译为 “收缩”
8.       “segregation” 推荐翻译为 “离析”
9.       “bleeding” 推荐翻译为 “泌水”
10.     “air-entraining admixture” 推荐翻译为 “引气剂”
11.     “freeze–thaw cycle” 推荐翻译为 “冻融循环”
12.     “sulfate attack” 推荐翻译为 “硫酸盐侵蚀”
13.     “alkali–silica reaction (ASR)” 推荐翻译为 “碱-硅反应”
14.     “modulus of elasticity” 推荐翻译为 “弹性模量”
15.     “map cracking” 推荐翻译为 “龟裂” 或 “龟甲裂缝”，不应该直译为“地图状裂缝”）
16.     “thermal cracking” 推荐翻译为 “热裂缝” 或 “温度应力裂缝”
17.     “rebar” 推荐翻译为 “钢筋”
18.     “creep strain” 推荐翻译为 “徐变应变”
19.     “impermeable concrete” / “dense concrete” 推荐翻译为 “低渗透性混凝土” 或 “致密混凝土”
20.     “passive film” 推荐翻译为 “钝化膜”
21.     “strength gain” 推荐翻译为 “强度发展” 或 “强度增长”
22.     “superplasticizer” 推荐翻译为 “超塑化剂” 或 “高效减水剂”