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.