Aerated Autoclaved Concrete

Noaina Hassan Khan---Tanzeel Ahmed--- Reema Khan

Nadia Ali --- Atia Khursheed

Introduction

Autoclaved aerated concrete (“AAC”), is one of the many building products being touted as “green” or “environmentally friendly.”

Developed in Sweden in the 1920s in response to increasing demands on timber supplies, AAC is a lightweight manufactured building stone.

. Comprised of all natural raw materials, AAC is used in a wide range of commercial, industrial, and residential applications.

AAC now accounts for over 40% of all construction in the United Kingdom and more than 60% of construction in Germany.

Manufacturing Process AAC is a precast product manufactured by combining silica

(either in the form of sand, or recycled fly ash), cement, lime, water, and an expansion agent - aluminium powder, and pouring it into a mould.

In structurally reinforced AAC products like lintels or roof panels, steel rebar or mesh is also placed in the mould.

Once added to the concrete, the aluminium powder reacts with the silica, resulting in the formation of millions of microscopic hydrogen bubbles. The hydrogen bubbles cause the concrete to expand to roughly five times its original volume.

The hydrogen subsequently evaporates, leaving a highly closed-cell aerated concrete, then cut into which are further steam and pressure cured in an autoclave.

Product Types AAC products consist of load bearing, non-load

bearing and cladding applications. Complete load bearing applications, however,

are limited to low-rise construction. Cladding and large panels are available to take

advantage of AAC’s insulative, fire proofing, and other benefits on mid- and high-rise projects.

The large wall, floor, and roof panels, measuring up to 20 feet long, 2 feet wide, and in various thicknesses, are also used in load-bearing capacities and are common in commercial and industrial applications.

The remainder of the AAC system consists of blocks, also called “units” which are stacked using thin-set mortar, as opposed to the traditional cement-based mortar.

Types of BlocksThe blocks are available in a variety of sizes and types. STANDARD BLOCKS - typically measuring 24 inches long, 8

inches high, and in thicknesses between 6 (non-load-bearing) and 12 inches

CORED BLOCKS – also known as O-blocks, which are used adjacent to corners and openings and have a centred, 4 inch vertical core at one end of the block to form a continuous vertical core through the wall that is then filled with rebar and concrete.

JUMBO BLOCKS - which reduce construction time. U-BLOCKS- which have a channel running the

length of the block that once filled with concrete, provides structural support as headers and on the top course of each floor (the “bond beam”). AAC lintels with integrated structural support are also manufactured and are an alternative to using the U-block system for headers.

Product Advantages

AAC reduces additional material use and minimizes waste and pollution. AAC is approximately 80% air, the finished product is up to 5 times the volume of

the raw material consumed in its production. AAC production raw materials are generally locally sourced and may consist of

recycled materials. AAC consumes approximately 50%-20% less energy than that needed to produce

CMUs. The manufacturing process, produces no waste or pollution and the final product is

completely recyclable. Due to its light weight and dimensional accuracy, AAC can be assembled with minimal waste and a reduced need for additional equipment.

The product is virtually maintenance free, eliminating the need for repair materials, pesticides, and chemical treatments, while also lowering operating costs

AAC Has High Thermal Efficiency AAC structures result in solid wall construction with

integrated insulation, preventing thermal bridging, creating an airtight building envelope, minimizing uncontrolled air changes.

AAC benefits from low thermal conductivity value.AAC Reduces Noise Pollution and Improves Indoor Air Quality Its millions of independent air cells dampen sound

transmission. Because AAC construction has very low air

infiltration and is mold resistant, indoor air quality of AAC structures is improved.

AAC Structures Are Price Competitive AAC buildings constructed near a supplier’s

manufacturing facilities cost approx. the same as traditional materials.

AAC’s light weight helps lower shipping costs. The higher initial cost must be balanced against savings

due to lower operating and maintenance costs, the structure’s longer lifespan.

Easily workable Less time consuming due to large size of blocks. Light weight also makes it easy to handle.

AAC Structures Are Well-Suited to Withstand Fires, Earthquakes, and Other Natural Disasters

It receives a 4 hr. UL fire rating and has a melting point of over 2900° F.

AAC is also termite and mold resistant, and nearly fireproof.

Product Disadvantages

Familiarity with Product Few contractors are a currently familiar with the product, and trained

masons must adjust to using thin-set mortar as opposed to traditional cement-based mortar, which requires less precision in its application.

Scarcity of Manufacturing Plants. Projects far from manufacturing facilities will

suffer from higher initial costs.

Design ConsiderationsGeneral considerations

AAC masonry components(block units) can be used to build load bearing or non load bearing walls.

O-block units used to build pilasters.

U-block units used to build bond beams and lintels.

Control joints on AAC reinforced walls must be placed at maximum 16 ft. o.c.

Installation Guide Check foundation.

Receiving and distribution of AAC wall units.

Installation requirements. Tools Equipment Other materials

Installing O-block for pilasters in first course.

Laying the first course (levelling course). Lay the first course over a semi-dry

cement mortar levelling bed- ½” to 2” thick.

Corner blocks are laid first and the first course should be completed before second course installation.

Once corner blocks are placed apply thin bed mortar, to the vertical joints for other blocks.

Thin bed mortar 1/16” inch to 1/8”

Anchoring vertical reinforcement to

foundation

Cutting blocks (adjustments and chases) A hand saw or band saw to cut the blocks to specific lengths.

Placing control joints in first course These are vertical joints taken through the full wall thickness, and

from bottom to top. 3/8” to ½” thick. Maximum spacing between control joints should be 15 ft.

Laying the subsequent courses For subsequent courses use only thin bed mortaring on all joints

between AAC blocks. Minimum overlapping of vertical joints between layers should be

4”. Metal strip ties should be placed every two courses at – 1)

connection of secondary walls to main walls – 2) connection of walls to concrete columns. Control joints in subsequent layers.

V-shaped metal strips should be set at every two courses unless there are two pilasters on both sides of control joints and less than 2” from the joint.

Once the wall is built fill the gap using backer rod and seal with caulking.

Fill up pilasters by pouring concrete. Building on site lintels using U-blocks.

Install temporary supports before putting U-blocks in place apply thin bed mortar to the vertical joints.

Once U-block are set, place rebars according to construction drawings and with concrete.

Installing U-blocks to build bond beams. Lay U-block course applying thin bed mortar on all joints. At each pilaster location, drill a hole in the bottom side so

the vertical bars can be attached in the bond beam. Before pouring concrete place rebar and anchor bolts

according to construction drawings.

Utilities installation after the walls are built: For electrical conduits and piping installation, a chase is cut

using an electrical router or a chasing tool. When required depth of chase is bigger than maximum

depth recommended, additional O-blocks are used to lodge the pipes or interrupt wall continuity.

After installation, the chase are filled with repair mortar or cement sand mortar.

Bond beam and pilaster connection

Renders and finishes Surface patching: rasp block joints and other areas where

AAC surface is out of plane. Surface must be cleaned using a scrub brush and any loose

or damaged material be removed. A rubber float is commonly used to smooth the wall surface. Fiber glass mesh: This should be installed directly over one

layer of render in all control joints, around windows, doors and utility locations.

Finishes: AAC masonry walls can be finished with stucco , acrylic

texture coats, or a combination of both, also laminated stones, ceramic or clay tiles, concrete pieces and ornamental products.

Anchoring to AAC: Fasteners: Anchors used with AAC shall be made of plastic or nylon.

Wood, fiber, lead, metal or expansion anchors are not recommended. Use power drills to make holes for fasteners. Percussion drilling or

inverting the rotation direction when drilling shall be avoided. The anchor shall penetrate tightly to avoid rotation when placing the

screw. Hebel AAC nail: Galvanized nails are designed specifically to provide

definitive anchorage in the AAC, and are directly hammered into it. Screws: minimum length of screw is defined by anchor length plus

thickness of the finish layer and the element to be fixed.