第1780期:What if cracks in concrete could fix themselves

Concrete is the most widely used construction material in the world. It can be found in swathes of city pavements, bridges that span vast rivers, and the tallest skyscrapers on earth. But this sturdy substance does have a weakness: it’s prone to catastrophic cracking that costs tens of billions of dollars to repair each year. But what if we could avoid that problem, by creating concrete that heals itself?

混凝土是世界上使用最广泛的建筑材料。它可以在大片的城市人行道、横跨浩瀚河流的桥梁以及地球上最高的摩天大楼中找到。但这种坚固的物质确实有一个弱点：它很容易发生灾难性的开裂，每年需要花费数百亿美元进行修复。但是，如果我们可以通过创造能够自我修复的混凝土来避免这个问题呢？

This idea isn’t as far-fetched as it may seem. It boils down to an understanding of how concrete forms, and how to exploit that process to our benefit. Concrete is a combination of coarse stone and sand particles, called aggregates, that mix with cement, a powdered blend of clay and limestone. When water gets added to this mix, the cement forms a paste and coats the aggregates, quickly hardening through a chemical reaction called hydration. Eventually, the resulting material grows strong enough to prop up buildings that climb hundreds of meters into the sky.

这个想法并不像看起来那么牵强。它归结为对具体形式的理解，以及如何利用该过程使我们受益。混凝土是粗石和沙粒的组合，称为骨料，与水泥混合，水泥是粘土和石灰石的粉末混合物。当向这种混合物中加入水时，水泥会形成糊状物并覆盖在骨料上，通过称为水合作用的化学反应迅速硬化。最终，由此产生的材料变得足够坚固，可以支撑数百米高的建筑物。

While people have been using a variety of recipes to produce cement for over 4,000 years, concrete itself has a surprisingly short lifespan. After 20 to 30 years, natural processes like concrete shrinkage, excessive freezing and thawing, and heavy loads can trigger cracking. And it’s not just big breaks that count: tiny cracks can be just as dangerous. Concrete is often used as a secondary support around steel reinforcements. In this concrete, even small cracks can channel water, oxygen, and carbon dioxide that corrode the steel and lead to disastrous collapse. On structures like bridges and highways that are constantly in use, detecting these problems before they lead to catastrophe becomes a huge and costly challenge. But not doing so would also endanger thousands of lives.

4000 多年来，人们一直在使用各种配方生产水泥，但混凝土本身的使用寿命却出奇地短。 20 到 30 年后，混凝土收缩、过度冻融和重载等自然过程会引发开裂。重要的不仅仅是大的裂缝：微小的裂缝也同样危险。混凝土通常用作钢筋周围的辅助支撑。在这种混凝土中，即使是很小的裂缝也会输送水、氧气和二氧化碳，腐蚀钢材并导致灾难性的倒塌。在经常使用的桥梁和高速公路等结构上，在导致灾难之前检测这些问题成为一项巨大且代价高昂的挑战。但不这样做也会危及成千上万人的生命。

Fortunately, we’re already experimenting with ways this material could start fixing itself. And some of these solutions are inspired by concrete’s natural self-healing mechanism. When water enters these tiny cracks, it hydrates the concrete’s calcium oxide. The resulting calcium hydroxide reacts with carbon dioxide in the air, starting a process called autogenous healing, where microscopic calcium carbonate crystals form and gradually fill the gap. Unfortunately, these crystals can only do so much, healing cracks that are less than 0.3mm wide.

幸运的是，我们已经在试验这种材料开始自我修复的方法。其中一些解决方案的灵感来自混凝土的自然自我修复机制。当水进入这些微小的裂缝时，它会水合混凝土中的氧化钙。生成的氢氧化钙与空气中的二氧化碳发生反应，开始一个称为自生愈合的过程，在此过程中会形成微小的碳酸钙晶体并逐渐填充间隙。不幸的是，这些晶体的作用有限，只能治愈宽度小于 0.3 毫米的裂缝。

Material scientists have figured out how to heal cracks up to twice that size by adding hidden glue into the concrete mix. If we put adhesive-filled fibers and tubes into the mixture, they’ll snap open when a crack forms, releasing their sticky contents and sealing the gap. But adhesive chemicals often behave very differently from concrete, and over time, these adhesives can lead to even worse cracks.

材料科学家已经想出如何通过在混凝土混合物中添加隐藏的胶水来修复两倍大小的裂缝。如果我们将充满粘合剂的纤维和管子放入混合物中，当裂缝形成时，它们会突然打开，释放出粘性物质并密封缝隙。但是粘合剂化学品的表现通常与混凝土非常不同，随着时间的推移，这些粘合剂会导致更严重的裂缝。

So perhaps the best way to heal large cracks is to give concrete the tools to help itself. Scientists have discovered that some bacteria and fungi can produce minerals, including the calcium carbonate found in autogenous healing. Experimental blends of concrete include these bacterial or fungal spores alongside nutrients in their concrete mix, where they could lie dormant for hundreds of years. When cracks finally appear and water trickles into the concrete, the spores germinate, grow, and consume the nutrient soup that surrounds them, modifying their local environment to create the perfect conditions for calcium carbonate to grow. These crystals gradually fill the gaps, and after roughly three weeks, the hard-working microbes can completely repair cracks up to almost 1mm wide. When the cracks seal, the bacteria or fungi will make spores and go dormant once more— ready to start a new cycle of self-healing when cracks form again.

因此，也许修复大裂缝的最佳方法是为混凝土提供自救工具。科学家们发现，一些细菌和真菌可以产生矿物质，包括在自体愈合中发现的碳酸钙。混凝土的实验混合物包括这些细菌或真菌孢子以及混凝土混合物中的营养物质，它们可以在那里休眠数百年。当裂缝最终出现并且水滴入混凝土时，孢子就会发芽、生长并消耗它们周围的营养液，从而改变它们的局部环境，为碳酸钙的生长创造完美的条件。这些晶体逐渐填补了空隙，大约三周后，辛勤工作的微生物可以完全修复近 1 毫米宽的裂缝。当裂缝闭合时，细菌或真菌会产生孢子并再次进入休眠状态——准备好在裂缝再次形成时开始新的自我修复循环。

Although this technique has been studied extensively, we still have a ways to go before incorporating it in the global production of concrete. But, these spores have huge potential to make concrete more resilient and long-lasting— which could drastically reduce the financial and environmental cost of concrete production. Eventually, these microorganisms may force us to reconsider the way we think about our cities, bringing our inanimate concrete jungles to life.

虽然这项技术已被广泛研究，但在将其纳入全球混凝土生产之前，我们还有一段路要走。但是，这些孢子具有使混凝土更具弹性和更持久的巨大潜力——这可以大大降低混凝土生产的财务和环境成本。最终，这些微生物可能会迫使我们重新考虑我们对城市的看法，让我们死气沉沉的混凝土丛林栩栩如生。