New technologies and mix design options are available now for sanitary precast concrete manholes.
By Evan Gurley
Concrete technologies and mix design options available for precast concrete products have made exciting advancements in recent years. Advancements have been largely accredited to concrete admixtures, both chemical and mineral. The versatility and performance characteristics of admixtures make them ideal for use in an ample range of applications including precast concrete products.
Admixtures are essential to concrete that is exposed to inclement weather and adverse conditions. One of the top-selling precast products today subjected to these conditions is the sanitary precast concrete manhole. Sanitary precast manholes are successfully helping convey sewer and stormwater while maintaining water quality. Typical admixtures that protect sanitary manholes have helped increase concrete quality and performance and have helped maintain a steady demand for precast manholes. Even with precast concrete’s superior quality, there is always room for new, innovative technologies that raise the bar in today’s concrete standards.
Before jumping into the new technologies and mix design options available, you may want to refresh your memory of what an admixture is. Please see the accompanying article “Admixtures Defined.”
One of the innovative admixtures to impact sanitary precast manholes is the next generation of polycarboxylates. Polycarboxylates are polymers high in molecular weight and used as partial replacements for polyphosphates. Polycarboxylate admixtures have been around since the early ’90s, but only recently have they gained wide acceptance as dispersants in admixtures.
Admixture producers say this new generation of polycarboxylates supports strong and durable concrete at lower admixture usage levels compared with traditional dispersants. Besides reducing the admixture usage levels and the amount of water needed to pour concrete by 10 percent to 15 percent, polycarboxylates add strength faster to the concrete mix and can also reduce the amount of energy needed to manufacture concrete (under the right conditions). Some have even gone so far as to say that polycarboxylates have transformed the concrete industry in terms of high early strength and the ability to lower the amount of water used in the mix.
This new generation of polycarboxylates also offers ready-mixed concrete producers and contractors superior slump retention, improved strength and setting characteristics, reduced in-place cost, increased workability and durability, superior unit consolidation, and extended life without retardation. These improvements greatly increase the quality and durability of sanitary precast concrete manholes. Polycarboxylates have also all but eliminated the problem in which accelerators and retardants are added to the mix to regulate the performance of the concrete.
Improved technologies in today’s concrete mixes allow lower water-to-cement (w/c) ratios. Concrete that is denser and less permeable usually has a lower w/c ratio, increasing the ability to resist chlorides through concrete. This is essential for sanitary precast concrete manholes, because they are often in contact with possibly highly reactive, damaging water. The American Concrete Institute suggests a w/c ratio of 0.45 or lower for concrete that is in contact with water and concrete located in harbors. A w/c ratio of 0.45 or lower can be relatively easy to accomplish using polycarboxylates in your mix.
Although polycarboxylates may have a slightly higher initial cost compared with others, producers have pointed out that the cost savings in the final project and reduced maintenance far outweigh this fee. One of the immediate paybacks comes because polycarboxylates allow the producer to work with certain grades of concrete that wouldn’t have been possible in the past.
Metakaolin enhances the properties of concrete and cement-based products in numerous ways. Metakaolin is a dehydroxylated form of the clay mineral kaolinite and is formed when virgin kaolin is heated to temperatures between 1,200 F and 1,750 F (650 C to 900 C). This treatment, also known as calcination, drastically modifies the particle structure, making it a highly reactive, amorphous pozzolan. Controlled blending of the kaolinite, processing and calcinations provides a constant, high purity, quality controlled product with exceptional physical and chemical properties. M etakaolin also has a smaller particle size than cement, resulting in savings through reduced cementitious material requirements. Metakaolin differs from other complementary cementitious materials like fly ash, slag or silica fume (microsilica), since it is not a byproduct of an industrial process and is manufactured for a specific purpose under controlled conditions.
Metakaolin is said to have twice the reactivity of most other pozzolans and is a valuable admixture for concrete/cement applications. Properties such as the filler effect, acceleration of ordinary portland cement hydration, and pozzolanic reactions are favorable engineering properties that occur by replacing the portland cement with up to 20 percent (by weight) metakaolin.
Metakaolin is ideal for use in sanitary precast concrete manholes for numerous reasons. Metakaolin increases the compressive and flexural strengths, reduces the permeability and efflorescence, increases the resistance to chemical attack and prevention of alkali silica reaction, reduces shrinkage (particle packing), increases durability, and improves the workability, finishability, color and appearance.
Metakaolin comes in two separate forms: clinker and powder. The metakaolin clinker has a good handling ability and resistance to deterioration. This form of metakaolin is ideal for intergrinding with portland cement clinker to form superior blended cement, ASTM C 150 Type IP. The powder form of metakaolin is produced by fine grinding the clinker to produce ASTM C 618 Type N pozzolan.
Metakaolin has a higher initial cost than most other aggregates that could be used in sanitary precast concrete manholes, but don’t let that deter you from considering it as an alternative. Just as strength and workability are not the only aspects considered in selecting a concrete design, initial cost is not the only monetary factor to consider when determining an admixture or filler material.
Limestone mineral filler
Another admixture is an innovative, ultrafine calcium carbonate addition and cement substitution for concrete applications. This new material is a type of limestone filler material. If applied to sanitary precast concrete manholes, this can have many beneficial results. “Limestone mineral filler is a good way to optimize a concrete mix in precast applications and to reduce costs of expensive chemical admixtures,” said Jean-Bernard Gélinas, sales representative of Omya Canada Inc. Reports have also shown that a number of customers using this mineral filler have increased production up to 700 yards in a single month.
Limestone has been used extensively in Europe for 20 years for such applications as ready-mixed, precast, shotcrete and self-consolidating concrete, but has just now started to become a larger player in the North American precast industry. Limestone mineral filler produces a consistently white product because of its pure calcium carbonate composition, making limestone filler ideal for precast or architectural cast-in-place concrete products. Limestone is commonly processed into two different grades – 3 and 10 – with particle sizes ranging from 1.4- 3.2 microns to 1.4-10 microns. Processing limestone filler into two separate grades makes it a suitable substitute/filler for fly ash and silica fume (microsilica). Limestone mineral filler particles are also smaller in diameter than the typical Type 1 portland cement diameter, resulting in savings through lesser cementitious material requirements.
There are many advantages to using limestone filler in place of cementitious materials. Using limestone as a replacement admixture has directly related to a large increase (up to 40 percent) in green concrete strength as compared to Type 1 mix. Using limestone also benefits the concrete mix design by helping to increase the mix fluidity and workability while operating under pumping conditions. Limestone filler reduces segregation and bleeding, which in return leads to improved particle packing. Another superior attribute this mineral filler offers due to its fine-grain particles is high early strength. Early age strength development in precast concrete applications is critical, so any percentage of cement replaced by limestone mineral filler is seen to be advantageous for performance and unit cost considerations.
Corrosion of steel reinforcement becomes an immense issue when dealing with sanitary precast concrete manholes. Using limestone as filler helps increase resistance to sodium chloride (salt) and helps prevent corrosion in steel-reinforced concrete. Limestone mineral filler helps prevent corrosion because of its hydrophobic calcites, which improve the anticorrosion and abstract properties of coatings.
Admixture manufacturers are continuing the search for superior concrete mix designs and technologies, which have produced promising results in the precast concrete industry.
Yet another innovative material impacting sanitary precast manhole design is a high-performing silica fume/microsilica. This admixture can be described as an improved microsilica admixture. One must have a general understanding of what typical microsilica is and how it behaves to see how this new technology is helping to make advancements in concrete sanitary precast concrete manhole technology.
Microsilica has been described by some as the most important new concrete technology product of the ’90s. Now imagine this technology with advancements and minor adjustments made to the microsilica incorporated into today’s sanitary precast concrete manhole products. The outcome could produce superior concrete products at a reduced cost.
Microsilica is the result of the reduction of high-purity quartz with coke in electric arc furnaces during the production of silicon and ferrosilicon alloys. Microsilica is made up of fine vitreous ( having extremely low or no porosity) particles with a surface area averaging 215,280 square feet per pound (20,000 square meters per kilogram). To find this average surface area, microsilica is measured by nitrogen absorption techniques with particles approximately 100 times smaller than the average cement particle.
Microsilica is a highly effective pozzolanic material in nature. This is because microsilica is extremely fine grained and high in silica content. Microsilica used as an admixture can create significant improvements in portland cement concretes, ranging from increased compressive strength and improved abrasion resistance to increased bond strengths. These improvements in the concrete properties can be attributed to both mechanical and pozzolanic reactions that occur when microsilica is added to the cement mix. The mechanical improvements occur when the actual fine powder is added to the cement mix, causing a reaction. The pozzolanic effect occurs when microsilica reacts with the free calcium hydroxide in the cement paste.
Sanitary precast concrete manholes need superior mix design since they come into contact with aggressive groundwater/wastewater conditions. The addition of microsilica can reduce the permeability of concrete chloride ions, which in turn will protect the reinforcing steel in the concrete from corrosion. Bridge construction, marine structures, parking structures, water supply and sewage facilities all benefit from the use of microsilica as an admixture. The special properties of microsilica produce benefits in fresh concrete properties, enhance rheology ( friction between liquids) and improve concrete pumping and stability of the concrete mix. High-strength concrete, lightweight concrete, shotcrete and low-permeability concrete exhibit superior qualities using microsilica.
A new type of microsilica is a highly reactive, very fine amorphous silica, which falls into the microsilica family of products. This new microsilica is processed from a natural white silica deposit found in New Zealand.
When added to precast and ready-mixed concrete, microsilica produces high-performance, high-strength concrete with an improved life span and structural economics. Other advantages include v ery low chloride ion intrusion and high electrical resistivity, which means that harmful chloride ions can not disperse or penetrate the concrete, preventing corrosion; increased compressive strength; high early strengths (assuring increased efficiency and greater cost effectiveness in production of prestressed and precast concrete); reduced water permeability (barring the ingress of moisture, chemicals and other contaminants); improved sulphate resistance; improved abrasion resistance, improved resistance to chemical attack; improved stability in geothermal environments; and reduction in efflorescence.
How to choose?
Numerous mix design options are available for sanitary precast concrete products. So how does the manufacturer determine which admixture, if any, to use in the design of the product? The solution is not as straightforward as one may think. Not all admixtures are economical to use on a particular project, and the chemistry of concrete admixtures is an intricate topic requiring in-depth understanding and experience. A general understanding of the options available for concrete admixtures is necessary to attain the right choice for the project, based on climatic conditions and job specifications. When it comes to choosing an appropriate admixture for any specific job, only an experienced specialist should be considered for making the key decisions. As much as admixtures can do for your concrete mix design and resulting product, alternatives to the use of admixtures should always be considered.
Some admixtures have been in production for a long time. Other admixtures are recent innovative technologies that represent a new expanding field. Regardless of the advancements in concrete mix design technology, admixtures do not compensate for bad practice and poor-quality materials. As technology continues to push forward, manufacturers of precast sanitary products need to determine which options work best for them, keeping in mind the standards and practices that make precast concrete a superior product.