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How Polycarboxylate Superplasticizer Dosage effect on the High Strength Concrete

High strength concrete (HSC) is widely adopted in marine structures, long-span bridges and high-rise buildings, where long-term impermeability and stable volume are critical to structural durability. Polycarboxylate superplasticizer(PCE) acts as a core functional admixture that directly alters cement dispersion, hydration kinetics and internal pore distribution. Improper PCE dosage can easily lead to excessive chloride penetration, severe self-shrinkage, or early cracking.

This laboratory research sets six gradient PCE dosages ranging from 0% to 1.5% for C70 concrete. A full set of test methods, including electric flux, seepage height ratio, non-contact shrinkage measurement, and mercury intrusion porosimetry (MIP), is used to quantify how PCE content affects impermeability, early self-shrinkage, long-term drying shrinkage, and pore microstructure. The study identifies the optimal PCE dosage to balance durability and volume stability, providing reliable mix design guidance for engineers and ready-mix producers.

Raw Materials & Test Mix Design

Raw Material Specifications

Cement: P·O 42.5 Portland cement, specific surface area 338 m²/kg, 28d compressive strength 55.3 MPa
Mineral admixtures: Class II fly ash, S95 GGBS, silica fume (SiO₂ 90.1%, SSA=15400 m²/kg)
Aggregates: Natural sand (fineness modulus 2.7, mud 1.1%), 5–20 mm continuous crushed stone (crushing value 6.9%)
PCE superplasticizer: 25.1% solid content, water reduction rate 26.6%
Tap water for mixing.

C70 Base Mix (per cubic meter)

Cement 390 kg, fly ash 90 kg, slag powder 100 kg, silica fume 40 kg, sand 690 kg, stone 960 kg, water 149 kg.
Six test groups with varying PCE dosages (by total binder mass):
PC-0 (0%), PC-0.3 (0.3%), PC-0.6 (0.6%), PC-0.9 (0.9%), PC-1.2 (1.2%), PC-1.5 (1.5%).

Test Standards & Methods

Impermeability: 28d electric flux test (chloride resistance), 7d seepage height ratio per GB/T 50082-2009
Shrinkage: Non-contact apparatus to measure 1d/3d/7d self-shrinkage; 14/28/60d drying shrinkage under 20±2 ℃, RH 60±5%
Microstructure: Mercury intrusion porosimetry (MIP) to test total porosity, average pore size and pore size distribution at 28d

Test Results & Mechanism Analysis

Effect of PCE Dosage on Concrete Impermeability

Group	PCE Dosage	28d Electric Flux ©	7d Seepage Height Ratio (%)
PC-0	0%	994	100
PC-0.3	0.3%	711	75
PC-0.6	0.6%	652	58
PC-0.9	0.9%	620	42
PC-1.2	1.2%	648	49
PC-1.5	1.5%	680	57

Impermeability improves continuously as PCE dosage rises from 0% to 0.9%. Without a superplasticizer, cement-aggregate mixtures severely aggregate, forming massive interconnected capillary pores that accelerate water and chloride-ion penetration. At a 0.9% dosage, the electric flux drops to 620 C, and the seepage ratio reaches 42%. The electrostatic repulsion and steric hindrance of PCE fully disperse cement flocs, cut the effective water-binder ratio and refine pore structures.

Once the dosage exceeds 0.9%, performance reverses. Excess PCE causes bleeding and segregation, damaging the aggregate-paste interfacial transition zone (ITZ) and generating new connected permeable channels. The delayed hydration effect also reduces early hydration product filling capacity, leading to higher electric flux and seepage height.

PCE Dosage Regulates Self & Drying Shrinkage

Shrinkage data (unit: ×10⁻⁶)

Group	1d Self	3d Self	7d Self	14d Dry	28d Dry	60d Dry
PC-0	280	390	450	480	520	550
PC-0.3	240	330	380	500	540	580
PC-0.6	210	290	340	510	560	610
PC-0.9	190	260	320	520	570	630
PC-1.2	220	290	350	540	590	650
PC-1.5	250	330	380	560	620	680

Early self-shrinkage (1–7d): Shows a trend of decreasing first, then increasing. The 0.9% group reaches the minimum 7d self-shrinkage of 320×10⁻⁶, a 33.3% reduction compared with the blank sample. Moderate PCE slows rapid early cement hydration, lowers capillary tension and mitigates volume contraction. Overdosage triggers bleeding and weak ITZ adhesion, leading to a sharp rebound in self-shrinkage.
Long-term drying shrinkage (14–60%): Gradually rises with higher PCE dosage. Finer pores generated by PCE increase capillary negative pressure during water evaporation, leading to larger long-term shrinkage deformation. The 0.9% group achieves a balanced trade-off: obvious early shrinkage suppression with controllable late drying shrinkage.

Pore Structure as the Core Intermediate Mechanism

Pore structure test results (MIP)

Group	Total Porosity	Avg Pore Size(nm)	<20nm harmless	20–50nm less harmful	>50nm harmful
PC-0	18.6%	58	32.5%	28.7%	38.8%
PC-0.3	16.2%	49	38.6%	32.4%	29.0%
PC-0.6	14.5%	41	45.2%	35.8%	19.0%
PC-0.9	12.3%	32	52.8%	37.5%	9.7%
PC-1.2	13.1%	36	48.6%	35.1%	16.3%
PC-1.5	14.5%	43	42.3%	32.7%	25.0

Pore structure directly controls concrete impermeability and shrinkage. Pores larger than 50 nm are classified as harmful permeable channels. At optimal 0.9% PCE dosage:

Total porosity minimized to 12.3%
Average pore size refined down to 32 nm.
Harmless micropores (<20 nm) account for over half of the total pores (52.8%)
Harmful macro-pores only occupy 9.7%

Excessive superplasticizer destroys this optimized pore grading, increases total porosity and the proportion of large, harmful pores, thereby simultaneously worsening anti-chloride performance and amplifying shrinkage deformation.

Core Research Conclusions

PCE dosage has a nonlinear effect on the impermeability of C70 concrete. Performance improves continuously within 0%–0.9% dosage and deteriorates above 0.9%. The optimal dosage is 0.9% of total binder mass, with an electric flux of 620 C and a 7d seepage height ratio of only 42%.
0.9% PCE delivers outstanding early self-shrinkage inhibition (7d shrinkage reduced by 33.3%), though long-term drying shrinkage slightly increases. This dosage realizes the best balance of volume stability for high-strength concrete.
The pore microstructure acts as an intermediate linking PCE dosage and macroscopic durability. 0.9% PCE minimizes total porosity, refines average pore size and drastically cuts harmful pore content, forming a dense hydration matrix to block penetration paths.
Overdosing PCE (>0.9%) causes concrete bleeding, segregation and ITZ damage, leading to coarser pores, higher chloride permeability and aggravated shrinkage cracking risks.

Engineering Application Guidance

C70 and similar high-strength structural concrete: Fix PCE dosage at approximately 0.9% to balance anti-permeability and shrinkage control.
Marine/chloride-exposed infrastructure: Strictly avoid PCE dosages above 1.0% to reduce the risk of increased ion penetration.
Precast components requiring early demolding: Adopt the 0.9% optimal formula to reduce early shrinkage cracking defects.
Quality control tips: Conduct small trial mixes before mass batching; adjust PCE content if raw material fineness or mineral admixture proportion changes.

Conclusion-How Polycarboxylate Superplasticizer Dosage effect on the High Strength Concrete

Polycarboxylate superplasticizer dosage is a decisive factor governing the impermeability and shrinkage performance of C70 high-strength concrete. The 0.9 dosage threshold divides performance trends: below 0.9%, PCE disperses cement, optimizes pore grading and enhances durability; above 0.9%, segregation and delayed hydration degrade pore structure and structural stability.

For bridges, marine engineering and high-rise HSC projects, controlling PCE admixture at 0.9% effectively lowers chloride penetration risk and suppresses early shrinkage cracking, greatly improving the long-term service life of concrete structures.