Green concrete roof

A green or vegetated roof provides the function of a conventional roof, while allowing plants to grow on the surface. A vegetated roof includes water proofing, a drainage system, filter layer, a lightweight growing medium and plants.

An ‘extensive’ green roof has a relatively shallow soil profile (25 to 125 mm) and is planted with ground cover plants that are adapted to the harsh conditions of the rooftop microclimate. ‘Intensive’ green roofs refer to more substantial roof gardens with deeper soil (150 mm or more) and are often planted with shrubs, and trees as well as ground cover.

Vegetated roofs are ideal for any place where people spend their day, including residential communities, office buildings, hospitals, day care centers, schools, recreational facilities, shopping centers and airports.

Concrete is the structural system of choice for vegetated roofs because it provides a continuous load-bearing surface for the potentially moist growing medium and plants. In addition, cast-in-place concrete provides a uniform surface. Projects constructed using waterproof concrete made with proprietary admixtures allow for the elimination of membranes and therefore simplify design, construction and maintenance processes. Lightweight concrete topping can be used to create the sloping deck of a vegetated roof system. In addition, structural lightweight aggregate can be used as a lightweight, absorptive portion of the growing medium. Lightweight applications reduce the dead load on the roof structure.

Read more on the subject in the application note by Portland Cement Association (PCA)

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Reducing carbon footprint of concrete

As I understand on date, there is no carbon neutral or carbon negative concrete mix design available, as all concrete contains cement. The production of one tonne of cement emits nearly the same amount of carbon dioxide in the atmosphere. It is interesting to know that fresh concrete absorbs some of that CO2 back during carbonation which takes place during the setting and hardening process. One effective way to reduce the carbon footprint of concrete is to minimise the use of cement, partially replacing it with the mineral admixtures like fly ash, blast furnace slag, rice husk ash, which are industrial and agricultural wastes. These admixtures, besides giving long term strength to concrete also enhance its durability.

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Green Buildings LEED Certification:

Many local or national institutions have put a variety of certification labels for green buildings (LEED, BREAMÒ, HQEÒ, Built Green, etc.), to help stakeholders guarantee a certain quality level and to promote best practices.

Maybe the most famous and well-established worldwide is the LEED (Leadership in Energy and Environmental Design) certification. The United States Green Building Council (USGBC), a

national non-profit entity, developed the Leadership in Energy and Environmental Design (LEED), a Green Building Rating System, used to rate new and existing commercial, institutional and high-rise residential buildings according to their environmental attributes

and sustainable features. The LEED system utilizes a list of 34 potential performance-based credits worth up to 69 points, as well as seven prerequisite criteria, divided into six categories:

· Sustainable Sites

· Water Efficiency

· Energy and Atmosphere

· Materials and Resources

· Indoor Environmental Quality

· Innovation and Design Process

LEED allows the project team to choose the most effective and appropriate sustainable building measures for a given location and/or project. These points are then tallied to determine the appropriate level of LEEDÒ certification. A full description of the LEED credits can be found on www.usgbc.org/leed/

Four levels of LEEDÒ certification are possible depending on the number of criteria met. They indicate increasingly sustainable building practices:

· LEED Certified 26-32 points

· LEED Silver 33-38 points

· LEED Gold 39-51 points

· LEED Platinum 52+ points

There is a general perception that LEED is becoming the standard for US green building design and probably for the world.

Green Buildings Economy:

Prima facie, green buildings cost more. However between three to five years, unlike a normal construction, a green building actually starts giving you returns. Since a green building conserves energy and also makes the most of sunlight the energy costs are slashed by 40 to 50 per cent. A US study, for example, found that certified green buildings cost 1.8% more to design and construct, but yield 20% cost savings over the life of the building. However the obstacles to achieving these benefits in the fragmented property sector are also well documented. The sustainable building design, green materials and green technology are often more expensive than normal building blocks and are often not easily available but the upside monetary advantages more than compensate for that. These buildings are also eligible for carbon credits, since they save the environment from carbon-dioxide emissions. That opens up another revenue stream for the developer. It is therefore no surprise that the projected growth potential for green buildings in India is Rs 2,000 crores.

The financial barriers, including high initial cost barriers and an inadequacy of traditional financing instruments, are a key element preventing private actors from engaging further towards making the residential building sector more energy efficient, according to a study from the International Energy Agency (IEA). Despite the proven cost-effectiveness of energy-efficient technologies, their potential remains untapped in the building sector "due to numerous market barriers", states the IEA, based on the results of case studies of the residential sector in France, Germany, the UK, Japan and the US.

The green buildings are important for India. In spite of India's per capita consumption of energy being far lower than western economies, it's building sector has climbed to an usage of nearly 30 percent energy, up from a low 14 percent in the 1970s. India's energy conservation laws for buildings are voluntary but this is one area in which the country is already greener than in many parts of the developed world. According to the Indian Green Building Council (IGBC) set up in 2000, the country's modest 25,000 square feet of green buildings in 2003 have grown to a phenomenal 25 million sq.ft in just 4 years, projected to grow to one billion sq. ft per year by 2010. One of the reasons for this rapid growth is India's high economic growth and emerging status as the global business giant, resulting in imminent demand for commercial, infrastructural and residential construction. The country seconds China in its growth and demand of infrastructural development. The IGBC's standard of green buildings is based on the U.S.' Leadership in Energy and Environmental Design (LEED). The savings in cost of energy and water alone are attracting more commercial builders into the IGBC

Condition air with tinted windows:

A new product may soon emerge from the race to create better materials for green building. Sage Electrochromics, based in Faribault, Minnesota, recently raised $20 million from investors to continue its development of tinted windows, which automatically shift from light to dark as environmental conditions change.

The electrochromic windows and skylights sense the change in surrounding temperature and respond accordingly to save energy. If it is warm, the windows darken to keep the building cool. When it is cool, the windows appear clear again. The technology aims to reduce the need for air conditioning, which is notoriously energy-inefficient and increasingly costly.

The windows are undeniably cool. But, as this article notes, the high-tech electrochromic windows may find it difficult to compete with other lower-tech, less expensive solutions already on the market:

Other companies, such as Denmark's PhotoSolar, make windows that block solar heat with a simple passive film inserted between two sheets of glass. The windows have a permanent, gray tint to them, but you can still see out of them. More importantly, they don not require any electronics or controls.

Communicating about formwork

COMMUNICATING ABOUT FORMWORK

Dr J D Bapat

This post is a summary of an excellent article by: Mary Bordner Tanck, Concrete International, January 2009, pp 53-56.

The communication among the architect, structural engineer, general contractor, formwork designer and contractor is one of the most important issues on any concrete construction project. The main goal of the formwork drawings is to convey to the contractor how to build it. Good communication on a project can take many forms but drawings are the principal means of communication. It can be a huge help, especially on fast-track jobs, if the CAD files on formwork layout are shared with the contractor. This has the benefit of expediting the formwork layout and eliminating the need for extensive shop drawing revisions. It is important that items such as slab edges, beam faces, work points and anchorages are properly dimensioned.

The design live loads, live load reductions and superimposed dead loads should be noted in the contract documents. The reshoring designer needs these values to produce a safe design. When a pour strip is required (for example, on a post-tensioned slab), the engineer should indicate whether the cantilevers on each side of the pour strip can support their self-weight and any superimposed dead and live loads or if they are required to be backshored until the pour strip is closed.

There are many details that need to be conveyed by the formwork designer for the contractor to construct a safe formwork system. Of particular importance are bracing sizes and locations as well as the specifications for the materials and components to be used. The more detail a designer provides, the lower the risk of errors, both in the design and in its realization in the field. The shop drawings should give field superintendents all the information they need, including the locations for potential problems, so the risk of missing a key item is significantly reduced. The drawings can also help the formwork designer and field personnel work together to address potential problems. Depending on the formwork designer’s scope of work, the location each beam edge relative to a grid line and relative to adjacent beams should be shown. These dimensions can help the contractor lay out the job so that material can be moved to the appropriate location the first time, saving the contractor time, money, and crane usage. The formwork designers need to very clearly communicate the materials required for shoring, reshoring or backshoring. When timber is used, the grade and species of each timber member should be clearly called out on the plan.

When responsibility is passed onto the contractor for critical items such as cantilever spans, framing over an opening or slab framing along column lines, the risk of problems increases significantly, as forms may be cobbled together in the field with little regard to connections, safety factors or back-span conditions. The sequencing can be crucial to the success of a shoring and reshoring design.

The members of the project team should share a common goal: to get the structure built safely, on time and within the budget.

The readers are advised to go through the original article to know more details.

Using building code to improve construction quality

In the last few years , two major changes have taken place in the cement and construction industry in India. Firstly, the emphasis of construction industry has shifted from high – strength to high – performance concrete . The realisation has come on account of the fact that nearly 65 % of the total cement sales in the country presently go towards the repairs of old structures , most of which are built with OPC . Secondly, the mineral admixtures, namely the fly ash (FA), silica fume (SF) and the blast furnace slag (BFS), are being increasingly used to improve the long term strength and the durability characteristics of cement and concrete. The need to build durable structures is felt not only from the point of view of economy , but also for the conservation of resources , energy and environment . The present study reviews the provisions of the Building Code IS 456 – 2000 towards building durable structures

The durability of concrete incorporates , besides strength , its capacity to resist the effect of the internal and external deteriorating factors , such as sulphate attack , chloride attack manifested in the corrosion of the reinforcement , carbonation , alkali – aggregate reaction , freezing and thawing , so as to give a satisfactory performance during the economic life , for which it is designed . Some of the important provisions in the Building Code, related to quality and durability are as follows :

a/ Clause 5.2: The following mineral admixtures, conforming to relevant Indian Standards, are permitted in the concrete: fly ash, silica fume, rice husk ash, metakaoline and ground granulated blast furnace slag (BFS).

b/ Clause 5.5 and 10.3.3: These two Clauses give various provisions on chemical admixtures. The dosage of retarders, plasticisers and superplasticisers has been restricted to 0.5, 1.0 and 2.0 % respectively by weight of cementitious materials.

c/ Clause 7: In the revised Code, the workability of concrete has been expressed in terms of slump only, unlike old Code wherein it was expressed in terms of Vee-bee time/compacting factor.

The expression in terms of compacting factor is recommended only in case of ‘very low’ workability category, such as pavement quality concrete. The ‘very high’ workability category has been newly introduced in the revised Code, applicable to tremie concrete and the workability measurement by flow has been recommended there. It will also be applicable to self-compacting concrete. It is well known that it is always better to express workability in one particular unit i.e. slump, compacting factor or flow, as these units are not always compatible with each other.

The maximum water-cement ratio for reinforced concrete has been reduced (example, 0.55 instead of 0.6, for M-20 concrete) and the assumed standard deviation (Clause 9.2.4.2) has also been reduced for higher grades of concrete. It is observed, in line with the reduced water-cement ratio, that the values of slump allowed by the Code are also low for normal type of construction.

The Code is thus indirectly expecting a change towards mechanisation of placing and consolidation of concrete.

d/ Clause 8.1.1: The permeability of concrete to the ingress of deleterious agents has been identified as one of the major characteristics affecting the durability. The factors influencing the durability have been delineated as environment , cover to embedded steel , type and quality of construction materials , cement content and water-cement ratio of the concrete , workmanship to obtain full compaction and efficient curing and shape and size of the member

e/ Clause 8.2.2.1: The general environment , to which the concrete will be exposed during its working life , is classified into five levels of severity , namely mild , moderate , severe , very severe and extreme. In comparison to the old Code, two more intermediate weather conditions, with less cement content, have been added in the revised Code.

f/ Clause 8.2.2.3: The air – entraining admixtures have been recommended for use in the concrete where freezing and thawing actions under wet conditions exist

g/ Clause 8.2.2.4: Recommendations have been given for the type of cement , maximum free water-cement ratio and minimum cement content , to develop adequate resistance in the concrete exposed to different sulphate concentrations. It is stated that PSC conforming to IS 455, with slag content more than 50 %, exhibits better sulphate resisting properties. Under conditions where chloride is encountered along with sulphates, the Code recommends the use of OPC with C₃A in the range of 5-8 % instead of SRC, PSC with more than 50 % slag or a blend of OPC and slag

h/ Clause 8.2.4.1: Recommendations have been given on the minimum cement content, maximum free water – cement ratio and minimum grade of concrete , for different exposure conditions. The minimum grade of concrete for footings or elements under non-aggressive soil or ground water (moderate environment) has been specified as M- 25. Thus the design mix is obligatory even for small buildings/structures.

i/ Clause 8.2.4.2: The upper limit of the cement content , not including FA and GGBS , has been kept at 450 kg / m³ , considering the increased risk of cracking due to drying shrinkage in thin sections or early thermal cracking and the increased risk of damage due to alkali-aggregate reaction, at the higher cement contents. It is hoped that the provisions for maximum cement content made in other standards, like IS 1343 on prestressed concrete structures, will also be brought in line with that in IS 456 – 2000.

j/ Clause 8.2.5.2: The total amount of chloride content ( as Cl ) in the concrete , at the time of placing , has been specified . The maximum chloride content of 0.6 kg/m³ of concrete has been stipulated, for reinforced or plain concrete containing embedded metal. The maximum limit has been rationalised and revised upward, as that specified in the earlier Code was difficult to realise in practice.

k/ Clause 8.2.5.3: The maximum total water – soluble sulphate content of the concrete mix , expressed as SO₃ , has been specified as 4 % by mass of the cement in the mix

l/ Clause 8.2.5.4: As a precaution against alkali – aggregate reaction , recommendations have been given on the constituent materials, like use of non-reactive aggregates and low alkali Portland cement ( < style=""> Na₂O equiv.) , partial replacement of cement with the mineral admixtures , use of impermeable membranes to reduce the degree of saturation of concrete during service and limiting the cement content of concrete .

m/ Clause 8.2.8: Recommendations have been given on the concrete constructions in sea - water or directly exposed along the sea - coast , with respect to the Grade of the concrete , type of cement , mix design and the use of pre-cast members . The use of slag or pozzolana cement has been recommended under such conditions.

n/ Clause 9.1.2: The Code stipulates the following information to be included while specifying a particular grade of concrete: type of mix (design or nominal), concrete grade, cement type, maximum nominal size of aggregate, minimum cement content for design mix, maximum w/c ratio, workability, mix proportions for nominal mix, exposure conditions (as per the Code), maximum placing temperature, method of placing and the degree of supervision

o/ Clause 10.1: This Clause on Quality Assurance Measures has been incorporated to emphasise the good concreting practices.

p/ Clause 10.2: The use of ready-mixed concrete or concrete from on/off site

batching and mixing plant has been recommended for large and medium size projects.

q/ Clause 10.3: The Clause on concrete mixing makes an important new provision. It stipulates that the concrete mixers shall be provided with water measuring (metering) devices. The provision will go a long way controlling water-cement ratio, especially in site mixed concrete. The old provision of hand mixing the concrete with 10 % extra cement in case of breakdown of mixer, work in remote areas or when concrete quantity is very small, has been removed in the revised Code.

r/ Clause 12.3.2: The old provision of negative tolerance on the cover has been removed. Under the new provision, use of PVC cover blocks has been permitted.

s/ Clause 13.4: The provisions under this Clause on “Construction Joints and Cold Joints” have been improved

t/ Clause 13.5: The use of proper and adequate curing techniques has been stressed, to reduce the permeability of the concrete and enhance its durability by extending the hydration of cement. The difference has been maintained between recommended minimum period of curing for concrete containing ordinary Portland cement and that containing cement with mineral admixtures

u/ Clause 16: The acceptance criteria for strength requirement has been totally revised and divided into two parts, namely that for (I) compressive strength and (II) flexural strength of concrete

v/ Clause 21: The concept of fire resistance of concrete has been newly introduced and discussed.

w/ Clause 26.4: The minimum values for the nominal cover (a new term introduced in the Code) have been specified , to meet the durability requirements. The minimum cover for footings has been specified as 50 mm.

While going through these provisions, it becomes clear that besides improving the construction quality, concrete strength and durability, the revised Code also puts emphasis on conservation of building materials.

Reference: Bapat J. D., “ IS – 456 : 2000 and Further ” Indian Cement Industry Desk Book 2002, December 2002, pp 30-39