IELTS Academic Reading Practice Test 18

Concrete is the most widely used construction material on Earth, and by some estimates the most heavily consumed substance of any kind apart from water. It is a deceptively simple mixture: sand and gravel or crushed stone, known collectively as aggregate, held together by cement and water. The cement is the crucial ingredient. Contrary to popular belief, concrete does not harden by drying out. Instead, the cement reacts chemically with water in a process called hydration, forming crystals that grow and interlock around the particles of aggregate and bind them into a kind of artificial rock. For this reason concrete can set even under water, and it continues to gain strength for months after it has been poured.

The Romans were not the first people to use mortars based on lime, but they turned concrete into a material capable of supporting monumental architecture. Their key discovery was that mixing lime with a volcanic ash found near the town of Pozzuoli, on the Bay of Naples, produced a mortar that hardened even in seawater.

With this material they built harbours, aqueducts and public baths across the empire, often pouring the concrete between outer faces of brick or stone that served as permanent moulds. The most celebrated example is the dome of the Pantheon in Rome, completed in the second century CE, which remains the largest unreinforced concrete dome in the world. Its builders reduced the load on the structure by using progressively lighter aggregate towards the top, including pumice, a volcanic stone full of air bubbles.

Many Roman structures have lasted for two thousand years, while some modern concrete begins to crumble within decades, and scientists have long tried to understand why. Studies of ancient harbour walls have shown that seawater seeping into the concrete reacted with the volcanic ash to form new minerals that reinforced the material over time. In 2023, researchers at the Massachusetts Institute of Technology reported that small white lumps of lime, once dismissed as a sign of careless mixing, may have allowed Roman concrete to repair itself: when water entered a crack, it dissolved some of the lime, which then recrystallised and sealed the gap. After the fall of the Western Roman Empire, however, the use of concrete declined sharply in Europe, and much Roman expertise was lost for centuries.

The revival began in eighteenth-century Britain. While designing a new lighthouse on the Eddystone rocks off the coast of Devon in the 1750s, the engineer John Smeaton carried out experiments showing that lime made from limestone containing clay would set under water. In 1824 Joseph Aspdin, a bricklayer from Leeds, patented a product he called Portland cement, because he thought it resembled the pale building stone quarried on the Isle of Portland. Aspdin's material differed considerably from the cement used today, which owes more to later manufacturers who burned the raw materials at much higher temperatures. His name for it, however, has survived.

Modern Portland cement is produced in several stages. Limestone and clay or shale are quarried, crushed and then ground together into a fine powder known as raw meal. This is fed into a long, slowly rotating kiln, where it is heated to about 1,450°C. At this temperature the limestone gives off carbon dioxide and the remaining minerals partly fuse, forming hard grey lumps called clinker, typically about the size of marbles.

After cooling, the clinker is ground into a very fine powder together with a small amount of gypsum, which is added to control how quickly the cement sets. On a building site, the cement is mixed with water and aggregate, poured into temporary moulds called formwork and kept moist while it cures. The proportion of water is critical: a wetter mixture is easier to pour, but it leaves the hardened concrete weaker and more porous.

Concrete has one important weakness. It resists being squeezed extremely well, but it is weak in tension, so a concrete beam that bends will crack on the side that is stretched. The solution was to embed iron, and later steel, inside it. Among the pioneers was Joseph Monier, a French gardener who in 1867 patented plant containers made of concrete strengthened with iron mesh, and who later applied the idea to pipes, panels and bridges. The combination works well partly because steel and concrete expand and contract by similar amounts when the temperature changes. In 1903 the sixteen-storey Ingalls Building in Cincinnati became the first skyscraper with a reinforced concrete frame, and the technique soon spread around the world.

Today the world produces around four billion tonnes of cement a year, and this scale carries a serious environmental cost. Cement manufacture is responsible for roughly eight per cent of global carbon dioxide emissions. Around half of these emissions come not from burning fuel but from the chemical reaction in the kiln that releases carbon dioxide from limestone, so they cannot be eliminated simply by switching to renewable energy. Manufacturers are therefore replacing part of the clinker with other materials, such as ash from coal-fired power stations, slag from steelworks and specially heated clay, and researchers are investigating ways of capturing the gas released by kilns. The material that shaped the modern world will have to change if it is to have a place in a low-carbon future.

Every evening, as daylight fades, insect-eating bats emerge to hunt in conditions that would leave most predators helpless. They locate their prey not mainly by sight but by sound, emitting rapid calls and listening to the echoes that return from objects around them. This ability, known as echolocation, allows a bat to detect an insect as small as a mosquito, judge its distance and direction, and intercept it in mid-air, often in complete darkness. Of the more than 1,400 known species of bat, the great majority echolocate, although most of the large fruit bats of Africa, Asia and Australia rely on vision and smell instead.

How bats navigate in darkness puzzled naturalists for a long time. In the 1790s the Italian scientist Lazzaro Spallanzani found that blinded bats could still fly around obstacles without difficulty, and experiments by the Swiss surgeon Louis Jurine showed that bats whose ears had been blocked collided with objects. Because nobody could hear any sound from the flying animals, however, the idea that bats found their way by hearing was widely doubted for well over a century. The mystery was solved in 1938, when Donald Griffin, then an undergraduate at Harvard University, took bats to the laboratory of the physicist George Pierce, who had built a device capable of detecting sounds above the range of human hearing. Working with Robert Galambos, Griffin showed that bats produce intense ultrasonic calls in flight, and in 1944 he coined the term 'echolocation'.

Most bat calls lie between about 20 and 200 kilohertz, well above the upper limit of human hearing at around 20 kilohertz. High frequencies have short wavelengths, which allows them to reflect off very small targets, but they also fade rapidly in air, so a bat can detect an insect only within a range of a few metres. To compensate, many species call extremely loudly, and some produce sounds that are among the most intense made by any land animal. To avoid deafening themselves, bats contract tiny muscles in the middle ear as each call is emitted, reducing their sensitivity for a fraction of a second, and then relax them in time to hear the returning echo.

The echoes carry a remarkable amount of information. The time delay between a call and its echo reveals the distance of the target, while slight differences in the timing and loudness of the echo at the two ears indicate its direction. The strength and structure of the echo give clues to the size and surface texture of the object. Horseshoe bats go further: they emit long calls at an almost constant frequency and detect the tiny changes in pitch, known as Doppler shifts, produced when sound reflects from the beating wings of an insect. Because their own flight also raises the pitch of returning echoes, these bats continually lower the frequency of their calls, so that the echoes arrive at the frequency to which their hearing is most sensitive.

A hunting bat changes its calls as it closes in on its prey. While searching, it calls relatively slowly, perhaps ten times a second. Once it has detected an insect, it calls more often in order to update its information, and in the final moments before capture it produces what researchers call the terminal buzz, in which calls are emitted at a rate of up to about 200 per second. In 2011 a team studying Daubenton's bats reported that this rate is made possible by 'superfast' muscles in the larynx, which can contract and relax far more quickly than ordinary muscles used for movement. Before then, muscles of this kind had been identified only in a few animals, such as the rattling tails of rattlesnakes and the vocal organs of songbirds.

Insects have not remained defenceless. Many moths have evolved simple ears that are tuned to the frequencies of bat calls; when they detect an approaching bat, they turn sharply away or drop towards the ground. Some tiger moths respond with clicks of their own, which in many species warn the bat that the moth has an unpleasant taste. One species, Bertholdia trigona, goes further. In a study published in 2009, researchers used high-speed infrared cameras and ultrasonic microphones to show that its rapid clicks interfere with the bat's ability to judge the moth's position, so that attacks narrowly miss. It was the first known case of an animal jamming the sonar of its predator. Other moths rely on more passive defences. Some have wing scales that absorb ultrasound, reducing the strength of the echo, while luna moths have long tails on their hindwings that spin as they fly, drawing the bat's attack away from the body.

Echolocation is not unique to bats. Dolphins and other toothed whales use it underwater, and a few birds, such as the oilbirds of South America, produce clicks audible to humans to find their way through dark caves. Genetic studies have shown that bats and dolphins share similar changes in a gene called prestin, which is involved in sensitive high-frequency hearing, even though their common ancestor did not echolocate – a striking example of evolution arriving independently at the same solution. Bat sonar has also inspired engineers developing sensors for robots, although no artificial system yet matches the speed and precision of a small bat hunting in a cluttered forest.

Children have been a target for advertisers for almost as long as advertising has existed, but the scale and sophistication of the effort have grown enormously. A child today encounters commercial messages not only in television breaks and on posters but also in video games, on social media and in videos made by young online celebrities whose income depends on promoting products. Whether, and how, such marketing should be restricted is one of the most contested questions in consumer policy. The case for firm limits is a strong one. However, the rules on which most countries rely were designed for an age of television, and they are increasingly ill-suited to the world in which children actually live.

The argument for treating children differently rests on research into how they understand advertising. Studies dating back to the 1970s have shown that young children often cannot reliably tell advertisements apart from programmes, and that the ability to recognise an attempt at persuasion develops only gradually. In 2004 a task force of the American Psychological Association concluded that most children under about eight years old do not grasp the persuasive intent of advertising and tend to accept its claims as truthful.

Much of the advertising aimed at children relies on free toys, games and popular cartoon characters, which build an emotional attachment to a brand rather than providing information about the product itself. Even older children, who can explain perfectly well what advertisements are for, do not necessarily apply that knowledge when they are absorbed in something entertaining. Knowing that someone is trying to sell you something, it turns out, is not the same as being able to resist the attempt.

Some governments have acted on this evidence. Quebec has prohibited commercial advertising directed at children under thirteen since 1980, and when commercial television began in Sweden in 1991, advertising aimed at children under twelve was banned from the start. Measuring the effect of such bans is difficult, because children may still see advertisements on channels broadcast from elsewhere. Quebec, however, offered economists an unusual natural experiment. French-speaking families there watched mainly local channels covered by the ban, while English-speaking families often watched channels from the United States. A study by Tirtha Dhar and Kathy Baylis that compared the two groups estimated that the ban had reduced weekly spending on fast food in French-speaking households by around 13 per cent.

Food has been the main focus of regulation, because of concern about rising rates of childhood obesity. Chile has introduced one of the most ambitious packages of measures. Under a law that took effect in 2016, foods high in sugar, salt, saturated fat or calories must carry black warning labels on the front of the packet, cannot be sold in schools and cannot be marketed to children under fourteen, a rule that led manufacturers to remove cartoon characters from the boxes of many breakfast cereals. A study published in 2020 found that purchases of sugary drinks carrying the warnings had fallen by almost a quarter. Because the advertising restrictions were introduced at the same time as the labels and the school rules, however, it is impossible to say how much of this change each measure produced.

Opponents of such rules make several arguments that deserve to be taken seriously. Advertisers point out that parents, not children, make most purchasing decisions, and that it is the job of families rather than governments to teach children to be sceptical consumers. They also argue that advertising revenue pays for children's television, so restrictions may simply reduce the quality of programmes available. Furthermore, the link between advertising and health is indirect, since children's diets are shaped by family income, food prices and countless other factors. Yet the first argument underestimates the influence that children exert on family spending, widely known as 'pester power', and the second assumes that commercial funding is the only way to support good programmes – an assumption that the long history of public broadcasting contradicts.

The more serious difficulty for regulators is that traditional rules are being bypassed. Television restrictions usually apply to programmes made for children or to times of day when many children are watching, but young people increasingly watch on video-sharing platforms and social media, where content is tailored to each individual and the boundary between entertainment and promotion is often invisible.

An online personality who unwraps toys or samples snacks in front of the camera may be paid by the manufacturer, yet a young viewer is even less likely to recognise this as advertising than a conventional commercial. Platforms can also collect data on children's viewing habits and use it to target them with great precision. In 2019 Google and its subsidiary YouTube agreed to pay $170 million to settle allegations by United States authorities that they had collected personal information from children without their parents' consent.

Some governments have begun to respond. In January 2026 the United Kingdom brought into force rules that prohibit television advertisements for less healthy food and drink before nine o'clock in the evening and ban paid online advertising of such products altogether. Whether these measures will reduce obesity remains to be seen, and enforcing national rules on global platforms will not be easy. Nevertheless, the principle behind them is sound. Rather than attempting to identify which programmes children watch, regulation should focus on what children are capable of understanding, and should require companies to show that their marketing does not exploit those limits. Children cannot be expected to defend themselves against persuasion techniques that many adults fail to notice.