Green Concrete

AUTHOR: AU YONG THEAN SENG

www.madisonvelocity.blogspot.com

TOPICS HIGHLIGHTED

OVERVIEW – WHEN GREY TURNS GREEN

MAIN OBJECTIVE

FOUR WAYS TO PRODUCE GREEN CONCRETE

COMPARISON BETWEEN CONVENTIONAL AND GREEN CONCRETE

DIFFERENT MIXTURE’S EFFECTS ON ENVIRONMENT

GREEN LIGHTWEIGHT AGGREGATE CONCRETE CASE STUDY

PERVIOUS CONCRETE – IT DRAINS WHEN IT RAINS



OVERVIEW – WHEN GREY TURNS GREEN There is a need for the reader to differentiate between green buildings with green concrete. Green building is a definition to describe the environmental effect caused from the building itself which includes architect’s design. Whereas the green concrete is a type of concrete which resembles the conventional concrete but the production or usage of such concrete requires minimum amount of energy and causes least harm to the environment. There is considerable knowledge about how to produce concrete with lower environmental impact, the so-called green concrete. The concrete industry realised at an early stage that it is a good idea to be in front with regard to documenting the actual environmental aspects and working on improving the environment, rather than being forced to deal with environmental aspects due to demands from authorities, customers and economic effects such as imposed taxes. Furthermore, some companies in concrete industry have recognised that reductions in production costs often go hand in hand with reductions in environmental impacts. Thus, environmental aspects are not only interesting from an ideological point of view, but also from an economic aspect. The knowledge and experience, about how to produce concrete with lower environmental impacts can be divided into two groups, concrete mix design and cement and concrete production: Concrete mix design: o using cement with reduced environmental impacts o minimising cement content o substituting cement with pozzolanic materials such as fly ash and micro

silica o recycling of aggregate o recycling of water Cement and concrete production: o environmental management Concrete mixture design The type and amount of cement has a major influence on the environmental properties of a concrete. An example of this is shown in figure 1, where the energy consumption in MJ/kg of a concrete edge beam through all the life cycle phases is illustrated. The energy consumption of cement production make up more than 90 % of the total energy consumption of all constituent materials and approx. 1/3 of the total life cycle energy consumption.



Figure 1 : Energy consumption of an edge beam design

By selecting a cement type with reduced environmental impacts and by minimising the amount of cement the concrete’s environmental properties are drastically changed. This must, however, be done whilst still taking account of the technical requirements of the concrete for the type and amount of cement. One method of minimising the cement content in a concrete mix is by using packing calculations to determine the optimum composition of the aggregate. A high level of aggregate packing reduces the cavities between the aggregates, and thereby the need for cement paste. This results in better concrete properties and a better environmental profile, due to a smaller amount of cement. When having experimentally determined the packing, the density, and the grain size distribution of each aggregate material, it is possible to calculate the packing of any combination of aggregates using computer program, Another way of minimising the cement content in a concrete is to substitute parts of the cement with other pozzolanic materials. In Denmark, it is common to produce concrete with fly ash and/or micro silica fume. Both of these materials are residual products (from production of electricity and production of silicon, respectively) and both have a pozzolanic effect. Thus, a material with large environmental impact, i.e. the cement, is substituted with materials with reduced environmental impacts. The restrictions on adding fly ash and micro silica fume laid down in the future concrete materials standard will be as shown in table 1.



Passive

environmental class

Moderate environmental

class

Average environmental

class

Extra aggressive

environmental class

Maximum content of F+M from C+F+M (%)

x 35 25 25

Maximum content of M from C+F+M (%)

x 10 10 10

C = Cement, F = Flyash, M = Micro Silica Fume Environmental classes: Passive: dry atmosphere with no risk of corrosion. Moderate: moist atmosphere, with no risk of frost combined with water saturation, and with no significant alkaline and/or chloride influence on the concrete surface. Aggressive: moist atmosphere, with significant alkaline and/or chloride influence on the concrete surface or where there is risk of water saturation combined with frost. Extra Aggressive: moist atmosphere, with significant alkaline or/and chloride influence or layering on the concrete surface

Table 1 : Requirements on the content of fly ash and micro silica according to the future

concrete materials standard.

In order to reduce the consumption of raw materials and to minimise the waste generated from demolished concrete structures, surplus, and production errors, crushed concrete can be reused as aggregate. It is expected that the use of recycled aggregate in concrete, for passive environmental class will be allowed. Also recycled water, initially used for washing out the aggregates from surplus concrete and cleaning the production equipment, is expected to be allowed in the new concrete standard. Cement and concrete production It is also possible to reduce a concrete’s environmental impact by reducing the environmental impacts in cement and concrete production. The cement manufacturer has many activities concerned with the reduction of environmental impacts. As regards concrete production, experience with reductions of primarily water consumption, energy consumption and waste production is available. Even though the contribution of concrete production to a concrete’s environmental profile is minor, it does give a contribution, and it is important - environmentally and economically - to the single concrete producer.



MAIN OBJECTIVE It has been defined that a number of alternative environmental requirements with which green concrete structures must comply to. These goals are in accordance with Danish environmental strategies, e.g. the goal for CO2 emission is in accordance with the Danish obligations at the Kyoto agreement (21% reduction before 2012 compared to the 1990 level): o CO2 emissions shall be reduced by at least 30%. o At least 20% of the concrete shall be residual products used as

aggregate. o Use of concrete industry’s own residual products. o Use of new types of residual products, previously land-filled or

disposed of in other ways. o CO2 neutral waste-derived fuels shall replace at least 10% of the fossil

fuels in cement production. The technical goals for the centre are to obtain the same technical properties for the green concrete compared to conventional concrete – or to determine in what way the properties differ. The compressive strength goals for the green concrete are: o Aggressive environmental class (outdoor, horizontal): 28-day strength

> 35 MPa and 56-day strength > 85% of the strength of a reference concrete.

o Passive environmental class (indoor): 28-day strength >12 MPa and 56-day strength > 85% of the strength of a reference concrete.



FOUR WAYS TO PRODUCE GREEN CONCRETE Four ways to produce green concrete are being investigated, see Figure 2: 1. To increase the use of conventional residual products, i.e. fly ash. 2. To use residual products from the concrete industry, i.e. stone dust (from

crushing of aggregate) and concrete slurry (from washing of mixers and other equipment).

3. To use residual products from other industries not traditionally used in concrete, i.e. fly ash from bio fuels and sewage sludge incineration ash (from sewage treatment plants).

4. To use new types of cement with reduced environmental impact.

Figure 2: Overview – concrete development in the Centre for Green Concrete. New types of

cement and binder can be utilised in combination with the residual products

All the above mentioned green concrete types will be tested for workability, changes in the workability after 30 minutes, air content, compressive strength development, E-modules, heat development, homogeneity, water separation, setting, density, and pump-ability. Furthermore, the water/cement ratio, water/binder ratio, and the chloride content will be calculated. From the tests, the most promising green concrete will be selected and exposed to more advanced testing.

Conventional

concrete,

conventional

cement, flyash,

and micro silica

Residual

products from the

concrete industry

Residual

products from

other industries

Cement with

reduced

environmental

impact

Conventional

cement, flyash

and micro silica

Sewage sludge

incineration ash

Flyash from bio fuels

Stone dust

Concrete slurry

Large quantities of fly

ash

Mineralised cement

Limestone addition

Waste-derived fuels



COMPARISON BETWEEN CONVENTIONAL AND GREEN CONCRETE

1): DENSIT® = Cement and micro silica based material providing high density and high compressive strength (150-300 MPa) 2): CRC® = Compact Reinforced Concrete which contains a high amount of steel fibre providing high ductility and compressive strength (150-400 MPa) *): without traditional waterproofing membrane **): designation used for the environmental screening

Table 2: Alternative designs with requires different maintenance/repair used for the

assessment of environmental effects of concrete bridges

An environmental screening has been performed for a column presenting the different design principles as described in Table 2 (green concrete columns defined as A, B, C). For comparison, the same environmental screening has been performed for a reference column (traditional concrete column defined as R), which is similar to column A, except that the green concrete type being substituted by a traditional concrete suitable for aggressive environment. The objective of the screening is to identify significant resource consumption and environmental loads of traditional concrete/design compared to green concrete/design occurring during the entire service life, this includes the environmentally viewed most critical maintenance/repair stage. The performed lifecycle screenings quantify material usage (consumption of concrete) as well as CO2 emissions generated at the involved stages during the lifecycle of the columns. The input data for the comparison are given in Table A1, Appendix A. In order to limit the analysis to a minimum, the



environmental screening comprises only those issues where the environmental impacts of the green concrete columns differ from those of the traditional one (see Table A1). The environmental parameters related to the working environment have not been included. The results of the environmental screening for the 3 green concrete columns (A, B, C) and the traditional concrete column (R) is presented in Table 3 with regard to the CO2-emission and in Table 4 with regard to the consumption of concrete.

Design solution Column R Column A Column B Column C

Traditional design + traditional concrete

Increased concrete cover + green concrete

Stainless steel reinforcement + green concrete

Stainless steel cladding + green

concrete

kg CO2 per year 300 200 86 80

Table 3 : CO2 emission for different designs of concrete column

Figure 3 : Sources of CO2 emission for four types of columns

Design solution Column

R Column

A Column

B Column

C

kg concrete for construction 5102 5733 5102 5102

kg concrete for maintenance/repair 1533 2442 0 0

kg concrete, total 6635 8175 5102 5102

Table 4 : Consumption of concrete for different designs of concrete columns



This comparison demonstrates that column B (stainless steel reinforcement) and column C (stainless steel cladding) present the most environmental-friendly design solutions both with regard to the CO2 emissions and the consumption of concrete. An even more environmental-friendly solution is if the selected concrete at column C would be substituted by a more environmental-friendly (greener) concrete type provided that the steel cladding assures the long-term protection of the reinforced concrete.







DIFFERENT MIXTURE’S EFFECTS ON ENVIRONMENT The investigation includes five green types of concrete. In addition, a reference concrete, AR, is included in the test programme. AR is a normal Danish concrete intended for aggressive environment. AR is produced with extra low alkali, highly sulphate resistant cement, a moderate fly ash content as well of a moderate content of silica fume. The five green types of concretes are: A0 Concrete with low alkali, moderate sulphate resistant cement. Change of

cement type lowers the energy consumption of cement production. The same cement type is used for A1 and A3. A0 fulfils DS481, but not the specification of the Danish Road Directorate.

A1 Concrete with a high amount of fly ash (40%) A3 Concrete with sewage sludge incineration ash instead of ordinary fly ash A5 Concrete with concrete slurry A6 Concrete with stone dust Environmental goal AO A1 A3 A5 A6

CO2 reduction (Yes) 27%

(Yes ) 52%

(Yes) 29%

Residual products as aggregates Yes

Residual products from the concrete industry Yes Yes

Residual products from other sources Yes

Waste-derived fuel (Yes) 9%

(Yes) 9%

(Yes ) 9%

Table 5 : Evaluation of environmental goals



GREEN LIGHTWEIGHT AGGREGATE CONCRETE CASE STUDY

A paper done by Tommy Y. Lo and H.Z. Cui from Department of Building and Construction, City University of Hong Kong, Hong Kong discusses the mechanical properties of a newly developed structural lightweight aggregate which is made from expanded clay. The aggregate is reinforced with a PFA rich surface coating applied at a later stage of firing. The experience of utilizing this green lightweight aggregate concrete in the prefabrication of structural element is also presented. The structural lightweight aggregate was used to develop precast concrete elements (façade) for green construction. The mix proportion used is given in Table 6.

Cement Water Sand Aggregate (pre-wetted) Admixture

420 175 715 630 1000ml

Table 6 : Mix proportion of green lightweight concrete (kg/m3)

Figure 4 : A good workable fresh concrete for concrete casting. The slump of lightweight concrete measured 30 minutes after batching was 50 mm.



Figure 5 : Protocol of finished lightweight concrete precast façade

Specification Façade quality

Unit weight 2275 kg 1590 kg

1 day strength 15MPa 14.5MPa

28 days 30MPa 34MPa

Slump 75mm 50mm

Density 2400kg.m3 1750kg/m3

Table 7 : Comparison of design requirement with conventional concrete production



PERVIOUS CONCRETE – IT DRAINS WHEN IT RAINS

One of the examples of a green concrete is pervious concrete. Known for its ability in storm water management, it’s one of the major breakthroughs in green concrete design. Pervious concrete is a mix of coarse aggregate, cement, water, and little to no sand. Also known as “no-fines” or porous concrete, this mixture creates an open-cell structure, allowing rainwater to filter through to underlying soil. By modelling natural ground cover, pervious concrete is an excellent choice for storm water management.

Pervious Concrete: The Environmentally Sound Choice Storm water runoff can send as much as 90% of the pollutant such as oil and other hydrocarbon liquids found on the surface of traditional parking lots directly into our rivers and streams. Pervious concrete has been recognized as a best management practice to address this most vital environmental concern. The open-cell structure of pervious concrete provides a medium for aerobic bacteria that break down many of the pollutants that seep from parked cars. Pervious concrete also contributes to enhanced air quality by lowering atmospheric heating through lighter colour and lower density, decreasing the impact of heat island effects. The heat island effect occurs when tree-covered areas are replaced with dark pavement surfaces, and is characterized by up to a 12-degree average temperature increase between an urban area and its surrounding countryside. This heat island effect increases ground level ozone production by as much as 30%. Concrete surfaces, both pervious and conventional, have a much higher albedo, a measure of reflectance than competitive paving materials. Specifications requiring a minimum surface albedo are becoming increasingly popular. The inherently light colour of concrete naturally reflects heat and light. Studies have shown as much as a 30% savings in lighting costs over other pavement types due to concrete pavement’s reflectivity.



Benefits of Pervious Concrete

o Reduces storm water runoff

o Eliminates the need for detention ponds and other costly storm water

management practices

o Replenishes water tables and aquifers

o Allows for more efficient land development

o Minimizes flash flooding and standing water

o Prevents warm and polluted water from entering streams

o Mitigates surface pollutants

“Storm water runoff occurs when rain falls. This runoff causes increased pollution in rivers and streams, flash floods, and loss of rainwater that could otherwise replenish water tables and aquifers. Pervious concrete has a 15-25% void structure and allows 3–8 gallons of water per minute to pass through each square foot—accounting for far more than is generated during most rain events. Pervious concrete puts rainwater back in the

ground where it belongs.”



What You Want to Know About Pervious Concrete? Q: What about freeze-thaw issues? A: Pervious concrete has been placed in freeze-thaw climates for over 15 years. Successful applications of pervious concrete in freeze-thaw environments have two common design features—the cement paste is air-entrained, and the pervious concrete is placed on 6–12 inches of drainable aggregate base (3/4” or larger clean gravel). Q: What about clogging? A: Clogging problems are mainly an issue of design. If a natural area with grass or exposed soil is allowed to drain storm water across a pervious concrete pavement, fine material can be introduced into the system causing localized clogging. Vegetative matter can collect on the surface of the pervious concrete causing some clogging, but routine sweeping or vacuuming will restore porosity. Studies have been conducted that indicate pressure washing will restore most of the porosity of clogged pervious concrete to nearly new conditions. Q: What other uses are there for pervious concrete? A: Pervious concrete has been successfully used for low volume streets, driveways, sidewalks, golf cart paths, retaining walls, slope protection, and French drains. Pervious concrete can be utilized in a variety of paving applications to provide landscape without altering hydrology of the land.

Green Concrete

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