A decision to use lightweight aggregates rather than normal-weight aggregates is not as difficult as one might imagine as long as there are enough facts on which to base the decision. There are many factors to consider when evaluating lightweight structural concrete for a precast project. Important considerations might include many of the following questions in order to make an informed business decision:
• How much weight reduction can be expected?
• Are there structural concerns?
• Will coarse lightweight aggregate cost more money?
• Will significant adjustments to batching and mixing procedures need to be made?
• How will using lightweight coarse aggregate affect current quality control practices?
Expected weight reduction
Aggregates in a normal-weight concrete mix account for 65 to 80% of the weight in a yard of concrete (using 145 lb/ft3 [2,320 kg/m3] as a baseline). If choosing a standard, readily available lightweight coarse aggregate replacement such as expanded shales, clays, slates and slag, one can expect to reduce the weight of a yard of concrete to approximately 95 lb/ft3 to 115 lb/ft3 (1,520 kg/m3 to 1,840 kg/m3) or a maximum potential reduction of approximately 35% by dry weight.
Advances in today’s concrete technology provide producers with many options for concrete mix designs that can overcome almost any perceived challenge relating to strength and durability. Precast producers should not be overly concerned that using lightweight aggregates will have significant structural issues – they can depend on suppliers of these products for assistance. However, there is no substitute for a complete engineering and structural analysis when facing these considerations; validation by an engineering professional is not optional for the decision process. Suppliers of these products can also provide assistance regarding any producer concerns.
One of the biggest advantages of using lightweight aggregates is the weight savings realized in the products themselves. Product weight reductions can save on shipping costs to the producer as fewer, more cost-efficient loads of product are shipped to the job site. Products receive a structural benefit as well by reducing the dead loads that the precast structure must support. Weight savings in the cast products can lead to reduced handling costs at the job site and in the plant by reducing crane capacity for lifting and setting the products.
Table 1 on the facing page shows a comparison of the shipping costs for two projects. The comparison shows that the savings in transportation costs well outweigh the additional expense of the lightweight aggregate. These savings were realized because additional product could be loaded on each truckload and, consequently, fewer hipments were needed. These examples show that saving money is possible. However, each precast concrete producer must analyze variables that would ultimately affect the overall production and shipping costs at his or her own plant.
Batching and mixing procedures
Using lightweight aggregates can present some unique production-related challenges compared with batching and mixing normal-weight aggregates. Plants making the decision to use the lightweight aggregates need to prepare for changes to their batching processes. Some lightweight aggregates can be highly absorptive; therefore, the water demand for the batching process may need some experimentation to determine if pre-wetting these aggregates is necessary to produce a consistent and sufficiently workable batch with the desired plastic and placing characteristics. Mix times may need to be extended to allow sufficient time to properly mix the batch constituents. The gradation and condition of the aggregates may create unique mixing challenges compared with conventional, normal-weight aggregate batching. Aggregate porosity and angularity will require the producer to carefully consider the appropriate gradations necessary to get the mix to perform and place as desired. While the producer may obtain valuable information on proportioning from ACI 211.2, “Standard Practice for Selecting Proportions for Structural Lightweight Concrete,” some suppliers will provide assistance and sample mixes as starting points.
Producers considering lightweight aggregates will also need to consider that current QC processes and testing parameters may need to be changed to accommodate this material. As an example, the air content of the mix cannot be obtained using conventional pressure meter methods. Plant QC personnel will need to follow ASTM C173, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method,” as opposed to ASTM C231, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method,” to evaluate the air content. The pressure method will be replaced by the volumetric method that uses a device commonly called a roll-a-meter. In addition, lightweight aggregates must meet the requirements of ASTM C330, “Standard Specification for Lightweight Aggregates for Structural Concrete.” Gradations and unit weights must be performed initially and then every 200 yd3 (150 m3) or once per month, whichever occurs first.
In the end, you will need to consider all the variables and make an informed business decision whether to use lightweight aggregates in your mix designs.
Phillip Cutler, P.E., is NPCA’s director of Technical Services.
Ali Akber says
How can I compare lightweight aggregates with normal weight aggregates?
Is there any code for lightweight aggregates? Where can I get those (if any)?
What are some of the demerits of lightweight aggregates over normal weight aggregates?
Claude Goguen says
Lightweight aggregates can weigh anywhere from 35 to 70 lbs/ft3 . Compare that to normal weight aggregates which can weigh from 75 to 110 lbs/ft3.
Codes and Standards for lightweight aggregates include ACI 213R-03: Guide for Structural Lightweight-Aggregate Concrete, and ASTM C330 / C330M – 09 Standard Specification for Lightweight Aggregates for Structural Concrete
The use of lightweight aggregates can be very beneficial and cost effective. However, when producing concrete using lightweight aggregates, there are several things you should be aware of. Fluctuations in the gradation, specific gravity, absorption, and moisture content are of particular concern. This means that additional quality control testing is necessary.
Air content testing cannot be performed by the pressure method with air meters typically used for normal concrete. Instead, air content testing must be performed according to ASTM C173 Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method.
The use of lightweight aggregates may have an impact on the compressive strength of the concrete.
Highly absorptive lightweight aggregate should be wetted at least 24 hours prior to use. It is not recommended that dry lightweight aggregate be directly batched and mixed because the aggregate particles can continue to absorb water from the mix. This can cause the mix to segregate or stiffen before it can be placed.
It may be necessary to extend mix times for lightweight concrete compared to conventional concrete to ensure that all of the mix constituents are properly mixed. Greater variations in workability should be expected, compared to conventional concrete with the same slump. Along those same lines, the amount of air-entraining admixture necessary to produce a constant amount of air content could also vary widely.
Depending on the porosity and the degree of the aggregate angularity, the concrete could be more difficult to place and finish. In some cases it is possible for the aggregate and the other mix constituents to separate, allowing the lightweight aggregate particles to float toward the concrete surface.
Finally, lightweight concretes may have increased tendency to experience drying shrinkage and creep (strain increase over long periods of time with a constant load). Steam curing effectively reduces the likelihood of both drying shrinkage and creep.
Ali Akber says
Thank you Mr.Claude Goguen for your generous reply and i hope this will help me a lot in my work.
Will post further comments after studying about the subject in a greater detail.
would highly appreciate if you can co ordinate with me on e mail too.
Looking forward to a positive reply .
Sir, is thiere any code available for structural light weight aggregate concrete mix design to achive target Strength..? Please answer me…
Neil D'Costa says
Light weight aggregate is really great product in construction. I really recommend it.