Concrete
Basic mix: Cements + Aggregate (gravel,sand) + Water
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Additives / admixtures:
A – Chemical (
http://www.concretenetwork.com/concrete/concrete_admixtures/chemical_admixtures.htm
)
Chemical admixtures reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.
B – Mineral (
http://www.concretenetwork.com/concrete/concrete_admixtures/mineral_admixtures.htm
)
Mineral admixtures make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties. They affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic ash used in Roman concrete), fly ash* and silica fume*.
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CHEMICAL ADMIXTURES
0/ WATER REDUCERS
Water reducers are concrete admixtures that reduces water content by 5-10%, decrease concrete porosity, allow for both improved workability and increased strength (by as much as 25%), reduce water permeability, increase durability, and result in a better finished surface.
first water reducers – lignosulfonates (wood and paper industry byproducts). Presently there are synthetic alternatives which have increased performance.
Products:
* Thiessen WRDA64 - A polymer based aqueous solution of complex organic compounds producing a concrete with lower water content (typically 8-10% reduction), greater plasticity and higher strength. Used in ready mix, job site and concrete paver plants; for normal and lightweight concrete; and in block, precast and prestressed work. Offers significant advantages over single component water reducers. Performs especially well in concrete containing fly ash and other pozzolans.
* Thiessen Zylar - A ready-to-use aqueous solution of organic compounds specifically designed as an admixture for Portland cement concrete. The ingredients are factory pre-mixed. ZYLA R is used to retard the initial and final set of concrete. Along with set retardation, ZYLA R typically provides 5% water reduction in a concrete.
* Thiessen Daracem 100 - A liquid admixture providing improved slump retention in flowing concrete. Ideal for low water/cement ratio concrete.
* Thiessen Daracem 19 - A liquid admixture for all applications that require highly flowable concrete where it is desirable to keep the water/cement ratio to a minimum while still achieving the degree of workability needed to provide easy placement, consolidation and high-early stripping strengths.
(
http://www.thiessenteam.com
)
1/ PLASTICIZERS – water reducers
Chemical admixtures that can be added to concrete mixtures to improve workability.
Plasticizers are often used when pozzolanic ash* (mineral additive) is added to the concrete mix to increase strength. Adding 1-2% plasticizer per unit weight of cement is usually sufficient. Adding an excessive amount of plasticizer will result in excessive segregation of concrete and is not advisable. Depending on the particular chemical used, use of too much plasticizer may result in a retarding effect. Plasticizers are commonly manufactured from pop lignosulfates, a by-product from the paper industry. Traditional lignosulfonate-based plasticizers (& napthalene and melamine sulfonate-based superplasticisers) disperse the flocculated cement particles through a mechanism of electrostatic repulsion. In normal plasticisers, the active substances are adsorbed on to the cement particles, giving them a negative charge, which leads to repulsion between particles.
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2/ SUPERPLASTICIZERS – high range water reducers
Superplasticizers reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction. The effect of superplasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability. As a result of the slump loss, superplasticizers are usually added to concrete at the jobsite.
They are chemicals used as admixtures where well-dispersed particle suspension are required. These polymers are used as dispersants to avoid particle aggregation, and to improve the flow characteristics (rheology) of suspensions. Their addition to concrete allows the reduction of the water to cement ratio, not affecting the workability of the mixture, and enables the production of self-consolidating concrete and high performance concrete. This effect drastically improves the performance of the hardening fresh paste. Indeed the strength of concrete increases whenever the amount of water used for the mix decreases. However, their working mechanisms lack a full understanding, revealing in certain cases cement-superplasticizer incompatibilities.
Superplasticizers have generally been manufactured from sulfonated napthalene condensate or sulfonated melamine formaldehyde, although newer products based on polycarboxylic ethers are now available. Lignin, napthalene and melamine sulfonate superplasticisers are organic polymers. The long molecules wrap themselves around the cement particles, giving them a highly negative charge so that they repel each other. Polycarboxylate ether superplasticizer (PCE) or just polycarboxylate (PC), work differently from sulfonate-based superplasticizers, giving cement dispersion by steric stabilisation, instead of electrostatic repulsion. This form of dispersion is more powerful in its effect and gives improved workability retention to the cementitious mix. With a relatively low dosage (0.5–1.5% by cement weight) they allow a water reduction up to 40%, due to their chemical structure which enables good particle dispersion.
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3/ HYDRO (Westroc Inc -
http://www.westrocinc.com/Concrete/Additives/Hydro/hydro.html
)
Hydro is a combination of organic and inorganic chemicals which react with Portland cement to produce more dense, hard and impermeable concrete.
* Greater hydration of portland cement (reduces the water soluble by-products of hydration).
* Increased water utilization (more of the mix water is utilized in the hydration process).
* Increased workability (Hydro imparts greater workability allowing for placement with reduced water requirements - reduce the water/cement ratio by 5%).
* High Early Strength (The increased hydration of Portland Cement with 1.5 gallons per yard dosage of HYDRO yields accelerated intitial setting times, 28 days strength in 7 days and 28 day strengths are increased 15% – 25%).
* Internal curing (Hydro concrete and mortars also tend to hold moisture longer due to the increased density of the mass).
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4/ AIR ENTRAINERS
Air entrainment in concrete is the creation of air bubbles.
- Surfactant – surface active substance
- Typically for exposed environments
- Air Entraining class of products includes some detergents
Effects:
* increases durability (beneficial for counteracting freeze-thaw cycle)
* reduced compressive strength – 1% additional air = -5% strength
* improved manipulation of plastic concrete (bubbles lubricate the much larger concrete particles)
* it takes 25:100 water to cement ratio for all cement particles to hydrate (however most concrete has w/c ratio between .45 and .60 to increase workability). When excess water evaporates, concrete becomes vulnerable to infiltration.
* Some of the stresses of freeze-thaw are absorbed by bubbles left from air entrainment process
Products:
* BASF Micro-Air 303 – ultra stable air entraining admixture for use in all types of concrete.
* BASF Micro-Air 940 – ready-to-use, liquid air entraining agent for use in all types of concrete. It is particularly recommended for use in concrete in which it has previously been difficult to maintain the desired air content. It is supplied as a ready-to-use aqueous solution, free of added chloride.
* BASF Micro-Air VR – Vinsol Resin based air entraining agent for use in all types of concrete and mortar where air entrainment is desired. It contains no chloride.
* Sika: Multi Air 25, Sika AIR, Sika AEA-14, Sika AEA-15
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5/ SET RETARDERS
- Slow hydration of concrete. Good for large pours.
- Prevents partial setting.
- Sugar, sucrose, sodium gluconate, glucose, citric acid, tartaric acid.
They slow the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting. High temperatures often cause an increased rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable during placement and delay the initial set of concrete. Most retarders also function as water reducers and may entrain some air in concrete.
Products:
* AXIM CATEXOL 100R – synthetically set-retarding admixture for concrete. CATEXOL 1000 R provides a more plastic and flowable concrete, greater uniformity, increased durability, and reduced shrinkage and permeability in your concrete. CATEXOL 1000 R facilitates placing and finishing by providing controlled retardation of the concrete’s setting time, preventing premature stiffening.
* Axim Stop-Set L – nonchloride liquid chemical system which controls the cement hydration process and rate-of-set of concrete. As a result of this control of the cement hydration process and rate-of-set, returned plastic concrete or wash water treated with STOP-SET L can be kept in a plastic or, in the case of wash water, fluid state for a few hours, overnight, or over a weekend in the drum of a ready-mix truck or central mixer.
(
http://www.aximconcrete.com
)
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6/ SET ACCELERATORS
- accelerates cement hydration
- shorter setting times
- increased early age strengths (+ for cooler temps)
They increase the rate of early strength development, reduce the time required for proper curing and protection, and speed up the start of finishing operations. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
Products:
Calcium chloride was the accelerator of choice for several years, but in fact it may be that the chloride has led to the corrosion of steel reinforcement in composite structures. As a result of corrosion, the reinforcing members expand, causing the concrete to spall and crack. In recent years synthetic alternatives have been developed.
* BASF POZZOLITH 555 - ready-to-use liquid admixture formulated to accelerate the setting time of concrete under cool winter conditions and to produce high-early strength in special application concrete. It does not contain added chloride and conforms to the most stringent chloride ion limits required by concrete industry standards. POZZOLITH 555 meets and exceeds AS 1478.1 Type Ac and ASTM C494 Type C requirements.
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7/ SPECIALTY ADDITIVES >
Chemical Manufacturers: Sika, Axim, BASF, Thiessen
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7A/ SPECIAL – CORROSION INHIBITORS
Corrosion inhibitors are used to slow corrosion of reinforcing steel in concrete. Corrosion inhibitors can be used as a defensive strategy for concrete structures, such as marine facilities, highway bridges, and parking garages, that will be exposed to high concentrations of chloride.
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7B/ SPECIAL – SHRINKAGE CONTROL
The shrinkage reducers are used to control drying shrinkage and minimize cracking.
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7C/ SPECIAL – ALKALI-SILICA REACTIVITY INHIBITORS
ASR inhibitors control durability problems associated with alkali-silica reactivity.
Products:
BASF ASRx 30 LN - a lithium-based, liquid admixture that is formulated for use in high alkali concrete containing reactive aggregates to inhibit and control Alkali-Silica Reactivity (ASR). Benefits include improved durability of the concrete and extended service life of concrete structures.
(
http://www.basf-admixtures.com/en/products/asr_inhibitor/asrx_30ln/Pages/default.aspx
)
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7D/ SPECIAL – COLORING
Coloring agents have become more commonly used, especially for patios and walkways. Most are surface applied and often have the additional effect of surface hardening. Such surface applied coloring admixtures generally should not be used on air-entrained concrete. Integrally colored concrete is also available.
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7E/ SPECIAL – BONDING ADMIXTURES
Bonding admixtures, including addition of compounds and materials such as polyvinyl chlorides and acetates, acrylics and butadiene-styrene co-polymers, can be used to assist in bonding new / fresh concrete with old / set concrete.
* ACRYL 60
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7F/ SPECIAL – WATERPROOFING ADMIXTURES
Waterproofing and damp proofing admixtures, including soaps, butyl stearate, mineral oil and asphalt emulsions, are used to decrease the amount of water penetration into the larger pores of concrete. “Antifreeze” admixtures typically are accelerators used in very high doses, with a corresponding high price, to achieve a very fast set-time, though they do not have properties to protect against freezing on their own. However, in general, these are not used for residential work. Damp proofing and waterproofing admixtures still have uncertain value and hazards.
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MINERAL ADMIXTURES
can account for 20 to 70 percent (by mass) of total cementitious material
These siliceous materials have pozzolanic and cementitious properties and can thus be used as a replacement for portland cement. Use of such byproducts as a concrete admixture is one of “high-value” when compared with other common uses such as roadbase construction.
Advantages- industry byproducts may or may not require further processing, where as dedicated processing of pozzolan material is typically an energy intensive process.
* Low-Calcium Fly ash – (pozzolanic) byproduct of coal-fired energy plants.
*High-Calcium Fly ash- has cementitious and pozzolanic properties
* Blast furnace slag – (cementitious) GGBFS or GGBS. Byproduct of steel production (reduces Portland cement by 80%)
* Silica fume – used to increase strength and durability of concrete, but because of higher surface to volume ratio, requires addition of superplasticizers for workability. Results in a dark grey color.
* Metakolin – Increases strength. Results in a bright white color.
As of the year 2000, fly ash was produced at a rate of 500 million tons per year, making it the largest industrial waste product in the world.
precautions: fly ash with >%5 carbon is undesirable due to increases in water (for workability) which results in the requirement of an air entraining admixture.
Portland cement largest single green gas producer in the world. Try to minimize.
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Reinforcement:
1/ FIBERMESH (Westroc Inc -
http://www.westrocinc.com/Concrete/Additives/Fibermesh/fibermesh.html
)
Fibermesh is a one-step fiber reinforcement system for concrete. Fibermesh provides a complete top-to-bottom, side-to-side, uniform micro reinforcement, as a cost-effective and superior alternate system to wire mesh.
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- Curing process for TECHNIQUE?
- use guidelines?
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Sources:
http://www.concretenetwork.com/concrete/concrete_admixtures/chemical_admixtures.htm
http://www.cement.org/basics/concretebasics_chemical.asp
http://www.toolbase.org/Technology-Inventory/Foundations/concrete-admixtures
http://www.aximconcrete.com/
