Rapid Set® Latex Modified Concrete (RSLMC) is a quick-setting concrete that is used primarily for bridge deck overlays. The latex improves an already very low permeability to protect the steel in the bridges. The rapid set cement in the concrete allows the bridge to return to normal surface and traffic use within 4 hours of final finishing.
Mixing Equipment – Continuous Cement Mixers, Volumetric Concrete Mixers or Cement Batch Plants
We use our custom designed cement batch plants when working with Rapid Set Latex Modified Concrete. View more information about our volumetric concrete or cement batch plant mixers.
The following is detailed information regarding Rapid Set Latex Modified Concrete:
What is Rapid Set Latex Modified Concrete?
- Fast Setting — Rapid Set Latex Modified Concrete sets in about one hour and is ready for traffic in 4 hours.
- Low Permeability — Rapid Set Latex Modified Concrete is nearly impervious to the compounds that deteriorate standard concrete.
- Low Shrinkage — The low or “non” shrink characteristic of Rapid Set Latex Modified Concrete results in better bonding and less cracking.
Materials to be used with Rapid Set Latex Modified Cement
- Rapid Set Cement is manufactured by CTS Cement Manufacturing Company in Cypress, California.
- Sand should be clean and conform to the requirements of ASTM C 33 for concrete sand.
- Coarse aggregate should be clean, sound, crushed stone or gravel meeting the general requirements of AASHTO M 80. The maximum size particle should not be larger than 1/2 the depth of the section to be placed.
- Latex emulsion should be Dow Modifier A as manufactured by Dow Chemical Company. Modifier A is a styrene butadiene polymeric emulsion. Stabilizers and an anti-foam agent have been added at the point of manufacture. Emulsion should weigh approximately 8.5-lb per gallon.
- Water used in the production of the Latex Modified Concrete should be clean and free from: salt, acid, oil, organic matter, or other substance injurious to the finished product.
- Set Control™ or Citric Acid (Food grade) can be used as a retarder to lengthen working time. Food grade citric acid should be dissolved into a solution with water. Solution should be added to the admixture tank, not containing latex emulsion, and dispensed as an admixture.
Surface Preparation when using Rapid Set Latex Modified Concrete
- Surface should be sound, clean, and free of any laitance that will be detrimental to achieving proper bond. Laitance is the top layer of concrete, consisting of cement, water and fine aggregates, and it may vary in thickness from 0.01″ to 0.1″ depending on several factors such as overworking the placed concrete or by excessive vibrating. This is generally done with mechanized equipment such as scarifiers and shot blasters or with high pressure water from hydro-demolition equipment.
- Surface preparation should be done within 24 hours to placing Rapid Set Latex Modified Concrete.
- Immediately prior to placement of Rapid Set Latex Modified Concrete the clean surface should be thoroughly wetted for a period of not less than 1 hour.
Accurate proportioning and thorough mixing are crucial requirements of a mixer of Rapid Set Latex Modified Concrete. The following guidelines are suggested:
- The capacity of the mixer and bulk materials handling system should be such that a minimum of 6 cubic yards per hour can be accurately proportioned, mixed, placed, and finished properly.
- A cement meter, with ticket printout is a desirable feature to assure uniformity of composition.
- All ingredients should be accurately and positively proportioned by weight with an approved scale.
- Mixing should be done with equipment that will not be aggravated by residual buildup of concrete. Ready-mix trucks are not recommended.
- Mixed material should be discharged on the deck in the sufficient time for the finishing operations to be completed. The working time of Rapid Set Latex Modified Concrete is largely dependent on temperature and as with regular Portland Latex Modified Concrete it can be as short as 10 minutes in hot, windy, dry weather.
Placing Rapid Set Latex Modified Concrete
Rapid Set Latex Modified Concrete or slurry should be brushed into the moist substrate just ahead of the pour. Care should be exercised to cover all vertical surfaces as well as horizontal surfaces with an even coating. Stones that accumulate from the brushing operation should be discarded.
Finishing Equipment used for Rapid Set Latex Modified Concrete
Since the Throop Company specializes in using Rapid Set Latex Modified Concrete, usually the general contractor doesn’t know the intricacies of working with this specialized cement. Therefore, the following is a list of tools and finishing equipment needed for most concrete placing projects:
- Shovels and brooms are used for placing and brushing-in freshly mixed modified concrete and for distributing it to approximate correct level. Hand operated vibrators and screeds should be used to place and finish small areas.
- A self propelled finishing machine, capable of forward and reverse movement should be used for finishing large areas of work.
- A suitable, portable, lightweight work bridge should be used behind the finishing operation for any brooming or tinning.
The Rapid Set Latex Modified Concrete should be placed as soon as possible over the brushed surface to prevent its drying out. The finishing machine should proceed over the pour as the material is placed. Deep sections should be vibrated with a spud vibrator. Edges will need to be finished by hand. Final finishing should be completed before the material has reached a final set. Water should NOT be applied to the surface to aid finishing. Working time can be extended by adding citric acid solution to an admixture tank. DO NOT ADD CITRIC ACID DIRECTLY TO DOW MODIFIER A.
Curing Rapid Set Latex Modified Concrete
The surface should be covered promptly after final finishing with a single, clean layer of wet burlap. Immediately following the covering of wet burlap, a layer of polyethylene (preferably white) film should be placed over the wet burlap. The coverings should remain until the Rapid Set Latex Modified Concrete has reached a strength desired for initial traffic loading. Depending on temperatures, and specified strength, this will usually be within 1 – 6 hours after the pour. The surface of the Rapid Set Latex Modified Concrete should remain wet for this time period. Additional water may need to be applied to the burlap during the curing, as the surface of the concrete will tend to dry out under the wet burlap.
Limitations of Rapid Set Latex Modified Concrete
- Traffic Loading: No vehicular traffic should be permitted on the Rapid Set Latex Modified Concrete until an early bearing strength has been achieved. This can be verified by making field cast test cylinders, and protecting them from adverse temperatures. An unheated foam curing box is recommended during a cold weather pour.
- Cold Weather (below 50o F) Limitations for Rapid Set Latex Modified Concrete: Rapid Set Latex Modified Concrete should not be placed if temperatures will not be above 45° F for the duration of the pour and initial curing period. Provisions will have to be made to ensure that the RSLMC has an initial temperature above 60° F. During the curing phase, the covering of polyethylene should be replaced with an approved insulation blanket. All other aspects of the curing section should be followed. Also see ACI 306, “Recommended Practice for Cold Weather Concreting” for further guidance.
- Hot Weather (above 80o) F Limitations for Rapid Set Latex Modified Concrete: During periods of hot weather, Rapid Set Latex Modified Concrete can be mixed with citric acid to extend working time. Consult your local CTS Representative for appropriate doses. When daytime conditions produce temperature in excess of 85° F, or when daytime conditions of temperature, wind, and humidity cause an evaporation rate greater than 0.15 lbs/sf/hour, consideration should be given to night placement. Also see ACI 305, “Recommended Practices for Hot Weather Concreting” for further guidance.
Set Control Admix for Rapid Set Latex Modified Concrete
The use of citric acid as a Set Control retarder when used in conjunction with Dow Modifier A Latex and Rapid Set Cement in the production of Rapid Set Latex Modified Concrete. CTS Cement Manufacturing Company and Dow Chemical Company recommend the use of citric acid (USP grade) as a Set Control Admixture in Rapid Set Latex Modified Concrete when mix temperatures rise above 70° F to increase the time for workability.
The combination of Dow Latex, Rapid Set Cement and citric acid as a Set Control retarder had been used on many projects in states that utilize Rapid Set Latex Modified Concrete. The amount of retarder needed is dependent on the Rapid Set Latex Modified Concrete mix temperature and should be adjusted according to job site conditions.