英文期刊文章

Fast and Green

Cement for rapid setting and strength-gaining applications is also

environmentally friendly

By leo KAhl

C

oncrete contractors sometimes refuse to substitute industrial byproducts for a portion of the portland cement in a mixture over fears that they will increase construction times by retarding setting and strength gain. CeraTech, Inc., has developed a line of products that not only contain between 30 and 95% industrial waste stream materials but also speed up setting times and early strength gain. One of the products, Great-White TM , is specially formulated for use with volumetric, mobile concrete mixers. The use of a mobile mixer allows concrete to be placed almost immediately after mixing so workers have the full 20 to 30 minutes to place,

consolidate, and finish the concrete before the material reaches initial set.

Durable anD sustainable

Minimizing the extraction of raw materials and extending the lives of existing structures are critical components of sustainability. Because of the flexible base chemistry used to make its cements, CeraTech is able to use locally available byproducts including fly ash, wood ash, crushed glass, and recycled concrete to produce its products. The raw materials used in its manufacture result in concrete that has a tan or brown color while fresh and that later fades to tan or gray.

GreatWhite contains 88% byproducts, but allows the return of repaired or replaced areas, such as roads, bridges, runways, or parking garages, to service in as little as 2 to 3 hours. The product has also been shown to reduce alkali-silica reaction, in some cases allowing the

use of local aggregates rather than having to transport acceptable aggregates from great distances.

The amount of water added to the concrete is mainly used to control workability. A proprietary activator solution is used to start the strength development process. CeraTech has also developed a modifier that works with the activator to accommodate a broad range of ambient and application temperatures.

A minimum section thickness of 3 in. (75 mm) is suggested when 1 in. (25 mm) coarse aggregate is used. This can be reduced to 1.5 in. (38 mm) when using 1/2 in. (13 mm) coarse aggregate. Concrete containing GreatWhite is proportioned similar to that of portland cement-based concrete mixtures. By varying the cement content, strengths from 1400 to 4500 psi (9.6 to 31 MPa) can be achieved in 2 hours. This gives volumetric concrete producers the opportunity to provide solutions across a broad spectrum of applications, from sidewalk replacement for store fronts on Main Street to slab replacements for runways and freeways.

A typical mixture containing the cement includes 1.5 to 2 parts fine aggregate and 2 to 3 parts 1 in. (25 mm)

nominal maximum size coarse aggregate for each part of GreatWhite cement by weight. The water content varies depending on the application and the amount of early strength required. Mixtures with proportions similar to these produce compressive strengths around 2800 psi (19 MPa) in 2 hours and 9800 psi (68 MPa) at 28 days. Flexural strength also develops rapidly, with values typically over 350 psi (2.4 MPa) in 2 hours and 980 psi (6.8 MPa) at 28 days. These mixtures are also very

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The deteriorated section of U.S. 101 in Gilroy, CA, shown in this photo was selected to be replaced using concrete containing GreatWhite TM starting at 8:00 p.m. so the 12 x 15 ft (3.7 x 4.6 m) section of 9 to cement. Traffic was diverted to the adjacent lane 11 in. (230 to 280 mm) thick concrete pavement could be sawcut into manageable chunks that were removed with a backhoe

Placement of the concrete began at 1:20 a.m. The mixer was calibrated off site prior to the placement. This is standard practice before the equipment is dispatched, and Caltrans has specific practices for calibrating volumetric equipment before any project is started. Because of the desire to gain significant concrete strength before the lane was reopened to traffic, the water content of the concrete was tightly controlled. Several

internal vibrators were used to ensure proper consolidation

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By 1:00 a.m., the existing pavement had been removed so the base could be prepared. Workers were installing a polyethylene vapor barrier to mitigate water intrusion from the base and act as a bond breaker between the adjoining slabs. At this time, the volumetric mixer containing the separated ingredients was

brought to the site. For this application, additional activator was brought in seperate containers to the site to top off the small

tank on the mixer

At 1:35 a.m., a vibrating screed was used to quickly strike off the concrete. Because the concrete reaches initial set in only 20 to 30 minutes, this process was started as quickly as possible—even before the entire panel of concrete was in place. Ambient temperature during the placement was 55 to 60 °F (13 to 16 °C)

By 1:45 a.m., concrete placement was complete and finishing operations were well underway. Although the fast setting time of the cement means crews must be ready to work quickly once placement begins, the use of the volumetric mixer adds a great degree of flexibility in the start time for the placement. There is no need to worry about the concrete arriving before the site has been prepared and also no waiting around for a concrete truck if

work goes smoother and faster than expected

durable, producing no mass loss after 50 cycles of the ASTM C672 scaling resistance test and retaining 100% of their dynamic modulus of elasticity after 300 cycles of the ASTM C666 rapid freezing-and-thawing resistance test. Other desirable properties for a repair material include a coefficient of thermal expansion complementary to the host concrete being repaired (typically between 4.5 and 5.5 x 10-6 in./in./°F [8.5 x 10–6 m/m/°C]) and a modulus of

elasticity of 5400 ksi (37.2 GPa). At a concrete temperature of 72 °F (22 °C), initial set is reached 20 to 30 minutes after mixing, with final set occurring 30 to 40 minutes after mixing. The material, however, can be applied on surfaces between 30 and 120 °F (–1 and 49 °C). It is self-curing, eliminating the need for curing compounds or polyethylene sheets under most ambient conditions, but it should be covered with curing blankets if ambient or surface temperatures dip below 32 °F (0 °C) or if exposed to other extreme conditions.

example appliCation

In September 2008, the California Department of

Transportation (Caltrans) used concrete produced with GreatWhite cement to replace a deteriorated section of U.S. 101 Southbound in Gilroy, CA. The cement was loaded in the volumetric mixer from 2200 lb (1000 kg)

The final finishing step was to use a grooving tool on the surface of the patch. This step was completed at 2:00 a.m., only 40 minutes after the start of placing concrete. Trial batches performed prior to the repair were used to establish the

concrete’s strength as a function of time. The repaired section of road was opened to traffic by 6:00 a.m., in time for the morning rush hour

Super Sacks, was transported to the site, and then mixed and dispensed into the repair section. The cement is

available in 75 lb (34 kg) waterproof bags for smaller uses or in bulk quantities for larger applications.

The accompanying series of photos and captions

illustrates the speed with which repairs such as these can be accomplished using this material.

Selected for reader interest by the editors.

—CeraTech, Inc.

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Leo Kahl is Vice president of Marketing for CeraTech, inc. he serves as the technical communications interface for the company and has over 30 years of industrial and consumer marketing and product develop-ment experience in industries ranging from defense systems, automotive, telecom,

manufacturing, and construction materials.

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