Hot-Weather Slab Pour and Water Addition
An 18 m³ suspended slab, 150 mm thick, is being site-mixed on a 34°C day. The design mix uses 350 kg of cement per m³ at a water/cement ratio of 0.50, with 700 kg sand and 1,150 kg coarse aggregate per m³. The sand has 5% moisture and the coarse aggregate 1%. At 11:00 the slump is 90 mm; by 13:00 it has fallen to 35 mm, and the crew begins adding about 20 litres of water to each 0.5 m³ batch to make placing easier. The pour is half complete, with a cold joint forming at a construction stop. No retarder or plasticiser is on site, and cube samples were taken only at the start. The client's clerk of works is not on site. You are the technician.
1. Calculate the free water in the aggregates and the correct added water per m³. Then assess the effect of the crew's extra 20 litres per 0.5 m³ batch.
Read the worked answer
Design water per m³ = 0.50 × 350 = 175 kg.
Free water in sand = 5% × 700 kg = 35 kg. Free water in coarse aggregate = 1% × 1,150 kg = 11.5 kg. Total free water in aggregates = 46.5 kg.
Correct added water = 175 − 46.5 = 128.5 kg, say 129 litres per m³.
The crew is adding 20 litres per 0.5 m³, which is 40 litres per m³.
If they have already added the uncorrected 175 litres per m³, total water = 175 + 40 + 46.5 = 261.5 kg, giving w/c = 261.5 ÷ 350 = 0.75.
If they had corrected the aggregate moisture and then added the extra 40 litres, total water = 128.5 + 40 + 46.5 = 215 kg, giving w/c = 215 ÷ 350 = 0.61.
Either case exceeds the design w/c of 0.50. At w/c 0.75 the strength loss is severe, and the risk of bleeding, shrinkage cracking and poor durability is high. The extra water should not be added.
2. What should you do about the half-complete pour and the forming cold joint?
Read the worked answer
Stop the uncontrolled water addition. The half-complete slab should be treated as a deliberate construction joint, not as a cold joint to be covered up.
Actions:
- stop placing at a defined line and compact the concrete already placed;
- remove laitance and loose aggregate at the joint, roughen the surface, and keep it damp;
- when work resumes, use an approved bonding method and a mix of the specified workability, without retempering old concrete;
- record the location of the joint and the time of the stoppage;
- take additional cube samples from the later concrete so the strength of both parts can be checked;
- inform the contractor and clerk of works, even though the clerk is absent, and do not conceal the joint.
If the concrete has stiffened beyond its useful life, it should be rejected rather than revived with water.
3. List the hot-weather and quality controls you would insist on for the remainder of the slab.
Read the worked answer
Controls for the remaining pour:
- Water control: measure water by container, correct for aggregate moisture, and prohibit unofficial water addition. Use an approved plasticiser or retarder if the mix needs higher workability.
- Temperature: shade cement and aggregates, use cool water, and avoid pouring in the hottest part of the day. Consider night or early-morning placement.
- Preparation: pre-wet formwork and reinforcement, but remove standing water.
- Delivery and placing: keep the time between mixing and placing short, place in a continuous sequence, and compact properly with a poker vibrator.
- Curing: start curing as soon as the surface can be worked without damage; keep the slab continuously moist or use an approved curing compound.
- Testing: take slump and cube samples at intervals through the pour, not only at the start; record batch times, water additions and the location of construction joints.
The technician should also report the earlier water addition and the cold joint, because strength testing may show that part of the slab needs assessment.