Recycled aggregate changes the data behind the batch
Recycled aggregate concrete needs source and moisture data that describe the material actually being batched. Recycled concrete aggregate, or RCA, can retain old mortar around its stone particles. Its absorption, density and grading can therefore differ from the aggregate it replaces. The useful question is how the approved mixture accounts for those differences, rather than how much extra cement should be added by default.
Keep the intended use explicit. An aggregate accepted as unbound road base has not thereby been approved for new concrete. A pavement mixture, a structural member and a small nonstructural product can have different acceptance requirements. Ask the producer or responsible designer to identify the applicable specification, approved source and replacement basis before changing an order or accepting a substitution.
Use the Cement Calculator only after the approved dosage is available. It estimates cement as an ingredient from concrete volume and a specified mass per cubic meter. A recycled-aggregate percentage is not a cement dosage, and a cement-bag result does not approve the mixture. Keep the laboratory report and mix identifier with the quantity worksheet so the arithmetic remains tied to the accepted material.
Separate source quality from a recycled-content label
FHWA’s 2007 Technical Advisory describes processing that removes steel, soil and other contaminants, and calls for evaluation of the original pavement’s distress. It also warns that RCA generally has lower density and higher absorption than the original aggregate. That guidance concerns recycled pavement used in new hydraulic-cement concrete pavement; it is not a universal specification for every demolition waste stream.
Ask for the source description, processing record, grading and relevant test reports. A delivery ticket saying ‘recycled’ does not distinguish clean crushed concrete from a mixed construction-waste product. Keep lot or stockpile identification with the report. If the supplier changes the source during a project, obtain the required acceptance before assuming the earlier data still describes the material.
For a proposed substitution, document who can approve it and which tests must be reviewed. Do not equate an ASTM C33 reference on a sales sheet with complete approval for the project’s exposure or design. Confirm the specification and edition named in the actual contract. For UK or European work, obtain the relevant national and project requirements instead of transferring a US pavement document as a local rule.
Absorption capacity and current moisture answer different questions
ACI explains free water by comparing wet-saturated aggregate mass with saturated-surface-dry mass. Its guidance identifies drying tests and calibrated moisture monitoring as ways to support water adjustment. The saturated-surface-dry, or SSD, condition has filled internal pores without free surface water. Record the test method, sample date and percentage basis, rather than using the word ‘wet’ as a numerical input.
Use a clearly labeled hypothetical laboratory ledger to see the distinction. Assume an oven-dry aggregate mass of 1,000 kg, absorption of 5% of oven-dry mass and current total moisture of 7% on that same basis. SSD mass is 1,050 kg; current wet mass is 1,070 kg. The difference above SSD is 20 kg, representing surface water in this simplified accounting example.
If the same hypothetical material instead has 2% current moisture, its wet mass is 1,020 kg and its difference below SSD is 30 kg. This is not a direction to pour 30 liters into a mixer. Absorption timing, sampling, preconditioning and the approved batching method affect the actual adjustment. The producer must translate the measured data into its accepted water-control procedure.
| Hypothetical condition | Mass for 1,000 kg oven-dry solids | Meaning of difference |
|---|---|---|
| Oven dry | 1,000 kg | Common reference for the assumed percentages |
| SSD at 5% absorption | 1,050 kg | Internal water included, no free surface water |
| Current moisture 7% | 1,070 kg | 20 kg above the assumed SSD mass |
| Current moisture 2% | 1,020 kg | 30 kg below the assumed SSD mass |
These are invented inputs for transparent arithmetic, not measured absorption data for a supplied product. Request the actual test report before using a value in production. A percentage expressed on wet mass is a different basis from a percentage on oven-dry mass. Copying the number without its denominator can reverse or misstate the adjustment even when the subtraction looks correct.
Keep workability evidence inside the approved mixture process
FHWA’s February 2022 tech brief explains that retained mortar increases absorption and that RCA mixtures can have greater water demand and workability loss. It discusses source characterization, stockpile moisture control and mixture development, and warns against uncontrolled retempering that exceeds the intended water ratio. Those findings support a measured production plan, not a blanket water-addition percentage for every RCA delivery.
The same brief reports more than 100 US pavement projects using full or partial RCA replacement over past decades. That is historical application evidence, not proof that any particular recycled stockpile is suitable or a forecast of current market adoption. Record the actual trial mixture, test ages, exposure requirements and acceptance results for the project being supplied.
If a delivered batch behaves differently, retain its batch ticket, aggregate source, moisture reading, sampling time and any authorized adjustment. Ask the producer to investigate against the approved mixture. A cement takeoff cannot diagnose slump loss, establish air content or replace strength and durability testing. Keep a request for further testing distinct from a decision to accept material already placed.
Calculate cement from the accepted binder basis
For a hypothetical approved quantity example, suppose finished concrete volume is 2.0 m³ and total cementitious dosage is 320 kg/m³. Total binder is 640 kg. If the approved binder contains 25% non-cement cementitious material by mass, Portland cement is 640 × 0.75 = 480 kg. Those assumptions demonstrate ingredient arithmetic; they do not specify a recommended RCA mixture.
Enter that basis in the Cement Calculator, selecting total binder dosage and the approved non-cement share. With hypothetical 50 kg cement bags, 480 ÷ 50 = 9.6, requiring ten whole bags to purchase before any separately justified reserve. Buying 500 kg does not authorize increasing the cement in the batch from its approved 480 kg. Purchase rounding and mixture proportions are different decisions.
If the dosage already describes Portland cement alone, do not subtract the non-cement share again. Keep the units kg/m³ and the final-yield volume explicit. A dry ingredient volume ratio cannot be substituted for an approved mass dosage. When buying ready-mixed concrete, the supplier’s accepted mix and ordered concrete volume remain the principal procurement records; bag arithmetic does not replace them.
Reconcile the delivery and change record before the next order
Keep recycled aggregate, binder and water records connected by the same mix identifier. A useful handoff tells the purchaser which source and proportions were accepted, tells the producer which moisture data applies, and tells the site which batch was delivered. Retain unsuccessful trials and rejected substitutions so an earlier problem is not repeated under a new order number.
- Obtain the permitted aggregate source and application.
- Retain actual grading, absorption and moisture reports with their test basis.
- Record the approved mixture, trial results and acceptance authority.
- Separate aggregate water accounting from cement purchase rounding.
- Review a source or process change before reusing the previous worksheet.
Use the Cement Calculator to reconcile the approved ingredient quantity after the mixture decision is settled. Compare quotes for equivalent accepted concrete, including testing, processing, delivery and quality-control responsibilities. A lower aggregate unit price alone does not establish a lower installed cost, and recycled content alone does not quantify a project’s environmental saving.
