Structural Lightweight Concrete
Structural lightweight concrete made with rotary kiln produced structural lightweight aggregate solves weight and durability problems in buildings and exposed structures. Structural lightweight concrete achieves strengths comparable to normal weight concrete, while being 20% to 30% lower in density.
Structural lightweight concrete offers design flexibility and substantial cost savings by providing reduced dead load, reduced seismic loading, longer spans, better fire ratings, thinner sections, decreased story height, smaller size structural members, less reinforcing steel, and lower foundations costs. Structural lightweight concrete precast elements have reduced trucking and placement costs. The excellent durability performance of structural lightweight concrete made with expanded shale, clay or slate (ESCS) structural lightweight aggregate is a result of the ceramic nature of the aggregate, and its exceptional bond to and elastic compatibility with the cementitious matrix.
Structural lightweight aggregate's cellular structure provides internal curing through water entrainment which is especially beneficial for high-performance concrete, (HPC). Internal curing improves the aggregate/paste contact zone which mitigates micro cracking.
Compared with ordinary concrete, lightweight concrete using ESCS lightweight aggregate has better thermal properties, higher fire ratings, reduced autogenous shrinkage, improved contact zone between aggregate and cement matrix, less micro-cracking as a result of better elastic compatibility, and better shock and sound absorption, high-performance lightweight aggregate concrete also has less cracking and improved skid resistance and is readily placed by pumping.
Uses of Structural Lightweight Concrete
- Floors in steel frame buildings (lightweight concrete on fire-rated steel deck assemblies)
- Concrete frame buildings & parking structures (all types, including post-tensioned floor systems)
- Bridge decks, piers & AASHTO girders
- Specified density concrete (concrete between 120 pcf and 135 pcf)
- Lightweight precast & prestressed concrete elements (beams, double-tees, tilt-up walls, raised access floor panel planks, hog slats, utility vaults, pipes, ornamentals, etc.)
- Marine structures, floating docks, ships, & offshore oil platforms
Lightweight Concrete Best Practices
- Mixes that are regularly used in a market area will be the most economical. Consult the lightweight aggregate suppliers for detailed mix design information and material capabilities (i.e., unit weight, strength, etc.).
- Specify 4 to 7 percent air entrainment for pumpability, workability, finishability, and durability.
- Specify the maximum size aggregate rather than specifying individual sizes.
- Allow higher slump into the pump to accommodate possible slump loss.
- Have the testing lab run design curves based on the maximum specified slump and air per ACI 301.
- Specify a pre-pump meeting with the following present: engineer, architect, contractor, ready-mix supplier, lightweight aggregate supplier, testing agency, admixture supplier, and pumping contractor.
- On large jobs, these same people should be present at the first concrete pump placement.
- Specify exactly where concrete should be tested, preferably at the end of the discharge line as per ACI 304.2R.
- Realize that absorbed water does not affect the water/cement ratio, as defined in ASTM C 125.
- Keep everyone communicating; this is a team effort!
- Use an experienced pumping contractor.
- Make arrangements for two ready-mix trucks to unload simultaneously.
- Designate a laborer to help the testing lab inspector.
- Provide a washout area for ready-mix trucks.
- Make use of the ready-mix truck radio when placement delays occur.
- Specify to the ready-mix supplier the number of yards needed per hour, not how many truckloads.
- Make an agreement with the ready-mix supplier as to how the quantity of concrete delivered will be determined.
- It is necessary to properly lubricate the pump line before placing concrete.
- Know the concrete unit weight being pumped.
- Order concrete to coincide with actual pumping time; not when the pump arrives at the job site.
- Maintain continuous placement.
- Operator should know the maximum slump allowed.
- Use 5" minimum clean steel lines; minimize rubber at the end of line; avoid reducers if possible.
- Keep the same pump and operator throughout the duration of the job.
- Use a pump whose piston size is as close as possible to the line size to maintain the best performance and least slump loss.
- The lightweight aggregate must be prewetted prior to batching using procedures recommended by the lightweight aggregate supplier.
- Check with the lightweight aggregate supplier for the recommended pump mix design and field correction procedures.
- The aggregate moisture content or unit weight should be checked every time bins are filled. This is necessary for concrete yield control.
- Make drivers aware of what admixtures are being used for slump control.
- Maintain a minimum 3" slump before the addition of “super-plasticizer.
- The field inspector shall be ACI Field Technician Grade I (or equivalent) per ASTM C 94.
- Make sure the inspector has the proper tools including a roll-a-meter for volumetric air tests and a proper strike-off plate for unit weight determination.
- On large jobs use the same inspector for all concrete placements.
- The inspector should know fresh unit weight limitations (min and max).
- Place test cylinders immediately upon casting in a curing box protected from vibration per ASTM C 31.
Lightweight Concrete Facts
Fact : Concrete pumps can be easily configured to accommodate lightweight concrete.
Fact : Independent testing and practical field experience have confirmed that structural lightweight concrete is as durable as normal weight concrete when exposed to freezing and thawing cycles.
Fact : Lightweight concrete is lightweight because of the inherently low density of the lightweight aggregate.
Fact : Modern technology has, for many years, produced lightweight concrete with similar or greater strengths than normal weight concrete.
Fact : The precast, prestressed concrete industry has been using Trinity Lightweight for structural members with great success for years.
When Costco decided to add an eighth level, and some 290 more parking spaces, to the parking structure at their headquarters in Issaquah, WA -- already designed and under construction -- lightweight aggregate from Trinity Lightweight helped provide the solution.
Cranes tower above Cobb County Georgia’s Cumberland District. Down below, SunTrust Park, the new home of the Atlanta Braves, is taking shape. The ballpark and mixed-use development is the first of its kind: a place where fans can shop, eat and even live long after the game ends.
Houston’s latest skyscraper, Market Square Tower, soars well above Preston Street. The stunning height of this residential high-rise is made possible by structural lightweight aggregate.
When constructing high-rise buildings, every floor has to be strong enough to support the loads imposed on each level, and also provide the fire resistance required by the building code.
The new Center for Science and Mathematics at California State University, better known as Cal Poly San Luis Obispo, is designed to be a distinctive landmark located in the geographic center of the campus. Under the specifications of the engineer, the concrete mix design for this new structure called for the use of HydroLite™.