Lightweight concrete solutions

1 Introduction to light weight concrete solutions
2 Lightweight Aggregate Concrete LWAC
3 Aerated Concrete AC
4 Autoclaved Aerated Concrete AAC
5 Reinforced Autoclaved Aerated Concrete RAAC
- 5.1
- 5.2 Contemporary issues with RAAC
6 Aircrete
7 Related articles on Designing Buildings
8 External links

[edit] Introduction to light weight concrete solutions

Lightweight concrete, sometimes referred to as cellular concrete is a general term covering a variety of concrete solutions with reduced weights, these products vary but in general will include lightweight aggregate concrete, aerated concrete, autoclaved aerated concrete and what is known as aircrete. Many variations on these products exist, adjusting ingredients and processes to improve performance, as well as introducing other elements such as for example Reinforced Autoclaved Aerated Concrete or RAAC, or significantly different approaches such as no-fines concrete.

[edit] Lightweight Aggregate Concrete LWAC

Lightweight concrete is a general term for various concrete solutions with reduced weight including air pockets. Lightweight aggregate concrete is a specific solid concrete solution that makes use of lightweight aggregates with minimal air voids. It is usually made with a mixture of lightweight coarse aggregates such as expanded shale or slate (heated to over 1000 degrees C), lightweight expanded clay aggregate (LECA or Hydroton), pumice stone, ash, perlite and some other minerals.

Some of the earliest lightweight concretes used pumice from Italy or Greece with burnt and hydrated lime. The Romans however, by removing the impurities of lime such as silica, alumina and iron oxides, started to produce stronger binders, known as grey lime, and stronger concrete, which today we refer to as Roman concrete. As part of this development, lightweight aggregates continued to be used but with the use of grey lime producing stronger lightweight concrete construction results.

In the early 1820's an English bricklayer named Joseph Aspdin, used pulverised impure siliceous materials and raw limestone, better known now as Portland cement, the strength possibilities of lightweight aggregate concrete were yet again significantly improved. By the 1840's this lightweight and effectively floating material was used to build the first concrete shi, a dinghy built by Joseph Louis Lambot in France and featured at the 1855 World Fair.

In the early 1900's, prior to the war, the introduction of heat applications to expand certain minerals such as slate, clay and shale, further opened opportunities. By the time the war came and as a result of material shortages, lightweight concrete received further consideration and investment for the building of ships to replace steel which was in short supply. In 1917 shortages of high-grade plate steel lead to US shipbuilding programs using materials other than steel, of which lightweight aggregate concrete was a forerunner, because it had the potential to reduce the deadweight of ships whilst maintaining the strength and durability required.

It was shortly after this that the benefits of lightweight aggregate concrete seen in shipbuilding were proposed as being advantageous to construction, reducing both the steel reinforcement and the concrete needed to construct towers through the reduced dead loads on the structures. As a result the first lightweight aggregate concrete building, a school in Kansas was constructed, others followed, notably the advantage of the product was applied to high rise buildings, effectively allowing buildings to be built taller due to reduced structural loads, with the first high rise, a 28 storey building, the Chase park Plaza in Chicago, and the first high rise built from light weight aggregate concrete being completed in 1928.

Since then lightweight aggregate concrete as a solution for the construction of towers, bridges and shelters continued. Today structural lightweight aggregate concrete continues to be used as a construction solution. Contemporary products vary but in-place densities of 90 to 115 lb/ft³, compared to regular weight concrete with ranges ranges from around 140 to 150 lb/ft³.

[edit] Aerated Concrete AC

Aerated concrete (AC) also called cellular concrete is often used to describe any concrete product that contains significant pockets of air in a controlled manner. Usually it is prepared with water, cement, a foaming agent and perhaps a fine light aggregate such as sand, but rarely with aggregates. The foaming agent introduces air bubbles into the finished product, making it lighter and having insulative properties.

When the air content within concrete is about 1.5% - 3% and bubbles are relatively large in size, over around 1mm and randomly located throughout the mix it is referred to as entrapped air, mostly an unintended consequence of mixing concrete, with insufficient agitation. When the air is uniformly distributed in microscopic bubbles of less than 1mm it is referred to as air entrainment. When it makes up between 3%-8% by volume of the finished product it is often done so to improve the performance under freeze thaw conditions. Aerated concrete or cellular concrete however normally refers to concrete products that have a higher content around 25-25% of air bubbles, which may be higher if no structural performance is required.

As early as 1889 Czech Hoffman tested and patented a method of aerating concrete with the use of carbon dioxide, whilst in 1914 Aylsworth and Dyer used aluminium powder and calcium hydroxide to create and patent a porous cementitious mixture. This might be considered as a very early form of aerated concrete, though much of the credit is associated with Johan Axel Eriksson who developed both aerated and autoclaved aerated concrete as described below.

[edit] Autoclaved Aerated Concrete AAC

Autoclaved Aerated Concrete (AAC) is essentially very similar to aerated concrete but it is cured under heat and pressure in an autoclave, to produce a lightweight fire and mould resistant product with insulative qualities as well as strength and durability. Composed of quartz sand, calcined gypsum, lime, Portland cement, water and aluminium powder it is used to form blocks, lintels, wall, floor and roof panels as well as cladding units.

The Architect and inventor Johan Axel Eriksson is generally credited with perfecting the technique of producing a limestone and ground slate, lime formula in the early 1920's with Professor Henrik Kreüger at the Royal Institute of Technology in Sweden. They found that the foamed concrete product could withstand the moisture and pressure of autoclaving and that this sped up the curing process without causing shrinkage, but improving performance. The technology was patented using ground slate or alum shale and started to be mass produced from the late 30's.

Other AAC manufacturers in Sweden and other countries started to produce variations on the product from around the 1940's up to the war. These products also included reinforced autoclaved aerated concrete (RAAC) using Eriksson's initial formula to develop panel, roofing and flooring systems with the goal of developing a whole house from the same base product.

Between 1945 and 1975 mass building programs across Europe searched for modern methods to construct buildings quickly and cost effectively, and as such innovative new products were employed extensively, with autoclaved aerated concrete being a key one. This alongside insulation products, glazing and window frame systems, and a variety of prefabricated and precast systems that often included lightweight elements from AAC or RAAC, collectively today are sometimes referred to as post-war building materials.

Sometime later in the 1970's it was discovered that natural uranium within the alum shale found in Sweden, caused some radioactive radon gas exposure, which led to a new recipe being developed. This formula contained quartz sand, calcined gypsum, lime(mineral), cement, water and aluminium powder, but no contaminated alum shale. It is the base recipe of many aerated concrete blocks that are produced today.

The aluminium powder is an air entraining agent and reacts with the calcium hydroxide formed on hydration of cement to produce hydrogen gas bubbles. The agent is mixed with the a fine aggregate (usually sand or fly ash), cement, lime, gypsum, and water and reacts on hydration creating pockets. Autoclaved aerated concrete (AAC) is essentially the same but once poured and setting, is placed in an autoclave which applies steam and pressure, to speed up the curing process and create a stronger product.

[edit] Reinforced Autoclaved Aerated Concrete RAAC

Reinforced autoclaved aerated concrete (RAAC) is essentially the same autoclaved aerated concrete(AAC) but containing reinforcement elements which improve tensile strength and allow larger, thinner panels to be made from the same aerated product.

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RAAC was first developed not long after the commercial development of autoclaved aerated concrete (AAC in the 1930's), the first reinforced AAC products were also developed in Sweden (where AAC was developed) not by the original manufacturer of AAC but by a competitor. The goal of developing reinforced elements was to be able to manufacture an entire building from AAC based systems, including panels and floor elements.

RAAC products were further improved by a German firm and began establish themselves in the market by the 1940's. Between 1945 and 1975 mass building programs across Europe searched for modern methods to construct buildings quickly and cost effectively, and as such many innovative new products were developed during this period (known collectively as post-war building materials). AAC and RAAC or lightweight concrete systems became key elements in many of these building programmes, alongside other products developed during the same period such as insulation products, glazing and window frame systems, and a variety of prefabricated and precast systems including lightweight concrete.

[edit] Contemporary issues with RAAC

Crumbling concrete describes failure where concrete parts fall off in chunks and the strength of the overall material is compromised. It can simply be the result of ageing but maybe worsened where the initial pour was not managed correctly, where reinforcement elements are beginning to corrode or where the material is exposed to harsh elements or mechanical damage. This type of failure has started to be associated more closely with RAAC, the reasons vary, with specific cases but some RAAC installations were built as early as the 1950's and so maybe seeing issues because the products have exceeded their expected lifespan. In many cases issues arise through the failure of waterproof coverings, such as flat roofs, allowing the external surface of the RAAC to become wet, once damp, over time, it can effectively soak up moisture, similar to a sponge, leading to corrosion of the reinforcement. Other related issues with newer product installations may be as a result of either poor design or fabrication / installation issues often relating to the location of the reinforcement too near the edges, relating again to moisture and corosion, and the concrete mix.

In December 2022 the UK Government published 'Reinforced autoclaved aerated concrete (RAAC): estates guidance'. The guidance set out a 5-stage approach for the identification and management of RAAC in educational buildings, where these may be present in floors, walls and roofs (pitched and flat) of buildings constructed or modified between the 1950s and mid-1990s. This guidance outlined initial steps that should be taken by those responsible for the management of educational buildings about how to procure building professional’s services when specialist advice is needed. It was designed for all parties involved in the identification and management of RAAC, including estates managers and those providing specialist advice.

In mid 2023 UK ministers Ministers launched a UK government-wide inquiry into the use of crumbling concrete, in particular occurring in reinforced autoclaved aerated concrete (RAAC). Initial indication is that many of the installations at risk are over 30 years old which may be beyond the expected lifespan of the product. Typically these are low-rise flat roofed structures built between the mid-1960s and mid-1990s primarily of RAAC blocks. At the end of the school summer break in September 2023, the Department for Education (DfE) announced 156 schools in England were affected by RAAC, of those 104 required urgent action and 56 had already undergone repair work. An estimated and unconfirmed number of 24 schools in England were told to close entirely, and thus were not open for te new school year.

[edit] Aircrete

Aircrete describes any concrete product that contains significant pockets of air. Usually it is prepared with water, cement, and a foaming agent, sometimes with sand but rarely with aggregates. The foaming agent in the case of aircrete, cellular concrete or foamed concrete as it is often called, is prepared into a foam which is then mixed into the slurry to introduce air bubbles into the finished product, making it lighter and having insulative properties. This is as opposed to other methods which introduce air entraining foam agents, such as aluminium powder which are introduced earlier and create bubbles through a chemical reaction with the concrete.

This product might also be referred to as aerated concrete (AC), air entrained concrete (where bubbles are less than about 1mm in size), air entrapped concrete (where bubbles are larger than about 1mm in size), aerated cellular concrete (ACC), aerated lightweight concrete (ALC) or non-autoclaved aerated concrete, as well as aerated concrete blocks, insulation blocks, breeze blocks or lightweight blocks.

Aircrete, might generally be considered to be the same process used to produce AC but without the autoclaving, ie it is cured naturally, normally in a sealed plastic sheet. The term aircrete itself is said to have been coined by the organisation DomeGaia much later in 2014, and generally refers to air cured aerated, cellular, foamed, or lightweight concrete. The organisation was set up by the Hajjar Gibran a longtime engineer and design / build contractor, who with Steve Areen, constructed original domed structures in Thailand as housing units. The business is now run by his son and her partner and continues to build across the globe, whilst supporting enthusiasts with other bespoke products such as foam machines and designs.